Evaluating the Role of the Rhyolite Ridge Fault System in the Desert Peak Geothermal Field with Robust Sensitivity Testing through Boundary Element Modeling and Likelihood Analysis

Abstract

Large faults provide critical conduits that host deep fluid circulation supporting an economic geothermal reservoir by forming and maintaining fracture networks in response to local stress changes resulting from their slip. These networks provide themajority of connected porosity constituting the geothermal reservoir. However, these structures are generally geometrically complex and key attributes of such fault and fracture networks are often poorly constrained. To address the uncertainty that results from these limitations, we model slip on the geometrically complex Rhyolite Ridge normal fault system and the resulting local stress concentrations in the Desert Peak Geothermal Field in Nevada using the boundary element method (BEM) implemented in Poly3D. We systematically explore and quantify the impact of uncertainty in the geometry of the Rhyolite Ridge fault system at depth and the tectonic stresses driving the slip on the predictions of fracture formation and slip in the surrounding volume due to local changes in stress. The affect of uncertainty in the input parameters to the model was evaluated through a sensitivity study in which: (1) models were run over a range of values for key input parameters such as fault height, dip, and the remote stress state; (2) the frequency distribution of predicted local stress states at an observation point in the reservoir was calculated; (3) the frequency distribution of model predictions was weighted by the probability distribution functions of the corresponding input parameter derived from field, laboratory measurements, or theoretical constraints. This analysis reveals that the complex geometry of the fault system leads to a high degree of variability in the locations experiencing stress states that promote fracture, but that such locations generally correlate with the main injection and production wells at Desert Peak. The spatial distribution of the model results are validated against recently induced seismicity at Desert Peak, and the resulting smaller displacement and slip tendency on the fault surface are compared to tracer tests that infer flow-paths between injection and production wells to show how clamped faults can inhibit flow at depth, while providing a lateral flow boundary for the reservoir.