Calculation Model of Shale Reserves Considering the Adsorption Layer Based on Molecular Simulation

In this paper, molecular simulation methods are used to construct nanopore models of organic matter, montmorillonite, and quartz. The occurrence state of CH₄ molecules in shale nanopores was simulated, and the distribution characteristics of CH₄ molecules at different temperatures, pressures, and pore widths were obtained. The thickness and density of the adsorption layer of CH₄ molecules at different temperatures and pressures were studied. On the basis of these, a calculation model of shale gas reserves considering adsorption is proposed. The results show that CH₄ molecules in shale nanopores present a nonuniform distribution. Two obvious wave peaks form in the space close to the surfaces of the shale nanopore, and the wave peaks increase with the increase of pressure. As the pressure increases, a second peak appears and gradually becomes larger. The adsorption layer formed on the surface of the medium has a certain thickness and density, which are affected by pressure and temperature. In the calculation example, the difference between the calculation results of the shale gas reserve calculation models considering and not considering the adsorption layer is about 26%. The higher the proportion of adsorbed gas, the greater the calculation error, which is related to pressure and adsorption capacity.