Preliminary Investigation of Reservoir Dynamics Monitored through Combined Surface Deformations and Micro-Earthquake Activity: Brady’s Geothermal Field, Nevada

Abstract

Fluid pressure change accompanying pumping in the Bradys Geothermal Field is associated with two complimentary deformation responses: (1) surface deformations sensible by InSAR corresponding to thermo-poroelastic deformations resulting in volume changes and (2) seismic slip on fractures induced by either changes in effective normal stress or solid stress with minimal impact to volume but potential impact on permeability structure due to dilation or compaction. We present an integrated data set that compares the impulse from pumping to the deformation response in order to investigate the coupling of these behaviors, and to constrain the geometry and rheology of the reservoir and surrounding crust. We find that surface deformation is strongly associated with the location of the production wells, with associated subsidence concentrated within a bend in the Bradys normal fault. A broader region of minor subsidence extends several kilometers to the NNE and SSW along the fault trend. Interestingly, MEQs are absent within the region of intense subsidence where effective stress stabilizing fractures are expected to be high but are broadly distributed in and around the margins of the region of minor subsidence below the reservoir depth. MEQs are also densely distributed along the Bradys fault from injectors to producers where effective stresses are expected to be lower. Initial modeling indicates that the pattern of subsidence is consistent with fluid extraction along a vertical conduit from shallow depths to approximately 1 km within the fault bend and then extraction at ~1 km along the entire length of the mapped Brady’s fault indicating a reservoir much larger than would be expected from the footprint of the production wells. The planned EGS stimulation well 15-12ST1 is located SSW of this permeable volume associated with production along the direction of SHmax , which is also the anticipated direction of stimulation growth, and east of the deeper reservoir volume in a low permeability volume adjacent to the reservoir. Although the spatial patterns provide relatively clear insights, we find that temporal correlations are complex and that both surface deformation and seismicity rates are highly variable in time, but that simple correlations to changes in pumping rate, which varies by ~15-20%, are not yet obvious. This complexity, and resulting limitations on predicting responses, is expected given the long history of stressing by production and geologic complexity and will be addressed in a future study modeling the pressure diffusion due to pumping and related deformation response.