Deformation of coal induced by methane adsorption at geological conditions

The quenched solid density functional theory (QSDFT) is employed to study methane adsorption on coal at geological conditions. The main focus is made on coal deformation in the course of adsorption that may result in either expansion/swelling or contraction, depending upon the pressure, temperature, and pore size. Two qualitatively different types of deformation behavior were found depending upon the pore width. Type I shows a monotonic expansion in the whole pressure range. This behavior is characteristic for the smallest pores <1.3σ_ff (0.5 nm) that cannot accommodate more than one layer of methane. Type II displays contraction at low pressures followed by expansion. Type II behavior was found for several groups of pores, which can accommodate dense packing with an integer number (from 2 to 6) of adsorbed layers. The results of the QSDFT model are compared to literature experimental data, and the model is employed to study the adsorption behavior of model coals at elevated pressures and temperatures. We established the relationships between the methane capacity and the solvation pressure that it exerts on the coal matrix and the depth of coal bed for pores of different sizes. We found that the coal deformation depends upon the bed depth, and at different depths, it either swells or contracts depending upon the pore size distribution. The implications of these findings for evaluating coal gas resources and coal mine recovery are discussed.