Experimental study of pressure sensitivity in shale rocks: Effects of pore shape and gas slippage

Abstract

Abstract The permeability and porosity significantly impact the flow and storage of fluids in shale reservoirs. In this work, permeability, porosity, gas slippage factors, and gas diffusion coefficients of different shale samples were systematically measured at varying confining pressures and pore pressures. Before the experiments, the scanning electron microscope (SEM), N2 adsorption, and X-Ray Diffraction (XRD) analyses were used to characterize the shale samples. Results indicate that while the pressure sensitivity coefficient is smaller than 1 for permeability, it is slightly larger than 1 for porosity. In addition, pore shape significantly impacts the pressure sensitivity of porosity and permeability. For cores with a large number of crack-like pores, the permeability was decreased by two to three orders of magnitude and the porosity was decreased by approximately 32%. However, for cores dominated by circular and irregular-shaped pores, the permeability varied by one order of magnitude, and the porosity was reduced by14% to 19%. Moreover, effective gas diffusion coefficients and slippage factors were measured at different pore pressure and confining pressure. The results show that gas diffusion coefficients varied from 0.95 to 3.17×10-7 m2/s, and gas slippage factors ranged from 0.64 to1.25 MPa. While the gas diffusion coefficient decreased exponentially with confining pressure, it increased exponentially with rock intrinsic permeability (k∞). The gas slippage factor increased exponentially with confining pressure, however, it decreased and exhibited a power relationship with k∞. Those experimental results provide a comprehensive understanding of pressure sensitivity, gas slippage factors, and gas diffusion coefficients in shale rocks.