Interplay between pore pressure and poroelastic stress induces US midcontinent seismicity

Seismicity in the US midcontinent has increased since 2009 due to high-rate wastewater disposal into the Arbuckle Group aquifer. Two mechanisms govern this seismicity: pressure diffusion and poroelastic stress transfer. Although the former is usually considered a first-order driving mechanism, recent modeling shows pressure diffusion drops significantly with distance, and at distances >15 km from an injection well, stress is the primary driving mechanism for fault failure. Stress also transfers more rapidly than pore pressure, reaching faults sooner during injection increases, and diminishing more rapidly when injection ceases. This study focuses on calculating pressure and stress fields at different locales in Kansas and Oklahoma to forecast seismicity rate. Kansas sites include Harper, Sumner, and Reno counties, where seismicity has increased recently. Oklahoma sites are within 70 km of the largest earthquakes, the M 5+ Prague, Pawnee, Fairview, and Cushing earthquakes. We use semi-analytical solutions for calculating pressure diffusion and stress transfer. The models incorporate well injection rates from 49 UIC (Underground Injection Control) Class I wells, for which there are detailed historical pressure records, and 4234 UIC Class II wells that inject into the Arbuckle Group. The models also incorporate diffusivity and transmissivity (derived from compressibility, thickness, porosity, and permeability), as well as poroelastic parameters (Biot coefficient and Poisson ratio). Hydrogeological parameters used are obtained by history matching Class I pressure data. The final pore pressure and radial and tangential stress at a point in time is calculated by superimposing pressure and stress changes caused by the different injection rates of each well. A declustered earthquake catalog is employed for associating pressure-stress changes and seismicity. The results suggest that the pressure-stress interplay can decrease the effective normal stress on the fault, unclamp the fault and trigger slip, which is also predicted by earthquake nucleation theory. The results also quantify pressure diffusion and stress transfer through the Arbuckle and the rate at which they change, which have implications for reducing seismicity risks and are useful for detecting areas or regions of high seismic concern.