Role of Barnett Production and Flow-Geomechanical Coupling on Stress Migration and Fault Stability in the 2013-2014 Azle Earthquake Sequence

Abstract:

Migration of production- and injection-induced stresses from reservoir layers to basement faults is not well understood, especially in reservoirs with hydraulically segregated production and injection units situated across low-permeability structural or stratigraphic boundaries. This is the challenge in understanding the 2013-2014 Azle-Reno earthquake sequence (maximum magnitude Mw 3.8) in the Fort Worth Basin of Texas, which occurred after billions of cubic meters of gas and brine was produced from Barnett Shale and millions of cubic meters of water was injected into Ellenburger dolomite below Barnett. The analysis of Barnett production on induced stress and reactivation of the synthetic-antithetic fault pair hosting the seismicity is missing from previous studies (Hornbach et al, 2015, 2016; Chen et al, 2019). The interaction between production-induced contraction, injection-induced expansion, and decrease in fault compression due to fluid diffusion from the injection layer has not been understood in previous studies that used pressure diffusion models uncoupled to geomechanics. The assumption that all the water produced from Barnett wells are coming from Ellenburger through vertical fractures has also affected the previous studies. Using a high-resolution coupled multiphase flow-geomechanics-fault reactivation model of the Azle region to integrate recently interpreted structural, seismic and well data in the region, we address this gap in understanding the Azle seismicity. In departure from previous studies, we find that Barnett production-induced stress played an important role in the Azle seismicity: in the footwall block of the antithetic fault, Barnett contraction-induced updip shear abets the updip shear due to Ellenburger's expansion. We also find that geomechanical coupling is important to understand the basement events: in the hanging wall block of the synthetic fault below Ellenburger, downdip shear from Ellenburger expansion colludes with the decrease in the fault compression due to injection to induce normal faulting. In a novel result, we quantified the impact of Barnett fracturing and subsequent pressure depletion on re-orientation of the principal stresses, which drive the induced seismicity. We find that the re-orientation is more severe in lower permeability Barnett compared to Ellenburger and discuss the value of this information in drilling new infill wells.