The Effect Of Coal Molecular Structure And Environment On The Formation And Migration Of Coalbed Gas

The formation and migration of coalbed gas are the critical scientific issuesfor the control and extraction of coalbed gas. The existence of tectonic coal isthe necessary factor for coal and gas outburst. Under the condition of tectonicstress, the molecular structure of coal is deformed, and then the nano-scalestructures even the molecular structures are changed. It ultimately affects theadsorption and diffusion of coalbed gas, and leads to the coalbed gas in tectoniccoal with high adsorption and diffusion capacity. Therefore, investigating theeffect of coal molecular structure and environment on the formation andmigration of coalbed gas, explaining the formation mechanism of coalbed gas,analyzing the micro-factors which affect the migration of coalbed gas, obtainingthe microcrystal structure parameters and the micro-model of tectonic coalcorresponding to its characteristics, and probing the influence of environment onthe migration of coalbed gas are not only has important theoretical meaning inunderstanding the mechanism of coal and gas outburst, but also has clearpractical significance in guiding the control and development of coalbed gas. In this study, the degradation and polycondensation formation mechanismof coalbed gas in different temperature have been obtained by using the densityfunctional theory calculation method. The effects of coal molecular structure onthe migration of coalbed gas have been studied by using the Monte Carlosimulation method, molecular dynamics simulation method anddispersion-corrected density functional theory calculation method. The influenceof environment on migration of coalbed gas has been discussed by using theMonte Carlo simulation method and molecular dynamics simulation method.The main conclusions are as follows:The formation mechanism of coalbed gas via degradation andpolycondensation during the coal evolution shows that, increasing reactiontemperature favors the acceleration of coalbed gas formation. Participation of Hradicals can significantly improve the ability of coalbed gas formation. Similarchemical environment at the position of bond breaking leads to little differencefor coalbed gas formation. The longer side chains in the macromolecularstructure of coal, such as–CH2CH3,–CH2OH, are more prone to degradation.The presences of–OH and–COOH in side chain are more prone topolycondensation, and the presences of–CH3and–OCH3in side chain causingthe ability of degradation and polycondensation are equal.The migration of coalbed gas under different molecular structure of coalhas been investigated. It indicates that the adsorption volume on unit mass ofcoal is essentially the same, while the diffusion coefficient of coalbed gas is increased with the increase of La. The adsorption volume on unit mass of coal isreduced with the increase of N, however, the diffusion coefficient is not amonotonic change, but a complex process of interaction of many factors. Theinteraction between coalbed gas and surfaces is enhanced under the presence ofsingle–defect and oxygen–containing functional groups, at the same time, therate of diffusion is increased under the presence of single–defect and–C=O. Thecoalbed gas is easier to diffusion in larger fracture of coal. Analyzing theadsorption volume, adsorption energy and diffusion coefficients of coalbed gascomprehensively, the microcrystal structure parameters of tectonic coal wereobtained: La=20.313nm, Lc=13.6–15.2nm, La/Lc=1.34–1.49, N=4and n=253.In addition, effects of environment on the migration of coalbed gas are alsoobtained. It can be concluded that the increase of equilibrium pressure favors theadsorption of coalbed gas, and the pressure which leading coalbed gas reachessaturation is higher than6MPa. The increase of temperature favors the diffusionof coalbed gas, and furthermore, the effects caused by temperature changes aremore significant at higher pressure. The presence of CO2and H2O molecules arenot conducive to the diffusion of coalbed gas, moreover, the influence of CO2isgreater than H2O.