Fault-Controlled Damage and Permeability at the Brady Geothermal System, Nevada, U.S.A.

Abstract

Identifying and locating permeable zones in geothermal fields is a critical step in determining reservoir potential and realizing energy production. Despite a general association with active faults, geothermal systems typically display heterogeneously distributed permeability that makes locating successful wells difficult. Faults are associated with complex distributions of secondary fractures, with variable attitude, fracture density, and connectivity – all of which can influence permeability. Simulations of the local stress state due to slip on a detailed model of the fault system at Brady Geothermal Field, NV, supported by models of key idealized fault geometries, are used to test the relationship between both productive wells or hydrothermal features and failed wells with stress states that promote or suppress fracture. These simulations show that hydrothermal features are generally associated with portions of faults best oriented to slip in the stress state measured at Brady. Critically, regions of enhanced coulomb stress (S_c^((max))) and reduced least compressive principal stress (σ3) that promote fractures occur at narrow, extensional relays and at intersections between faults; at Brady such locations correlate with the locations of production wells and hydrothermal surface manifestations. Despite this positive correlation, several of these structures do not host evidence of hydrothermal flow due to a lack of persistence along the dip of the fault necessary to connect to the heat source at depth. In contrast, regions of reduced S_c^((max)) and enhanced σ3 correspond to volumes that lie near the interior of faults, including at bends and at contractional relays. These locations are generally associated with failed wells; however, major production wells occur at a clear bend in a large fault at Brady. This may reflect the origin of the bend as breached relay and warrants further investigation.