Factors influencing the production of coalbed methane from deep reservoirs

Abstract

ObjectiveThe production characteristics of coalbed methane (CBM) from deep reservoirs differ significantly from those of CBM from shallow reservoirs. Key challenges in deep CBM production include maintaining reservoir permeability or minimizing permeability loss, enhancing CBM (CH4) desorption efficiency, and accurately predicting the laws of CH4 diffusion. There is an urgent need to overcome these challenges through technological innovation and theoretical research. MethodsThis study systematically analyzed the advances in domestic and international research on coal reservoir permeability, CBM desorption, and CBM diffusion. By integrating the classification of production stages of deep coal reservoirs with the dominant CBM migration mechanisms of varying stages, this study summarized the mechanisms and influential factors of deep CBM production. Results and Conclusions The results indicate that deep CBM production can be divided into four stages: rapid production increase, relatively stable production, gradual production decrease, and low production. During the former two stages, reservoir pressure remains high, free gas serves as a primary gas source, and methane migration is dominated by seepage flow. Key influential factors of both stages include coal structure, developmental degrees of pores and fractures, reservoir temperature, in situ stress, and effective stress. At these stages, minimizing permeability loss is crucial, and direct fracturing should be avoided in reservoirs with a high proportion of granulated and mylonite coals. After the relatively stable production phase, an increase in the reservoir permeability caused by reservoir temperature will gradually increase with an enhancement in the slip effect. Controlling pressure drop and slow production can help to slow down the decline of reservoir permeability. In the low-production stage, the permeability loss rate caused by both primary and artificially induced fractures approaches 100%. However, the irreversible permeability loss rate remains significantly lower than that of shallow coal reservoirs, suggesting the feasibility of secondary reservoir stimulation for increased production. From the rapid production increase stage to the relatively stable production stage, the adsorbed gas begins to undergo gradual desorption. In this case, the primary objectives are to expand the desorption range, ensure the opening of seepage channels, and enhance the productivity of CBM wells. Compared to shallow reservoirs, the desorption of adsorbed gas in deep coal reservoirs occurs over a prolonged period, with the critical desorption pressure being challenging to determine accurately. Furthermore, the pathways for gas migration are prone to be compressed and close, leading to a limited desorption range. To achieve precise estimations of CBM recovery rates, it is necessary to adopt a stepwise depressurization desorption method in experimental research. Specifically, achieving a gradual decrease in the reservoir pressure using control measures during CBM production can effectively enhance the desorption rate of adsorbed gas in micropores. In the low-production stage, gas production primarily originates from desorbed gas in remote well areas. In this stage, the production of CBM wells is determined by methane diffusion, with the accurate measurement of the diffusion coefficient and the development of dynamic diffusion models playing a crucial role. Notably, the diffusion coefficient exhibits significant anisotropy, yet current CH4 diffusion models seldom account for the anisotropic characteristics of coal structure. It is necessary to develop a time-varying CH4 diffusion model while considering the CH4 diffusion patterns across multi-scale pores and microfractures in coals. Experiments on the fine-scale characterization of multi-scale pores and fractures, combined with high-temperature with high-pressure nuclear magnetic resonance imaging, allow for the characterization of variations in CH4 density across different pore sizes. This systematic review integrates theories and practice, further laying a theoretical foundation for deep CBM recovery.

Keywords

• coalbed methane (cbm)
• permeability
• desorption
• diffusion
• deep coal reservoir