Effect of water on methane and carbon dioxide sorption in clay minerals by Monte Carlo simulations

Shale is mainly composed of: (1) inorganic, and (2) organic materials. As an important constituent of inorganic matter, clay minerals may affect gas-in-place of shale permeable media. Clay minerals are hydrophilic. Hydrophilicity may affect pore space saturation with water in shale media. In this work, we investigate the effect of water on methane and CO2 sorption in clay minerals by using grand canonical Monte Carlo simulations. Our investigation reveals that water may significantly reduce methane and CO2 sorption in clay nanopores. In small pores (1 nm), water and CO2, and water and methane adsorbed in the same layer. In large pores (>2 nm), water molecules adsorb on the first layer, and CO2 and methane form a weak second layer adsorption. CO2 unlike methane both in pure and low water content conditions may form multi-layer adsorption at high pressure. Multilayer adsorption results in significant increase of sorption with pressure. Langmuir adsorption model cannot be used for such descriptions. Our study on sorption of CO2 and water mixtures in clay minerals shows that with a small amount of water in the domain outside the nanopores, CO2 sorption is significantly reduced. In larger pores (>2 nm), gas molecules mainly accumulate in the middle of the pores.