Effects of CO₂ adsorption on coal deformation during geological sequestration

Adsorption-induced deformation of coal during carbon dioxide sequestration in coal seams at elevated pressures and temperatures is studied with the quenched solid density functional theory (QSDFT) model. Two types of deformation behaviors in pores of different sizes can be identified from the dependence of the solvation pressure on the CO₂ pressure. The smallest micropores (<0.5 nm, at 360 K) monotonically expand as the pressure increases. The larger pores (>0.5 nm) tend to contract at low pressures (1-10 MPa); however, this low-pressure contraction is followed by expansion as the pressure increases further. Comparison with methane adsorption under the same geological conditions shows that the adsorption capacity of carbon dioxide is larger than that of methane. The difference in volumetric strain induced by adsorption of carbon dioxide and methane is most pronounced for micropores (2 nm), where the volumetric strain difference can be as large as 1.7% in the case of a 0.7 nm pore at 100 m depth, which could cause a significant reduction in permeability of the reservoir due to coal deformation. The contrast between the adsorption stress, resulting from the displacement of methane by CO₂, decreases to 0.6% at 5 nm pores with increasing pore size and gradually diminishes in larger mesopores. The conclusions of the QSDFT model are validated by comparison with experimental data from the available literature and can be used for quantitative estimates of the effects of coal deformation.