Temperature effect on methane sorption and diffusion in coal: application for thermal recovery from coal seam gas reservoirs

Abstract

Investigating the effects of in situ thermal treatment on coal seams requires adequate knowledge of gas sorption and its kinet¬ics in coal at various temperatures. Methane sorption onto two Australian coal samples (high-volatile bituminous) at dry state and different temperatures was measured. Methane adsorption isotherms were measured at pressures up to 7 MPa by the gas ad¬sorption manometric method. Adsorption isotherms data at two temperatures were used to investigate the effects of in situ thermal treatment on critical desorption pressure, ultimate gas recovery and the diffusion coefficient in coal. An increase of experimental temperature from 308 to 348 K resulted in a 50% reduction in the adsorption affinity of the coal sample and an insignificant reduction in the saturation capacity of the isotherms. At higher ex¬perimental temperatures, Langmuir isotherms exhibit downward shift with the initial gas content of the coal seam being constant, resulting in critical gas desorption pressure increase. According to the measured Langmuir isotherms at different temperatures, an increase in reservoir temperature by 1 K leads to a 2% and 1.2% increase in total recovery for the tested coal seams. Gas left in the coal seam at the abandonment pressure can only be recovered at a higher reservoir temperature. Diffusion coefficients of coal seam samples were calculated for different experimental temperatures. Fractional uptakes of the first coal sample show a good agreement with the results obtained using the unipore diffusion model with the diffusion coefficient to be 4.7 × 10–12 m2/s at 348 K. For the second coal sample, the unipore diffusion model fairly matches the uptake data. A bidisperse diffusion model was also applied to measure the adsorp¬tion kinetics of the second coal sample, resulting in an improved agreement with the experimental uptake data. Both coal samples exhibited a reduction of the diffusion coefficient with an increase in equilibrium pressure; this effect was more pronounced at equilibrium pressures below 0.045 MPa. It was observed that the diffusion coefficient change with pressure becomes flat at high pressures, with the pressure effect diminishing much faster at lower temperatures.