A Preliminary Experimental Study On Coalbed Methane Displacement With Carbon Dioxide Injection In Fuxin Coalfield

Based on the reservoir geological characteristics of coals in Fuxin Coalfield, a preliminary laboratory experimental study on coalbed methane （CBM） displacement with carbon dioxide injection is carried out in Liujia CBM pilot-test area and Dongliang mine using an IS-100 isothermal adsorption apparatus. A series of measurements including monocomponent gases （CO₂, CH₄ and N₂） adsorption, binary component gases （80%CO₂+20%CH₄, 20%CO₂+80%CH₄, 50%CO₂+50%N₂） adsorption/desorption, and also with CH₄ displacement by pure and mixed CO₂ gases （100%CO₂, 80%CO₂+20%N₂, 20%CO₂+80%N₂） are performed. During the experiments, all gas saturations （e.g., CO₂, CH₄） at a separate phase are tested using a gas chromatography （GC）, at the same time all the gas satrations at an adsorbed phase are calculated based on the Langmuir equation and phase disengagement method. As a result the mechanisms of adsorption, desorption and displacement of these gases are concluded, which can contribute greatly to the theory of enhanced coalbed methane recovery （ECBM） and CO₂ sequestration by CO₂ injection.Results from the monocomponent gases adsorption measurements show that the adsorption capacity of coals for gases is as follows: CO₂>CH₄> N₂. The higher adsorption capability of CO₂ comparing to CH₄ is the basical principle for displacement experiments. The Liujia coal has a higher adsorption capacity for all gases than the Dongliang coal, because it has a higher vitrinite and exinite compositions and also with a lower ash and minerals contents.Results from the binary component gases adsorption/desorption measurements show that, （1） the gas with higher adsorption capacity is generally first adsorbed from the coal matrix surface during the processes of gas adsorption, but lastly desorbed from the coal surface during the processes of gas desorption, （2） the evident CH₄ displacement with CO₂ is messured during the processes of adsorption and desorption.Comparing with the Liujia and Dongliang coals, it is found that the desorption rate and the desorption ratio of gases in the binary component （CH₄ and CO₂） are relatively higher in the coals with higher separation factor of CO₂ VS CH₄, and also relatively higher with higher proportion of CO₂ in the gas components.In conclusion, there are four main influence factors of coalbed methane displacement with carbon dioxide. Firstly, the higher proportion of CO₂ in the multicomponens is, the more favorable to the displacement is. The displacement should take the best efficiency for pure CO₂ gas, and should take the distinctly good efficiency when the proportion of CO₂ > 80% in the multicomponents. Secondly, the injection pressure of CO₂ or multicomponents should be lower than the critical pressure for liquefaction. It is about 5.6MPa for the experiment in this study. Thirdly, an intermittent CO₂-injection mode is obviously better than a continuous CO₂-injection mode. Finally, the higher separation factor of CO₂ VS CH₄ is, the more favorable for displacement implements of coal is.