| The heat injection technology is one of the effective means of increasing coalbed methane production.Most of the reservoir conditions exceed the critical temperature and critical pressure of methane,so most of the methane in the coal seam is in the supercritical state.When the heat is increased,the first problem is how to replace the supercritical methane in adsorbed state.When the water vapor is injected into the coal seam,the water vapor also adsorbs on the surface of the coal pore fissure,breaking the adsorption equilibrium of adsorption of the coal and supercritical methane.Since the supercritical methane in the coal seam is mostly present in the adsorbed state,it is important to study the adsorption of methane and water vapor in the supercritical state.Based on the multidisciplinary theory of thermodynamics,physical chemistry,organic chemistry and quantum chemistry,the adsorption characteristics of methane in supercritical state are analyzed by theoretical analysis,numerical calculation and quantum chemical simulation,and the effects of water vapor with the supercritical methane are discussed,the main conclusions are as follows:Firstly,the supercritical methane adsorption model was established according to the Potential Adsorption Theory,and compared with the isothermal adsorption line predicted by the isosteric heat.Using the theoretical model of adsorption potential theory and the method of isosteric heat,the supercritical methane adsorption capacity can be calculated according to the isothermal adsorption test data under different temperature and pressure conditions.The adsorption isotherms at 40℃ are predicted by the adsorption isotherms at 27.5℃ and 50℃.Compared with the experimental ones,the prediction results of the isosteric heat are similar to the predicted results of the adsorption potentials,but the heoretical calculation process of Adsorption Potential Theory is simple.Secondly,based on the calculation of quantum chemistry,the Gaussian software was used to calculate and simulate the coal molecules,which are respectively represented by lignitous coal molecules,high-volatile bituminous coal and anthracite.Between the coal molecules and CH4 molecules,and the adsorption between water molecules calculations proves that:（1）The adsorption sites of CH4 adsorbed on coal in different coal rank are different.The most stable sites of CH4 adsorption in lignitous coal are 3.9683×10-10 m above the lignite,and the adsorption energy is-20.92kJ/mol.The most stable adsorption site of the high-volatile bituminous coal molecule is above the molecules.The distance between the molecules is 4.596×10-10 m and the adsorption energy is-27.79kJ/mol.The most stable site of anthracite adsorption is at 4.6107×10-10 m above the anthracite coal molecule,and the range distances of adsorption of CH4 are within the range of van der Waals force,indicating that no matter what the coal rank is,the adsorption is physical adsorption,and with the degree of metamorphic increase,adsorption of CH4 can increase,that is,the higher the coal rank,the more stable adsorption.（2）When the water molecules adsorbed equilibrium,H2O and lignite coal molecule,high-volatile bituminous coal molecule and anthracite hydroxyl molecules in the formation of hydrogen bonds,through the Mulliken charge layout analysis,that the coal molecules in the hydroxyl hydrogen is donor,water molecule is the hydrogen bond acceptor,and the hydroxyl bond energy in the coal molecule and the hydroxyl bond energy in the water molecule are changed due to the charge shift.There are many kinds of oxygen functional groups in the lignite coal,and the adsorption capacity of carboxyl groups is the strongest,and the adsorption structure is the most stable.By comparing the adsorption energy of adsorption of CH4 and the adsorption energy of H2O,the adsorption capacity of different coal coal ions to H2O is stronger than that of CH4,and the hydrogen bond is stronger than van der Waals force,which proves that the feasibility of heat injection.（3）When CH4 and H2O are present in the adsorption system,H2O will seize the adsorption sites of CH4,and the distance between H2O and coal molecules is shorter than that of CH4.It shows that there is competitive adsorption between H2O and CH4,and the ability to be stronger than the adsorption of CH4,consistent with the previous calculation results.Finally,in order to maximize the replacement of supercritical methane in the adsorbed state without injecting excess water vapor,resulting in waste of energy,combined with N-A equation and the relation between adsorption capacity and adsorption energy,given the consideration of water vapor and supercritical methane adsorption adsorption optimum ratio,the heating process has a certain reference significance. |
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