The Role of Fault-Zone Architectural Elements and Basal Altered Zones on Downward Pore Pressure Propagation and Induced Seismicity in the Crystalline Basemen

Interest in reducing the risk of induced seismic events associated with unconventional oil/gas brine reinjection wells, supercritical CO2 sequestration, and other deep waste disposal activities has heightened with the six-fold increase of earthquake frequency in the midcontinent region of the U.S. during the past decade. Most prior modeling efforts of fluid-fault interactions represent fault zones as either conduits or barriers by assigning bulk permeability values. This study assesses the consequences of representing realistic fault zones in models with multiple architectural components, each associated with characteristic permeability structures. We incorporate observations from recent geologic mapping of fault zone architecture at the contact between the crystalline basement and basal reservoirs. Our recent field investigations have revealed, in some instances, evidence of a highly altered/weathered discontinuous basement layer, hereafter referred to as altered zone, exhibiting ductile behavior. Enhanced clay content observed with this altered zone suggests lower permeability than unaltered bedrock. The present research integrates these new field observations with hydrogeologic modeling to understand what geologic settings promote downward fluid pressure propagation in crystalline basement rock to depths consistent with observed seismic events. Geologic field observations and core sampling have helped to constrain the host rock and fault-zone permeability architecture and inform numerical modeling efforts.;We present a suite of simulations that use an idealized three-dimensional hydrogeologic model domain to assess what fault-zone properties promote or deter the downward propagation of anomalous fluid pressures from an injection well into crystalline basement. We found that realistic, multi-component representations of conduit-barrier fault zones were able to effectively transmit elevated pore pressures to depths of ~2.5 km within the crystalline basement while still compartmentalizing fluid flow within the injection reservoir. Furthermore, our results indicate that the presence of a relatively low-permeability altered zone (kweathered layer = 0.1 x kbasement) can reduce the penetration depth of the same pressure front to ~500 m. We also developed simple models of hydromechanical failure and permeability increases using a transient two-dimensional finite difference code. Permeability was increased step-wise up to 100-fold in response to excess pressure thresholds of 0.005 and 0.01 MPa. Permeability enhancement was found to increase depth of pressure front migration by factors of 1.8 to 4 depending on the assigned values of enhancement parameters. The present research highlights several important factors in controlling the depth of pressure migration into the crystalline basement: 1) presence of continuous conduit-barrier faults connecting the basement and injection horizon, 2) presence of a confining altered zone beneath the injection reservoir horizon, and 3) hydromechanical failure to help explain seismicity at depths of >10 km after a few years of pumping such as is observed within the Permian Basin, NM.