Fault Reactivation in Response to Saltwater Disposal and Hydrocarbon Production for the Venus, TX, Mw 4.0 Earthquake Sequence.

The combined effects of oil and gas production and saltwater disposal in stacked reservoirs can result in poroelastic-stress changes that affect fault stability and induced seismicity but that are not captured by models that consider disposal only. While the significance of these combined effects has been demonstrated in site-generic geomechanical simulations, their significance is yet to be quantified for specific sites of observed induced seismicity. We conducted 3D monolithically coupled poroelastic finite-element simulations for a site-specific geomechanical analysis to assess the potential for reactivation of basement-rooted faults in response to saltwater injection and hydrocarbon production near Venus, Johnson County, Texas. Earthquake activity with magnitudes as high as Mw 4.0 primarily occurred in the basement section of a listric normal fault extending from basement across the Ellenburger disposal reservoir and into the overlying gas-producing Barnett Shale. We find that using the best estimates of in-situ stress and fault orientation, and fault frictional coefficient of 0.6, do not hindcast fault reactivation. Increasing the maximum horizontal stress azimuth by 10° and the basement fault dip by 5°, both within the uncertainty space of the input parameters, and lowering the friction coefficient of the fault in basement to 0.35, leads to fault reactivation in basement. Using the same model geometry but a friction coefficient of 0.6 leads to fault reactivation within the Ellenburger disposal reservoir, which is inconsistent with observed hypocenter depths. Including the effects of production from Barnett reduces the potential for fault reactivation compared to simulations of disposal only. Comparing simulations with only five disposal wells to results of simulating 35 wells, we demonstrate the sensitivity of fault reactivation to selected number of wells. In addition to showing the sensitivity of simulation outcomes on the availability of high-quality field parameters, these results demonstrate the need for coupled poroelastic simulations unlike common hydrogeological and reservoir engineering simulations that may significantly over- or under-estimate the potential for fault reactivation and thus for induced-seismicity hazard. Highlights: We developed fully coupled poroelastic models for the Barnett-Shale and Ellenburger reservoirs to assess causative mechanisms for MW 4.0 earthquake sequence near Venus, TX. Models using a most favorable stress state require a fault friction coefficient of 0.35 for slip in the basement consistent with observed hypocenter depths. Models using the best estimates of stress azimuths, fault dip, and a 0.6 friction coefficient fail to hindcast fault slip. Model domains should be at least four times the distance between the closest injections and the earthquake epicenter to minimize boundary effects and include more disposal wells. [ABSTRACT FROM AUTHOR]