SEISMOGENIC PERMEABILITY, ks

Abstract

The temporal and spatial pattern of seismicity associated with reservoir water level fluctuations, injection of high-pressure fluids in deep boreholes, and seasonal groundwater recharge provide a unique setting to study the hydrological properties of the seismogenic fractures. Pore pressure diffusion is primarily responsible for the build up of fluid pressures and the onset of seismicity. The hydrologic property controlling pore pressure diffusion is hydraulic diffusivity c, which is directly related to intrinsic permeability k. By analyzing more than 90 case histories of induced seismicity, we determined the hydraulic diffusivity value of fractures associated with seismicity to lie between 0.1 and 10 m2/s. This range of values of c corresponds to a range of intrinsic permeability values between 5 × 10−16 and 5 × 10−14 m2. We call this range the seismogenic permeability ks. Fractures with ks were found to be associated with Darcian flow. Fractures with permeability less than ks were aseismic, as the pore pressure increase was negligible. In fractures with permeability larger than ks, aseismic non-Darcian flow was observed. Seismicity was uniquely associated with fractures with seismogenic permeability. Thus seismogenic permeability is an intrinsic property of fractures where pore pressure diffusion is associated with seismicity.