An inversion for fluid transport properties of three-dimensionally heterogeneous rocks using induced microseismicity

Abstract

In this paper an approach is proposed for a 3-D mapping of the permeability tensor of heterogeneous reservoirs and aquifers. This approach uses the seismic emission (microseismicity) induced in rocks by fluid injection (e.g. borehole hydraulic tests). The approach is based on the hypothesis that the triggering front of the hydraulic-induced microseismicity propagates in the same manner as a diffusion wave of the process of the pore-pressure relaxation. If the dominant frequency of the fluid-injection-related perturbation of the pore pressure is much lower than the global flow critical frequency of Biot's theory, then this diffusion wave is identical to the second-type compressional Biot wave. In the case of a heterogeneous anisotropic medium, assuming that the wavelength of the triggering diffusion wave is shorter than the typical permeability heterogeneity, we derive a differential equation of the triggering front. This equation describes kinematic aspects of the propagation of the triggering front in a way similar to the eikonal equation for seismic wave fronts. In the case of isotropic heterogeneous media the inversion for the hydraulic properties of rocks follows from a direct application of this equation. In the case of an anisotropic heterogeneous medium only the magnitude of a global effective permeability tensor can be mapped in a 3-D spatial domain.