The Generic CO2 Geological Storage FEP Database, Version 1.0.1
Preamble
Abstract
This is the Generic CO2 Geological Storage Database of Features, Events and Processes (FEPs). It provides a tool to support the assessment of long-term safety and performance of geological carbon dioxide (CO2) storage.
This category of FEPs determines the 'boundary conditions' for any assessment, specifying the spatial and temporal domain of the system. The Assessment Basis determines what is being assessed and why, so that those FEPs that need to be considered in the analysis can be defined, and those which can be 'screened out' as being outside the scope of the assessment can be identified.
The purpose of the assessment of geological CO2 storage.
Relevance to performance and safety:
The general purpose of an assessment of geological CO2 storage is to determine the performance of the storage system. In any specific case, however, the purpose of conducting an assessment may vary from simple calculations to test initial ideas for storage concepts, to support for an application for regulatory approval requiring detailed, site-specific performance assessment against relevant criteria. The level of complexity and comprehensiveness will vary according to the use to which it will be put. Additionally, the assessment endpoints of interest may not only vary in type, depending on the assessment purpose, but also in the level of rigour required for compliance demonstration.
The structure and composition of an assessment will tend to reflect the endpoints that are required to be assessed. These in turn, will reflect the criteria that are adopted to judge the overall performance of the storage system. Thus, for example, an assessment may be constrained to considering the degree of containment within a geological feature, alternatively, it may need to address potential near-surface impacts. Invariably, a combination of endpoints will be required.
The spatial domain of interest will be dependent on the site context, which may vary from generic assessments to site specific assessments, the storage concept and the endpoints of interest. The spatial domain will contribute to determining the information requirements and modelling capabilities that may be required.
Timescales over which the assessment will be performed will constrain processes which must be considered in the assessment. In general terms, there are two timescales of interest for geological storage of carbon dioxide. Firstly, there is that over which isolation of carbon dioxide from the atmosphere is necessary to mitigate climate change. This timescale is likely to be in the order of a few hundred years at most. The second timescale of interest is potentially much longer and is that pertaining to the assessment of potential hazard to humans and the environment. This timescale could be in the order of thousands to tens of thousands of years.
High level assumptions concerning the storage system(s) of relevance to the assessment. For example, the quantity of CO2 stored, the method of injection and information concerning the assumed performance of the storage system.
Illustration of geological storage concepts, from USGS CO2 sequestration website
Relevance to performance and safety:
Provides a background to the storage technique adopted. Note that more detailed consideration of the CO2 storage system is provided in subsequent FEP categories.
The assumptions made in the assessment concerning general boundary conditions for assessing future human action.
Relevance to performance and safety:
For example, it can be expected that human technology and society will develop over the timescales of relevance for the assessment of CO2 storage systems, however, this development is unpredictable. Therefore it may be necessary to make some assumptions in order to constrain the range of future human actions that are considered, such as assuming that only present-day technologies, or technologies practised in the past, will be considered.
The legal and regulatory framework within which the assessment takes place.
Relevance to performance and safety:
In undertaking an assessment it is vital to consider the appropriate regulatory framework requirements. At one extreme these may be specific, prescriptive quantitative requirements, at the other they could be non-prescriptive or may not have been fully developed.
The legal and regulatory framework can shape various aspects of an assessment, such as the required assessment endpoints, timescales of interest and assumptions concerning future human actions.
General methodological issues affecting the assessment modelling process and use of data
Relevance to performance and safety:
Examples of general model and data issues include the treatment of uncertainty; the method for handling site specific data; and the reduction/simplification of models and data.
This category of FEPs describes natural or human factors that are outside the system domain. These FEPs are most important in determining scenarios for the future evolution of the system, and are often referred to as EFEPs (External FEPs). Three classes of FEPs are considered. Geological Factors and Climatic Factors are concerned with natural processes and events, whilst Future Human Actions is concerned with those human activities that can directly affect the storage system.
Neotectonics is the study of crustal movements that both occurred in the Earth's recent past and are continuing at the present day. These movements, which are driven directly or indirectly by global plate motions (tectonics), result in the vertical and horizontal warping, folding or faulting of the Earth's surface.
As the oceanic and continental crust converge, the more dense oceanic crust sinks below the less dense continental crust. From Wikipedia
Relevance to performance and safety:
Neotectonic events have the potential to cause sudden changes in the physical properties of rocks due to stress changes and induced hydrogeological changes.
Magma is molten, mobile rock material, generated below and within the Earth's crust, which gives rise to igneous rocks when solidified. A volcano is a vent or fissure in the Earth's surface through which molten or part-molten materials (lava) may flow, and ash and hot gases be expelled.
Crater activity in the Pu`u `O`o volcano (Hawaii). Photograph by J. Kauahikaua courtesy of U.S. Geological Survey
Relevance to performance and safety:
The high temperatures associated with volcanic and magmatic activity may result in permanent changes in the surrounding rocks, either directly, or through circulating high temperature fluids. This FEP is relevant to CO2 storage in areas of potential magmatic activity, e.g. Japan.
Events and processes related to seismic events and also the potential for seismic events. A seismic event is caused by rapid relative movements within the Earth's crust usually along existing faults. The accompanying release of energy may result in rock movement and/or rupture, e.g. earthquakes.
From USGS website
Relevance to performance and safety:
Seismic events may result in changes in the physical properties of rocks due to stress changes and induced hydrogeological changes. Seismic events are most common in tectonically active or volcanically active regions at crustal plate margins.
Processes associated with high temperature groundwaters, and hydrothermal alteration of minerals in the rocks through which the high temperature groundwater flows. Hydrothermal activity may be directly associated with volcanic and magmatic activity.
Hot springs, geysers and submarine hydrothermal vents provide evidence of hydrothermal activity (see pictures below).
Geyser image, from USGS Yellowstone Observatory website
Relevance to performance and safety:
Can result in the hydrothermal alteration of rocks or minerals by the reaction of hot water (and other fluids) with pre-existing rocks.
Processes arising from large-scale geological changes. These processes include changes in hydrodynamic boundary conditions, the effects of changes in the physical properties of geological units (e.g., porosity, permeability etc.), and variations in potential gradients driving water flow (e.g., topographical gradients). These changes are caused by rock stress accompanied by brittle and/or plastic deformation of the rock units.
Relevance to performance and safety:
Flow of groundwater within and surrounding a CO2 storage reservoir will influence the extent to which the CO2 remains in one location. If the CO2 does migrate, then groundwater flow will influence the routes taken and the extent to which the CO2 is dissolved and diluted. Physical changes to the rock units caused by geological changes may also create or destroy potential pathways for CO2 migration. In and below low-permeability geological formations, hydrogeological conditions may evolve very slowly and often reflect past geological conditions, i.e. be in a state of disequilibrium.
Processes related to the large scale (geological) removal of rocks and sediments, with associated changes in topography and geological/hydrogeological conditions of the system.
The Grand Canyon, original image from the National Park Service website
Relevance to performance and safety:
Potential to modify the geological and hydrogeological environment.
An extraterrestrial body in the 1-10-km size range, which impacts the earth at high velocity, explodes upon impact, and creates a large crater.
Crater caused by bolide impact, from USGS website
Relevance to performance and safety:
A low probability, high consequence event that has the potential to substantially disrupt the CO2 storage system. Often screened out on the basis that the impact of the bolide will greatly exceed that of the disruption caused to the storage system.
The process of global climate change due to natural and/or anthropogenic causes. The last two million years of the Quaternary have been characterised by glacial/interglacial cycling. According to the Milankovitch Theory, the Quaternary glacial/interglacial cycles are caused by long-term changes in seasonal and latitudinal distribution of incoming solar radiation which are due to the periodic variations of the Earth's orbit about the Sun (Milankovitch cycles).
Evidence suggests that the Earth is presently in a period of global warming (see the figure below). The anthropogenic release of gases into the atmosphere may be increasing the rate of global warming by enhancing the natural 'greenhouse effect', a process by which longwave radiation emitted from the Earth is trapped in the atmosphere by 'greenhouse gases' such as CO2.
Changes in temperature, sea level and Northern Hemisphere snow cover from IPCC report Climate Change 2007: The Physical Science Basis, Summary for Policymakers (Solomon et al., 2007)
Relevance to performance and safety:
Changes in the global climate are likely to impact the CO2 storage system in a number of ways. For example, through it\'s affect on sea levels and the local and regional climate.
Processes related to the possible future changes, and evidence for past changes, of climate at a storage site. This is likely to occur in response to global climate change, but the changes will be specific to situation, and may include short term fluctuations.
Climate is characterised by a range of factors including temperature, precipitation and pressure as well as other components of the climate system such as oceans, ice and snow, biota and the land surface. The Earth's climate varies by location and for convenience broad climate types can be distinguished, e.g. tropical, savannah, Mediterranean, temperate, boreal and tundra. Climatic changes lasting only a few decades may be referred to as climatic fluctuations. These are unpredictable at the current state of knowledge although historical evidence indicates the degree of past fluctuations.
The response of regional and local scale climate to global climate change have been considered for its implication for the geological disposal of radioactive waste in the EC's BIOCLIM programme (see link below).
Areal extent of Chacaltaya Glacier, Bolivia, from 1940 to 2005 from IPCC report Climate Change 2007: Impacts, Adaptation and Vulnerability (Parry et al., 2007)
Relevance to performance and safety:
Changes in the regional and local climate could affect the CO2 storage system in a number of ways. For example, changes in groundwater recharge could affect regional hydrogeology and hence the transport of CO2 dissolved in groundwater. It may also alter the near-surface environment to which some of the stored CO2 may migrate.
Processes related to changes in sea level which may occur as a result of global (eustatic) change and/or regional geological change, e.g. isostatic movements.
The component of sea-level change involving the interchange of water between land ice and the sea is referred to as eustatic change. As ice sheets melt so the ocean volume increases and sea levels rise. Sea level at a given location will also be affected by vertical movement of the land mass, e.g. depression and rebound due to glacial loading and unloading, referred to as isostatic change.
Annual averages of the global mean sea level from IPCC report Climate Change 2007: The Physical Science Basis, Technical Summary (Solomon et al., 2007)
Relevance to performance and safety:
Sea level change may affect the storage system through its impact on the near surface environment and the regional or local hydrogeological regime.
Related to the physical processes and associated landforms in cold but ice-sheet-free environments.
Taliks image from Physical Geography.net
Relevance to performance and safety:
An important characteristic of periglacial environments is the seasonal change from winter freezing to summer thaw with large water movements and potential for erosion. Frozen sub-soils are referred to as permafrost. Meltwater from seasonal thaw is unable to percolate downwards due to permafrost and saturates the surface materials. Permafrost layers may isolate the deep hydrogeological regime from surface hydrology, or flow may be focused at \"taliks\" (localised unfrozen zones, e.g. under lakes, large rivers or at regions of groundwater discharge).
Processes related to the effects of glaciers and ice sheets within the region of a storage site. This is distinct from the effects of large ice masses on global and regional climate.
The ice sheet itself and the accompanying frozen ground (permafrost) beneath the majority of the ice sheet (the cold-based portion) may constitute a barrier to groundwater flow and to heat loss. If the basal transmissivity of the till and bedrock below the ice is low, water pressures may rise to levels equalling the ice pressure inducing the formation of major conduits in the subglacial material. The central parts of the ice sheet are likely to be warm-based and could permit groundwater recharge to take place, possible to great depths if high groundwater heads are generated at the base of the ice sheet. Discharge of groundwater is likely to take place close to and beyond the frontal parts of the ice sheet. Its location being determined, in part, by the presence, or otherwise, of permafrost and its thickness. Excessive recharge at the margin of the ice sheet could provide direct recharge of oxidising water to considerable depths in conductive fracture zones. If the permeability at and beyond the rim of the ice is low, e.g. due to permafrost, the water pressures may again build up resulting in hydrofracturing of the ice or the rock mass. As the ice sheet advances, these induced fractures may increase their aperture and depth due to freezing of subglacial meltwater.
Image of the Muir Glacier from the NASA Glacier Bay website, courtesy of NASA/Goddard Space Flight Center
Relevance to performance and safety:
Erosional processes (abrasion, overdeepening) associated with glacial action, especially advancing glaciers and ice sheets, and with glacial meltwaters beneath the ice mass and at the margins, can lead to morphological changes in the environment, e.g. U-shaped valleys, hanging valleys, fjords and drumlins. Depositional features associated with glaciers and ice sheets include moraines and eskers. The pressure of the ice mass on the landscape may result in significant hydrogeological effects and even depression of the regional crustal plate.
Processes related to warm tropical and desert climates, including seasonal effects, and meteorological and geomorphological effects special to these climates.
Multi-spectral image of Hurricane Floyd, from the National Oceanic and Atmospheric Administration website
Relevance to performance and safety:
Regions with a tropical climate may experience extreme weather patterns (monsoons, hurricanes) that could result in flooding, storm surges, high winds etc. with implications for erosion and hydrogeology. The high temperatures and humidity associated with tropical climates result in rapid biological degradation and soils are generally thin. In arid climates, total rainfall, erosion and recharge may be dominated by infrequent storm events.
Processes related to changes in hydrology and hydrogeology, e.g. recharge, sediment load and seasonality, in response to climate change in a region.
Relevance to performance and safety:
The hydrology and hydrogeology of a region is closely coupled to climate. Climate controls the amount of precipitation and evaporation, seasonal ice cover, and thus the soil water balance, extent of soil saturation, surface runoff and groundwater recharge. Vegetation and human actions may modify these responses.
Processes relating to changes in ecology and human behaviour in response to climate change.
Climate affects the abundance and availability of natural resources such as water, as well as ecology, including the types of crops that can be grown by humans. Human responses, such as changes in habits, diet and the size of communities, are closely linked to climate and ecology. The more extreme a climate, the greater the extent of human control over these resources necessary to maintain agricultural productivity.
Projected appreciable changes in terrestrial ecosystems by 2100 relative to 2000. From IPCC report Climate Change 2007: Impacts, Adaptation and Vulnerability (Parry et al., 2007)
Relevance to performance and safety:
Changes in ecology and human behaviour in response to climate change will influence the relevant FEPs to be considered in the surface environment as an assessment extends into the future.
Processes related to human activities that could affect the change of climate either globally or in a region.
Original image: IPCC website
Relevance to performance and safety:
Anthropogenic emissions of \'greenhouse\' gases such as CO2 and CH4 have been implicated as factors in global warming. One of the primary aims of CO2 storage is to reduce the quantity of CO2 that is discharged to the atmosphere.
Regionally, climate can be modified by human activities such as de-forestation.
Events and processes related to the degree of knowledge of the existence, location and/or nature of the storage site. Also, reasons for deliberate interference with, or intrusion into, a CO2 storage site after closure with complete or incomplete knowledge.
Knowledge of the storage site may be regained through post-closure airborne, geophysical or other surface-based non-intrusive investigation of a storage site. Such investigations might occur after information of the location of the storage system has been lost and therefore excludes monitoring of the storage system, but includes activities such as prospecting for geological resources. The evidence of the storage, such as injection boreholes, may itself prompt investigation.
Relevance to performance and safety:
Some future human actions could directly impact upon performance of the storage system. The following could be distinguished:
- inadvertent actions, which are actions taken without knowledge or awareness of the storage site, and
- deliberate actions, which are actions that are taken with knowledge of the storage system\'s existence and location, e.g. deliberate attempts to retrieve any hydrocarbons associated with the CO2, malicious intrusion and sabotage.
Intermediate cases, of intrusion with incomplete knowledge, could also occur.
Events and processes related to changes in social patterns and degree of local government, planning and regulation.
Potentially significant social and institutional developments include:
- changes in planning controls and environmental legislation;
- demographic change and urban development;
- changes in land use;
- loss of archives/records, loss/degradation of societal memory.
Relevance to performance and safety:
Social and institutional developments have the potential to affect motivation and knowledge issues, human use of the surface and sub-surface environments and the type of impacts that may be considered.
Events and processes related to future developments in human technology and changes in the capacity and motivation to implement technologies. This may include retrograde developments, e.g. loss of capacity to implement a technology.
Relevance to performance and safety:
Of interest are those technologies that might change the capacity of humans to intrude deliberately or otherwise into a storage site, to cause changes that would affect the movement of CO2 and associated contaminants, or that may otherwise affect the performance and safety of the storage system. Technological developments are likely but may not be predictable especially at longer times into the future.
Events related to any type of drilling activity in the vicinity of the CO2 storage system. These may be taken with or without knowledge of the storage and may include activities such as:
- exploratory and/or exploitation drilling for natural resources;
- attempted recovery of residual hydrocarbon resources;
- drilling for water resources;
- drilling for site characterisation or research;
- drilling for further storage; and
- drilling for hydrothermal resources.
Drilling rig image from the MinePro website
Relevance to performance and safety:
Has the potential to disrupt geological features that provide a barrier to CO2 migration and provide a relatively quick migration pathway to the near-surface.
Events related to any type of mining or excavation activity carried out in the vicinity of the storage site. These may be taken with or without knowledge of the site.
Mining and other excavation activities include:
- resource mining;
- excavation for industry;
- excavation for storage or disposal;
- excavation for military purposes;
- geothermal energy production;
- injection of liquid wastes and other fluids;
- scientific or archaeological investigation;
- shaft construction, underground construction and tunnelling;
- underground nuclear testing;
- malicious intrusion, sabotage or war;
- recovery of materials associated with CO2 injection (e.g. hydrocarbons).
Underground mine image from the Australian Coal Association website
Relevance to performance and safety:
Mining and other underground activities have the potential to disrupt the geosphere (storage reservoir, surrounding and overlying rock) and near-surface environment. They therefore have the potential to significantly affect the migration and distribution of stored CO2.
Events and processes related to any type of human activities that may be carried out in the surface environment that can potentially affect the performance of the storage system, or leakage pathways, excepting those FEPs related to water management which are described elsewhere.
Examples include:
- quarrying, trenching;
- excavation for construction;
- residential, industrial, transport and road construction;
- pollution of surface environment and groundwater.
Excavation works, from Tullamarine-Calder website
Relevance to performance and safety:
Human activities in the surface environment have the potential to affect CO2 release processes, should leakage occur. They may also determine the types of impact to be considered.
Events and processes related to groundwater and surface water management including water extraction, reservoirs, dams, and river management.
Water is a valuable resource and water extraction and management schemes provide increased control over its distribution and availability through construction of dams, barrages, canals, pumping stations and pipelines. Groundwater and surface water may be extracted for human domestic use (e.g. drinking water, washing), agricultural uses (e.g. irrigation, animal consumption) and industrial uses.
From Bristol University website
Relevance to performance and safety:
Extraction and management of water may affect the movement of CO2 or associated contaminants to and in the surface environment.
The presence of injected CO2 may hinder future extractive operations by obscuring seismic traces or by making the drilling process more difficult. Conversely, the presence of CO2 locally, might allow for more economic future enhanced oil recovery operations. This FEP assumes that future technical advances might identify useable resources in the vicinity of previous CO2 injection.
Relevance to performance and safety:
Drilling through a formation filled with supercritical CO2 might cause \'blowouts\' or loss of CO2 along the wellbore. A CO2 \'bubble\' will change the velocity of seismic waves, distorting the \'image\' of the underlying formations, and reducing confidence in the understanding of this structure.
Events related to deliberate or accidental explosions and crashes such as might have some impact on a closed storage site, e.g. underground nuclear testing, an aircraft crash on the site, acts of war, marine collisions or trawler damage to exposed sea-bed structures.
Aerial image of underground nuclear test site, from Wikipedia
Relevance to performance and safety:
Explosions and crashes are likely to be low probability events that could have a significant impact on the performance of the storage system by disrupting the expected evolution of the system.
This category of FEPs specifies details of the storage concept under consideration. It is split into two classes for the pre- and post-closure periods.
Details of the storage concept, the fluids injected, and factors for the design, construction, operation and decommissioning phases.
Relevance to performance and safety:
The details of the storage concept are fundamental to determining which FEPs in other categories need to be considered in a given assessment. Some details of storage operations may affect the post-closure performance.
Features related to the concept of storage, such as whether a closure exists (storage in an abandoned oil or gas field) or whether isolation of CO2 is dependent upon slow diffusion rates through an areally extensive open structure (saline aquifer storage).
Illustration of geological storage concepts, from USGS CO2 sequestration website
Relevance to performance and safety:
Different processes will be relevant to different storage concepts. For example, the rate of CO2 migration in an open aquifer will be relevant to safety assessment of saline aquifer storage, but less relevant to storage in a closed geological structure.
Features related to the amounts of CO2 injected and their rate of injection into the storage aquifer/reservoir.
Design options for putative monitoring wells at Sleipner, from Best Practice for the Storage of CO2 in Saline Aquifers
Relevance to performance and safety:
High rates of CO2 injection could have adverse affects, such as hydrofracturing.
For fast injection rates, displacement of oil/water will not be efficient during enhanced oil recovery; instead one will recover CO2; premature break through is undesirable from an economic perspective.
The composition and physical state (liquid, supercritical fluid etc.) of injected CO2, with contents of impurities etc. Temperature and pressure of injected fluid are also relevant.
During CO2 storage operations, the principal injected gas is CO2 captured and concentrated from human activity sources. However, the gas that is injected into a reservoir may not be 100% CO2, especially if there is some recycling of gas (in the case of enhanced oil recovery). Impurities can include: H2S, CH4, N2, NOx, SO2 and mercaptans. These may be present either intentionally or because it could be particularly difficult or superfluous to separate them from CO2.
Relevance to performance and safety:
The presence of even small amounts of other gases has a strong effect on the phase behaviour of CO2-dominated gases. High-pressure equations of state for CO2-dominated gas mixtures are required to take into account changes in critical pressures and temperatures caused by the presence of other gases. Impurities will reduce the critical temperature which, in turn, has effects on interfacial tension.
Amongst the possible companion gases, NOx and SO2 are particularly relevant because:
- NOx and SO2 are polluting gases that are generated by the same power plants that generate large amounts of CO2 and attract emission taxes in certain countries (e.g. Italy). Their injection, in smaller amounts, with CO2 could therefore help the economics of storage.
Impurities may affect pore water chemistry (pH and redox conditions, for example) depending on the impurities involved. Special care is needed when considering corrosive gases, such as H2S.
Microbiological contamination of injected fluid. Contamination of supercritical CO2 is considered unlikely due to its being a very good solvent. However, other fluids may be injected into the storage site that may be contaminated with microbes.
Relevance to performance and safety:
The introduction of microbes into the storage system may affect both the performance of the storage system and the endpoints considered.
Features related to the sequence of events and activities occurring during storage system design, CO2 injection and sealing. This could include enhanced oil recovery processes.
Gantt Chart, from Wikipedia
Relevance to performance and safety:
Relevant events may include phased drilling of wells and emplacement of CO2, sealing of boreholes, and monitoring activities to provide data on the transient behaviour of the system or to provide input to the final assessment. The sequence of events and time between events may have implications for long term performance, e.g. chemical and hydraulic changes during a prolonged injection phase.
Features related to measures to control events at or around the storage site during the design, construction, operation and decommissioning phases. The type of administrative control may vary depending on the stage in the storage system lifetime.
Relevance to performance and safety:
The pre-closure administrative control will influence the quantity and quality of information about the storage project that is available post-closure, therefore helping to determine societal memory. The better the amount and quality of information available, the lower the possibility of inadvertent intrusion.
Processes related to any monitoring undertaken during the operational and closure phase. The extent and requirement for such monitoring activities may be determined by issues such as storage concept, geological setting, regulations, or public pressure.
A number of monitoring techniques exist including seismic data, electrical resistance, soil gas and isotopic characteristics.
Seismic sections through the Sleipner injection site, original image from Zweigel et al, 2001
Relevance to performance and safety:
Monitoring during the operational phase contributes towards the amount and quality of information initially available after closure concerning the behaviour and distribution of stored CO2.
Features related to quality control procedures and tests during the design, and operation of the storage system.
It could be expected that a range of quality control measures would be applied during operation of the storage system and supply of CO2 to be stored. There may be specific regulations governing quality control procedures, objectives and criteria.
Relevance to performance and safety:
The degree of quality control during the design, construction, operation and decommissioning of a storage system can affect the post-closure performance by influencing the integrity of the engineered parts of the system (borehole seals, for example) and by influencing the quantity, quality and accuracy of records.
Events related to accidents and unplanned events during site investigation, CO2 emplacement and closure which might have an impact on long-term performance or safety.
Accidents are events that are outside the range of normal operations although the possibility that certain types of accident may occur should be anticipated in operational planning. Unplanned events include accidents but could also include deliberate deviations from operational plans, e.g. in response to an accident, unexpected geological events or unexpected aspects of CO2 quality and injection arising during operations.
Relevance to performance and safety:
Accidents and unplanned events may affect the post-closure performance of the storage system. One example may be the incomplete sealing of an injection borehole that may subsequently provide a pathway for CO2 migration.
The CO2 injection process is greatly influenced by the target reservoir formation pressure (pore pressure).
The formation pressure is considered overpressure if it is above the normal hydrostatic pressure for the given depth, i.e. above the pressure at the bottom of a water column equal to the reservoir depth below surface. Overpressuring may occur at any depth, naturally or artificially. Man-made overpressure could be accidental or deliberate.
An example of deliberate overpressure is injecting gaseous CO2 in a shallow aquifer at a faster rate than water can drain from the reservoir zone. As more and more CO2 is injected, the CO2 gas column below the seal at the top of the reservoir increases and so the formation pressure at the top of the gas column. In such a case an artificial overpressure is created intentionally. The maximum tolerable overpressure is calculated as a function of the desired storage volume and of a chosen safety factor below the critical fracturing gradient of the top seal rock.
An example of accidental overpressure is given by the unforeseen depressurisation (water production, CO2 leakage) of a storage reservoir where the CO2 is in liquid form, below but near the critical phase change pressure. The change of phase to gas creates a gas column that could exercise an unforeseen overpressure at the top of the gas column.
Relevance to performance and safety:
Deliberate or accidental overpressuring during the operational phase will affect the initial geosphere conditions for a post-closure assessment. It has the potential to cause fractures in the sealing formation and hence provide migration pathways for the stored CO2.
Administrative control of the storage site after closure of the project.
The administrative control of the post-closure site may differ from that of the pre-closure site with subsequent implications for the resources available for administrative control, the degree of access and availability of information etc.
Relevance to performance and safety:
There may be potential for loss of information in any transfer of administrative control. A lack of awareness about the details of a CO2 storage project could result in inadvertent disruption in the future.
FEPs related to any monitoring undertaken during the post-closure phase. This includes monitoring of parameters related to the long-term safety and performance. The extent and requirement for such monitoring activities may be determined by issues such as storage concept, geological setting, regulations, or public pressure.
A number of monitoring techniques exist including seismic data, electrical resistance, soil gas and isotopic characteristics.
Design options for putative monitoring wells at Sleipner, from Best Practice for the Storage of CO2 in Saline Aquifers
Relevance to performance and safety:
Post-closure monitoring will provide information regarding the performance of the storage project and may trigger post-closure remedial actions, if necessary.
Features related to the retention of records of the content and nature of a CO2 storage site after closure and also the placing of permanent markers at or near the site.
Archive image fom Wikipedia
Relevance to performance and safety:
It is expected that records will be kept to allow future generations to recall the existence and nature of the storage reservoir/aquifer following closure.
The degree to which the stored CO2 could be deliberately removed, if required. Either as an extreme remedial action, because the CO2 is required as a resource, or because its presence is impeding access to other geological resources.
Relevance to performance and safety:
The degree to which reversibility is considered in the design of a storage system may influence its long-term performance, by leaving viable boreholes in-situ after closure, for example.
Events and processes related to actions that might be taken following closure of a storage site to remedy problems that, either, are associated with its not performing to the standards required, result from disruption by some natural event or process, or result from inadvertent or deliberate damage by human actions.
Some potential remedial measures, from IPCC Special Report on Carbon Dioxide Capture and Storage (Metz et al., 2005)
Relevance to performance and safety:
The aim of possible future remedial actions will be to modify the performance and safety of the CO2 storage system.
This category of FEPs is concerned with those Features, Events and Processes that are relevant to the fate of the stored fluid. Carbon dioxide's properties can vary greatly between conditions at depth and near surface, and a wide range of physical and chemical reactions can be important. The category is divided into three classes for the properties, interactions and transport of carbon dioxide.
The fundamental physical and chemical properties of carbon dioxide, taking into account impurities.
CO2 Molecule, from Wikipedia
Relevance to performance and safety:
Carbon dioxide\'s properties can vary greatly with pressure, temperature and impurities, and an understanding of these properties is essential before the fate of stored fluid can be assessed.
Physical properties of CO2 including density, viscosity, interfacial tension and thermal conductivity and their dependence on pressure and temperature.
Variation of CO2 density over a range of typical reservoir temperatures and pressures (Chadwick et al., 2006)
Relevance to performance and safety:
The physical properties of CO2 determine the way in which it will behave in the environment once injected.
FEPs related to the phase behaviour (gas, liquid, supercritical fluid) of CO2. The presence of contaminants in the injected CO2 (e.g. N2) and gas and hydrocarbons in the reservoir will affect the phase behaviour and partition of CO2 between different physical states.
A temperature - pressure diagram for CO2. From IPCC Special Report on Carbon Dioxide Capture and Storage (Metz et al., 2005)
Relevance to performance and safety:
CO2 phase behaviour is a primary consideration for modelling CO2 migration.
CO2 solubility is the amount of CO2 that can dissolve for given conditions in water. It can vary as a function of temperature, pressure and precise composition of the fluid (e.g. salinity, dissolved species/complexes, presence of hydrocarbons). Changing temperature and pressure accompanying migration of CO2 can therefore influence CO2 solubility, potentially leading to gas exsolution.
CO2 is present in the aqueous phase as: aqueous CO2; carbonic acid (H2CO3); bicarbonate (HCO3-); and carbonate (CO3--). Note that other dissolved ions (Na+, Mg++, Ca++, etc.) are also involved and provide an array of linked, reversible reactions.
Relevance to performance and safety:
CO2 solubility has an impact on the chemical composition of formation fluids, pressure distribution, \'sorption\' processes, mineral-fluid reactions and the overall storage capacity of CO2. Dissolved CO2 may migrate in a different manner to \'free\' supercritical CO2. CO2 solubility in water and its partition between aqueous, gaseous, and organic phases controls the efficiency of diffusive transport and successive mineral reactions.
The aqueous speciation of dissolved CO2 will impact upon the mobility of stored CO2 and associated contaminants.
Potential interactions of carbon dioxide with solid, liquid or gaseous media.
Relevance to performance and safety:
A wide range of physical and chemical reactions can be important, and an understanding of these interactions is essential for the assessment of potential impacts.
A storage reservoir will experience enhanced pressure due to injection of CO2. This may exceed original 'natural' pressurisation due to hydrocarbon emplacement, or clay mineral transformations during diagenesis.
Relevance to performance and safety:
\'Overpressuring\' of the reservoir may involve leakage of CO2 through the cap rock due to fracturing or enhanced interactions with CO2.
Increased pressurisation caused by the injection of supercritcal CO2 will affect the behaviour of other fluids within the reservoir.
Injection of CO2 for enhanced oil recovery, from IPCC Special Report on Carbon Dioxide Capture and Storage (Metz et al., 2005)
Relevance to performance and safety:
The potential importance of increased pressurisation to enhanced oil and gas recovery indicates that it can modify the mobility of other fluids in the receiving reservoir.
Hydrocarbons could be mobilised by CO2, by miscible displacement for example, and transported to the near-surface. This is of particular relevance if enhanced oil recovery is an additional aim of the storage concept. Kolak and Burruss (2003) demonstrate that polyaromatic hydrocarbons (PAHs) can be mobilised by storage in deep coal beds.
Stored CO2 can also precipitate asphaltenes from crude oil under certain conditions of composition, temperature and pressure. Such precipitation in the vicinity of injection wells can lead to loss of injectivity and even plugging of the wells.
Relevance to performance and safety:
Mobilised hydrocarbons may migrate to the near-surface environment.
Precipitated asphaltenes can clog pores, reducing permeability and affecting fluid flow paths.
Injection of CO2 into a geologic formation may result in displacement of saline formation fluids into potable water supplies. Limitations on the pressure in a formation (for seal integrity) will mean that existing fluids are displaced/replaced. Displaced fluids are highly likely to be saline. Because the pressure wave created by injection travels much further than the physical CO2 front, displacement of saline formation fluids can occur at locations outside the CO2 storage area. Inter-connection of aquifer systems may enable saline fluids to enter potable water formations.
Relevance to performance and safety:
Displaced saline formation fluids may contaminate near-surface aquifers with subsequent impacts, such as contamination of potable water supplies.
Features and processes related to the mechanical processes and conditions resulting from the injection of CO2 that affect the rock, boreholes and other engineered features, and the overall mechanical evolution with time. This includes the effects of hydraulic, mechanical and thermal loads imposed on the rock by the injected CO2. Injection of CO2 into a reservoir can cause (directly or indirectly) changes of the geomechanical properties of the reservoir rock. Direct changes can be due to change of reservoir pressure and temperature (PVT system). Indirect changes (of rock properties) might result from geochemical and mineralogical changes after storage of CO2.
Relevance to performance and safety:
Mechanical changes of the reservoir resulting from CO2 injection (such as generation of fractures, reactivation of fractures/faults, changes of bulk elastic properties and effective reservoir) could lead to subsidence/uplift (at surface), induced seismicity, changes in migration pathways, even burst/leakage of the seal. Examples of other relevant processes are: borehole lining collapse; rock volume changes, leading to cracking
Injection of CO2 may cause and trigger seismic events and earthquake hazards through processes such as reducing friction at existing faults. This may occur both in seismically active areas and in areas characterised by a low background seismicity (reactivation of ancient fault planes, changes in the orientation, fluid-pockets occurrence). This FEP includes microseismicity.
From USGS website
Relevance to performance and safety:
Seismicity can introduce sudden physical changes to the storage system and may expose any local population to earthquake hazards.
Injecting the CO2 may cause acidification of formation water, leading to mineral dissolution and subsidence. This is of particular relevance to shallow storage sites.
Injection of large quantities of CO2 into a confined aquifer may increase pore pressure and 'lift' the overlying rocks upwards.
Relevance to performance and safety:
Deformation may affect geological processes and may result in impacts of concern at the surface.
Water phase geochemistry of stored CO2. This includes the solubility trapping of CO2 in water (H2O) to form carbonic acid (H2CO3). Subsequent ionic trapping of carbonic acid with hydroxide ions (OH-) forms bicarbonate ions (HCO3-), which can react in turn with further hydroxide ions to form carbonate (CO3).
The formation of carbonic acid, based on images from Wikipedia
Relevance to performance and safety:
Modification of the water phase geochemistry can disturb the equilibria between the water and solid phase of the reservoir and result in further geochemical (for example, solid phase geochemistry) and physical changes with resulting implications for the long-term performance of the storage system.
Chemical barriers (pH, Eh-pH, ion exchange) may exist in aquifers to retard the migration of CO2 from depth. The precipitation of CO2 bearing solids may result from such interactions.
Sleipner plume reactions over 10000 years (Chadwick et al., 2007)
Relevance to performance and safety:
Such barriers will affect the rate of migration of CO2 from depth.
The sorption and desorption of CO2 on geological materials. Sorption onto coal and the displacement of methane (CH4) is the primary mechanism behind the enhanced coalbed methane recovery (ECBM) method for geological CO2 storage.
Pure gas absolute absorption on Tiffany Coals from IPCC Special Report on Carbon Dioxide Capture and Storage (Metz et al., 2005)
Relevance to performance and safety:
The rate of sorption and desorption of CO2 on geological materials affects its mobility and therefore the performance of the storage system.
Heavy metal ions may be dissolved in formation fluids or sorbed on rock/mineral surfaces. Complexation may occur between CO2 and heavy metals dissolved in formation fluids. The influence of dissolved CO2 on pore water chemistry can also reduce the pH and change the equilibrium between sorption/desorption of metals, thereby resulting in significant release of these metals.
Relevance to performance and safety:
This process has the potential to release heavy metals, which may then migrate to the near-surface environment with resulting impacts of interest. These heavy metals will also change pore water chemistry, which could impact on carbonate complexation.
Geochemistry of the mineral phase relevant to stored CO2, including ion exchange and mineral dissolution.
Relevance to performance and safety:
Geochemical reactions between stored CO2 and the mineral phase of the storage system will affect the evolution of the system and the sorption (and therefore mobility) of the CO2.
The dissolution of minerals due to the addition of CO2 (an 'acid gas') to the geochemistry and precipitation. For example, the dissolution of albite and precipitation of calcite modelled for the Sleipner site by Gaus et al. (2003).
Relevance to performance and safety:
CO2 reaction with the host rock will modify: the porosity and permeability of the reservoir; fluid flow (direction or velocity); mechanical properties (e.g. strength); and CO2 storage capacity.
The process of exchanging one ion in the liquid phase for another ion on a charged, solid substrate.
Injected CO2 may perturb ion exchange equilibria between relevant minerals (such as sheet silicates) and the pore fluid. Some cations may be released to the pore fluid and others fixed as a consequence.
Relevance to performance and safety:
Disturbance of the rock-pore fluid equilibria may affect the capacity of the rock to store CO2.
CO2 is likely to be dried to prevent corrosion during transport. Injection of dry CO2 will cause it to take up water from the pores of the host formation and overlying rocks. It has the potential to 'suck' water out of an overlying clay.
Relevance to performance and safety:
If clay dehydrates, it will shrink and crack. This might aid CO2 migration upwards.
Gases such as CO2, methane and H2S, will occur naturally in the geosphere, either sorbed onto minerals, dissolved in formation fluids or as a free gas phase. Gas solubility will depend upon pressure, temperature and the salinity of the formation fluid.
Relevance to performance and safety:
Gases naturally present in the geosphere could affect the behaviour of CO2 injected into a storage reservoir and could accompany CO2 along potential migration paths.
CO2 migration through the reservoir and into the overlying barrier sequence could result in the CO2 stripping other gases entrained within the sediments. These gases could include radon, methane (CH4) and hydrogen sulphide (H2S).
Relevance to performance and safety:
The presence of other gases in a leaking CO2 gas stream is important in deciding the level of CO2 leakage that can be tolerated and may constitute an important hazard.
Gas hydrates are 'ice-like' solids that form at low temperatures and high pressures. They are formed of 'cages' of water molecules surrounding a gas molecule.
Relevance to performance and safety:
Cooling of the reservoir (e.g. by injecting cold CO2 or through adiabatic expansion) well below normal in-situ temperatures might stabilize gas hydrates. Their growth might seal fluid flow pathways (at least temporarily).
If CO2 is injected below deep water or permafrost then rising CO2 might hit the hydrate stability zone before escaping to the ocean or air, so hydrates could act as a secondary chemical barrier. Similarly, storage could be focused on actively forming CO2 hydrate as a stable, immobile phase to lock up the CO2 (Koide et al., 1997).
Features and processes related to the biological/biochemical processes that affect the CO2, borehole seals and rock/pore fluid, and the overall biological/biochemical evolution with time. This includes the effects of biological/biochemical influences on the CO2 and engineered components by the surrounding geology. Microbes exist in the subsurface and are used in hydrocarbon operations to improve hydrocarbon recovery. Microbes can also catalyse geochemical reactions, including methanogenesis, but the latter reaction is thermodynamically unfavourable and is unlikely.
Microbe image from Wikipedia
Relevance to performance and safety:
Examples of relevant processes are:
- microbial growth;
- microbially/biologically mediated processes; and
- microbial/biological effects of evolution of redox (Eh) and acidity/alkalinity (pH) , etc.
This FEP has probably low relevance to the safety/fate of CO2. However, CO2 releases may affect/impact microbe populations being used in independent hydrocarbon-recovery enhancement projects.
Microbes can metabolise CO2, for example, methanogenic microbes use H2 to reduce CO2 to methane (CH4), a process called methanogenesis. These microbes need anaerobic conditions.
Relevance to performance and safety:
Methanogenesis, if it occurs, could affect the pressure distribution of CO2. The fate and impact of the CH4 produced may be an endpoint of interest in itself.
Transport processes that may affect stored carbon dioxide and associated impurities.
Relevance to performance and safety:
An understanding of those processes that could transport carbon dioxide, and associated impurities, within the geosphere, near-surface and surface environments is fundamental to the assessment of long-term performance and safety.
Advection of free CO2 occurs in response to differences in pressure. The pressure difference may be due to differences in the pressure of injected CO2 and formation pressures.
The rate and direction of advection is affected by the physical properties of the rock, such as porosity and permeability.
Advection may also occur though fractures. Fracture flow will be episodic with high transport efficiencies. Resealing of fractures (for example by cementation) will reduce and ultimately block fluid flow.
Relevance to performance and safety:
Advective flow is a key transport process for migrating CO2, and associated contaminants, in the geosphere (reservoir, surrounding and overlying rock), near-surface and surface environments.
Fault valving is a process resulting from gradual build up of pore pressure due to fluid generation, causing the subsequent opening of a fault along with fluid escape towards surface. This mechanism has been recognised as causing earthquakes in many parts of the world, as a result of hydrocarbon generation or infiltration of other fluids.
Fault valving illustration from Bodmer (1994).
Relevance to performance and safety:
Large releases of pore fluids may occur during fault valving episodes.
Different relative densities of fluids in a geological system will result in buoyancy-driven flows as less dense fluids will have a tendency to flow upwards. The density of fluids will depend on its temperature and pressure.
Buoyancy forces acting on the crest of the structural closure, from Chadwick et al. (2006)
Relevance to performance and safety:
Carbon dioxide can be less dense than water, which may cause injected CO2 to flow upwards and accumulate above the water phase below the cap rock of a reservoir. Water with dissolved CO2 is more dense than water, which can result in stratification of water bodies into which CO2 may leak, if conditions are suitable.
This depends on interfacial tension and capillary pressure.
Capillary pressure is the pressure difference existing across the interface separating two immiscible fluids due to interfacial tension. The interfacial tension itself is caused by the imbalance in the molecular forces of attraction experienced by the molecules at the surface and is a function of temperature and pressure.
At a given pressure, increased interfacial tension values between water and CO2 will make larger pores accessible to CO2 (this is only valid for water-wet systems). The change from a water-wet system to a CO2-wet system has an effect on capillary forces (i.e. displacement of water by enhanced pressure versus CO2 injection with less capillary pressure) and the displacement capacity (i.e. as a non-wetting fluid, CO2 will have less displacement capacity). If the injection velocity is high, effects of capillary forces are small.
Relevance to performance and safety:
Interfacial tension and capillary pressure determine the location of CO2 within the pore spaces of the reservoir and the displacement capacity of the reservoir.
The process of dissolution of CO2 in formation fluids. The rate of dissolution depends on factors such as the interfacial area between the CO2 and the formation fluids and temperature.
Simulations of CO2 dissolution in the Froan Basin area (Chadwick et al., 2006)
Relevance to performance and safety:
Dissolution in formation fluids can be an important process in determining the period that free CO2 remains in the reservoir.
Processes related to the transport of CO2, and associated contaminants, in groundwater and surface water, including advection, dispersion and molecular diffusion.
Advection is the process by which CO2, and associated contaminants, are transported by the bulk movement of the water in which they are dissolved. Advective groundwater flow can occur along connected porous regions, such as fractures and faults. Processes that affect the movement of groundwater, such as fault valving, will also affect the migration of dissolved CO2, and associated contaminants.
Dispersion is the collective name for the consequences of a number of processes that cause 'spreading-out' of CO2, and associated contaminants, dissolved in water in all directions, superimposed on the bulk movement predicted by a simple advection model. It results in a spatially distributed contaminant plume. Spreading of the solute plume can occur in the direction of advection, in which case it is known as longitudinal dispersion, or it can occur perpendicular to the direction of advection, in which case it is known as transverse dispersion.
Diffusion is the process whereby chemical species move under the influence of a chemical potential gradient (usually a concentration gradient). In the storage domain, diffusion of CO2 might be significant in the cap rock and low permeability sedimentary host rock environments where advective transport does not occur or is limited, and diffusion of other chemical species may give rise to chemical regimes in parts of the system that inhibit or enhance the transport of CO2.
Relevance to performance and safety:
The transport of CO2, and associated contaminants, within groundwater is likely to be a key migration process and therefore an important consideration in determining performance and safety.
Processes by which CO2 is lost from the storage system. Once in the near-surface, changes in pressure and temperature result in the potential for phase changes and degassing, with resulting changes in the transport properties of stored CO2. Examples of CO2 release processes include:
- surface and undersea blowouts;
- CO2 geysers (such as Crystal Geyser in Utah, see picture below);
- submarine gas release (see picture below).
Natural gas seepages from hydrothermal vents provide an example of seepages in the marine environment (see picture below). Seepage of CO2 into the marine environment will reduce pH as dissolved CO2 will form bicarbonate and carbonate ions.
CO2 erupting from a natural reservoir at Crystal Geyser, Utah. Image from James Riding, British Geological Survey (BGS)
Relevance to performance and safety:
The potential for CO2 to be lost from a storage system will determine the performance of that system.
The rapid turnover and degassing of CO2 from a surface water body.
Due to the high solubility of CO2 in water, a lake can dissolve a volume of CO2 that, in gaseous form, is more than five times its volume. CO2 rich water is denser that pure water which can result in an unstable stratification. A drop in temperature will reduce the solubility of CO2 in water. If the water reaches its solubility limit as a result, bubbles will nucleate. As the bubbles rise and grow, a chain reaction occurs where a sudden ex-solution of CO2 can result in a rapid degassing of the water body with an eruption of rising, expanding bubbles.
Limnic eruptions can be triggered by events such as landslides that disturb the unstable stratification.
Relevance to performance and safety:
Limnic eruption provides a release mechanism for CO2, migrating from a storage system to the atmosphere.
Natural limnic eruption events can be catastrophic. On the 29th of August 1986, a massive limnic eruption from Lake Nyos in Cameroon resulted in 1800 deaths as the CO2 smothered local villages. The CO2, which is volcanic in origin, seeps through the lake bed sediments and builds up in the lower strata of the water column.
Surface seepages of CO2 may contain significant amounts of other gases due to co-migration, such as hydrogen sulphide (H2S). This is especially the case where the CO2 reservoir is a reducing chemical environment. H2S is derived from the hydration of sulphide minerals (e.g. FeS) or from the chemical reduction of aqueous sulphate species. H2S is highly toxic and therefore potentially harmful to the biosphere. Even in low concentrations it is deleterious to long-term health (human/animal), in addition to being extremely unpleasant.
Relevance to performance and safety:
The co-migration of other gases to the surface environment may cause areas of leakage to become uninhabitable - at least temporarily.
The detection of H2S provides a quick (non-analytical/aesthetic) test for CO2 escape and thus can be used as a \'marker\' gas.
This category of FEPs is concerned with the geology, hydrogeology and geochemistry of the geosphere. The category covers the reservoir, overburden and surrounding rock up to the near-surface which is considered in a separate FEP category.
Taken together, the FEPs in this category describe what is known about the natural system prior to storage operations commencing. The category is divided into three classes: Geology, Fluids, and Geochemistry.
The geographic location of a CO2 storage reservoir will influence the type of impacts to consider, e.g. continental or sub-marine, in the vicinity of a volcano, or tectonic activity, etc. In addition, proximity to human populations will increase importance of any release to the surface.
The Sleipner injection operation (Chadwick et al., 2006)
Relevance to performance and safety:
Proximity to natural hazards will increase their importance in being considered in the assessment. Proximity to human populations places more emphasis on the significance of near-surface releases.
Natural resources within the geosphere including solid mineralogical resources, such as coal or minerals, fluid and gaseous resources, such as hydrocarbons or water, and other resources such as geothermal or microbial resources.
Relevance to performance and safety:
The presence of natural resources may mean that future human exploitation of the system cannot be ignored in assessing long-term performance since they may increase the possibility of future human intrusion.
The generic type of reservoir being considered for storage of CO2. For example:
- oil reservoir (such as the Weyburn project);
- gas reservoir (such as the Coal-Seq project);
- aquifer (such as at Sleipner); and
- coal beds (such as the Coal-Seq and RECOPOL projects).
Illustration of geological storage concepts, from USGS CO2 sequestration website
Relevance to performance and safety:
The generic reservoir type will provide a high-level indication of the geological characteristics of the storage location. It will also contribute towards the extent and type of historical exploitation of any geological resources.
Geometry of the CO2 storage reservoir including the spatial distribution, depth and the topography of the top.
Depth map of the Schweinrich structure and the surrounding areas (Chadwick et al., 2007)
Relevance to performance and safety:
The geometry of the storage reservoir helps to determine the capacity of the geosphere.
The geometry of the top is particularly important because supercritical CO2 is buoyant and will therefore migrate to the top of a reservoir. Once at the top of the reservoir, it will migrate according to the precise topography of the top. Local \'highs\' could produce small-scale traps within the overall aquifer; bigger structures would produce bigger traps.
Spill points are determined by the lowest point that can retain the stored CO2.
Degree to which geological resources (such as oil and gas) have been exploited prior to the injection of CO2.
Oil and gas wells in South Liberty field (Knox et al., 2003)
Relevance to performance and safety:
The extent of previous exploitation will help to determine the initial state of a storage reservoir. For example:
- the existence and nature of boreholes; and
- the presence of geological resources, such as oil and gas.
Previous exploitation of the storage reservoir area will improve the amount of historical information available concerning the reservoir characteristics.
The nature of the relatively impermeable layer of rock overlying the storage reservoir that forms a barrier to the upward migration of buoyant fluids, such as stored CO2.
Photograph of the Utsira caprock core (Chadwick et al., 2006)
Relevance to performance and safety:
The cap rock or sealing formation plays a key roll in preventing the stored CO2 from migrating to the surface environment.
This concerns the concept of successive lithological, hydraulic and/or chemical barriers acting successively to prevent fluid escape to surface environments. From a geological point of view the permeability barrier is probably the most important, in comparison with other types of traps. However, it may be necessary to consider a sequence of traps in CO2 migration models in addition to the conventional low permeability barriers.
For example, within the Weyburn storage project, the primary cap rock is the Watrous formation, however, low permeability formations at higher stratigraphical layers provide potential additional seals, preventing upward migration of stored CO2.
Relevance to performance and safety:
Features with the potential to retard or prevent CO2 migration in the geosphere are important considerations when determining the performance of a storage project.
The systematic description of rocks in terms of their mineral assemblage and texture.
The lithology of the geosphere (including both the reservoir and the caprock) determines the reservoir physical and transport properties (including porosity and permeability). It concerns the mineralogical composition, texture (grain size, sorting) and fabric (sedimentary structures, vertical and horizontal heterogeneities). Potential reservoir lithologies include sandstones and limestones.
Relevance to performance and safety:
The physical and chemical and mineralogical properties of the reservoir rocks affect: the capacity to store CO2; fluid flow and CO2 migration; determine which water-rock reactions can take place; and influence the rock strength and elastic properties (such as compressibility, shear strength, Poisson\'s ratio etc).
The slow physical, chemical and/or biological processes by which unconsolidated sediments become sedimentary rock. These processes can result in changes to the original mineralogy.
Relevance to performance and safety:
The state of lithification/diagenesis contributes to determining the physical and chemical characteristics of sedimentary rock. For example, porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitisation.
Structure and density of discrete voids within the rock (pores).
SEM images of the Utsira Sand a) Reflected light b) Transmitted light (Chadwick et al., 2006)
Relevance to performance and safety:
The pore architecture determines the porosity and permeability of the rock, which are key features when considering the mobility of fluids and gases within the rock.
Geological surfaces separating older from younger rocks and representing a gap in the geologic record. Such a surface might result from a hiatus in deposition of sediments, possibly in combination with erosion, or deformation such as faulting. An angular unconformity separates younger strata from eroded, dipping older strata. A disconformity represents a time of nondeposition, possibly combined with erosion, and can be difficult to distinguish within a series of parallel strata. A nonconformity separates overlying strata from eroded, older igneous or metamorphic rocks.
Relevance to performance and safety:
Unconformities can act both as potential seals or lateral migration pathways for fluids. For example, the impermeable barrier resulting from the widespread development of diagenic anhydritised carbonate associated with the unconformity between the Mississipian beds and overlying Triassic Watrous Formation in the vicinity of the Weyburn pool. Stratigraphic section showing the reservoir units at the Weyburn Pool, and the relationship of the Mississipian strata to the sub-mesozoic unconformity, from Whittaker and Rostron, 2001.
Heterogeneities are variations in the rock properties of a geological formation.
Relevance to performance and safety:
Heterogeneities can result in directional variations in permeability, which affects the mobility of fluids and gases in the rock. For example, experience from the Saline Aquifer CO2 Storage project (SACS) has shown that both stratigraphical and structural local permeability heterogeneities have the potential to profoundly affect CO2 distribution and migration (Chadwick et al., 2003).
The image below shows seismic sections through the Sleipner injection site from Zweigel et al. (2001). The strong amplitudes are taken to be CO2 accumulations.
Seismic sections through the Sleipner injection site, original image from Zweigel et al, 2001
Fractures are cracks or breaks in rock. Fractures along which displacement has occurred are called faults. Fractures and faults can occur over a wide range of scales.
Fault picture from Georgia Perimeter College website
Relevance to performance and safety:
Fractures can enhance conductivity, for example, by a conductive fracture connecting permeable regions together. They can also act as seals, by bringing a relatively permeable region into contact with low conductivity rock, for example.
Natural or man-made features within the geological environment that may not be detected during site investigations.
Examples of possible undetected features are fracture/fault zones, the presence of brines or old mine workings. Some physical features of the storage environment may remain undetected during site surveys and even during preliminary borehole drilling. The nature of the geological environment will indicate the likelihood that certain types of undetected features may be present and the site investigation may be able to place bounds on the maximum size or minimum proximity to such features.
Relevance to performance and safety:
Undetected features could significantly affect the performance of a storage system. For example, local permeability hererogeneities with the potential to profoundly affect the distribution and migration of CO2 at Sleipner (within the Saline Aquifer CO2 Storage project, SACS) were only discovered after effectively being \'illuminated\' by the stored CO2 (Chadwick et al., 2003).
Temperature profile in the geosphere prior to the injection of CO2.
The critical temperature for CO2 is 31.1 degrees centigrade. The average geothermal gradient is approximately 2.4 degrees centigrade per kilometre. If the surface temperature is 10 degrees centigrade, the critical temperature will be reached at a depth of 840 metres. However, a considerable variation in geothermal gradients and sub-surface temperatures can be expected at a depth of 1000 metres. For example, in Europe temperatures at 1000 m range from 20 to 75 degrees centigrade, with local temperatures of more than 200 degrees centigrade in volcanic areas.
Relevance to performance and safety:
Relevant to temperature dependent physical/chemical/biological/hydraulic processes, such as CO2 phase behaviour.
The pressure of fluids within the pores of a formation, normally hydrostatic pressure, or the pressure exerted by a column of water from the formations depth to the sea level prior to the injection of CO2.
The critical pressure of CO2 is 7.38 mega-Pascals. The average underground hydrostatic pressure increases with depth by approximately 10.5 mega-Pascals per kilometre for aquifers that are in open communication with surface water. Applying this average gradient, the critical pressure of CO2 will be reached at a depth of around 690 metres. However, aquifers or hydrocarbon reservoirs that are sealed off from the rest of the sub-surface may be under- or overpressured.
Relevance to performance and safety:
Contributes towards determining the mobility of stored CO2.
Petrophysical properties of the geosphere prior to the injection of CO2. This includes features such as permeability, porosity, residual saturation, capillary pressure and wettability.
Relevance to performance and safety:
Petrophysical properties influence how injected CO2 will migrate in the geosphere. For example, permeability influences the direction and rate of CO2 movement, and porosity and residual saturation influence the dimensions of the CO2 plume.
Details of fluids in the geosphere, which comprises of the reservoir, overburden and surrounding rock prior to the injection of CO2. Water will generally be present, but other fluids, particularly hydrocarbons, may be important, dependent on the storage concept.
Relevance to performance and safety:
Water and other fluids in the storage system will affect the transport and interactions of injected carbon dioxide.
Natural formation water flow pathways (directions, velocities) in the geosphere will be important in determining the long-term migration paths for CO2. This depends on factors including: hydraulic heads, permeability and porosity distribution, the existence of fracture networks, connection between aquifers, position of the recharge and discharge areas.
Direction and rate of ground-water movement, from USGS report on 'Ground Water and the Rural Homeowner'
Relevance to performance and safety:
This will affect the migration of dissolved CO2 in the reservoir and the geosphere (direction, timing), the position of the interface between supercritical CO2 and aquifer water (inclined interface). There may also be a possible effect on overlying aquifers used for drinking water.
The presence and distribution of hydrocarbons, such as oil and/or gas, within the storage system.
Hydrocarbon molecule, from Wikipedia
Relevance to performance and safety:
Hydrocarbons have potentially important implications for a storage system, by both influencing the likelihood of previous geological exploitation of the area, and by being an important component of the system with which stored CO2 can interact.
This category of FEPs is concerned with the way that activity by humans alters the natural system. Both boreholes used in the storage operations and those drilled for other purposes are relevant to the long-term performance of the system. The category is divided into two classes for the drilling process, and sealing and abandonment.
Alteration of the far-field or virgin characteristics of a formation, usually by exposure to drilling fluids. Fracturing associated with formation damage can increase porosity, whereas the water or solid particles in the drilling fluids, or both, can decrease the pore volume and effective permeability of the formation in the near-wellbore region.
A number of mechanisms can result in a decrease in porosity, including:
- solid particles from the drilling fluid physically plug or bridge across flowpaths in the porous formation;
- when water contacts certain clay minerals in the formation, the clay typically swells, increasing in volume and decreasing the pore volume; and
- chemical reactions between the drilling fluid and the formation rock and fluids can precipitate solids or semisolids that plug pore spaces.
Relevance to performance and safety:
Formation damage has a number of potential implications for assessing CO2 storage:
- it can make information from affected boreholes non-representative of the true characteristics of the damaged formations; and
- damaged regions themselves may provide flowpaths for CO2 migration, particularly if damage results in fracturing.
At the time of drilling, boreholes are lined with a metal casing. Cement is pumped downhole inside the casing string, and it is pushed upward under and outside the casing lower end, between the casing and the rock wall. In multi-stage cement jobs, cement is squeezed between the casing and the rock wall though purpose made perforations. The cement could be pushed behind casing from the bottom hole to the surface, or to a predetermined depth. In such a case, the height of cement outside the casing is checked with the Cement Bound Log (CBL).
Curing of cement is the process of maintaining the proper temperature and moisture conditions to promote optimum cement hydration immediately after placement. Proper moisture conditions are critical because water is necessary for the hydration of cementitious materials. As cement hydrates, strength increases and permeability decreases. When hydration stops, strength gain ceases. Therefore, proper hydration of the cement is important in the fabrication of strong, durable concrete. The degree of curing of cement used in boreholes determines their properties, including strength, permeability and durability.
Alteration of borehole linings (metal and cement/concrete) will occur with time, depending on the natural fluid composition of the deep reservoir and the input of high concentrations of CO2 carrying natural H2S, which may accelerate corrosion.
Relevance to performance and safety:
Borehole lining and completion will contribute to determining the performance of a borehole both during its operational and post-closure phases. This is important from the perspective of CO2 storage, since boreholes may provide preferential short circuits to the surface with potential release of CO2 and contamination of upper aquifers.
The process of performing major maintenance or remedial treatments on a borehole often associated with the re-use of existing boreholes. Workover techniques include flushing of the formation and the removal and replacement of the borehole lining.
Relevance to performance and safety:
Workover will result in modified borehole properties that may need consideration within an assessment.
Often, monitoring wells are needed to monitor the physical conditions (pressure, temperature, etc.) of the storage reservoir, both inside and/or outside the area immediately affected by the storage operations, or above the storage reservoir.
Monitoring wells could be 'adopted', by using existing wells to host the appropriate instrumentation (piezometers, pressure gauges, thermometers, etc.), or they could be purpose drilled anew.
Design options for putative monitoring wells at Sleipner, from Best Practice for the Storage of CO2 in Saline Aquifers
Relevance to performance and safety:
Observation or monitoring wells may provide an accidental leakage route for the stored CO2, particularly wells drilled inside the area of storage.
The drilling of boreholes (wells) for site investigation, resource exploitation and/or CO2 injection will result in many documents being generated in paper or digital form. Typical well records include location co-ordinates, depth, electric logs, mud logs, drilling parameter logs, composite log, testing reports (if applicable), coring report (if applicable), and a final report. Physical records from cutting samples, cores and fluid samples will also be documented.
The principal tool to gain knowledge about the sequence of drilled rock formations is Borehole Logging. This is achieved by lowering an electro-mechanical tool to the bottom of the hole at the end of a steel rope (with connecting electrical cables) and by measuring petrophysical parameters on the way up. Common measures include hole diameter, natural gamma ray response, spontaneous electrical potential, rock resistivity, velocity of acoustic waves through rock, neutron susceptibility, etc. Borehole logging can be extended to include focussed fluid testing over short intervals or at specific points along the borehole.
The documents originating at the time of drilling are often the most accurate records of the succession of events associated with the drilling of the well and represent (particularly after years and decades) a precious source of information. The curation of such unique records is an invaluable tool to pass knowledge to future generations.
Map of oil and gas wells drilling around the site of the pilot project for injection into a saline aquifer in the Frio formation, Houston (Knox et al., 2003)
Relevance to performance and safety:
Well records provide a key source of baseline information regarding the storage site. They provide information concerning the nature of the rocks drilled by the well and their petrophysical characteristics in terms of sealing potential and reservoir potential. Additionally, records from wells drilled over a period of time can give a picture of how the system is evolving either naturally, or as a result of the exploitation of geological resources. This baseline information is an important input both into the initial conditions relevant to the system to be assessed, as well as providing an indication of the likely importance of other FEPs.
Incomplete well records equate to a potential gap in understanding of the storage system and may result in potentially important features being overlooked in the assessment process.
FEPs relevant to the closure of boreholes drilled within the system domain.
Relevance to performance and safety:
The way that boreholes are closed and sealed is directly relevant to the likelihood that they could act as \'short circuits\' for carbon dioxide transport.
Features related to the cessation of CO2 injection operations at a site and the sealing of injection and monitoring wells.
When a borehole is drilled to the potential storage reservoir, it creates communication with possible overlying reservoirs and with the surface. Cementing and abandonment procedures are designed to permanently plug such communication channel. At the time of well abandonment, cement plugs tens to hundreds of metres thick are placed at intervals inside the well casing.
The cement plugs are commonly located across potential problem spots (e.g. perforations, casing shoes, top of liner, etc.), to minimise leaking risks. Particular attention should be paid to the quality of the original cement job behind the casing string. If uncemented space is detected, known or suspected behind casing, depending on the lithology across such interval, it may be important to squeeze extra cement between the rock face and the casing to complement the final abandonment plugs inside the casing.
Individual boreholes may be closed in sequence, but closure refers to final closure of the whole system, and may include removal of surface installations. The schedule and procedure for sealing and closure may need to be considered in the assessment.
Examples of how cased and uncased wells are abandoned. From IPCC Special Report on Carbon Dioxide Capture and Storage (Metz et al., 2005)
Relevance to performance and safety:
The intention of borehole sealing is to prevent human access to the stored CO2 and to prevent the borehole from providing a migration pathway for the CO2. Correct cementing and abandonment operations are essential to achieve restoration of pristine sealing above the designed storage reservoir formation.
Degradation of borehole linings (metal and cement) will occur with time, depending on the natural fluid composition of the deep reservoir and the input of high concentrations of CO2. Any H2S present may accelerate corrosion of metal linings. Cement will be attacked by high partial pressures of CO2, low pH and appreciable concentrations of sulphate, chloride, and magnesium ions in the formation fluids. Seal failure will occur once liners have degraded and corroded.
Possible leakage pathways in an abandoned well. From IPCC Special Report on Carbon Dioxide Capture and Storage (Metz et al., 2005)
Relevance to performance and safety:
Seal failure may provide preferential short circuits to the surface with potential release of CO2 and associated contaminants to the surface or near-surface environment. The failure may provide a preferential pathway either through the borehole annulus or around the outside of the casing.
CO2 storage projects may be part of an Enhanced Oil Recovery (EOR) project or stand-alone deep saline aquifer projects. Either way, it is likely that the target geological structure has been the object of past exploration efforts, possibly involving drilling wells.
The existence of old wells could be obvious if the wells are still active, but could be overlooked if the old (orphan) wells have been long cemented and abandoned. Technical details of such abandoned vintage wells may in fact not be readily available, or altogether lost. In such a case, the old cementing (sealing) job could be substandard.
Oil and gas wells in South Liberty field (Knox et al., 2003)
Relevance to performance and safety:
Old substandard plugged well could provide a potential CO2 leakage route to the surface or to possible reservoirs above the designed CO2 storage reservoir.
There is little chance of detecting a substandard well abandonment before the beginning of CO2 injection to the designed reservoir, particularly if the existence of an old well has been overlooked. If old abandoned wells are known in the area of the CO2 injection operations, the risk is minimised by carefully check any potential CO2 leak to the surface at the old well head location.
If old wells are unknown and not suspected, it is good practice to run a baseline soil gas survey (if applicable) and successive soil gas surveys at intervals after the beginning of CO2 injection.
The slow downward gravitational movement of soil around boreholes.
Relevance to performance and safety:
This process results in changing properties of the soil around borehole casings after abandonment. It may either increase or decrease the degree of sealing and therefore the potential for the region immediately around the borehole to act as a migration pathway for CO2 and/or associated contaminants.
This category of FEPs is concerned with factors that can be important if stored carbon dioxide returns to the accessible environment. The environment could be terrestrial or marine, and human behaviour in that environment needs to be described. The category is divided into three classes: Terrestrial Environment; Marine Environment; and Human Behaviour.
This class of FEPs is concerned with factors that can be important if stored carbon dioxide returns to the accessible terrestrial environment.
Relevance to performance and safety:
The near-surface environment is where most potential impacts would be incurred. The FEPs in this class are relevant if that environment is terrestrial.
Features related to the relief and shape of the surface environment and its evolution.
Relevance to performance and safety:
This FEP refers to local land form and land form changes with implications for the surface environment, e.g. plains, hills, valleys, and effects of river and glacial erosion thereon. In the long term, such changes may occur as a response to other geological changes.
Features related to the characteristics of the soils and sediments and their evolution.
Different soil and sediment types, e.g. characterised by mineralogy, particle-size distribution and organic content, will have different properties with respect to erosion/deposition, sorption etc.
Chernozem soil profile, image from PhysicalGeography.net
Relevance to performance and safety:
Soil and sediment characteristics will influence the type of vegetation and land use. They will also determine relevant processes to consider should CO2 and/or associated contaminants migrate to the terrestrial surface environment.
FEPs related to all the erosional and depositional processes that operate in the surface environment, and their evolution with time. Relevant processes may include fluvial and glacial erosion and deposition, denudation, aeolian erosion and deposition. These processes will be controlled by factors such as the climate, vegetation, topography and geomorphology.
The picture below illustrates the relationship between fluvial stream flow velocity and particle erosion, transport, and deposition.
Relationship between fluvial stream flow velocity and particle erosion, transport, and deposition, from PhysicalGeography.net
Relevance to performance and safety:
Erosional and depositional processes will influence the way in which the surface environment has evolved and will evolve over the time-scale of interest.
FEPs related to the characteristics of the atmosphere, weather and climate, and their evolution with time. In case of CO2 leakage to the surface, the weather is a relevant factor determining the dispersion or the displacement of the gas: currents, evolution of the concentration of the gas, gas accumulations.
Atmospheric characteristics include physical transport of gas, aerosols and dust in the atmosphere and chemical and photochemical reactions.
Meteorology is characterised by atmospheric precipitation, temperature, pressure, wind speed and direction. The variability in meteorology should be included so that extremes such as drought, flooding, storms and snow melt are identified.
Aerial photograph of clouds, from Wikipedia
Relevance to performance and safety:
This information will determine the behaviour of CO2 should it reach the atmosphere and is therefore an important factor when considering exposure of the local population and of the local environment.
Meteorological characteristics also influence the near-surface hydrological regime with its subsequent consequences for CO2 migration.
Processes related to near-surface hydrology at a catchment scale and also soil water balance, and their evolution with time.
The hydrological regime is a description of the movement of water through the surface and near-surface environment. It includes the movement of materials associated with the water such as gas or particulates and extremes such as drought, flooding, storms and snow melt.
Direction and rate of ground-water movement, from USGS report on 'Ground Water and the Rural Homeowner'
Relevance to performance and safety:
The hydrological regime and water balance will influence the way in which CO2 migrates should it reach the near-surface environment.
Features related to the characteristics of aquifers and water-bearing features and their evolution.
Glendalough Lake in Ireland, from Wikipedia
Relevance to performance and safety:
Shallow Aquifers
Aquifers may yield significant amounts of water to wells or surface springs and may thus be a flowpath for CO2 to the surface environment. The presence of aquifers and other water-bearing features will be determined by the geological, hydrological and climatic factors.
Shallow aquifers will be able to dissolve CO2, reducing further upward migration. The amount of CO2 that can dissolve will depend on factors such as the location of the water table, the chemical composition of pore waters, CO2 flux rates, and hydrogeology. The water table location is important in determining the water-body thickness available to interact with any CO2 rising from depth. The lowering of shallow aquifer piezometric levels reduces the thickness of shallow CO2-buffering water-bodies. The consequence is a reduced capacity to buffer CO2 migrating from depth.
Surface Water Bodies
Streams, rivers and lakes often act as boundaries of hydrogeological systems. They may represent a significant source of dilution for CO2 and contaminants entering these systems, or act as reservoirs. In hot dry environments, where evaporation dominates, concentration or exsolution of gas is possible. Water with dissolved CO2 is denser than pure water, which can result in stratification and the potential for limnic eruption.
Features and processes related to the characteristics of terrestrial and freshwater flora and fauna, and their evolution. Includes plants, animals, fungi, algae and microbes.
Dragonfly image from Wikipedia
Relevance to performance and safety:
Flora and fauna may be affected by concentrations of CO2 in the near-surface environment and may be indicators of CO2 leakage.
Features and processes related to interactions between terrestrial and freshwater populations of animals, plants, algae, fungi, microbes and their evolution.
Characteristics of the ecological system include the vegetation regime, and natural cycles such as forest fires or flash floods that influence the development of the ecology. The plant, animal, algal, fungal and microbial populations occupying the surface environment are an intrinsic component of its ecology. Their behaviour and population dynamics are regulated by the wide range of processes that define the ecological system. Human activities have significantly altered the natural ecology of most environments.
Relevance to performance and safety:
The ecology of the terrestrial near-surface environment determines the types of organisms present and their inter-dependencies. These can influence the types of impact of interest and can provide a mechanism for monitoring CO2 leakage.
Features and processes related to the characteristics of coasts and the near shore, and their evolution. Coastal features include headlands, bays, beaches, spits, cliffs and estuaries.
Coastal image from Wikipedia
Relevance to performance and safety:
The processes operating on these features, e.g. active erosion, deposition, longshore transport, may affect mechanisms for the migration of CO2, and associated contaminants, entering the surface environment.
Features and processes related to the characteristics of seas and oceans and their evolution. This includes the topography and morphology of the seabed; thermal stratification and salinity gradients; and marine currents.
Relevance to performance and safety:
The local oceanographic features and processes determine the potential for dilution or accumulation of CO2, or associated contaminants in the marine environment.
Features and processes associated with sediments in the marine environment. This includes both the physical and chemical characteristics of the sediments, along with sedimentation and resuspension processes.
Relevance to performance and safety:
Marine bed sediment characteristics will influence the ecology of the marine environment. They will also determine relevant processes to consider should CO2 and/or associated contaminants migrate to the marine environment.
Features and processes related to the characteristics of marine flora and fauna, and their evolution. Includes plants, animals, fungi, algae and microbes.
Colourful reef fish of the Northwestern Hawaiian Islands, from Wikipedia
Relevance to performance and safety:
Flora and fauna may be affected by concentrations of CO2 in the marine environment and may be indicators of CO2 leakage.
Features and processes related to interactions between populations of algae, animals, microbes and their evolution.
Characteristics of the ecological system. The algal, animal and microbial populations occupying the marine environment are an intrinsic component of its ecology. Their behaviour and population dynamics are regulated by the wide range of processes that define the ecological system.
Colourful reef fish of the Northwestern Hawaiian Islands, from Wikipedia
Relevance to performance and safety:
The ecology of the marine environment determines the types of organisms present and their inter-dependencies. These can influence the types of impact of interest and can provide a mechanism for monitoring CO2 leakage.
Features and processes related to characteristics, e.g. physiology, metabolism, of individual humans. This includes considerations of variability, in individual humans, of physiology and metabolism due to age and other variations.
Physiology refers to body and organ form and function. Metabolism refers to the chemical and biochemical reactions which occur within an organism, or part of an organism, in connection with the production and use of energy.
Children and infants, although similar to adults, often have characteristic differences, e.g. metabolism and respiratory rates, which may lead to different characteristics of exposure to CO2 or contaminants.
Relevance to performance and safety:
Human physiology and metabolism determine the affect of exposure to CO2 and associated contaminants.
Features related to intake of food and water by individual humans and the compositions and origin of intake. This includes considerations of how diets, of individual humans, may vary with age and other variations (ingestion of soil by infants, for example).
This FEP also includes processes related to the treatment of foodstuffs and water between origin and consumption. For example, once a crop is harvested it may be subject to a variety of storage, processing and preparational activities prior to human or livestock consumption. Water sources may be treated prior to human or livestock consumption, e.g. chemical treatment and/or filtration.
Relevance to performance and safety:
The human diet provides a potentially important exposure pathway to contaminants released into the foodchain as a result of the CO2 storage system.
Food preparation processes may change the distribution and content of CO2 and/or associated contaminants in the product.
Features related to non-diet related behaviour of individual humans, including time spent in various environments, pursuit of activities and uses of materials. This includes consideration of variability of the habits of individuals due to age and other factors.
The human habits refer to the time spent in different environments in pursuit of different activities and other uses of materials. The diet and habits will be influenced by agricultural practices and human factors such as culture, religion, economics and technology.
Relevance to performance and safety:
Human habits will determine the exposure pathways of interest in an assessment. Smoking, ploughing, fishing, and swimming are examples of behaviour that might give rise to particular modes of exposure to CO2 and/or contaminants mobilised as a result of the CO2 storage system.
FEPs related to land and water use by humans in the near-surface environment and the resulting implications for CO2 leakage and contaminant transport and exposure pathways. This includes consideration of:
- the use of natural or semi-natural tracts of land and water such as forest, brush, rivers, lakes and the sea;
- rural and agricultural land and water use (including freshwater and marine fisheries);
- urban and industrial land and water use related to developments, including transport, and their effects on hydrology; and
- leisure and other uses of environment.
Relevance to performance and safety:
These FEPs can influence the potential transport and exposure pathways for CO2 and its associated contaminants as well as the potential evolution of the system during the timescales of interest. Particular considerations are relevant for each type of land use addressed, for example:
- special foodstuffs and resources may be gathered from natural land and water which may lead to significant modes of exposure to CO2 or contaminants;
- an important set of processes are those related to agricultural practices, their effects on land form, hydrology and natural ecology, and also their impact in determining contaminant uptake through food chains and other exposure paths;
- human populations are concentrated in urban areas in modern societies. Significant areas of land may be devoted to industrial activities. Water resources may be diverted over considerable distances to serve urban and/or industrial requirements;
- significant areas of land, water, and coastal areas may be devoted to leisure activities. e.g. water bodies for recreational uses, mountains/wilderness areas for hiking and camping activities.
Features related to characteristics, behaviour and lifestyle of groups of humans that might be considered as target groups in an assessment.
Relevant characteristics might be the size of a group and degree of self-sufficiency in food stuffs/diet. Associated with this is a consideration of the amount of resources required to meet the needs of the community.
Relevance to performance and safety:
This FEP involves a consideration of aggregated human behaviour in order to consider their dependency on and interactions with their environment. It therefore provides input to a consideration of potential human interference with the CO2 storage system as well as input to considering potential exposure pathways.
Features related to houses, or other structures or shelters, in which humans spend time.
Relevance to performance and safety:
The structure or materials used in building construction be significant factors for determining potential exposure pathways to CO2 or contaminants. For example, given that CO2 is denser than air, it may accumulate in the basements/cellars of dwellings.
Illustration of CO2 accumulating in a basement, from the USGS Mammoth Mountain website
This category of FEPs is concerned with any endpoint that could be of interest in an assessment of performance and safety. The classes of impact considered are: Impacts to Humans; Impacts to Flora and Fauna; and Impacts to the Physical Environment. Note that:
- financial impacts are assumed to be implicitly considered within each of the impact FEPs; and
- unless stated, the FEPs refer to both CO2 and mobilised contaminants (minerals, heavy metals, hydrocarbons, gases).
Loss of stored CO2 from the intended storage reservoir. Loss includes both consideration of loss to other parts of the geosphere and to the near-surface and surface environments, such as loss to marine water and surface water bodies, where CO2 may result in stratification or pooling.
Relevance to performance and safety:
Loss of containment may be an endpoint of interest to the assessment. For example, the assessment context may dictate that near-surface or surface processes are outside the scope of the assessment.
FEPs relevant to adverse impacts on the physical environment. Note that these may be endpoints of interest in themselves, but may also cause other impacts of interest.
Relevance to performance and safety:
Adverse impacts on the environment can be postulated as a result of storage operations, even if there are no associated impacts to humans or on flora and fauna.
The existence of water aquifers may be important if they are subject to CO2-induced chemical changes or CO2-induced saline intrusion. The migration of CO2 into an aquifer will result in the acidification of the water. Depending on the mineralogical composition of the aquifer and the chemical composition of the water, chemical reactions may occur which release heavy metals from the solid phase. The mechanisms which may cause this release include dissolution of metal oxides or oxyhydroxides, the reaction/diagenesis of clay minerals, and the desorption of metals that are adsorbed on clay surfaces or organic complexes.
Relevance to performance and safety:
Contamination of groundwater resources may result in impacts on flora, fauna and/or humans if the water is abstracted or flows to the surface environment. These potential impacts are considered in the subsequent FEP classes, however, contamination of groundwater may be an endpoint of interest in itself.
Soils and sediments may have elevated concentrations of CO2, should it leak from the storage system, and/or other contaminants, such as heavy metals, hydrocarbons or even increased salinity resulting from CO2 storage.
For example, natural CO2 leaking from a trapped reservoir near Mammoth Mountain, in California, has resulted in soil gas concentrations of 20 to 90%.
Migration of CO2 to contaminate soil at Mammoth Mountain, from USGS website
Relevance to performance and safety:
Increased CO2 concentrations and/or contamination of soils and sediments with associated substances may be sufficient to modify the ecology and/or use of the impacted area by humans.
Release of CO2 to the atmosphere from the storage system or other contaminants, such as radon or methane, mobilised as a result of the storage.
Relevance to performance and safety:
Release of stored CO2 or mobilised methane to the atmosphere will reduce the effectiveness of the storage system at preventing greenhouse gases from being emitted to the atmosphere.
Atmospheric contamination could also lead to health impacts on humans and wildlife.
The impact of CO2 storage on the exploitation of natural resources such as oil and gas.
Enhanced oil recovery (EOR) and enhanced coal bed methane (ECBM) recovery projects involving the injection of CO2 are based on improved recovery of oil and methane respectively resulting from CO2 storage. However, CO2 storage may result in the contamination of geological resources (such as hydrocarbons and minerals) or inhibit their recovery or future exploitation.
Note that the potential impact on groundwater resources is considered in 'Impacts on groundwater'.
Relevance to performance and safety:
Impacts on the exploitation of natural resources can be positive (such EOR and ECBM) and/or negative (such as inhibited recovery). These may result in other (for example, financial) impacts, or may be endpoints of interest in themselves.
The injection of CO2 may result in modifications to both the deep hydrogeology and near-surface hydrology.
Relevance to performance and safety:
Changes in the deep hydrogeology or near-surface hydrology may affect aquifer abstraction or even surface hydrology for groundwater driven features. These impacts may be either positive or negative.
The injection of CO2 will modify the geochemistry of the storage system. This may be confined to the immediate vicinity of the storage location, or, through leakage from the reservoir, may affect other locations.
Relevance to performance and safety:
The extent of geochemical modifications may be an endpoint of interest, due to resulting changes to geological processes. For example, the acidification of the geochemical regime may cause minerals to be dissolved, with potential implications for the porosity and stability of the geological formations.
Injection of CO2 into a geological formation may induce seismic events and processes.
Relevance to performance and safety:
Induced seismicity may be an endpoint of interest in itself, or it can result in other impacts, such as physical disruption of the surface environment.
The gradual or sudden sinking (subsidence) or elevation (uplift) of the topography of the terrestrial surface or marine sea-bed.
Relevance to performance and safety:
Deformation of the terrestrial surface or sea-bed may be an endpoint of interest in itself, or may result in other impacts, such as damage to property.
Addition of CO2 in a limestone or carbonate-rich aquifer could result in dissolution of the rock matrix and the enlargement of voids. If this process takes place at relatively shallow depth collapse may result in subsidence at the surface and sinkhole formation.
For example, CO2 leakage around a borehole drilled to extract natural CO2 from a reservoir in Florina, Greece, resulted in subsidence around the borehole that filled up with water (see image below).
Florina sinkhole produced as a result of CO2 leakage, image reproduced with permission from George Hatziyannis, Institute of Geology and Mineral Exploration (IGME), Greece
Relevance to performance and safety:
Large scale collapse structures may cause significant change to surface topography and possible CO2 migration paths. Sinkholes can provide locations where leaking CO2 can accumulate.
Asphyxiation effects of CO2 on terrestrial and aquatic fauna.
Oxygen is an essential requirement for respiration in animals and is therefore needed to sustain life. High concentrations of CO2 in air or water will lead to suffocation of terrestrial and aquatic animals due to a lack of oxygen reaching the blood stream.
If meteorological conditions do not disperse CO2 released to the atmosphere, it can gather close to the surface and remain in depressions, such as natural hollows. This property allows CO2 to reach high concentrations if released in sufficient quantities under particular atmospheric conditions.
The 1986 Lake Nyos disaster in Cameroon provides a graphic example of the potential effects of high atmospheric concentrations of CO2. A large limnic eruption resulted in the death of wildlife (see picture below) and approximately 1800 people in the surrounding area and up to 27 km away.
In a similar way to gaseous CO2 being denser than air, water containing high concentrations of dissolved CO2 is denser than pure water, a factor that contributes to the limnic eruption phenomenon observed at Lake Nyos. In addition to the possibility of dissolved CO2 causing asphyxiation in aquatic organisms, if gaseous CO2 forms a layer at the surface of water bodies, it can prevent the oxygenation of the water and lead to a reduction of the O2 concentration thereby contributing to asphyxiation of aquatic organisms.
Image of animals that died due to suffocation by CO2 released during the limnic eruption event at Lake Nyos in 1986, from Volcano World website
Relevance to performance and safety:
Levels of CO2 from a storage project sufficient to cause asphyxiation of fauna would be an endpoint of concern in assessing the geological storage of CO2.
Plants and algae (in both terrestrial and marine environments) use energy in sunlight to photosynthesise carbohydrates from CO2 and water (H2O). Increasing concentrations of CO2 around the photosynthetic tissues, increases the rate of photosynthesis and therefore growth and productivity in terrestrial and aquatic plants and algae.
However, the roots of most plants need oxygen to breakdown carbohydrates to provide energy for root growth and healthy metabolism, a process called aerobic respiration. High concentrations of CO2 in the soil reduces the availability of O2 and can cause roots and therefore plants to die.
At Mammoth Mountain, in California, CO2 has accounted for up to 95% of the gas concentration in soil at the edge of Horseshoe Lake due to release from natural geological CO2 reservoirs caused by volcanic activity. These high soil concentrations have resulted in areas of forest being killed (see picture below).
Areas of trees killed by high concentrations of CO2 in the soil at the edge of Horseshoe Lake, Mammoth Mountain, California, from USGS Mammoth Mountain website.
Relevance to performance and safety:
Concentrations of CO2 in the soil, atmosphere and/or water sufficient to impact on the growth of plants and algae would be an endpoint of interest in assessing the geological storage of CO2.
Contaminants other than CO2 may be introduced to the biosphere as a result of geological CO2 storage due to:
- impurities associated with the storage fluid, such as hydrogen sulphide (H2S), methane (CH4), nitrogen oxides (NOx) and mercaptans;
- the mobilisation of substances in the geological environment due to the storage of CO2, such as hydrocarbons, brine, CH4 and heavy metals.
These substances may have a toxic effect on organisms in the biosphere, including plants, animals, algae and fungi.
Relevance to performance and safety:
The potential for contaminants associated with and/or mobilised by CO2 storage to have a toxic effect on organisms should they migrate to the biosphere will be of interest in an assessment of the geological storage of CO2.
Geological storage of CO2 may have an impact on the biosphere at a community, population and/or ecological level, with subsequent implications for biodiversity. The potential impact of releases of CO2 and associated or mobilised contaminants into the biosphere may disrupt biological interactions sufficiently to modify the terrestrial or marine ecosystems affected.
For example, the sudden release of natural CO2 due to the limnic eruption at Lake Nyos, Cameroon, resulted in the sudden death of wildlife, but was unlikely to affect the ecology in the longer term. However, the continued gradual release of natural CO2 into soil near Mammoth Mountain, California, has been sufficient to kill trees and damage the local ecosystem since 1996 until the present day.
Relevance to performance and safety:
The degree of potential ecological disruption resulting from geological CO2 storage may be an endpoint of interest, especially if the ecosystem affected is considered valuable and/or sensitive to perturbations.
Microbes will be present in the geosphere as well as in the terrestrial and/or marine environments above the CO2 storage reservoir. CO2 storage may disrupt microbial aerobic respiration but may enhance anaerobic respiration, with subsequent implications for the processes in which the microbes are involved.
Microbes play an important roll in all terrestrial and marine ecosystems, including those associated with extreme environments, such as deep sea hydrothermal vents.
Relevance to performance and safety:
The potential impact of CO2 storage on aerobic and anaerobic microbial respiration in the geosphere, terrestrial and marine biosphere may be an endpoint of interest.
Elevated atmospheric concentrations of CO2 can result in both acute and chronic health effects in humans. If meteorological conditions do not disperse CO2 released to the atmosphere, it can gather close to the surface and remain in depressions, such as the basement of buildings or at the surface of lakes. This property allows CO2 to reach high concentrations if released in sufficient quantities under particular atmospheric conditions.
The primary health effect of concern is asphyxiation. Oxygen is an essential requirement for respiration in humans and is therefore needed to sustain life. High concentrations of CO2 in air will lead to suffocation of humans due to a lack of oxygen reaching the blood stream. Asphyxiation can occur once atmospheric concentrations reach approximately 10% CO2.
Other health effects include those directly associated with elevated concentrations of CO2 in the blood stream, such as acidosis (acidification), and physiological responses to the elevated blood CO2, such as stimulation of the sympathetic nervous system and the release of catecholamines (such as adrenaline).
The 1986 Lake Nyos disaster in Cameroon provides a graphic example of the potential effects of high atmospheric concentrations of CO2. A large limnic eruption resulted in the death of approximately 1800 people in the surrounding area and up to 27 km away.
It is estimated that 1800 people died due to suffocation by CO2 released during the limnic eruption event at Lake Nyos in 1986, from Volcano World website
Relevance to performance and safety:
The potential for CO2 to be released to the atmosphere in sufficient quantities to cause health effects in humans will be an endpoint of interest in assessing the geological storage of CO2.
Contaminants other than CO2 may be introduced to the biosphere as a result of geological CO2 storage due to:
- impurities associated with the storage fluid, such as hydrogen sulphide (H2S), methane (CH4), nitrogen oxides (NOx) and mercaptans;
- the mobilisation of substances in the geological environment due to the storage of CO2, such as hydrocarbons, CH4 and heavy metals.
Such contaminants may be toxic to humans and could cause harm if exposure pathways exist.
The toxicity will depend on: the form of exposure, e.g. ingestion or inhalation, leading to internal exposure or proximity to concentrations of contaminants leading to external exposure; the metabolism of the contaminant and physico-chemical form if inhaled or ingested, which will determine the extent to which the element/species may be taken up and retained in body tissues.
Relevance to performance and safety:
The potential for contaminants associated with an/or mobilised by CO2 storage to cause harm to humans is an endpoint of interest when assessing the geological storage of CO2.
Impacts on humans due to physical disruption of the environment caused by geological CO2 storage. For example, damage to buildings due to induced seismicity, damage to farmland due to subsidence or uplift.
Relevance to performance and safety:
Physical disruption of the environment caused by CO2 storage may have a detrimental impact on humans.
Impacts on humans due to ecological modification. These may be negative (for example, reduced timber yields due to damage caused to trees by CO2 in the soil) or positive (for example, increased crop yields due to higher atmospheric CO2).
Relevance to performance and safety:
Ecological modification caused by CO2 storage may have a positive or negative impact on humans.
Experimental Geochemical Studies Relevant to Carbon Sequestration
Publication
Online paper presented at the First National Conference on Carbon Sequestration
Date
2001
Publisher
National Energy and Technology Laboratory, US Department of Energy
Description
Describes 3 on-going CO2 sequestration research studies at the Oak Ridge National Laboratory: 1) isotopic and tracer studies in support of the GEO-SEQ project; 2) isotopic partitioning in carbonate-brine systems under subsurface conditions; and 3) volumetric properties and phase relations of CO2-CH4-H20 fluids.
Bruant R G, Giammar D E, Myneni S C B and Peters C A
Title
Effects of Pressure, Temperature and Aqueous Carbon Dioxide Concentration on Mineral Weathering as applied to Geologic Storage of Carbon Dioxide
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
2
Pages
1609--1612
Description
Describes experiments to investigate the effects of pressure, temperature and aqueous solution composition on rates and mechanisms of silicate mineral dissolution and carbonate precipitation
Legal Aspects of Underground CO2 Storage: Summary of Developments under the London Convention and North Sea Conference
Date
2001-12-14
Publisher
The Fridtjof Nansen Institute
Description
A report by The Fridtjof Nansen Institute on behalf of Statoil evaluating the position of CO2 storage in light of the institutional framework of the London and OSPAR Conventions.
CO2 Solubility in Water and Brine under Reservoir Conditions
Publication
Chem. Eng. Comm.
Date
1990
Publisher
Gordon and Breach Science Publishers S.A.
Volume number
90
Pages
23--33
Description
Paper describing the determination of the reference Henry's constant from the literature, along with a correlation for the A parameter from the Krichevsky-Ilinskaya equation.
Houghton J T, Ding Y, Griggs D J, Noguer M, van der Linden P J and Xiaosu D (Eds.)
Title
Climate Change 2001: The Scientific Basis
Date
2001
Publisher
Cambridge University Press
Description
Part of the Third Assessment Report (TAR), has been produced by Working Group I of the Intergovernmental Panel on Climate Change and focuses on the science of climate change. It covers the physical climate system, the factors that drive climate change, analyses of past climate and projections of future climate change, and detection and attribution of human influences on recent climate.
Hoversten G M, Gritto R, Daley T M, Majer E L and Myer L R
Title
Crosswell Seismic and Electromagnetic Monitoring of CO2 Sequestration
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
1
Pages
371--376
Description
Demonstrating a methodology for jointly interpreting crosswell seismic and electromagnetic data, in conjunction with detailed constitutive relations between geophysical and reservoir parameters.
Optimal Geological Environments for Carbon Dioxide Disposal in Brine Formations (Saline Aquifers) in the United States - Pilot Experiment in the Frio Formation, Houston Area
Date
2003-04
Publisher
Bureau of Economic Geology, The University of Texas at Austin
Description
for U.S. Department of Energy, National Energy Technology Laboratory
Sensitivity and Cost of Monitoring Geologic Sequestration using Geophysics
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
1
Pages
2003
Description
Rock physics models were used to calculate anticipated contrasts in seismic velocity and impedance in brine saturated rock when CO2 is introduced. Results are presented from a model based on Texas Gulf coast geology. Results indicate monitoring costs may be only a small percentage of overall geologic sequestration costs.
Features, Events and Processes (FEPs) for Geologic Disposal of Radioactive Waste
Date
2000-08-01
Publisher
OECD
Description
Safety assessments of disposal sites for radioactive waste involve analyses of potential releases of radionuclides from the disposed waste and subsequent transport to the human environment. An important stage of assessment is the identification and documentation of all the features, events and processes (FEPs) that may be relevant to long-term safety. This report provides an international compilation of FEPs as well as a basis for selecting the FEPs that should be included in safety analyses.
Monitoring Carbon Dioxide Sequestration using Electrical Resistance Tomography (ERT): A Minimally Invasive Method
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
1
Pages
353--538
Description
A paper describing numerical simulations and laboratory experiments to determine the potential of ERT methods to detect and monitor CO2 in the subsurface.
Effects of Supercritical CO2 on the Integrity of Cap Rock
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
1
Pages
483--488
Description
Paper concerning an investigation of the effects of supercritical CO2 on the integrity of cap rock by using samples of siltstone cap rock from an injection site in Japan.
Paper SPE-66537, presented at SPE/EPA/DOE Exploration and Production Environmental Conference, San Antonio, Texas
Date
2001-02
Publisher
GEO-SEQ, a National Energy Technology Laboratory (NETL) sponsored project
Description
This paper presents scoping studies of the amounts of CO2 that can be trapped into the various phases (gas, aqueous, and solid) for a range of conditions that may be encountered in typical disposal aquifers.
In search of evidence of deep fluid discharges and pore pressure evolution in the crust to explain the seismicity style of the Umbria-Marche 1997-1998 seismic sequence (Central Italy)
Raistrick M, Shevalier M, Mayer B, Durocher K, Perez R, Hutcheon I, Perkins E and Gunter B
Title
Using carbon isotope ratios and chemical data to trace the fate of injected CO2 in a hydrocarbon reservoir at the IEA Weyburn Greenhouse Gas Monitoring and Storage Project, Saskatchewan, Canada
Publication
8th International Conference on Greenhouse Gas Control Technologies
Date
2006-06-22
Publisher
Elsevier
Description
Proceedings from GHGT6 Trondheim, Norway, 19-22 June 2006
Geological Sequestration of CO2 in Coalseams: Reservoir Mechanisms, Field Performance and Economics
Publication
Greenhouse Gas Control Technologies: Proceedings of the 5th International Conference on Greenhouse Gas Control Technologies
Date
2001
Publisher
CSIRO Publishing
Volume number
1
Pages
593--598
Description
Describes a joint U.S. Department of Energy and industry project to study the reservoir mechanisms, field performance and economics of CO2 sequestration in coalseams.
Issues Related to Seismic Activity Induced by the Injection of CO2 in Deep Saline Aquifers
Publication
Journal of Energy and Environmental Research
Date
2002-02
Publisher
National Energy Technology Laboratory, US Department of Energy
Volume number
2
Pages
32--47
Description
Case studies, theory, regulation, and special considerations regarding the disposal of carbon
dioxide (CO2) into deep saline aquifers are investigated to assess the potential for induced
seismic activity.
Strutt M H, Beaubien S E, Beaubron J C, Brach M, Cardellini C, Granieri R, Jones D G, Lombardi S, Penner L, Quattrocchi F and Voltatorni N
Title
Soil Gas as a Monitoring Tool of Deep Geological Sequestration of Carbon Dioxide: Preliminary Results from the EnCana EOR Project in Weyburn, Saskatchewan (Canada)
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
1
Pages
391--396
Description
Natural background levels and concentration distributions have been established for measured soil gases for comparison with future data sets to estimate CO2 storage integrity for the reservoir rocks.
Vomvoris S, Scholtis A, Waber H N, Pearson F J, Voborny O, Schindler M and Vinard P
Title
Lessons learned from the use of hydrochemical data for the evaluation of the groundwater flow models developed within the Swiss L/ILW programme
Publication
Use of hydrogeochemical information in testing groundwater flow models - technical summary and proceedings of a workshop organized by the NEA Coordinating Group on Site Evaluation and Design of Experiments for Radioactive Waste Disposal (SEDE), Sweden
A cold geyser appears to be a contradiction in terms. But a combination of carbon dioxide, effervescing groundwater and a fortuitous oil exploration well can create a very spectacular water fountain.
An EU 5th Euratom Framework Programme considering how to represent long-term climate and environmental change within assessments of deep geological radioactive waste disposal
The IPCC has been established by WMO and UNEP to assess scientific, technical and socio- economic information relevant for the understanding of climate change, its potential impacts and options for adaptation and mitigation.
The RECOPOL project is an EU funded combined research and demonstration project to investigate the possibility of permanent subsurface storage of CO2 in coal.
The Hadley Centre for climate prediction and research, which is part of the Met Office, provides a focus in the United Kingdom for the scientific issues associated with climate change.
.JPG)
