Molecular Simulation Study Of Methane Desorption And Diffusion Process In Coalbed Methane Drainage

Zhaozhuang 3# coal seam is a typical soft,low-permeability coal seam,where Coalbed methane(CBM)wells have lower productivity and are more difficult to develop.In order to find the cause of this phenomenon and obtain its output law,the grand canonical Monte Carlo(GCMC)and molecular dynamics(MD)were systematically used to study the adsorption and diffusion process of methane in the pores of Zhaozhuang 3# coal and compare with its adsorption and diffusion process in Chengzhuang 3# coal.At the same time,the effect of different conditions on the adsorption of methane in Zhaozhuang 3 # coal and its diffusion law were discussed,and the methane in Zhaozhuang 3# coal under various influencing factors Empirical formula was obtained.The main contents are as follows:Based on Zhaozhuang 3# coal and Chengzhuang 3# coal macromolecular models,Zhaozhuang 3# coal and Chengzhuang 3# coal structure models were constructed.The GCMC method was used to calculate the adsorption isotherms of methane in two coals at a series of temperatures.The calculation results showed that Zhaozhuang 3# coal has lower methane adsorption capacity than Chengzhuang 3# coal at the same temperature.By comparison,it was found that the presence of aromatic rings is beneficial to the adsorption of methane molecules,and the larger the oxygen content,the more unfavorable is the adsorption of methane molecules.By studying the effects of temperature,moisture content,and stress/strain on methane adsorption,it could be seen that as the temperature increases,the amount of methane adsorption decreases regularly in the studied temperature range.An empirical formula for methane adsorption capacity under the influence of temperature was obtained.Because water molecules are easier to adsorb than methane molecules,water molecules preferentially occupy the adsorption sites on the coal surface,which reduces the amount of methane adsorption.The larger the moisture content,the more obvious the reduction of methane adsorption.An empirical formula for methane adsorption capacity under the influence of moisture content was obtained.As the stress/strain increases,the more the corresponding adsorption sites are,the more the methane adsorption capacity accordingly rises.An empirical formula for methane adsorption capacity under the influence of stress/strain was obtained.Based on the MD method,the diffusion situations of methane in Zhaozhuang 3# coal and Chengzhuang 3# coal at 298 K were compared,and the diffusion coefficients of methane in Zhaozhuang 3# coal and Chengzhuang 3# coal at a series of temperatures were calculated by analyzing the mean square displacement(MSD).Meanwhile,the activation energy of methane diffusion in Zhaozhuang 3# coal and Chengzhuang 3# coal was obtained.It can be seen that the methane diffusion process of Zhaozhuang 3# coal is much harder.It is also the direct reason for the low production and difficult development of Zhaozhuang 3# CBM well.