A Model for apparent permeability of organic slit nanopores in shale gas based on GCMC molecular simulation

Shale gas reservoirs exhibit the characteristics of low porosity, low permeability, and widespread development of nanopores. Organic matter is an important place for the development of nanopores, the Study of gas seepage characteristics in organic nanopores is of great significance for the stable and efficient development of shale gas reservoirs. In this paper, the adsorption of methane in organic slits of different widths was firstly simulated by Monte Carlo method. The results show that when the slit width is 1 nm, the adsorption layer in the pore is a single-layer adsorption. At this time, the adsorption layer in the pore does not conform to the Langmuir adsorption model, but presents an exponential adsorption model. When the slit width is greater than or equal to 1.5 nm and less than 3 nm, double-layer adsorption of methane molecules occurs in the pores. Each layer satisfies Langmuir adsorption law and has different adsorption parameters. When the slit width is greater than or equal to 3 nm, the density distribution of methane adsorption layer in the slits with different slit widths shows consistency under the same pressure condition, and the methane adsorption layer is in a bilayer adsorption state. Each layer satisfies the same parameter of the Langmuir monolayer adsorption law, respectively. Based on this, a shale gas organic slit flow model considering double adsorption layer surface diffusion, Knudsen diffusion, slip flow and nanopore deformation was established. Calculations proves that: <Circled Digit One> When the pore size is as small as 1-2nm, the apparent permeability decreases with the increase of pore size. When the pore size is larger than 3nm, the apparent permeability increases with the increase of pore size. <Circled Digit Two> As the pore size gradually increases, the contribution of surface diffusion of the adsorption layer to the overall apparent permeability becomes smaller and smaller, while the contribution of free gas becomes larger and larger; The contribution of Knudsen diffusion to free gas flow decreases gradually, while slip flow contributes increasingly to free gas flow. <Circled Digit Three> The larger the outer aspect ratio of the slit, the larger the overall apparent permeability, the smaller the share of adsorbed gas surface diffusion, and the larger the share occupied by the free gas phase.