New insights into the mechanisms of seismicity in the Azle area, North Texas

Abstract

We have performed a site-specific study of the mechanics of induced seismicity in the Azle area, North Texas, using a coupled 3D fluid flow and poroelastic simulation model, extending from the overburden into the crystalline basement. The distinguishing feature of our study is that we account for the combined impact of water disposal injection and gas and water production on the pore pressure and stress distribution in this area. The model is calibrated using observed injection wellhead pressures and the location, timing, and magnitude of seismic events. We used a stochastic multiobjective optimization approach to obtain estimated ranges of fluid flow and poroelastic parameters, calibrated to the pressure, rate, and seismic event data. Mechanisms for induced seismicity were examined using these calibrated models. The calibrated models indicate no fluid movement or pressure increase in the crystalline basement, although there is plastic strain accumulation for the weaker elements along the fault in the basement. The accumulation of strain change appears to be caused by the unbalanced loading on different sides of the fault due to the differential in fluid injection and production. Previous studies ignored the produced gas volume, which is almost an order of magnitude larger than the produced water volume under reservoir conditions and which significantly impacts the pore pressure in the sedimentary formations and the stress distribution in the basement. A quantitative analysis indicates that the poroelastic stress changes dominate in the basement with no noticeable change in pore pressure. Even though the low-permeability faults in the basement are not in pressure communication with the Ellenburger formation, the poroelastic stresses transmitted to the basement can trigger seismicity without elevated pore pressure.