Simultaneous determination of the effective stress coefficients for permeability and volumetric strain on a tight sandstone

Gas transport processes in low-permeable (tight) sandstones at elevated pore pressures involve a combination of fluid-dynamic (slip flow) and poro-elastic or rock-mechanical stress (pore and bulk compressibility) effects. To assess the interdependency of both processes, permeability experiments with gas (argon) were combined with volumetric strain measurements. The flow studies were performed under controlled effective stress (10, 20 and 30 MPa) with pore and confining pressures up to 30 and 50 MPa, respectively. The results of the permeability and strain measurements could only be described by the modified effective stress laws for apparent permeability coefficients (chi = 1.25) and volumetric strain (Biot alpha = 0.7), respectively. Measured apparent permeability coefficients (similar to 10-(18) m(2)) were implemented into a permeability model considering both gas and stress effects individually. A "clay-shell" pore structure model was used to describe the interrelated changes in permeability and bulk volume with respect to pore and confining pressure.