Experiment Studies On Displacing Deep Coalbed CH₄by Gas Injection And Characteristic Trace After Displacement

CO₂contributes the most to the greenhouse effect of the earth. CO₂sequestrationin deepunminable coal seam is a potential management option for greenhouse gas emissions,reducing CO₂emission meanwhile enhancing coalbed methane recovery, which has has greatpotential for development. And coal seam permeability is generally in China lower,traditional recovery methods of methane by depressurization of coalbeds yield only20～60%of CH₄in place and generally produce large volume of water at the same. There is aconsiderable number of methane in the coal seam. In order to recovery this part of theexploitation of resources, on the basis of increasing coal seam permeability, the technologythat CH₄is recovered by injecting another gas. But this technology is still in the exploratorystage. Based on the theories and methods of surface chemistry, fluid flow mechanics, finiteelement numerical analysis, CO₂flooding for deep coal CH₄dynamic process carried out bysystematic experiment and simulation study. The coal samples and gas samples were selectedcontinuously. Laboratory and field measured methods were used to carry out the gascomponent concentration and carbon isotope change, evolution of the physical and chemicalstructure characteristics of the coal. Coal-gas structure evolution was obtained in the originalseam away from the injection points over the geological time.Single gas adsorption isotherms and binary gas adsorption and desorption isotherms oncoal samples from Haishiwan coal mine in the Yaojie coalfield, were experimentally observed.The adsorption amount of CO₂on coal was significantly greater than the adsorption amountof CH₄on the coal, approximately1.36～2.1times. The total amount of adsorption ofCO₂-CH₄mixed gas on coal samples is between adsorption amount of pure CO₂andadsorption amount of pure CH₄, with the higher the proportion of CO₂gradually, the totalamount of adsorption of mixed gas on the coal gradually increases. According to theseparation information of binary gas composition both in free phase and adsorptive phase,CH₄tends to be desorbed preferentially, which however slows down with desorptionproceeding. On the contrary, CO₂adsorbs preferentially on coal. But the velocity of CO₂adsorption slows down as well. Above findings are regarded as the fundamental theories forcompetitive adsorption of binary gas and displacing the coalbed methane by CO₂injection.Experimental platform was developed independently to use to displace coalbed CH₄by gasinjection. Using this platform, we carry out displacing coalbed CH₄by CO₂or N₂the underdifferent injection pressure （1.5,1.8,2.2MPa） and different stress conditions （14,19MPa）.Theresearch results show that after injection of0.5displaced volume, N₂break through outlet, as the injection pressure increases, the breakthrough time gradually advance, after N₂breakthrough the gas outlet, its concentration increased slowly to100%. In the CO₂displacing CH₄experiments, there was high efficiency of displacement, the coalbed CH₄canbe all drive completely, after1.4～2.4replacement volume being injected, the CO₂breaksthrough gas outlet, and CO₂concentration quickly reach100%, with increasing the injectionpressure, CO₂breakthrough time is reduced. After increasing stress, the permeability reduces,which results in lower flow velocity of CO₂in the coal, CO₂can be sufficiently adsorbed oncoals. The breakthrough time is only1.4displaced volume under the stress of14MPa,however the breakthrough time increases to2.4displaced volume under stress of19MPa. Thestress significantly influences on the gas displacement.The mathematicoal model was established, which consists of the competitive adsorptionof binary gas, the competitive proliferation, the gas flow equation; COMSOLMultiphysicswas adopted to solve and built numerical simulation platform, CO₂, N₂respectively wasinjected into the coal seam to displace coalbed CH₄in this numerical simulation platform.Under same conditions, N₂first breaks through outlet, but the coalbed CH₄was recoveriedcompletely by CO₂injection first. With the improvement of the gas injection pressure,seepage velocity increases, shortening the displacing time.Good geological background of Yaojie coalfield provides a unique perspective for thestudy that displacing CH₄by CO₂injection. The physical structure of the coal was analysedby CO₂adsorption and mercury porosimetry. Gas composition and concentration wasdetermined using mass spectrometry and chromatography. The chemical structure of the coalwere analysed by fourier transform infrared spectroscopy. The results show that away fromthe injection points, the pore volumes of micropores, mecropores and macropores decreasegradually, and the CO₂concentration decreased, whereas the CH₄concentration increasedgradually. According to the FIR spectra of series of coal samples, the absorption peakschange significantly in the2800～3000cm-1,2270～2413cm-1, and1000～1200cm-1regions. Injection of CO₂into deep coal beds, particularly at conditions near the CO₂criticalpoint, may displace aromatic hydrocarbons from the coal matrix during migration, resultingin the absorption peak intensity weakened, and changing the corresponding to the number offunctional groups, which indicates the chemical structure of coal changed.