Response of Induced Seismicity to Injection Rate Reduction: Models of Delay, Decay, Quiescence, Recovery, and Oklahoma

Abstract

When injection‐induced seismicity poses a risk to communities, it is common to reduce the injection rate or halt operations. This applies both to individual wells and well clusters, such as those within the Area of Interest for Triggered Seismicity in Western Oklahoma, where in 2016 a 40% volume reduction mandate was imposed by the state regulator. Here we quantify how induced seismicity responds to an injection reduction. We introduce models of pressure diffusion in idealized geometries coupled to steady state pressurization and rate‐state models of earthquake triggering. We find that the delay in seismicity onset and then postreduction behavior—decay, sometimes quiescence, and recovery of the seismicity rate—depend on the critical triggering pressure and on diffusion and rate‐state parameters. We have adapted our model to replicate the timing of onset, peak, and recent pace of decline of seismicity triggered by wastewater injection in Western Oklahoma. Our analysis implies that diffusivity in the Arbuckle formation is high (between 44 and 277 m2/s). The critical triggering pressure is inferred to be between 0.021 and 0.077 MPa, and fluid overpressure at 4.5‐km depth in the basement is estimated to have increased by as much as 0.190 MPa. We simulate future seismicity out to 2025 for three scenarios. Fixing the 2018 injection rate, already less than the limit imposed by the state regulator, we find a high likelihood of further M ≥ 5 earthquakes. This suggests that the volume reduction mandate in Western Oklahoma is, at present levels, inadequate.

Pore-pressure diffusion, enhanced by poroelastic stresses, controls induced seismicity in Oklahoma

“…Our model incorporates coupled porous media flow modeling and a physics-based seismicity rate model to statistically forecast the magnitude–time distribution of induced earthquakes as well as to calculate the earthquake magnitude exceedance probability, which is physically different from other approaches that predict the induced seismicity frequency (15–18). Seismogenic index theory, which considers injection rates to reconstruct a Gutenberg–Richter law, has been applied to M3+ induced earthquakes in Oklahoma (15).…”

“…Ignoring fluid diffusion also prevents the model from forecasting future seismicity rate. A recent model (18) combines fluid-diffusion modeling and seismicity rate theory in WO, but the distributed injectors are superimposed and simplified to a broad zone of a single injection. Since the diffusion length and time are related to hydraulic diffusivity, and the length scale is greatly increased, this simplified model requires a diffusivity of 44–277 m 2 /s, which is 1–2 orders of magnitude larger than that in the Arbuckle layer.…”

“…The elevated rate of seismicity and chance of damaging earthquakes raise broad societal concerns among the public and regulators (1). Despite significant efforts in improving the monitoring capability for detecting induced seismicity (5) and understanding the underlying mechanism (4, 10–13), much more needs to be done to successfully quantify and forecast the associated time-varying seismic hazard (14–18).…”

“…Despite injection reduction and decline of seismicity rate, seismic moment release soared within the injection-regulation zones, culminating in several major events in late 2016, such as the Pawnee and Cushing earthquakes (19). This observation indicates that the relation between fluid injection and the associated induced seismic hazard is complex and that the underlying physics associated with the process of induced seismicity is not captured in the current induced earthquake-forecasting models (14–18).…”

Effect of the Injection Scenario on the Rate and Magnitude Content of Injection‐Induced Seismicity: Case of a Heterogeneous Fault

“…On the other hand, Langenbruch and Zoback () proposed a diffusion model for the seismicity in Oklahoma that can explain the previous induced seismic sequences and found that by decreasing the injection rate, the pore pressure rate at 3‐km depth can decrease and can lead to a drop in the seismicity rate. Dempsey and Riffault () proposed a numerical model for the seismicity rate in Oklahoma and also showed that a reduction in injection rate may lead to a decrease in the seismicity rate, while the characteristic time to reach the background seismicity level depends on the magnitude of the injection rate reduction.…”