Quantitative evaluation of the effect of gas adsorption and desorption hysteresis on gas flow in coal

Coal mining methane (CMM) is a major hazard source and a typical greenhouse gas in the coal industry. Gas extraction is an effective solution to eliminate coal seam gas disasters and reduce greenhouse gas emissions. Multi-field coupling model of gas flow in coal (MCMGF) is an important theoretical basis for guiding gas extraction, and the adsorption model is an important component of MCMGF. Numerous studies have shown that there is a hysteresis effect in the desorption of gas in coal compared to the adsorption process. Since gas extraction is a depressurized desorption process, HE should be taken into account when building the adsorption model. However, most of the current MCMGFs ignore the hysteresis effect, resulting in an overestimation of the amount of gas involved in the desorption process. Therefore, in this paper, the calculation method of the degree of hysteresis index (DHI) is revised and the adsorption model is modified based on systematically sorting out the adsorption–desorption hysteresis phenomenon. On this basis, a coupled gas flow model considering the hysteresis effect of adsorption and desorption was constructed, and the reliability of the model was verified by isothermal adsorption and desorption experiments on coal pillars. Eventually, the influence of different hysteresis degrees (DHI = 0, 0.25 and 0.59) on gas extraction was quantitatively evaluated by using the new model. The results show that hysteresis effect has a significant influence on gas extraction, and the larger DHI value is, the smaller the daily extraction is. Ignoring the influence of adsorption–desorption hysteresis will make the gas extraction capacity of the borehole overestimated. After 160 d of extraction, the difference between the residual gas content when adsorption–desorption hysteresis is considered and when it is ignored is up to 2.05 m3/t. This is because the adsorption–desorption hysteresis limits the ultimate amount of gas that can participate in the flow. In our previous studies, it has been found that the additional resistance to gas desorption is intrinsic to the hysteresis. With the increase of the mining depth, the in-situ stress increases rapidly, therefore the influence of coal adsorption–desorption hysteresis on gas extraction under high stress is one of the problems that need to be considered emphatically.