Risk-based assessment of geothermal reservoir integrity

Several potential hazards are linked to geothermal operations, including hazards arising when reservoir integrity is compromised. This contribution provides a framework for assessing the potential risk of loss of integrity of a geothermal reservoir. Focus is on low enthalpy, matrix permeable sedimentary systems at depths of approximately 1500-3000 m, typical for the Netherlands, but the framework can also be applied to other geothermal plays. Loss of reservoir integrity via a number of leakage pathways is considered, such as newly formed fractures in the caprock induced by thermal cooling or increased fluid pressures, abandoned wells through the caprock, pre-existing faults in the caprock and through the matrix of the caprock. Possible effects of loss of reservoir integrity that are taken into account include groundwater contamination, subsidence/uplift and interference of operations with those in other neighbouring permit areas. With the proposed framework, a qualitative assessment can be made of the risk related to each of these effects, leading to a general evaluation of the risk of loss of reservoir integrity. This framework can be used by geothermal operators in their exploration for suitable locations for geothermal systems and for underpinning the safety of their application for permits for these operations. It can also be used by regulators in their evaluation of geothermal permit applications.