Gas slippage in anisotropically-stressed shale: An experimental study

Due to the extensive presence of nanopores, the slippage effect is frequently invoked in shale to enhance gas transport, therefore, causing the more than expected gas production. We conducted laboratory experiments on cubic shale samples (drilled from the Longmaxi Formation in southwestern China) to study the gas slippage effect with respect to anisotropic stresses and low pore pressures. The pore size distribution (PSD) of Longmaxi shale demonstrates that pore sizes mostly lie in the range of 8-400 nm and that gas transport in shale generally exists within the slip flow regime (0.001 < Knudsen number (K-n) < 0.1) given its distribution of pore sizes and pressures. The permeability dependence on effective stress became less significant when the deviatoric stress dropped. As a result of the gas slippage effect, the apparent permeability increased more rapidly as the pore pressure declined and this relationship was more prominent at low pore pressures. Also, shale permeability decreased with increasing stress anisotropy, with this effect dropping gradually. An almost 10- to 50-percent in permeability reduction response can be attributed to the stress anisotropy effect, reflecting a need for paying attention to this effect in permeability predictions. The gas slippage effect became significant with increasing effective stress and it is interesting that decreasing deviatoric stress enhanced the gas slippage effect dramatically. Additionally, we introduced a new relationship between the intrinsic permeability k(infinity) and gas slippage factor b for shale. The gas slippage factor first increased slightly and then significantly with decreasing intrinsic permeability. Moreover, the simultaneous effects of rising effective stress and diminished pore pressure (as commonly encountered during commercial gas production) both contribute to the deviation of the apparent permeability from that predicted according to Darcy flux. We hypothesize this new insight may be able to shed light upon the more than expected gas production in the petroleum industry, especially for shale gas. Finally, a theoretical model was introduced to describe permeability evolution by combining the stress and gas slippage effects. The model was in good accordance with the experimental results.