Modeling Seismic Wave Propagation During Fluid Injection in a Fractured Network: Effects of Pore Fluid Pressure on Time-Lapse Seismic Signatures

Fluid flow in the earth's crust plays an important role in a number of geologic processes. In carbonate reservoirs, fluid flow is thought to be controlled by open macrofractures. The movement of fluids in the fractured media results in changes in the pore pressure and consequently causes changes in the effective stress, traction, and elastic properties. Many recent examples in time-lapse or 4D seismic surveys have demonstrated that seismic waves can be used to monitor changes in oil or gas reservoirs as a function of time (e.g., Landro, 2002; Angerer et al., 2002). During production from a reservoir, the movement of fluids is accompanied by substantial change in the pore pressure field. As fluids drain, pore pressure decreases, which increases the effective pressure on fractures, grain boundaries, and microcracks. Higher static load on these surfaces decreases their compliance nonlinearly and decreases fracture opening and/or pore throat size, thus increasing the stiffness of the rock (by increasing compressional and shear velocities) and decreasing permeability (Schoenberg, 2002). Conversely, pore pressure buildup due to injection leads to a decrease in effective pressure and an increase in rock compliance. Fractured rock is often modeled as a relatively rigid, defect-free, “background” medium with embedded sets of linear slip interfaces. A linear slip interface is a surface across which anomalously large strain occurs due to the passage of a wave. In linear slip deformation theory, the large strain is approximated by a displacement discontinuity across the surface that is linearly related to the dynamic traction acting on the interface (to the first order). The dynamic elastic properties of the rock are determined by adding the compliance tensor of the background to an excess fracture compliance tensor associated with the fractures (e.g., Liu et al., 2000). The linear parameters governing the infinitesimal slip on these planes have …