Laboratory measurements under hydrostatic pressure demonstrate both permeability and specific storage are dependent upon effective confining pressure, where both properties decrease as effective hydrostatic pressure is increased. Within the limits of experimental reproducibility, values of permeability in a single effective pressure loading cycle agree with the effective stress concept, i.e. that permeability is approximately constant for a given Peff irrespective of the particular combination of Pcon and Pflu used to achieve the given effective pressure. However, permeability and specific storage are found to be non-linearly related to effective pressure over the effective pressure range experienced by the sample (20 MPa - 80 MPa), and Biot's a parameter for permeability and specific storage varies as a function of effective pressure. For a given sample of Tennessee sandstone values of a for permeability range from 1.1 at low effective pressures (relatively high permeability) to 0.5 at relatively high effective pressures (relatively low permeability). Therefore, the simple effective stress law, where Peff = Pcon - aPflu and where a = 1 does not hold for permeability and specific storage over the whole range of test effective pressures, inferring that the stress dependence of the a parameter for permeability and specific storage should be characterized for individual materials.
1. INTRODUCTION
A wide range of rock properties and processes, including permeability and fluid specific storage, depend on effective pressure. Effective pressure can be defined as Peff = Pcon - a Pflu where Peff is the effective confining pressure, Pcon the external confining pressure, Pflu the internal pore pressure, and a a poro-elastic multiplier termed the Biot effective stress parameter. The 'simple' effective pressure law, taken to be the difference between Pcon and Pflu with a = 1, applies for some rock properties but does not hold universally. Not only is a specific to a particular property, but it is also markedly sensitive to the magnitude of applied effective pressure.
Hydrostatic compression experiments have been conducted to investigate the effective pressure sensitivity of both permeability and specific storage in a low porosity tight reservoir sandstone analogue (Tennessee sandstone). The effective stress a parameter for both permeability and specific storage has been determined. A novel, single-ended transient pulse permeability measurement technique was implemented to enable synchronous measurement of fluid flow and specific storage. Derivation of the a parameter for specific storage is unique to this study.
Laboratory measurements under hydrostatic pressure demonstrate both permeability and specific storage are dependent upon effective confining pressure, where both properties decrease as effective hydrostatic pressure is increased. Within the limits of experimental reproducibility, values of permeability in a single effective pressure loading cycle agree with the effective stress concept, i.e. that permeability is approximately constant for a given Peff irrespective of the particular combination of Pcon and Pflu used to achieve the given effective pressure. However, permeability and specific storage are found to be non-linearly related to effective pressure over the effective pressure range experienced by the sample (20 MPa - 80 MPa), ie Biot's a parameter for permeability and specific storage varies as a function of effective pressure. For a given sample of Tennessee sandstone value
