A Study on the Factors Influencing the Production of Deep Coalbed Methane

[Objective] The production of deep coalbed methane (CBM) differs significantly from that of shallow CBM. Maintaining reservoir permeability or minimizing permeability loss, enhancing the desorption efficiency of CBM (CH₄), and accurately predicting the diffusion patterns are major challenges faced in the production of deep CBM. These issues urgently require breakthroughs through technological innovation and theoretical research. [Methods] Systematically analyze and summarize the progress in fundamental research on coal reservoir permeability, desorption, and diffusion around the world. [Results and Conclusions] Taking into account the characteristics of deep coal reservoirs and the various forms of coalbed gas migration during different production stages, the following conclusions are drawn: (1) the production of deep CBM can be divided into four stages: rapid production increase, relatively stable production, slow decline in production, and low-production stage. (2) During the rapid production increase and relatively stable production stages, the reservoir pressure is high, and the gas source is primarily free gas. Methane migration is dominated by seepage, with key influencing factors including coal structure, the development of pores and fractures, reservoir temperature, in-situ stress, and effective stress. During this period, it is essential to minimize the loss of permeability as much as possible, and avoid directly fracturing reservoirs with a high proportion of ruptured coal and mylonite coal. After the relatively stable production phase, the permeability increase due to reservoir temperature will gradually increase with the enhancement of the slip effect. Controlling pressure drop and slow production can help to slow down the decline of reservoir permeability. Although in the low-production stage, the permeability loss in primary and artificial fractures approaches 100%, the irreversible permeability loss is much lower than that in shallow coal reservoirs, indicating the feasibility of reservoir secondary enhancement for increased production. (3) As production increases rapidly and reaches a relatively stable stage, the adsorbed gas begins to desorb gradually. Expanding the desorption range, ensuring the continuity of seepage channels, and enhancing CBM well productivity become the primary objectives. Desorption in deep coal reservoirs takes significantly longer than in shallow ones, and the critical desorption pressure is difficult to determine accurately. The transport channels are prone to compression and closure, limiting the desorption range. Experimental studies should adopt a stepwise depressurization desorption method to precisely predict the methane recovery rate. In production engineering, controlling the reservoir pressure to decrease slowly can effectively enhance the desorption rate of adsorbed gas in micropores. (4) During the low-production stage, methane supply is mainly provided by desorbed gas from far-well area, and methane diffusion dictates the CBM well output. Accurate measurement of diffusion coefficients and construction of dynamic models are key factors. The anisotropic characteristics of the diffusion coefficient are significant, and current CH₄ diffusion models rarely consider the anisotropic characteristics of coal structure. A time-varying diffusion model for CH₄ should be developed, taking into account the diffusion patterns in coal's multi-scale pore and micro-fracture systems. Combined with fine-scale characterization experiments of coal's multi-scale pores and fractures and high-temperature, high-pressure nuclear magnetic resonance imaging analysis, the density changes of CH₄ between different pore sizes can be represented. This systematic summary and understanding combine theory with production practice, further improving the theoretical foundation for deep CBM development.