Abstract
Hydraulic stimulation treatments required to produce deep geothermal reservoirs present the risk of generating induced seismicity. Understanding the processes that operate during the stimulation phase is critical for minimising and preventing the uncertainties associated with the exploitation of these reservoirs. It is especially important to understand how the phenomenon of induced seismicity is related to the pressurisation of networks of discrete fractures. In this study, we use the numerical simulator CFRAC to analyse pressure drops commonly observed during stimulation of deep geothermal wells. We develop a conceptual model of a fractured geothermal reservoir to analyse the conditions required to produce pressure drops and their consequences on the evolution of seismicity, fluid pressure, and fracture permeability throughout the system. For this, we combine two fracture sets, one able to be stimulated by shear-mode fracturing and another one able to be stimulated by opening-mode fracturing. With this combination, the pressure drop can be triggered by a seismic event in the shear-stimulated fracture that is hydraulically connected with an opening-mode fracture. Our results indicate that pressure drops are not produced by the new volume created by shear dilatancy, but by the opening of the conjugated tensile fractures. Finally, our results reveal that natural fracture/splay fracture interaction can potentially explain the observed pressure drops at the Rittershoffen geothermal site.
Highlights
Geothermal energy development, either for electricity generation or for direct applications of geothermal heat, can be carried out in a wide range of geological settings
We present numerical simulations based on simple fracture geometries, avoiding complex fracture networks, to investigate how pressure drops are related to stimulation and induced seismicity
Using simple fracture geometry configurations, we investigated different hypotheses for the occurrence of fluid pressure drops associated with hydraulic stimulation in Engineered Geothermal Systems (EGS)
Summary
Geothermal energy development, either for electricity generation or for direct applications of geothermal heat, can be carried out in a wide range of geological settings. The exploration and exploitation of deep geothermal reservoirs have significantly increased worldwide (e.g. Tester et al 2006; Breede et al 2013; Király et al 2015). In this context, geothermal projects focusing on. Piris et al Geotherm Energy (2018) 6:24 heat distribution (low and medium enthalpy) have mainly targeted crystalline basement rocks or large and deep sedimentary basins, such as intracratonic basins and foredeep orogenic belts, as well as continental rifts. Projects focusing on power generation in high-temperature, low-permeability settings generally need to be developed as Enhanced Geothermal Systems (EGS), either in fractured crystalline basement rocks, or in sedimentary and volcanic rocks (Zimmermann and Reinicke 2010; Elders et al 2014)
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