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Due to the nanometer scale pore size and extremely low permeability of a shale matrix, flow and Knudsen diffusion. Three transport models modified by the Darcy equation with apparent permeability are used to describe the combined gas transport mechanisms in ultra-tight porous media, porosity model with consideration of the combined gas transport mechanisms are developed to evaluate with consideration of Knudsen diffusion and the tighter the shale matrix, the greater difference of the gas production estimates. In addition, the numerical simulation results indicate that shale fractures have a great impact on shale gas production. Shale gas cannot be produced economically without fractures.