Effect of confining pressure on sorption parameters and structure of coal in terms of analyzing the possibility of underground CO2 storage with CH4 recovery.

An original apparatus was constructed, allowing observation of sorption processes occurring in hard coal briquettes. The investigations were carried out on four hard coals from Poland and France differing in the degree of coalification. The coal briquettes were subjected to confining pressure with values in the range of 1.5-30 MPa. The pressure of sorbates (CO2, CH4) was regulated and controlled in the range up to 1MPa. With the measurement system, the effect of swelling of coals on the sorption processes was observed. Successive sorption points were recorded, from which isotherms were extrapolated and kinetics of combined processes of sorption and gas transport were analyzed. All the coals studied showed a decrease in the amplitude of the Langmuir isotherm for CH4, with increasing confining pressure. This decrease ranges from 0.2 to 0.5 cm3CH4/g for each 10 MPa change in confining pressure. For one of the coals tested, the decrease in the amplitude of the Langmuir isotherm with increasing confining pressure was higher than the others and was about 2 cm3CH4/g per 10 MPa change in confining pressure. The swelling of the coal material examined at the last point of the CH4 sorption isotherm (0.8 MPa) ranged from 0.45% to 0.8% of the initial volume for a confining pressure of 1.5 MPa. A high confining pressure of 30MPa resulted in a reduction of swelling to between 0.1% and 0.2%. The reduction in sorption capacity of coal relative to CO2 after sorption exchange was also analyzed. The relative reduction of the CO2 sorption capacity at an ambient pressure of 30 MPa versus 1.5 MPa ranged from about 4% to 8%. Before and after the sorption processes, the structural parameters of the coal briquettes were investigated using porosimetric and optical methods. Parameters such as volume and distribution of micropores and specific surface area were determined. Performing sorption tests on samples subjected to such high confining pressure, at sorbate pressures up to 1MPa, corresponds to the parameters of the rock-gas system that occur in typical hard coal seams.