Evaluation of adsorbed and free gas in the coal matrix during desorption processes: Insights from experimental and numerical methods

Investigating desorption and diffusion characteristics of coalbed methane holds significant importance in enhancing methane recovery efficiency and preventing gas disasters. There are still challenges in monitoring and evaluating both adsorbed and free gas content. This work systematically explores the dynamic behaviors of gas desorption with the coal matrix through desorption experiments by the volumetric method and nuclear magnetic resonance technique. The relationship between gas desorption capacity and time is investigated, while the proportion of free gas and adsorbed gas are analyzed under varying pressures. Furthermore, a mathematical model of gas desorption is developed based on density gradient and numerically solved using the finite volume method. The gas desorption process in coal matrix is visualized by a self-developed simulator, and the changes in gas content and pressure are analyzed under different initial desorption conditions. The results show that: (i) The reciprocal of cumulative gas desorption capacity (1/Q) Q ) is in line with the desorption time (t t-n ). The relationship between adsorbed gas content and pressure can be described by the Langmuir equation, while the relationship between free gas content and pressure is linear. (ii) Relative errors between desorption results obtained through the volumetric method and NMR method remain below 5%. The proportion of free gas to total gas content increases with the increasing pressure. (iii) Higher initial gas pressure exerts a more pronounced driving effect on the gas desorption process, thereby advancing the onset time of gas content reduction. This research aims to provide a comprehensive perspective for an accurate assessment of the gas desorption process.