Lattice Boltzmann method simulations about shale gas flow in contracting nano-channels

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Shale matrix consists of numerous interconnected nanoscale slit pores with size ranging from 4 nm to 200 nm, naturally resulting in variable cross-section scenarios for shale gas transport. Understanding the behavior of shale gas flow in variable cross-section channels would have practical benefits to further reveal the gas flow mechanism. Herein, using multiple-relaxation time lattice Boltzmann method (LBM) simulations, we investigate the flow characteristic of shale gas in sudden and gradual contraction channels. The influences of Knudsen number and cross-section shrinking coefficient on the shale gas transport are discussed in details. We find that the relationship between centerline velocity and Knudsen number is dependent on the gas flow regime. The nonlinear pressure deviation shows a negative correlation with the Knudsen number. The slip mass flux and the second-order permeability correlation factor are found to have positive correlations with the cross-section shrinking coefficient, demonstrating that the shale gas apparent permeability would be underestimated without considering the influence of cross-section contraction. Moreover, we reveal that the shrinking of pore channel has a great influence on Knudsen minimum effect. A high cross-section shrinking coefficient and dramatically varied cross-section type will lead to a low critical Knudsen number of Knudsen minimum effect. The LBM results and insight obtained here would be conductive to understanding shale gas transport in contracting nano-channels.