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Numerical Modeling of Adsorption and Roughness Effects on Gas Transport in Shale Using the Lattice Boltzmann Method

Abstract

Advanced techniques have greatly promoted the exploitation of shale gas from shale matrix with low permeability. But the gas transport mechanism in shale is still unclear. Due to the multi-scale pore systems in shale reservoir, viscous flow, slip flow, Knudsen diffusion and adsorption/desorption should be applied to the flowing equation so that it can properly predict gas flow dynamics. Multi-scale simulations are performed to investigate shale transport mechanism in organic matters by LBM. Firstly, a generalized model with roughness surface was proposed to investigate the competitive effects of gas slippage, roughness surface and adsorption layer on permeability of shale matrix. A pressure-dependent adsorption thickness and relative roughness were adopted in the present study. Then a multiple-scale integration method was proposed to upscale the pore-scale simulations to field-scale problems. Decline curve analysis was studied to predict the production of shale gas. The results showed that the surface roughness has remarkable effects on gas flow. The streamlines near rough surface are distorted and the axial velocity is strengthened. This is because gas molecular and rough surface is undergoing multi-collisions under roughness diastema region. Shale tends to adsorb on the organic matter surface, so the roughness surface also affects the adsorption. Adsorbed layer affects the gas flow in organic matters through two ways: reducing the volume of void space and changing the slippage. The existence of adsorption will reduce the actual pore size and the higher the pressure, the lower the actual pore size. The positive effect of slippage overwhelms the negative effect of adsorption. Under lower pressure, slippage is stronger and the volume of adsorption is less, leading to higher apparent permeability than intrinsic permeability. In DCA, the rarefaction effect is found to be dominant at the early stage while the compressibility effect becomes dominant at late stage, while respectively results in an overestimation and an underestimation of the gas production. The permeability estimation is also compared with the experimental data and a good match is achieved. This paper adopted a pressure-dependent adsorption thickness and a novel boundary scheme, reduces the mass flux error near the solid surface, to conduct further studies of the microcosmic transport mechanism.