Why an indirect estimate of the unjacketed pore modulus may not work

The unjacketed pore modulus is a measure of changes in pore volume due to the variation in pore pressure for constant Terzaghi effective pressure. In the oil and gas industry, this parameter is commonly used in linking the undrained bulk modulus of a porous saturated rock to the drained bulk modulus and the properties of the fluid and solid components through Gassmann's equation. For an ideal porous rock, the unjacketed pore modulus should be identical to the solid bulk modulus of the major mineral constituent. While direct measurements confirm this prediction, previous indirect estimates suggest that this parameter may have values close to the bulk modulus of the fluid rather than the bulk modulus of the grains. These indirect results demonstrate that the estimate of the undrained bulk modulus, due to substitution of different fluids in the rock, can change considerably if the unjacketed pore modulus is equal to the bulk modulus of the fluid. In this paper, we first briefly provide the results of the direct approach and then present laboratory measurements of Skempton's pore pressure coefficient for a quartz arenite sandstone and use these data to indirectly estimate the unjacketed pore modulus. We show that the difficulties associated with the measurement of the Skempton coefficient clearly contribute to the underestimation from the indirect approach.