Jacketed and unjacketed stress-strain data, jacketed and unjacketed bulk moduli, and porosity calculations, all as a function of confining pressure, are presented for a suite of igneous, metamorphic, and sedimentary rocks. The permeability of rocks to combinations of confining pressure and pore pressure is investigated. Simple elasticity considerations, which exclude irreversible effects such as hysteresis, indicate that differential pressure, the difference between confining and pore pressure, should determine the permeability in homogeneous porous materials in which the solid phase has a bulk modulus of typical rock-forming minerals. Experimental measurements of permeability for a sample of Chelmsford granite at two different pore pressures support this conclusion since the differential pressure determines permeability to within experimental error. The effect is observed with a 50,000 ppm NaCl solution, but only on the first cycle of variation in pore and confining pressures.
Further cycles result in reduction and perhaps disappearance of the effect.
Interpretation of permeability measurements and pore pressure effects are complicated by the pressure of natural hydrocarbons in Berea sandstone and by its water sensitivity. Simple considerations of rock elasticity similar to those in permeability analysis indicate that differential pressure, the difference between confining pressure and pore pressure, should determine velocity in the absence of effects due to pore fluid properties and hysteresis. Pore fluids used include nitrogen gas, benzene, and water. In general, however, the determination of velocities by differential pressure can only be considered an approximation, although probably a good one in water-saturated rocks. Measurements were made at ambient laboratory temperatures.