Molecular dynamics simulation of methane adsorption in shale matrix

With the increasing demand of natural gas, shale gas gains growing attention as a new type of unconventional natural gas resources. Shale gas is natural gas stored in shale formations in adsorbed, free and dissolved gas form, whose main component is methane. The adsorbed gas is mainly adsorbed on the surface of organics and mineral grains, and the free gas is stored in the slit-pore between the organics and rock grain. The adsorption of methane in the slit-pore has significant effects on the reservoir engineering and transport of shale gas. In order to understand the adsorption mechanism of methane in shale, molecular dynamics simulations of methane adsorption in pores of different solid materials under different pressure and temperature are performed. Results reveal that adsorbed layers are formed near the solid walls due to the adsorption effects of the solid wall, the density of which is higher than that of the free gas region in the middle of the pores where pore pressure is defined. Under relatively low pressures, only one adsorbed layer is observed. As the pressure increases, a second adsorbed layer is gradually formed. When the width of a pore is less than 1.0 nm, methane molecules are always under the influence of adsorption from the pore wall, and adsorption with three density peaks can be observed. The densities in the adsorbed layer and the free gas region achieve constant values if the pore width is wider than a critical value under a certain pressure. Methane is more likely to be adsorbed on the organic matter surface. Besides, the crystal surface of calcite also affects the adsorption.