| Abstract |
The behavior of joints and other discontinuities plays an important role in heat extraction from enhanced geothermal systems. The fracture permeability changes in response to fluid injection/extraction, rock cooling, variations of the in-situ stress field, and mineral dissolution/precipitation. Deformation of discontinuities also contributes to reservoir seismicity. Thermal stress is believed to be an important factor in the dynamics of enhanced geothermal systems. In fact, the variation of injectivity with water temperature, and injection-related seismicity in geothermal fields can be attributed to thermal stresses. In this paper we couple a 3D heat extraction/stress analysis to a 3D elastic displacement discontinuity method to obtain the fracture opening and slip in response to fluid pressure and rock cooling under a given in-situ stress field. Using this approach, we estimate the effect of each mechanism on fracture slip with 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 distribution of the thermal stresses also suggests that their contribution to rock mass deformation will not stop upon cessation of water injection and can be a factor in delayed seismic activity. |