The Effect of Correlated Permeability on Fluid‐Induced Seismicity.

One of the challenges associated with subsurface high‐pressure fluid injections is the estimation of the seismic hazard and its spatial footprint. Field data have shown that the spatial footprint typically varies significantly between injections into the basement and injections above basement. Here, we show that varying degrees of spatial correlations in porosity or log(permeability) can explain this observation. Using high‐resolution well‐log data, we first show that porosity within the basement tends to follow a power‐law scaling, S(k) ∼ 1/kβ, with β ≈ 0.9, while above basement β > 1.4. Using this in a novel conceptual model, we show that β controls the spatial footprint of fluid‐induced seismicity such that large values of β lead to more seismic activity at large distances and a higher variability in the spatio‐temporal migration of seismic events, hence, explaining the field observations. Our findings indicate that correlations in log(permeability) need to be incorporated in the seismic hazard assessment. Plain Language Summary: Fluid‐induced earthquakes are a side effect of industrial operations such as hydraulic fracturing and enhanced geothermal systems, where high‐pressure fluids are pumped into the earth's crust to increase oil and/or gas flow to a well from petroleum‐bearing rock formations or to improve permeability in underground geothermal reservoirs. The induced seismicity has become a concern for industry and nearby residents. One of the challenges associated with this seismic hazard is answering the question of how far away from the injection site seismicity can be induced. Previous observations showed that if the fluids are pumped into the crystalline basement, the occurrence of the seismicity is typically very localized in space, while otherwise it is not. Here, we introduce a novel model that explains this behavior as a consequence of a varying degree of correlations in permeability. By analyzing high‐quality permeability data, we directly show that the degree of correlations in the crystalline basement is indeed significantly different from formations above the basement. Our findings can be directly incorporated in any seismic hazard assessment to improve the safety of workers and residents. Key Points: Spatial correlations in permeability can control the spatial localization of fluid‐induced seismicitySpatial correlations in permeability tend to be different within the basement and above basementThe variability in the migration profile of fluid‐induced seismicity is controlled by the permeability field [ABSTRACT FROM AUTHOR]