Molecular simulation of the adsorption-induced deformation during CO2 sequestration in shale and coal carbon slit pores

The swelling induced by CO2 adsorption is a major concern for CO2 sequestration in coal and shale. Deeper insight into the mechanism of adsorption-induced swelling is essential for the CO2 geological storage. In this work, we have used grand canonical Monte Carlo simulation to study the adsorption-induced deformation strain based on the deformable organic carbon slit pore models. In particular, we studied the microstructure and distribution of the adsorbed molecules to obtain the adsorption location effect on swelling. The results showed the deformations of both swelling and shrinkage are sensitive to the pore size. The pores below 0.55 nm have no deformation. The 0.55-0.6 nm pores show the maximum swelling, corresponding to the largest adsorption density. The shrinkage occurs in the 0.65-0.80 nm range depending on pressure and temperature. When the pressure increases, the swelling is enhanced while the shrinkage is mitigated, while the pore size for the maximum swelling or shrinkage remains the same. When the temperature increases, both swelling and shrinkage decrease due to decreased adsorption density. The results showed the CO2 molecules adsorbed in different locations exert heterogeneous solvation pressures across the pore. Generally, the molecules close to pore walls are parallel, tending to swell the pore, which plays a dominant role in the deformation. However, when the pore size increases, some molecules exert a negative solvation pressure, with inclined and vertical orientations, tending to contract the pore although swelling is the total deformation.