Stress Effects on Wave Velocities of Rocks: Contribution of Crack Closure, Squirt Flow and Acoustoelasticity

The elastic properties of rocks depend on the mineral constituents, pore structure, saturating fluids, and stress (loading) conditions. To study these properties, we measured ultrasonic P- and S-wave velocities as a function of the differential (confining minus pore) pressure and propose an unrelaxed double-porosity acoustoelasticity model, which generalizes the single-porosity one. The new approach includes the effects of crack closure, based on the David-Zimmerman model, and the squirt-flow mechanism, based on the Gurevich model. When cracks are open at low differential pressures, their properties dominate the wave velocity variations, followed by the squirt-flow mechanism. Then, a transition occurs, where cracks partially close, and the squirt-flow effect vanishes. At high pressures, cracks close and acoustoelasticity effects prevail. This behavior is observed in sedimentary rocks, whereas in granites, which have a low crack content, the acoustoelastic effect is dominant at all pressures.