A 3-D study of the effects of thermomechanical loads on fracture slip in enhanced geothermal reservoirs

Heat extraction from enhanced geothermal systems (EGS) can greatly affect the behavior of joints and other discontinuities in the reservoir. Fracture permeability can change in response to fluid injection/extraction, rock cooling, variations of stress field, and mineral dissolution/precipitation. The reduction in effective stress caused by pore pressure increase can cause the slippage of discontinuities, thus inducing seismicity. Studies have shown that thermal stresses generated by cold water injection have a similar effect. In order to assess the influence of thermal stresses on fracture opening and slippage, a 3-D coupled heat extraction/thermal stress/elastic displacement discontinuity model is used in this study. The effects of each mechanism on fracture slip is estimated with particular reference to the Coso geothermal field. The results indicate that under typical field conditions, a substantial increase in fracture slip is observed when thermal stresses are taken into account. The temporal evolution of the thermal stresses suggests that the rock mass deformation will not cease upon stoppage of water injection, which can be a cause of delayed seismic activity.