Research On Coalbed Methane Desorption And Fractionation Characteristics In Zhengzhuang Block, Jincheng Area

Coalbed methane(CBM) is generated and stored in coal by adsorption, and belongs to nonconventional gas. The results of canister desorption of coal indicate that the desorption rate of CBM in Zhengzhuang block, Jincheng area is high, and ranges from 65.0% to 96.2%. The desorption rate has a positive correlation with gas content in coal, and has a negative correlation with ash constituent. The changes of desorption rate scaled to coal rank can be divided into two stages. The adsorption time is 0.33～8d, which indicates that the CBM desorption tempo in this area varies largely, the time to reach high production is different as well ,however , short as a whole. The adsorption time of coal sample has a positive correlation with gas content and fracture growth.In desorption process, the tendency of carbon isotope growing to higher can be divided into faster and slower stages. However,the results of Changji area are different from it. It is because that the coring operation and matrix shrinkage during desorption damaged the original structure of coal, which have influence on the normal isotope fractionation effects. The relative abundance of micropores increases along with coal maturity, so isotope fractionation via diffusion through microporous intensifies, and the fractionation effects are more apparent to high-rank coal samples.The carbon isotopic fractionation effect of CH4 during gas desorption in coal reservoir may result in theδ13C1 value in well head fluctuates in a narrow range for unfracture well. Longer period of fluctuation means better exploitation stability and higher production. Through the correlation analysis between desorption amount from the coal sample and the carbon isotope of methane, the total gas desorption predicting equation was regressed. The producible reserves of the CBM well can be predicted with this equation. Theδ13C1 value in well head changes littlely during production test, which may due to the impact on isotope fractionation of the crack caused by fracturing for fracture well. So,we should try to avoid using the values measured in well head when studyδ13C1 value.In canister desorption of coal, the～50% desorption midpoint is suggested for taking representative entire gas samples for carbon isotopic analysis. Since gas kinetic desorption from canister may be mirrored as the production of CBM well, the desorption midpoint'sδ13C1 value can represent CBM well gas'sδ13C1 value. The realδ13C1 value of CBM in this area is lower than theδ13C1 value of simulation test, but the differences are not very large. Secondary gas generation, unnormal hit event, and methane aqueous fusion makeδ13C1 value light, desorption fractionation effect make remained gasδ13C1 value heavy. The realδ13C1 value is a compositive reflection of all the influential factors. Theoretically, the heaver methaneδ13C1 value is existent.