USING BOUNDARY ELEMENT MODELING OF FAULT SLIP TO PREDICT PATTERNS OF STRESS PERTURBATION AND RELATED FRACTURES IN GEOTHERMAL RESERVOIRS AND EXPLORE PARAMETER UNCERTAINTY

摘要

Geologic structures at the surface such as fault tips, intersections, and fault relays are known to concentrate stress that can promote increased fracture density or reactivation in response to fault slip that facilitate fluid flow. The stress state driving slip determines where along the fault slip occurs that will induce deformation in the adjacent rock volume to promote or inhibit fracture formation or reactivation. Unfortunately, both the geometry and stress state have significant uncertainties that will impact the usefulness of this information in prospecting for geothermal resources, developing a reservoir model, distinguishing between competing models, or quantifying the probability of model validity. The impact of these uncertainties on the reservoir model can be quantitatively evaluated through efficient numerical modeling that iteratively explores the range of uncertainty in the controlling parameter space. The boundary element method is used to model fault slip (or opening) and calculate stress/strain variation. The Rhyolite Ridge fault system located at the Desert Peak Geothermal Field in Nevada is modeled in this study as an example of a blind geothermal field associated with a geometrically complex fault, which is poorly constrained at depth. We explore the impact of that uncertainty to better constrain the fault geometry at depth, evaluate the impact of uncertainty in tectonic stresses predictions of deformation, and thus the predicted pattern of fracture slip and formation. We find that the level of uncertainty in fault geometry leads to a high degree of variability in the locations experiencing stress states that promote fracture. But for instance, only a narrow subset of fault heights and dips approach the observed stress state in well DP27-15. In addition, the most stable stress concentrations occur within relays between unconnected fault segments.