Master role conversion between diffusion and seepage on coalbed methane production: Implications for adjusting suction pressure on extraction borehole

Diffusion and seepage control coalbed methane (CBM) production jointly, and their controlling roles differ significantly in different stages of extraction. Although many CBM models have well been established, but these models cannot better quantify the controlling degree of diffusion and seepage on CBM production at a certain moment and determine when the master role between them happens to convert. In this paper, the conversion between different forms of methane and the underlying controlling mechanisms were analyzed firstly. According to contributions of different forms of methane to total production, a theoretical master role conversion model between diffusion and seepage on CBM production was established. And the theoretical model can better distribute the contribution degree of in-situ adsorbed methane to the total CBM production. The model was employed to analyze the time nodes when master role conversion occurs with different initial permeabilities and diffusion coefficients. Results show that higher coal permeability/diffusion coefficient results in earlier the conversion time node. The same conclusion also applies to the time node when the diffusion effect almost completely controls the CBM production. Besides, the change curves of daily methane production indicate that the production is primarily controlled by seepage in early stage of extraction, while controlled by diffusion in later stage. Methane extraction efficiency and concentration are closely related to drainage pressure. For ensuring underground mining safety, constant low-pressure extraction strategy is employed frequently to insure the extraction efficiency, which, however, leads to low methane concentration and utilization rate. This paper proposes a new extraction pressure regulating method, namely, time-based pressure regulating method that selected time node in accordance with the master role conversion theory. The simulation results prove that the new method can not only ensure extraction efficiency, but also guarantee extraction concentration as well as utilization, thus reducing the natural emissions of methane and amount of greenhouse gas.