The Delaware Basin in Texas, one of the largest oil and gas production sites in the US, has been impacted by widespread seismicity in recent years. The M5.0 earthquake that occurred in March 2020 near the town of Mentone is one of the largest induced earthquakes recorded in this region. Characterizing the source parameters and triggering mechanism of this major event is imperative to assess and mitigate future hazard risk. A former study showed that this event may be attributed to the deep injection nearby. Interestingly, the earthquake is located in proximity to shallow injection wells with much larger total injection volume. In this study, we investigate the role of these shallow injection wells in the triggering of the M5.0 event despite their farther distance from the mainshock. We perform source-parameter inversion and earthquake relocation to determine the precise orientation of the south-facing normal fault plane where the mainshock occurred, followed by fully coupled poroelastic stress modeling of the change of Coulomb Failure Stress (ΔCFS) on the fitted fault plane caused by shallow injection in the region. Results show that shallow wells caused up to 20 kPa of ΔCFS near the mainshock location, dominated by positive poroelastic stress change. Such perturbation surpasses the general triggering threshold of faults that are well aligned with the local stress field and suggests the nonnegligible role of these shallow wells in the triggering of the mainshock. We also discuss the complex effect of poroelastic stress perturbation in the subsurface and highlight the importance of detailed geomechanical evaluation of the reservoir when developing relevant operational and safety policies.
Numerous studies have reported the occurrence of aseismic slips or slow slip events along faults induced by fluid injection. However, the underlying physical mechanism and its impact on induced seismicity remain unclear. In this study, we develop a numerical model that incorporates rate-and-state friction fault and fluid injection to simulate the coupled processes of pore pressure diffusion, aseismic slip, and dynamic rupture. We establish a field-scale model to emulate the induced seismicity near the Dallas-Fort Worth Airport, Texas, where events with lower stress drops have been observed. Our numerical calculations reveal that the diffusion of fluid pressure induces aseismic slips and advances or delays seismic ruptures. Furthermore, the stress drops associated with aseismic slips indicate lower values (< 1 MPa), which may explain the observed variation in stress drops near the Airport. Simulations encompassing diverse injection operations and fault frictional parameters show that the interplay between the amount of pore pressure perturbations and stress states during the interseismic period influences the initiation, quantity, recurrence intervals, and source parameters of aseismic slips. However, the scaling relationship of moment (M0) with ruptured domain (r0) for all simulated events follows an unusual trend, M0∝r04.3, similar to M0∝r04.7 observed in the Airport sequence. Based on the consistent scaling, we hypothesize that the lower stress drop events in the Airport may be less dynamic ruptures, similar to aseismic slips as illustrated in our simulations.
In the context of fluid-induced seismicity, various injection parameters have been shown to affect fault behaviour differently, although existing studies about their effects sometimes show contradictory results. Aseismic slip is also known to affect seismicity, but its exact contribution remains elusive. To address these, we perform numerical modelling to understand the effects of injection volume and rate on long-term seismic and aseismic fault slip behavior. Our results suggest that both parameters can affect various aspects of fault behaviour to different extents and their roles are interdependent, thus they should be examined simultaneously. We observe the fault predominantly releasing aseismic energy, which plays a significant role in altering the timing of triggered earthquakes that follow and exhibit lasting impacts in subsequent seismic cycles. In terms of seismic responses, the number of events in the triggered cluster is primarily controlled by the injection rate, and the seismicity rate by the injected volume.