Contrasts between deformation accommodated by induced seismic and aseismic processes revealed by combined monitoring of seismicity and surface deformations: Brady Geothermal Field, Nevada, USA

Fluid pressure change accompanying pumping in the Brady Geothermal Field is associated with two easily measureable deformation responses: (1) surface deformations and 2) seismic slip. Surface deformation can be imaged by InSAR and appears to correspond to volume change at depth. Seismic slip on fractures is likely induced by either changes in effective normal stress or solid stress with minimal impact to volume. Both responses have potential impact on permeability structure due to dilation or compaction along natural fractures. We present an integrated data set that compares pumping records with these deformation responses to investigate their coupling and to constrain the geometry and rheology of the reservoir and surrounding crust. We also seek to clarify the relationship between induced seismicity and pumping. Currently, the dominant pumping signal is pressure reduction resulting from on-going production since 1992. Surface subsidence extends over a region of approximately 5 km by 2 km with the long axis along the strike of the major normal faults associated with the reservoir. Smaller approximately 1 km length-scale regions of intense subsidence are associated bends or intersections among individual normal fault segments. Modeling of the deformation source indicates that the broader subsidence pattern is consistent with the majority of fluid extraction from a reservoir at a depth of approximately 1 km and extending along the entire length of the mapped Brady normal fault. The more intense subsidence is consistent with fluid extraction along steep conduits from shallower depths that extend to the main reservoir. These results indicate a reservoir much larger than would be expected from the footprint of the production wells. In contrast, seismicity is primarily concentrated along a narrow path between injecting and producing wells, but outside the regions of most intense subsidence. Overall, seismicity represents only a small fraction of the strain energy associated with subsidence. During the latter half of 2013, Enhanced Geothermal Systems demonstration well 15-12ST1, located immediately SSW of the current production area along the direction of the structural trend and SHmax, will be stimulated to expand the reservoir, providing the opportunity to evaluate injection dominated behavior. Along with a geomechanical model of the stress state from borehole observations and well testing, modeling of the deformation and pumping history provides a key means to characterize the stress state and pre-stimulation reservoir properties. Continued monitoring of surface deformation and seismicity accompanying production and injection following stimulation will test if the stimulation significantly pre-existing patterns.