Experimental Investigation about Gas Transport in Tight Shales: An Improved Relationship between Gas Slippage and Petrophysical Properties

This study presents both experimental and theoretical investigations about gas transport in shales. Gas apparent permeability coefficients and Klinkenberg slippage factors were determined on Longmaxi shales using He, Ar, N-2, CH4, and CO2. Then, a model was developed to interpret the experimentally determined gas slippage factor, considering the effects of intrinsic permeability, porosity, tortuosity, and gas physical properties. The proposed model is verified by correlating Klinkenberg-corrected permeabilities and gas slippage factors of shales probed by He, Ar, N-2, CH4, and CO2 at different confining pressures. The model can quantitatively describe the gas dependence of slippage factors (He > Ar > N-2 > CH4 > CO2). According to the model presented, the slippage factor increases proportionally to the ratio of the characteristic gas parameter (C) to tortuosity. The model also leads to a practicable approach to determine the effective tortuosity of tight rocks at in situ reservoir stress state. Effective tortuosity of shales determined using helium slippage measurements are far larger than the generally assumed values. Another advantage of the model is its ability to quantitatively account for the variation in permeability values at similar gas slippage and the counterintuitive reduction in gas slippage during compaction observed in previous experiments. The proposed model correctly matches a set of gas slippage measurements and provides insight into gas transport in tight porous medium.