Research on the Evaluation of Negative Pressure Drainage for Enhancing Coalbed Methane Recovery Efficiency

Coalbed methane primarily exists in coal seams in both adsorbed and free states. Due to factors such as formation pressure, coal properties, and reservoir conditions, shallow CBM reservoirs are dominated by adsorbed gas, with minimal free gas content. In contrast, deep coal seams contain more than 50% adsorbed gas. Therefore, the efficient extraction of adsorbed gas plays a crucial role in determining CBM development effectiveness and recovery rates. Negative-pressure drainage has proven to be an effective stimulation technique for improving CBM production performance. To evaluate how negative-pressure drainage enhances the desorption efficiency of adsorbed gas in coal seams, this study first analyzed the influence of pressure reduction on desorption behavior under different conditions using isothermal adsorption curves. Subsequently, a laboratory simulation experiment was designed to investigate variations in desorbed gas volume under different negative-pressure conditions. Based on these experimental results, a numerical simulation model for negative-pressure drainage was established to assess its impact on CBM recovery rates and optimize operational parameters. The study indicates that there exists a critical desorption pressure for coalbed methane adsorption-desorption. Below this critical pressure, the volume of desorbed gas increases rapidly. The application of negative pressure extraction can significantly enhance desorbed gas volume during the late-stage development of coalbed methane. Numerical simulation evaluations suggest that a negative pressure range of 0.2–0.4 MPa notably improves desorption efficiency. As a stimulation measure for late-stage coalbed methane development, negative pressure extraction can markedly improve the desorption efficiency of adsorbed gas, enhance coalbed methane recovery rates, and optimize development outcomes.