New Insights from Supercritical Methane Adsorption in Coal: Gas Resource Estimation, Thermodynamics, and Engineering Application

Accurate coal-bed methane (CBM) resource estimation becomes significant for CBM extraction through ground CBM wells and underground coal mines, and adsorbed gas accounts for more than 80% of the total CBM in place resource in subsurface coal seams. However, the CBM resource estimation presents many challenges, especially the inappropriate adsorbed gas estimation method and the oversimplified adsorption thermodynamics description. This work tackles both issues by utilizing a dual-site Langmuir model to describe supercritical methane adsorption behavior in anthracite and analyze the corresponding thermodynamic characteristics. The proposed model not only accurately describes measured adsorption isotherms under elevated pressures (up to 17 MPa) and temperatures (up to 352.55 K) and interprets all observed adsorption phenomena, but also can extrapolate the adsorbed gas content and the total gas content at subsurface conditions beyond test conditions. The estimated density of the adsorbed methane is found to be temperature and pressure dependent, which is higher than the gas density but lower than the liquid methane density. The isosteric enthalpy of adsorption for methane in coal shows adsorption uptake dependence and temperature dependence behavior. Using the classic simplified Clausius-Clapeyron equation overestimates the isosteric enthalpy of adsorption, which cannot reveal the associated temperature dependence behavior. Furthermore, the proposed method is applied for estimating deep CBM in place resource and understanding coal and gas outburst prediction technology by using the temperature measurement approach.