The Twente region of the Netherlands has a large number or redundant salt caverns that were developed by brine production. Some of these caverns are unstable and have the potential to collapse leading to the formation of sinkholes at the ground surface. AkzoNobel Industrial Chemical’s B.V. undertook the Pilot Stabilisation Caverns Twente (PSCT) project, to determine the feasibility of stabilising these caverns. The intention was to use residual materials from a local waste-to-energy plant operated by Twence B.V., thereby reusing residual materials that would otherwise have to be managed by other means.
Under contract to AkzoNobel, Quintessa undertook a staged generic risk assessment to build confidence that the caverns can be successfully stabilised using the proposed approach, without contaminating groundwater or the surface environment. The work involved:
1) developing a site conceptual model;
2) detailed modelling and analysis (geochemical, hydrogeological and geotechnical);
3) scenario analysis to describe the expected evolution of the system and less likely alternatives;
4) contaminant transport modelling and impact assessment for each scenario; and
5) integration of the multiple lines of evidence within the risk assessment using Evidence Support Logic (ESL).
The risk assessment calculations considered the coupled physical processes (mechanical processes, migration through fractures, leaky boreholes seals, etc) and chemical processes (solubility limitation, sorption, etc) that affect the long-term migration of potential contaminants. The results of the generic risk assessment have built confidence that the caverns can be successfully backfilled and stabilised, without adverse environmental impacts.
Image of a sinkhole that developed above a collapsed salt cavern, as seen in 1991. Courtesy of AkzoNobel.