Molecular dynamics simulation of micro mechanisms of gas and water migration and diffusion in shale reservoirs

Clarifying the fluid transport mechanism in nanoporous shale media for shale gas extraction is crucial. In this study, the microscopic mechanism of gas and water migration and diffusion in the nanopores of shale clay minerals is obtained through molecular dynamics simulation. Because of the different compositions and structures of different clay minerals in shale, the adsorption heats of different clay mineral surfaces with CH4 and H2O molecules are different. As the pressure increases, the adsorption heat of the clay mineral surfaces with CH4 and H2O molecules increases. At the same temperature and pressure, the adsorption heat of the clay mineral surfaces with H2O molecules is greater than that with CH4 molecules, and the high-energy adsorption sites on the surface of clay minerals are mainly occupied by H2O molecules, leading to H2O molecules significantly aggregating on the shale clay mineral surfaces, while CH4 molecules are dispersed in an disordered manner in shale clay minerals. The higher the pressure or the lower the temperature is in shale formations, the higher is the degree of aggregation of H2O molecules on the surface of clay minerals. The migration speed and fluidity of gas and water increase with the increase in temperature and decrease with the increase in pressure. At the same temperature and pressure, the migration speed of CH4 molecules is greater than that of H2O molecules. This is because H2O molecules are polar and have stronger intermolecular forces compared to CH4 molecules, resulting in weaker mobility of H2O molecules. The diffusion coefficient of CH4 molecules in kaolinite is the highest, followed by that in illite, and that in montmorillonite is the lowest. When the diffusion coefficient of H2O molecules in shale clay minerals is lower, the corresponding diffusion coefficient of CH4 molecules is higher.