Concepts of soft stimulation treatments in geothermal reservoirs

Enhanced geothermal systems (EGS) allow for widespread use of enormous untapped geothermal energy potential. EGS measures are generally intended to improve productivity (or injectivity) of a geothermal reservoir by increasing the overall transmissivity of the reservoir rocks. Soft stimulation approaches are under development to enhance the reservoir performance with treatments that negate potential hazards such as induced seismicity or environmental contamination. Scientific progress has been made on topics such as fluid-rock interaction on a local scale, determination of the stress field on local and regional scale and analysis of induced seismicity. Hydraulic treatment concepts including mitigation of triggered or induced seismic events were also proposed as an outcome of the GEISER-project. A hydraulic stimulation treatment, designed to incorporate the results of this scientific progress, has been performed in a granodioritic hard rock reservoir, including complete flowback of the injected fluid volume, at Pohang (South Korea), with the result that no earthquake of moment magnitude ≥2.0 was induced. However, results of this controlled treatment demonstrated insufficient impact on enhancement of system productivity so far. Experience at Klaipeda (Lithuania) indicates that sandstone reservoirs require a complex analysis of the interaction of physical, chemical and biological processes during operation. Based on this, we have developed an iterative approach with a sequence of treatment, analysis, feedback, ranking of induced processes and final measures. Our approach resulted in comprehensive guidance for adequate treatment of unproductive sandstone reservoirs. These various methods are dependent on the geological system, comprising the rocks, the rock structures, the tectonic situation and the stress field. Hydraulic treatments sometimes induce seismic events that can, in some cases, be felt at the surface and jeopardize public acceptance of a project. Hydraulic, thermal, and chemical treatments are options to address EGS requirements in different geological settings with different geothermal exploitation strategies. These issues are addressed by the DESTRESS (Demonstration of soft stimulation treatments in geothermal reservoirs) project.