Numerical description of coalbed methane desorption stages based on isothermal adsorption experiment

Abstract

Quantitative description of desorption stages of coalbed methane is an important basis to objectively understand the production of coalbed methane well, to diagnose the production state, and to optimize the management of draining and collection of coalbed methane. A series of isothermal adsorption experiments were carried out with 12 anthracite samples from 6 coalbed methane wells located in the south of the Qinshui Basin, based on the results of isothermal adsorption experiments, and an analytical model was developed based on the Langmuir sorption theory. With the model, a numerical method that adopts equivalent desorption rate and its curve was established, which can be used to characterize the staged desorption of coalbed methane. According to the experimental and numerical characterizations, three key pressure points determined by the equivalent desorption rate curvature that defines pressure-declining desorption stage, have been proposed and confirmed, namely, start-up pressure, transition pressure and sensitive pressure. By using these three key pressure points, the process of coalbed methane desorption associated with isothermal adsorption experiments can be divided into four stages, i.e., zero desorption stage, slow desorption stage, transition desorption stage, and sensitive desorption stage. According to analogy analysis, there are differences and similarities between the processes of coalbed methane desorption identified by isothermal adsorption experiments and observed in gas production. Moreover, it has been found that larger Langmuir volume and ratio of Langmuir constants are beneficial to earlier advent of steady production stage, whereas it is also possible that the declining production stage may occur ahead of schedule.