||S Tabrez ALI, Nicholas C DAVATZES, Robert J MELLORS, William FOXALL, Peter S DRAKOS, Ezra ZEMACH, Corne KREEMER, Herbert F WANG, Kurt L FEIGL
||EGS, InSAR, numerical modeling
||Stanford Geothermal Workshop
||Enhanced Geothermal Systems
||InSAR images acquired by the ERS and TerraSAR-X satellites between 1997 and 2013 over the Brady Hot Springs geothermal field delineate subsidence on the order of a few centimeters per year over an elliptically shaped area roughly 5 km long by 2 km wide. This subsiding area is centered adjacent to a prominent bend in the fault system where the successful production wells are located. The long axis of the deforming region parallels the north-northeast strike of the predominant normal fault system. Within this broad bowl of subsidence, the interference pattern shows several smaller features with length scales of the order of ~1 km. Inverse modeling suggests that these small scale features are a result of contraction, following pressure decline, in shallow laterally confined reservoirs. Using poroelastic models we demonstrate how highly permeable conduits, associated with faults, can channel fluids from the shallow reservoirs to the deep reservoir which is tapped by the production wells. Such structurally controlled, high permeability conduits are consistent with relatively recent fault slip evidenced by scarps in late Pleistocene Lake Lahontan sediments and spatially associated surface hydrothermal features that predate production at Brady. In contrast, Desert Peak, a "blind" geothermal field, located less than 7 km away from Brady, shows little or no deformation in the InSAR dataset, although the two fields are otherwise similar in spatial extent, structural setting, and geothermal production. Desert Peak exhibits neither hydrothermal features nor any evidence of recent surficial fault slip, however, suggesting that the "plumbing" associated with the fault system there is deeper and more isolated from the surface than at Brady.