Laboratory measurement of Biot's coefficient and pore pressure influence on poroelastic rock behaviour

Abstract

Understanding rock behaviour as a function of pore pressure and confining pressure is crucial for petroleum and geomechanical analysis. Indeed, deformation and local stress variations within hydrocarbon reservoirs and their surroundings occur due to pore pressure changes. Theoretically, pore pressure changes coupled with stress variations in hydrocarbon reservoirs are a function of the Biot’s coefficient, the elastic properties of the rock and the reservoir shape. Thus, in this study, the Biot’s coefficient was measured as a function of porosity, permeability, and volumetric strain for five Gosford sandstone samples. A triaxial loading system was used to measure rock volumetric strain while pore pressure and confining pressure were varied. The constant deformation technique was employed for these experiments; i.e. the variation of pore pressure created a volumetric strain, and the confining pressure required to restore the original volumetric strain was measured to calculate Biot’s coefficient. For the investigated samples, measured liquid permeabilities were in the range of 7–10 mD and Biot’s coefficients were 0.84–0.91. This is consistent with similar investigations by other researchers in which measured Biot’s coefficients were in the range of 0.65–0.90. This study thus illustrates how liquid permeability and the Biot’s coefficient decrease as a function of confining pressure.