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.
The goal of hydraulic stimulation is to increase formation permeability in the near vicinity of a well. However, there remain technical challenges around measuring the outcome of this operation. During two enhanced geothermal system stimulations in South Australia, Paralana in 2011 and Habanero in 2003, extensive catalogs of microseismicity were recovered. It is often assumed that shear failure of existing fractures is the main mechanism behind both the induced earthquakes and any permeability enhancement. This underpins a common notion that the seismically active volume is also the stimulated reservoir. Here we compute the density of earthquake hypocenters and provide evidence that, under certain conditions, this spatiotemporal quantity is a reasonable proxy for pore pressure increase. We then apply an inverse modeling approach that uses the earthquake observations and a modified reservoir simulator to estimate the parameters of a permeability evolution relation. The regime implied by the data indicates that most permeability enhancement occurred very near to the wellbore and was not coincident with the bulk of the seismicity, whose volume was about 2 orders of magnitude larger. Thus, we conclude that, in some cases, it is possible for permeability enhancement and induced seismicity to be decoupled, in which case the seismically active volume is a poor indicator of the stimulated reservoir. Our results raise serious questions about the effectiveness of hydroshearing as a stimulation mechanism in enhanced geothermal system. This study extends our understanding of the complex processes linking earthquakes, fluid pressure, and permeability in both natural and engineered settings.
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