The effect of confining pressure on elastic wave velocities and dynamic to static Young's modulus ratio

We carried out laboratory experiments under dry conditions on limestone core specimens of Sarvak formation obtained from an oil well in the southwest of Iran. Our objective was to study the effect of confining pressure on the compressional and shear wave velocities (V-P, V-S), and on the dynamic to static Young's modulus ratio (E-d/E-s). Furthermore, we made attempts to predict V-P and V-S at atmospheric pressure based on the same velocities at various confining pressures. These analyses revealed that, below a critical pressure with an increase in confinement V-P and V-S increased exponentially, representing a poroelastic regime. Above a critical pressure, however, the trend was linear. Likewise, we observed that with an increase in confinement, E-d/E-s initially decreased exponentially, followed by a linear decreasing trend above the critical pressure. This indicated that E-s is more responsive than E-d. Accordingly, these observations infer that it is possible to predict E-s based on E-d at different confining stresses. This is an important improvement for geomechanical modeling of hydrocarbon and geothermal reservoirs because static parameters are more realistic input parameters. Besides, we derived the coefficients of the velocity-pressure equation for Sarvak limestone using least square regression analysis. More interestingly, we predicted V-P and V-S at atmospheric pressure based on these coefficients. Good agreement was observed between measured and predicted velocities at atmospheric pressure. Analysis of similar published experiments on oceanic basalts strongly confirmed these observations.