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 kinetics 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 adsorption 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 experimental 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 adsorption 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.

Feasibility Study of Thermally Enhanced Gas Recovery of Coal Seam Gas Reservoirs Using Geothermal Resources

“…Figure shows methane storage capacity for a coal seam at two different temperatures . All experimental data are well fitted with the Langmuir equation.…”

“…Figure 5 shows methane storage capacity for a coal seam at two different temperatures. 15 All experimental data are well fitted with the Langmuir equation. The Langmuir isotherm at 75 °C was measured experimentally and also approximated using the Arrhenius rate equation.…”

Coalbed methane emissions and drainage methods in underground mining for mining safety and environmental benefits: A review

“…The effects of temperature on coal bed properties, e.g. methane sorption, heat flow is investigated (Salmachi and Haghighi, 2012b). Compared with the conventional stimulation method, thermal treatment could lead to 58% more gas in a production period of 12 years (Salmachi and Haghighi, 2012a).…”

Evolutions of Pore and Crack Structure of Coal under Hot Steam Heating

“…Salmachi et al 24 measured the methane adsorption of two types of bituminous coal in dry state and at different temperatures. When the experimental temperature increased from 308 to 348 K, the adsorption affinity of coal sample decreased by 50% Wang et al 15 studied high-temperature N 2 injection into coal seams by experiment and numerical simulation.…”