Nature of Anisotropic Response of Fluid Saturated Medium to Surface Seismic Wave Propagation

Monitoring of pore pressure or water level changes in observation wells shows significant variations both during the passage of P and Rayleigh waves and during the passage of S and Love waves. Recent borehole measurements have shown an azimuthal dependence of pore pressure variations on the stress orientation and strike direction of the fault zone. In the active fault zone, the fracture-induced anisotropy corresponds to the preferred orientation of microcracks and other discontinuities in the medium. This paper is devoted to the development of a modified Skempton equation for a quantitative description of surface wave induced pore pressure variations in a reservoir, related to the orientation and principal values of the stress tensor and rock damage (fracturing). The developed relationships allow the azimuthal dependence of the pore pressure response to be described by a dimensionless parameter defined as the ratio of the amplitudes of the pressure variations caused by the shear component and the volumetric strain. According to the proposed theoretical model, the maximum poroelastic response of the reservoir to the passage of a seismic wave is manifested in the case of subparallelism of the directions of predominant rock fracturing and maximum horizontal stress. Pore pressure monitoring data from the Arbuckle wastewater disposal reservoir (Oklahoma, USA) are used to verify the proposed theoretical model. It is shown that the observed diversity of pore pressure response in wells located in the vicinity of a fault zone intersecting the reservoir to the passage of seismic waves from seismic events at different distances is described with high accuracy by the developed model.

poroelastic response, hydrogeological effects, damage-induced anisotropy, nonlinear mechanics, damage tensor