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

Abstract

Abstract Coalbed methane (CBM) recovery is a process of desorption rather than adsorption. Traditional experimental studies of methane adsorption/desorption on moist coal samples illustrate that, there is little hysteresis between adsorption and desorption processes, and these two behavior could be described by Langmuir equation. However, it is widely believed that coal porosity is normally occupied by water, and few researches focus on the gas sorption mechanism in condition of pore systems saturated with water. In this condition, there is an equilibrium between methane molecular dissolution into pore water and adsorption on (or desorption from) pore surface. And it is a typical liquid-solid internal interaction rather than gas-solid internal interaction. This paper presents experiments with methane adsorption/desorption on coal samples saturated with water and illustrates the significant difference between liquid-solid adsorption and gas-solid adsorption. In this work, we carry out three sets of experiments with coal samples immersed in water, and the equilibrium pressure is P=5.445 MPa, 5.220 MPa and 2.965 MPa respectively. Subsequently, we conduct desorption experiments based on the former adsorption experiments to simulate the production process in aqueous environment with decreasing pressure. During desorption process, we are surprised to find that desorption amount changes slightly with different pressure drops, and desorption ratio for four sets of experiments is only 3.36%, 4.98% and 4.21% respectively. This dramatic results indicate that the desorption amount from saturated aqueous solution is not sensitive to pressure, which differs considerably from gas-phase desorption case. Furthermore, we analyze the Gibbs free energy change during adsorption/desortion processes in these two different systems: gas-solid and liquid-solid interface systems. It should be noted that, in an ideal gas-solid interface system (such as adsorption/desortion on flat surface case without the effect of pore structure), the Gibbs free energy change in adsorption are approximately equal to that in desortion processes, thus, the phase transition between absorbed layer and bulk is quasi-reversible, and no hysteresis exist in these two processes. However, in the liquid-solid interface system, due to the additional energy consumed by nucleation, it requires more energy for unit molar of methane during desorption process than adsorption process, which indictes that adsoption/desorption of liquid-solid interface are unreversible. Based on this theory, we established a hysteretic model to describe the hysteresis phenomenon during adsoption/desorption from aqueous environment, and this model can be perfectly verified by our experiment results. Our result demonstrates that the process of the methane desorption from micropores under water saturated condition is hysteretic seriously. This meaningful finding indicates that when almost no free gas exists in matrix pores, i.e., only absorbed gas and dissolved gas exist, and adsorbed gas almost cannot desorb and gas production of CBM wells will be very low. In other words, the larger amount of adsorbed gas doesn't always yield the larger gas productivity, and free gas in matrix pores could induce adsorbed gas to desorb.