Many shale/tight gas reservoirs can have pore scale values in the range from one to hundreds of nanometer. And the flow in nano-scale deviate the Darcy's law. Knudsen diffusion and/or gas slippage effects usually have modeled to character the non-Darcy flow mechanisms by many authors.
In this paper, we investigate the non-Darcy flow mechanisms in unconventional gas reservoirs, and classify these various mechanisms based on different pore scale and pressure. Then, based on the change of pore scale and pressure, the models of gas flow that consider the absorption, desorption, slip flow, transition flow, Knudsen diffusion and continuous flow in nano-pore have been proposed to evaluate the flow character. Then, the relationship between the absorbed layers and pressure or Langmuir coefficient has been built and the influences of absorption of gas molecule have been studied on the permeability change. Compared with experimental value, the model could agree with the experimental value very well. And, desorption of the absorbed layers make the pore diameter become larger. When the thickness of the absorbed layers and the pore diameter ratio is larger than 0.1, the effect of adsorbed layer becomes very significant.
With this study, the change of permeability and the gas rate on entire long term production performance could be understood better and predicted, and it is very important for the optimization of production performance and adjustment.