Variation Characteristics Of Energy In The Process Of Coalbed Methane Adsorption And Desorption

The main component of coalbed methane(CBM)is methane,and more than 80%of methane is stored in coal in an adsorbed state.Therefore,understanding coal adsorption/desorption characteristics of methane is of great significance to the evaluation of coalbed methane content and the productivity prediction of coalbed methane wells.In this paper,taking coal with different coal rank in China as the research object.Through a series of experiments such as vitrinite reflectance analysis,industrial analysis,microscopic composition analysis,and isothermal adsorption/desorption experiment,the basic characteristics of coal samples with different coal rank are analyzed.And combining the theories of adsorption heat,adsorption potential and surface free energy,reveals the variation characteristics of energy in the process of methane adsorption and desorption.(1)Determined the pore size distribution and pore morphology characteristics of different rank coals.Based on the low-temperature N₂ experiment on the microscopic pore structure of coal,it is found that the specific surface area of coal first decreases and then increases with the increase of coal rank(R_o,max is between 0.5%and 2.54%).Micropores have the highest contribution to surface area.High-rank coals generally has higher pore volume than low and medium-rank coals.Through the analysis of N₂ adsorption loops,it is known that low-rank coals are mainly open pore,medium-rank coals are dominated by impermeable pores closed at one end,and high-rank coals mainly contains bottleneck pore.(2)The influence of coal rank and temperature on coal adsorption/desorption of methane is clarified,and its influence mechanism is revealed.Among the different adsorption/desorption models,the D-A equation and Weibull function are the best,but from the perspective of the physical meaning of the formula parameters and the calculation of the experimental pressure,the Langmuir equation and the desorption equation are better.The maximum adsorption capacity of coal to methane first decreases and then increases with the increase of coal rank,and reaches the minimum at R_o,m=1.18%.As the temperature increases,the maximum methane adsorption capacity decreases.Comparing the delayed desorption capacity,theoretical desorption rate and recovery factor of four typical coal samples at different temperatures,it is found that the methane production rate in low rank coal is higher.(3)Quantitatively calculating the changes of the isothermic adsorption heat and adsorption potential of coal during the adsorption/desorption process,and explaining the difference principle of coal adsorption/desorption characteristics under different conditions from the perspective of adsorption heat and adsorption potential.The isosteric heat of adsorption of coal is proportional to the adsorption amount.The order of the value is different under different adsorption amounts.The reason is that the higher the coverage of methane molecules on the coal matrix,the stronger the force between the methane molecules and the greater the value of isometric heat.The adsorption capacity is fixed,and the isometric heat of adsorption in the desorption process is greater than that in the adsorption process.It explains the mechanism of adsorption hysteresis from the energy point of view or reveals the existence of"re-adsorption".The adsorption potential of coal has a negative correlation with the adsorption volume.When the adsorption volume is constant,high-rank coal has higher adsorption potential value,and the adsorption potential of the desorption process is higher than that of the adsorption process.The adsorption potential of coal samples decreases with the increase of water content,and the more micropores in the coal,the greater the adsorption potential.(4)Clarified the change of the surface free energy of coal and the control effect of temperature during the process of methane adsorption/desorption.The cumulative decrease value of the surface free energy of coal increases when the pressure become higher,but the rate of increasing slows down,corresponding to the reduction of surface free energy change value at each pressure point.The change of surface free energy at each pressure point decreases with the increase of temperature,indicating that high temperature has an inhibitory effect on adsorption.When the pressure is the same,the surface free energy change value of the adsorption process is greater than that of the desorption process,showing that the amount of methane gas desorbed is less than the amount of adsorbed gas,and the difference between the two processes decreases with the increase of temperature,that is,high temperature promotes desorption.