Mechanism of Liquid-Phase Adsorption and Desorption in Coalbed Methane Systems: A New Insight into an Old Problem

Coalbed-methane (CBM) recovery is a process of desorption rather than adsorption. Traditional experimental studies of methane adsorption/desorption on dry or moist coal samples illustrate little hysteresis between adsorption and desorption isotherms. However, coal porosity is always occupied by water in actual conditions, and few researches focus on the gas sorption from water-saturated coals. In such conditions, there is an equilibrium between methane-molecule dissolution into pore water and adsorption on (or desorption from) pore surface, which indicates a typical liquid/solid sorption process rather than gas/solid sorption process. This paper presents experiments with methane adsorption/desorption on coal samples saturated with water. To our surprise, the desorption amount changes slightly with different pressure drops, and the desorption ratio for each of our three sets of experiments is only 3.36, 4.98, and 4.21%, respectively. This dramatic result indicates that sorption from saturated aqueous solution presents significant hysteresis, which considerably differs from a gas-phase sorption case (e.g., sorption on dry or moist samples). On the basis of our theoretical analysis, this hysteresis is caused mainly by additional energy consumed by phase behavior (e.g., nucleation) and capillary resistance in a gas/water/solid system. Further, a unified model is established instead of the traditional Langmuir equation to describe the hysteresis phenomenon between adsorption and desorption from an aqueous environment. Our present work demonstrates that pore water will play a significant role in resisting methane desorption, and the high water saturation in the CBM system will lead to extremely low recovery. Therefore, using the traditional sorption-isotherm curve to predict deliverability of a gas well may result in a large deviation in some cases.