Shear-Wave Anisotropy and Fracture Criticality: The Potential for Monitoring Hydrocarbon Production

Improving hydrocarbon production beyond the 30% typically achieved depends on gaiming a better understanding of the detailed internal structure of the reservoir rock. Seismic shear waves carry much more information about the internal structure of the rockmass than P-waves. In particular, shear waves invariably display shear-wave splitting indicating the effective anisotropy of the stress-aligned fluid-filled voids (fractures, microcracks, and preferentially oriented pore-space) along the raypath. Moreover the amount of splitting (the percentage of anisotropy) suggests that most rocks, including mos. reservoir rocks, are in a state near to fracture criticality, where any further deformation could lead to the loss ,of shear strength, the dispersion of pore fluids, and the Iikelihood of through-going fractures, (Over-pressurized fluidcompartments are likely to be particularly close to fracture criticallty.) This compliancy of the rockmass means that the geometry of the stress-aligned pore space (voids) throughout the reservoir is very sensitive to any change in the external and internal conditions affecting the reservoir, so that minor modifications to conditions may induce significant changes to the effective crack geometry. Since shear waves are very sensitive to this internal crack geometry, they can be used to monitor such changes to the crack geometry. Thus shear waves (and guided waves) have the potential for monitoring hydrocarbon production and enhanced oil recovery. This talk briefly review the current understanding of shear-wave propagation and discuss potential applications.