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.

Automated summary

The elastic properties of rocks depend on various factors, including mineral constituents, pore structure, saturating fluids, and stress conditions. A new acoustoelasticity model has been proposed to study these properties, considering the effects of crack closure and squirt-flow mechanism. The behavior of wave velocity variations in rocks is observed to be dominated by crack properties and then transitioning to acoustoelastic effects as pressure increases.