Methane adsorption characteristics and adsorbed gas content of low-rank coal in China

Coalbed methane (CBM) reserve of low-rank coal (R < 0.65), occupying a proportion of more than 40% of the total CBM reserve of China, is 14.7 × 10 m. The exploration degree, particularly in situ gas content estimation of low-rank CBM reservoirs in China, is extremely low. Meanwhile, adsorption characteristics of low-rank coal reservoirs have not been revealed comprehensively. In this study, methane isothermal adsorption experiments of 30 low-rank coal samples collected from typical low-rank coal-bearing basins and coalfields in China were performed under different temperatures and moisture-equilibrated conditions. The results showed that the Langmuir volume, which was found to be relatively low when the inertinite content was above 50% and relatively high when the vitrinite content was above 50%, was positively correlated with the maximum vitrinite reflectance and generally decreased with the increase of the equilibrium moisture content. The adsorbed gas content decreased as the temperature increased, while the relationship between methane adsorption decrement and pressure was described by logarithmic form. The in situ adsorbed gas content of low-rank coal reservoirs increased slightly or almost unchanged with the increase of the burial depth. These characteristics were quite different from those of middle- or high-rank coal reservoirs, and the calculation method for estimation of the in situ absorbed gas content was validated to be correct based on a comparative analysis between the calculated results and actual test results. Findings in this work are helpful to further understand the adsorption characteristics as well as adsorbed gas content of low-rank coal to improve the evaluation technique of the CBM reserve of low-rank coal reservoirs