Effects of Pressure and Temperature on Gas Diffusion and Flow for Primary and Enhanced Coalbed Methane Recovery

Abstract

Due to the rapid increase of coalbed methane (CBM) exploration and development activities in China, gas adsorption and flow behavior for Chinese coals are of great interest for the industry and research community. How pressure and temperature affect the gas adsorption and flow on different rank coals are not only important for CBM recovery but also important for CO2 or N2 enhanced CBM recovery, since gases are often injected at a temperature different to the reservoir temperature. In this work, gas adsorption and permeability of three different rank Chinese coals are measured using CH4, N2 and CO2 at three temperatures, 20°C, 35°C and 50°C. Gas diffusivity and permeability with respect to gas species, pore pressure, effective stress and temperature are studied. The three coals are SQB-1 from Southern Qinshui Basin, JB-1 from Junggar Basin and OB-1 from Ordos Basin. Gas adsorption results show that both pressure and temperature have significant impact on adsorption behavior for SQB-1 and JB-1 using CH4. For higher rank coal SQB-1, adsorption isotherm tends to reach adsorption capacity quicker with respect to pressure. However, the maximum adsorption capacity is higher for the lower rank coal JB-1. Moreover, temperature has a stronger effect on reducing adsorption capacity for lower rank coal. Gas diffusivity results for OB-1 and JB-1 show that CO2 diffusivity is generally higher than that of CH4 and then N2. This could be related with their different kinetic diameters and their interaction with the coal. Both pressure and temperature have impact on gas diffusivity. In general, gas diffusivities increase with pressure and temperature. Permeability results show that it varies greatly with respect to coal rank with highest rank coal having the lowest permeability. Permeability is also strongly sensitive to effective stress and pore pressure. Temperature has a noticeable impact on permeability change. Permeability changes differently with temperature increase for the different rank coal samples studied. This may be attributed to the combined effect of coal strain change due to gas adsorption and thermal expansion. These results have significant implications for the design of enhanced CBM recovery and CO2 storage for different rank coals as injecting gas at different temperature and pressure would affect the CO2 injectivity and the CBM production rate.