Rapid evaluation of induced seismicity using slip tendency analysis and well hydraulics

<span">Three ingredients for inducing seismicity are (i) fault or faults capable of slipping seismically, (ii) non-hydrostatic stress state, and (iii) pore fluid pressure perturbation. Slip tendency is the ratio of shear to normal stress for any surface experiencing a three-dimensional stress state. Its value depends on the relative magnitudes of the principal stresses and the orientation of the surface of interest; it is a measure of the likelihood that a surface will experience slip. The value of slip tendency that generates slip is variable and depends on rock type and fault zone characteristics. We investigate geologic scenarios coupling slip tendency analysis with established models of well hydraulics to generate pressure histories throughout a rock volume. Because it is sensitive to both stress and fault geometry, slip tendency is an efficient measure of the propensity of a fault to slip, and is a better predictive tool for induced seismicity than pore fluid pressure, or pore fluid pressure change. Our modeling suggests reasons why the most energetic earthquakes that are linked to subsurface fluid injection in the United States are likely the result of reactivated basement faults. Low permeabilities tend to increase the magnitude of the pore pressure perturbation, which in turn increases slip tendency. For much of the USA, "basement" connotes crystalline rock with low intrinsic permeability, and hence fault and fracture zones are often the primary conductive features within a low permeability host. Many such faults are long-lived, may have experienced repeated reactivation and have been modified by chemically active fluids, they are thus likely to have developed characteristics such as foliation and phyllosilicate mineralogies that reduce their frictional strength. The combination of high slip tendency and low friction renders these faults susceptible to relatively small perturbations in pore fluid pressure.