Sorption Capacity and Sorption Kinetic Measurements of CO2and CH4in Confined and Unconfined Bituminous Coal

Carbon dioxide injection into coal formations provides an opportunity to sequester carbon while simultaneously enhancing methane recovery. Although powdered coal samples provide a quick indication of the gas sorption capacity, underground storage takes place within compact coal monoliths, and therefore, it may be necessary to account for in situ conditions, specifically confining stress, for meaningful estimates. This study presents the sorption rates and sorption capacities of CO2 and CH4 for a bituminous coal sample in a whole sample and in pulverized form. The impact of confining stress on these sorption capacities of coal cores is evaluated with a multiple-point isotherm over a prolonged time period. The kinetics of the complex, heterogeneous processes occurring in a bituminous coal sample are quantified while under confining stress. Sorption capacities for a powdered sample are 1.17 and 0.66 mmol/g for CO2 and CH4, respectively. The application of 6.9 and 13.8 MPa of confining stress contributed to 39 and 64% CO2 sorption capacity reduction. Similarly, 85 and 91% CH4 uptake capacity reduction is observed at those confining stresses. The time-dependent gas diffusion parameters are quantified using the volumetric method with a mathematical analysis of the pressure-decay data. Carbon dioxide diffused through the coal faster than CH4. Initial exposure over a few days showed a rapid reduction in diffusion presumably as the macro- and mesopores filled. With longer exposure, 10 additional days, a steady slower diffusion is observed for CO2. The steady-state slower diffusion is achieved within a few days for CH4. It was found that the overall gas movement, specifically diffusion, is hindered by confining stresses and takes place at rates significantly less than in unconfined powder coal.