| Abstract |
In many cases, geothermal systems in the Basin and Range tectonic province have no surface expression that reveals the resource or that can guide the siting of boreholes. These geothermal systems are generally controlled by the active, extensional normal faults characteristic of the region that offset bedrock and modern sediments and provide vertically extensive conduits necessary to bring hot water to shallow depth by groundwater circulation. However, they show a complex relationship to these faults, with resources localized along limited portions of the faults, at fault bends and terminations, or in some regions of fault intersection or overlap. The conceptual model is that these large faults distort the adjacent volume of rock to produce fractures and maintain open fracture networks that provide the necessary conduits for fluid flow as well as sufficient storativity and heat exchange area to support geothermal production. The Rhyolite Ridge fault system adjacent to the Desert Peak Geothermal field, Nevada, has been identified as a key structural control on the geothermal reservoir and possesses a typically complex fault trace. EGS well DP27-15 is located along the northernmost N-NE striking fault trace with the injecting and producing wells located within ~0.5 to ~2 km to the SW, just beyond the apparent tip of the fault or within an extensional step or bend. Investigation of the structural control of the Rhyolite Ridge fault system on fracture formation and slip in Desert Peak is conducted through boundary element modeling of the Rhyolite Ridge fault system and resulting distortion in the adjacent volume. We use Poly3D software, which simulates fault slip as a displacement discontinuity on a triangularly discritized surface in response to a far-field stress and calculates the related stress/strain perturbation in the surrounding three dimensional, quasi-static, homogeneous, isotropic, linear elastic medium. A heuristic modeling approach is adopted to investigate the sensitivity of this elastic distortion to details of fault geometry and stress state within uncertainties derived from mapping and borehole studies and reasonable frictional constraints on the state of stress in the surrounding crustal volume. The coefficient of friction explored ranges from 1.0 to 0.6, between the upper and lower limits of Byerlee friction, as well as incorporating constraints from laboratory measurements of friction from triaxial compression tests of representative core which have a mean friction of ~0.8. Initial models allow for complete shear stress drop on the Rhyolite Ridge system; these results are compared to models of partial stress drop derived using two friction solvers: (1) an iterative solver incorporated into Poly3D, and (2) a linear complementarity friction solver. For simple fault geometries, varying the shear stress drop primarily reduces the volume experiencing large stress magnitude changes and principal stress rotations. However, for geometries more consistent with the complexity of the mapped trace of the Rhyolite Ridge fault, the inclusion of finite residual frictional strength changes the location of some of the volumes experiencing locally enhanced coulomb stress indicative of fracture formation or reactivation. Within this framework, the potential variation in the stress state along the DP 27-15 borehole and in the vicinity of the injection and production boreholes at Desert Peak is then explored by systematically combining variations in modeling parameters through hundreds of simulations. Then the dependence of the local state |