Experimental and theoretical characterization of methane and CO₂ sorption hysteresis in coals based on Langmuir desorption

Sorption hysteresis is important for primary gas production and CO₂ sequestration in coalbed methane (CBM) reservoirs. We represent the degree of hysteresis using an areal hysteresis index (AHI) method incorporating a hysteresis parameter β, representing the ratio of available sorption sites for desorption relative to adsorption. This approach was applied to quantify the gas-coal sorption hysteresis to both methane and CO₂ on sub-bituminous and bituminous coals and on anthracite. A theoretical desorption model describes hysteresis based on the molecular dynamic equilibrium between gas adsorption and desorption rates. Volumetric excess ad-/desorption isotherms of methane and CO₂ show that adsorption capacities for anthracite are higher than those for the sub-bituminous and bituminous coals over the entire experimental pressure range. Hysteresis for CO₂ is greater than for methane for all four coal samples investigated. The relationships between β and Langmuir pressure, Langmuir volume and final equilibrium pressure are weakly negative with null relationships between β and coal properties. Sorption hysteresis is caused by Langmuir pressure and gas type based on theoretical and experimental analyses although its physical mechanism remains unclear. A β_CH4 − β_CO2 plot is proposed as a screening tool to determine likely gas substitution contents for enhanced coalbed methane (ECBM) and CO₂ sequestration in coals.