Abstract: Shale gas is affected by many factors during nanopore transport, including pore size and pressure, pore wall surface roughness, pore mechanics reaction, adsorption-induced expansion reaction, and weighting factors. Therefore, the effects that these factors and the space occupied by the adsorbed gas molecules in the pores have on the gas flow must be considered. This is necessary to clarify the contribution to the total gas flow in the nanopores resulting from different migration mechanisms of shale gas (surface diffusion, slip flow, Knudsen diffusion, and viscous flow) based on different pore sizes and pressures. First, physical descriptions and mathematical characterizations of different migration mechanisms of shale gas are provided. A mathematical gas transport coupling model for shale gas is then developed that considers pore wall surface roughness, pore mechanics reaction, adsorption-induced expansion reaction, and weighting factors. The reliability of the model is verified by the lattice Boltzmann method. The results show that when the pore diameter is less than 10 nm, the total flow in the nanopores mainly consists of surface diffusion flux. In addition, the smaller the pore size, the greater is the surface diffusion flux. When the pore diameter is 40~250 nm at low pressure, the slip flow and Knudsen diffusion have a considerable effect on gas transport. When the pore diameter is longer than 10 μm, the total flow in the nanopores is primarily viscous.

Key words: shale gas, surface diffusion, pore mechanics reaction, adsorption-induced expansion reaction, weighting factor