New Model for Gas Transport in Microfractures in Shale Reservoirs: Integration of Effective Stress, Gas Adsorption, and Second-Order Slippage

The natural microfractures, which are widely present in shale rock, are the dominant channels for shale gas seepage. However, most of the existing studies on gas seepage in shale microfractures have not considered the second-order slip effect, and there is a lack of detailed discussion on the quantitative characterization of the slip factor and the influencing factors. In this study, a dynamic apparent permeability model for shale microfractures is proposed considering the second-order slip correction by quantifying the microfracture width evolution through the geometric relationship between pore size and microfracture, considering the influence of real gas effect, effective stress, and gas adsorption, and then deriving the expression for the slip factor in microfractures. The accuracy of the developed model has also been verified by using publicly published experimental permeability data under various conditions, with the effects of effective stress and adsorption on the slip factor and apparent permeability under different stress conditions being analyzed. Finally, the conversion laws relating to the slip factors of each order in association with shale's apparent permeability in microfractures under the influence of different factors have been discussed. And parameter sensitivity analysis was carried out to reveal the effects of the internal swelling coefficient, the compressibility coefficient, and adsorption deformation parameters on gas transmission. The results of this work should contribute to a better understanding of changes in the slip factors and apparent permeability in shale microfractures, which should be of great importance for the accurate assessment of the production potential and economic development of shale reservoirs.