Direct measurement of the unjacketed pore modulus of porous solids

Abstract

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 .