Seismic characterization of multiple fracture sets at Rulison Field, Colorado

Abstract

Conventional fracture-characterization methods assume the presence of a single set of aligned, vertical cracks in the subsur-face. We relax this assumption and demonstrate the feasibility of seismic characterization of multiple fracture sets. Our technique relies on recent numerical findings indicating that multiple, differently oriented, possibly intersecting planar cracks embedded in an otherwise isotropic host rock result in a nearly orthorhombic (or orthotropic) effective medium. Here, the governing parameters of crack-induced orthotropy are estimated from 3D, wide-azimuth, multicomponent seismic reflection data acquired over the tight-gas Rulison Field in Colorado. We translate strong azimuthal variations of the normal-moveout velocities intointerval crack densities, fracture orientations, type of fluid infill, and velocities of P- and S-waves in an unfractured rock. Our inversion procedure identifies a set of cracks aligned in approximately west northwest-east southeast direction in the western part of the study area and multiple, likely intersecting fractures in its eastern part. We validate both our underlying theoretical model and the obtained estimates by two independent measurements: (1) the estimated fluid-infill parameter indicates dry cracks as expected for the gas-producing sandstones at Rulison; and (2) the obtained crack orientations are supported by well observations. As a by-product of fracture characterization, we build an anisotropic velocity model of the Rulison reservoir which, we believe, is the first orthorhombic velocity field constructed from surface seismic data.