Pore structure in sandstones from velocities with increasing pore pressure

SUMMARY Pore structure is an important parameter controlling the storage capacity and transport properties of porous rocks and determining their pressure-dependent elastic properties. However, pore structure is predominantly inverted from velocities with increasing confining pressure and it remains unclear whether the pore structure from velocities with increasing pore pressure differs. We develop an improved pore-structure inversion method that incorporates the linear reduction of stiff porosity with pressure to extract the complete aspect ratio distribution of compliant cracks from pressure-dependent velocities. We also measure the compressional and shear wave velocities and porosity of two dry Berea sandstone samples as a function of both increasing confining pressure and increasing pore pressure. The pore-structure inversion method is applied to the two samples to obtain and compare their pore structures from the velocities measured with different pressure paths. The results show systematically higher velocities and lower porosities for the increasing pore pressure path at equivalent differential pressures. The inverted pore structures show a substantially greater cumulative crack porosity and density from velocities with increasing confining pressure, and reveal a markedly smaller population of compliant cracks, albeit distributed over a slightly broader range of lower aspect ratios from velocities with increasing confining pressure. The difference in the pore structures from velocities with different pressure paths is explained in terms of the crack hysteresis mechanism. The results have helped to explain the greater velocities and smaller porosity of the samples measured with increasing pore pressure, and would help for the estimation of capacity and permeability of CO2 and hydrogen stored reservoirs and for the more accurate prediction of pore pressure in hydrocarbon generated overpressure zones.