Gas Flow Transport in Shale Matrix: Simultaneous Effects of Stress and Slippage on Matrix Permeability

Gas flow modelling in shale and tight gas reservoirs is still challenging mainly due to different pore-scale flow regimes present in micro- and nano-pores of these reservoirs. The effect of geomechanical stress also significantly affect the measurement and prediction of apparent matrix permeability. In this study, series of experiments were designed and performed on three shale samples to study the simultaneous effects of slippage and stress at five different pore pressures and four net stresses. The experimental data were used to obtain a general slip plot, which quantifies the effect of slippage on matrix permeability. Then, the stress effect was taken into account by modifying the average pore size and non-slip permeability at each net stress based on the experimental observations. It is found that the matrix non-slip permeability and average pore size follow an exponential behaviour when changing the net stress. These two relationships are then proposed to be incorporated into the corresponding slip flow model in order to capture the effects of slippage and stress at the same time. The validity of the proposed model was also investigated (using published data in the literature), which shows that the proposed technique is able to capture the intensity of permeability reduction and enhancement due to stress and slippage, respectively. The outcomes of this study increase our knowledge of rarefied flow dynamic inside micro- and nano-pores under confining stress, which is necessary for accurate predictions of the apparent matrix permeability in unconventional reservoirs.