Study On Coalbed Methane Desorption Hysteresis Effect

The characteristics of coalbed methane adsorption desorption is an important factor todetermine the CBM reservoir with gas content and the potential for coalbed methanedevelopment. Most people of CBM industry think that the coalbed methane desorption is thereverse process of adsorption. But a large number of adsorption desorption experimentalresults and the spot production data of coalbed methane wells show that adsorption processand desorption process is not fully reversible, and desorption process lags behind theadsorption process; The adsorption desorption reversible standpoint causes the misjudgmentof the critical desorption pressure and the control errors of production system, which lead to adirect impact on the production of coalbed methane wells and steady production. Theisothermal adsorption/desorption experiments of different coal ranks，which come from theactive regions of the domestic coal bed methane exploration and development in current weredesigned. Meanwhile the experiments of coal analysis, pore analysis, scanning electronmicroscopy, low temperature nitrogen adsorption and mercury penetration etc.were conducted.Combined with the coalbed methane geology, physics and chemistry etc. related disciplinesand the spot production data, the comprehensive analysis was conducted to study themechanism of coalbed methane desorption hysteresis effect.The results show that the main pore of low rank and medium rank coal are holes andmicroporous, and the ratio of the holes is higher than the proportion of micropores. The mainpore of high rank is microporous.The main contribution of specific surface area is themicropore, followed by the holes, and then the mesoporous and macroporous.The experimentof macroscopic lithotypes separation found the pore connectivity of HuangLing vitrain andTieFa coal is the best, followed by the Sihe durain and the LiuLin coal, and the Sihe vitrainand Huangling durain is poor.With the coal rank increasing, the temperature reduction, theparticle size decreasing, the moisture content reducing, the adsorption capacity of methane inthe coal matrix was heightening and the adsorption rate and desorption rate was increasing.There is unconspicuous influence on coalbed methane adsorption desorption forwater,temperature and particle size differences of low rank coal.Coalbed methane adsorptionprocess and desorption process is not fully reversible, and desorption process lags behind theadsorption process. The main controlling factors that influence the coalbed methanedesorption hysteresis are the CBM desorption heat, the characteristics of the pore throat andmoisture content of the coal.Combined with the results of previous studies, expanded research on the mechanism ofcoalbed methane desorption, put forward the microscopic desorption and macroscopicdesorption of coalbed methane and coalbed methane desorption hysteresis effect concept, theresearch results apply to the field production wells, proposed a judgment method of CBMcritical desorption pressure and an evaluation method of the coalbed methane desorptionhysteresis effects.The smaller the CBM reservoir gas saturation and the lower the permeability,the more obvious desorption hysteresis effect, the greater influence on the yield of coal bedmethane wells, which is more obvious in high rank coal reservoirs than in the lower rankcoal’s. Based on the research results, there is an important guiding significance forestablishing the coalbed methane drainage system, forecasting the production of coalbedmethane well and formulating the development planning.