Study On The Desorption Characteristics And Control Mechanism Of Coalbed Methane Under The Action Of Multiple Factors

CBM resources are abundant,but the recovery rate in actual production is low.Studying the desorption of methane in coal is not only beneficial to improve the recovery efficiency of coalbed methane,but also has important guiding significance for the co-production of coal and coalbed methane.In this dissertation,by selecting coal samples from southern margin of Junggar basin,eastern margin of Ordos Basin and Qinshui Basin,and adopting the experimental test methods of maceral analysis,industrial analysis,liquid nitrogen adsorption / desorption and methane isothermal adsorption / desorption,the characteristics and main controlling factors of methane desorption under the constraints of metamorphic degree,temperature and moisture content of coal were determined,the inefficient desorption stage,slow desorption stage,fast desorption stage and sensitive desorption stage were divided,the desorption formula of coalbed methane under multi factor constraints is constructed.The detailed results are as follows:(1)Determined the pore size distribution,pore morphology and heterogeneity characteristics of different rank coals.With the help of liquid nitrogen adsorption experiments,it is found that the specific surface area and pore volume of the coal sample show a trend of first decreasing and then increasing with the increase of the degree of metamorphism,and the proportion of micropores is the largest.Based on the analysis of liquid nitrogen adsorption loops,it can be seen that the low rank coal samples are mainly open pores,the middle rank samples are mainly impermeable pores closed at one end,and the high rank samples are mainly ink bottle pores.Pore heterogeneity is mainly controlled by multiple factors such as microscopic composition,material composition and pore structure by FHH fractal model.(2)the control mechanism of temperature,pressure,water content,pore structure and coal quality on the desorption capacity of methane in coal reservoirs was clarified by using methane adsorption/desorption experiments.The results shows that the higher the content of micropores in coal,the more difficult it is to desorb methane.The difficulty of desorbing methane from coal with different pore shapes is from high to low: ink bottle holes,open pores,and impermeable pores closed at one end;the desorption capacity of methane increases with the increase of temperature,and decreases with the increase of pressure and water content;Meanwhile,the degree of metamorphism and microscopic composition will also affect the desorption capacity of coalbed methane.Among them,the pore structure is the main controlling factor affecting methane desorption.(3)By introducing the concept of desorption efficiency,the initial pressure,transition pressure and sensitive pressure are clarified,and then the methane desorption process is divided into four desorption stages: inefficient desorption stage,slow desorption stage,fast desorption stage and sensitive desorption stage stage.Among them,the critical pressure is greatly affected by temperature.(4)Based on the characteristics of methane desorption under single factor constraints,the formulas of methane desorption rate under multi-factor constraints such as temperature-pressure,temperature-water content and temperature-pressure-water content are calculated and deduced,and the mechanism of coalbed methane desorption under multi-factor constraints is clarified.