High hydraulic diffusivities revealed from in-situ measurements in the Delaware Basin: Implications for Coulomb Stressing and Induced Seismicity

Seismicity rates in west Texas and southeast New Mexico have increased over the last nine years and are in large part driven by subsurface wastewater disposal associated with oil-gas operations. Injection-induced seismicity is often explained as the result of fault weakening from fluid pressurization. However, fluid injection also induces poroelastic stresses from fluid-rock coupling, which in some cases are larger than the perturbations induced from pore pressure alone. In this work, we model in-zone changes in pore pressure and poroelastic stressing along four major fault zones that have hosted moderate-large ML 4 – 5 earthquakes in the Delaware Basin (DB) of west Texas. We leverage high-quality downhole pore-pressure measurements to constrain the in-situ hydraulic diffusivity and storativity. The data show that the deep injection interval has a storativity of ∼ 5×10−5 and a diffusivity between 23-65 m2/s, suggesting that this interval can pressurize rapidly and transmit fluid pressure efficiently. We view these as local hydraulic properties and use an ensemble modeling approach that accounts for a large range in diffusivities and storativities to model changes in Coulomb Failure Stresses (CFS). Our 2D fully coupled poroelastic models show that deep subsurface fluid injection can induce between ∼1-1000 kPa in CFS along basement rooted faults that penetrate the injection interval, with the largest values occurring in models that use the hydraulic properties inferred from our in-situ measurements. However, the induced changes in CFS are much smaller (∼ 20-30 kPa) when averaging over a large range in hydraulic properties. Irrespective of the model parameterization, the in-zone perturbations in CFS are dominated by changes in pore pressure, even at distances as far as 20-30 km from the nearest injection source. Our results highlight the importance of obtaining in-situ poromechanical measurements and indicate that such high-resolution measurements are critical to understanding subsurface stressing associated with fluid injection.