Pressure decline caused by the extraction of oil from deep sedimentary layers depends on the pore modulus K pp , a poroelastic parameter that characterizes the effect of pressure change on pore volume under constant mean stress. Measurement of K pp is difficult, however, as it requires calibration to account for fluid compressibility and compliance of the testing system. Nevertheless, knowing the easily measurable drained pore modulus K p and adopting an assumption on the unjacketed pore modulus K s , it is possible to determine K pp because these pore moduli are related. Previous work on indirectly estimating K s claimed that K s is strongly dependent on Terzaghi effective pressure P and therefore not a constant; also, K s might be different from K s , the solid bulk modulus of the major mineral constituent. We overcome the limitations of the indirect approach by directly measuring K s . The experiments reveal that K s is indeed a constant and that for an ideal porous rock, the assumption of K s = K s holds. Furthermore, a constant K s implies that K p and K pp are functions of Terzaghi effective pressure only. These results provide a framework to accurately determine the Skempton coefficient B.
The volumetric response of fluid-saturated and linearly elastic rock due to a change of either mean stress or pore pressure is characterized by three independent material parameters. The unjacketed bulk modulus is a convenient choice because it can be directly measured in a laboratory test under a loading that preserves the difference between the mean stress and the pore pressure constant. For a monomineralic rock, the measurement of the unjacketed bulk modulus is ignored because it is assumed to be equal to the bulk modulus of the solid phase. To examine this assumption, we tested porous sandstones and limestones mainly composed of quartz and calcite, respectively, under the unjacketed condition. Special attention was dedicated to reaching full saturation ensuring the transmission of the pore pressure to the solid frame. The presence of microscale inhomogeneities, in the form of nonconnected (occluded) pores, was shown to cause a considerable difference between the unjacketed bulk modulus and the bulk modulus of the solid phase. Furthermore, we found the unjacketed bulk modulus to be independent of the unjacketed pressure and Terzaghi effective pressure and therefore a constant.
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