The Fractionation Mechanism Of Coalbed Gas

The investigation on coalbed gas fractionation can not only explain the geological effect of coalbed gas fractionation, but also offer theory support for the prediction of coalbed gas accumulation. Based on adsorption potential theory, ab initio calculation theory, Henry Law, and other relational theories, coalbed gas fractionation mechanisms were systemically discussed. There have four kinds of coalbed gas fractionation mechanism: desorption-diffusion-migration fractionation, solution-migration fractionation, microorganism fractionation, and high temperature cracking fractionation, which are described as follow: (1) the mechanism of coalbed gas desorption-diffusion-migration fractionation can be interpreted by adsorption potential theory. The coal is preferential adsorption of 13CH4 over 12CH4 and preferential desorption of 12CH₄ over 13CH₄ because the adsorption potential of 13CH4 on coal is higher than that of 12CH4; so the migration-fractionation of 12CH4 is more easily occurred. The coal is preferential adsorption of CO₂ over CH4 and preferential desorption of CH4 over CO₂ when compared the adsorption capacity of CO₂ and CH₄, so methane fractionation effect is stronger, especially at high pressure. It provides an approach to quantitative assessment of injection of CO₂ to enhance CH₄ recovery technique. The acting forces between the coal molecule and other gas molecules are calculation by ab initio calculation theory, which explained the adsorption/desorption fractionation effect from the quanta angle. (2) The solution of groundwater which takes place in transmission zone made CO2 preferential to be dissolved, migrate to the stagnant zone and accumulate, which caused the CO2 content is higher than in-situ. Methane carbon isotope has same fractionation effect. That is to say, the 13CH4 content is higher than in-situ. (3) The biogenic methane is produced by two main ways. One is CH₃COOH ferment, the other is CO2 reduction. CO₂ reduction is often quantitatively more important than the aceticlastic ferment. The two biogenic methane all results in methane carbon isotope light. (4) The pyrolysis gas causes methane carbon isotope to heavy.