Effect of Coupled Porothermoelastic Stress on Shear Stimulation of Enhanced Geothermal Systems

Abstract

Cost-effective extraction of heat from enhanced geothermal systems (EGS) depends highly on the successful hydraulic stimulation of geothermal reservoirs. Wide field observations of microseismicity during hydraulic stimulation of EGS when the injection pressure is far below the magnitude of least in situ stress suggest that hydroshearing may be the primary mechanism of induced permeability enhancement. Long time delay between the start of hydraulic stimulation and the onset of seismicity highlights the importance of incorporating thermal stress in stability analysis of preexisting fractures and faults. To better understand the mechanism of shear stimulation in EGS and to address the role of thermal stress in changing shear potential of fractures, with superposition technique we analyzed the stress and pore pressure changes due to injection of cold fluid into a hot geothermal reservoir under different stress regimes incorporating fully coupled porothermoelasticity. The results show that at early time, thermoelastic stress due to the shrinkage of rock matrix is mainly confined to the vicinity of injection wells, whereas poroelastic stress duo to matrix deformation influences a larger region. The temperature front lags behind the pore pressure front. Cooling induced thermoelastic stress counteracts to the poroelastic stress. With increasing time of injection, thermoelastic stress plays a more dominant role in shearing fractures. Depending on the relative orientation of fractures with respect to the in situ stress and magnitude of in situ stress, the direction of shear migration is controlled by the transient competition between poroelastic stress and thermoelastic stress.