Division of the stages of coalbed methane desorption based on the Langmuir adsorption isotherm

An analytical theory of stages division of coalbed methane (CBM) desorption is proposed and established based on the Langmuir adsorption isothermal model. The concept of desorbed efficiency (eta) is proposed to quantitatively characterize the CBM desorbed rate. According to key nodes on mathematical curves, the starting pressure (P-st), turning pressure (P-tu), and sensitive pressure (P-se) are defined. The Langmuir desorption isotherm is divided into four stages, i.e., inefficient, slow, fast, and sensitive desorbed stages. Then, the theory is tested by methane adsorption-desorption isothermal experiments. It is noted that inefficient and slow desorbed stages have little contribution to CBM productivity, while fast and sensitive desorbed stages are very significant to CBM productivity. The adsorbed capacity and the reservoir pressure are two key factors for CBM desorption. Higher adsorbed capacity and relatively shallower burial depths are more favorable to CBM development.