THE HYDROLOGIC CONNECTION BETWEEN BASAL RESERVOIR INJECTION, CRYSTALLINE BASEMENT FAULT ZONES, AND INDUCED SEISMICITY

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. Recent geologic mapping along the contact between the crystalline basement and basal reservoirs has revealed, in some instances, evidence of a highly weathered zone exhibiting ductile behavior. The present research integrates these new geologic field observations and hydrogeologic modeling to understand under what geologic settings downward fluid pressure propagation into crystalline basement occurs. 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 8 km x 8 km x 10 km idealized three-dimensional hydrogeologic model to assess what fault-zone properties promote or deter the downward propagation of anomalous fluid pressures from an injection well into crystalline basement. The model includes an 80 m-thick reservoir underlain by 20 m of weathered basement and approximately 9.9 km of crystalline basement. We include one pumping well located 75 m from the fault zone (Q = 5000 m³/day). We used representative permeability levels for each layer while varying the permeability of the fault core and damage zones. We found that recent faulting within both the reservoir and basement rock layers had the greatest impact on pressure propagation, increasing the depth of significant (0.07 MPa) pressure migration to over 1 km depth, 5 times deeper than in the unfaulted reservoir condition. We also found that the presence of a relatively lower-permeability weathered basement horizon (k_{weathered zone} = 0.1 k_basement) reduced the depth of the same pressure front by 350 m, and the depth of the 2.5 MPa pressure contour by a factor of 2. Thus, the present research highlights two dominant factors in controlling the depth of pressure migration into the crystalline basement: 1) continuous faulting in both basement and injection horizon (i.e. recent faulting), and 2) presence of a weathered basement confining unit beneath the injection reservoir horizon.