Pressure dependence of elastic wave velocities in sandstones.

Summary Rocks response to stress is governed by its microstructure and constituent minerals. This is manifested in pressure dependence of elastic waves velocity, i.e. velocity behavior as a function of effective stress. In the lower pressure regime, the increase in velocity with increasing pressure is very steep and nonlinear. This is due to the closure of microcracks of low aspect ratios, which dramatically affects the elastic properties of rock and thereby the velocities. In the higher pressure regime, the increase in velocity (with increasing effective pressure) is moderate and somewhat linear in nature, as fewer numbers of cracks with high aspect ratios get closed. Several efforts have been made in the past to model the pressure dependence of velocity. However, there is a little attempt to relate this pressure dependence with petrophysical parameters like rock composition and microstructure. In this study, we have tried to correlate the pressure dependence of velocity with rock microstructure and composition. A simple functional form was developed and fitted to the compressional and shear wave velocities for 145 different rock samples (both saturated and unsaturated) from six different formations. V = A – C exp (-DPe)