Methane isotope fractionation characteristics of shale gas and its significance as a productivity indicator

Carbon isotopes of methane in shale gas provide an important index for studying isotope fractionation characteristics and the gas production process. The carbon isotope variation of shale gas during in situ gas content measurement was studied for 10 shale gas wells of Wufeng-Longmaxi formations in 6 large blocks in the southeastern Sichuan Basin and the basin margin transition zone (the basin margin-transition zone of southeastern Chongqing). The fractionation characteristics of methane under different pressure systems and different subformations were studied, and the relationships between the isotopes of shale gas and its physical and gas-bearing properties were discussed. The production stages of typical shale gas wells were divided by drainage data.δ¹³C₁ gradually increased during shale gas desorption, while the carbon isotope of methane in overpressure shale gas was smaller. From the outside of the basin margin to the inside, the carbon isotope of methane gradually became lighter, and in the longitudinal direction, with the increase of depth, the methane isotope becomes lighter. Greater porosity of shale, higher content of free gas and better preservation conditions of shale correlate with less fractionation of carbon isotopes. Finally, the desorption stage of shale gas was divided by methane isotope fractionation, and the isotopes of production gas was compared with that of core desorption gas of well L in the Wulong syncline as an example. The δ¹³C value of gas released corresponded to that of core sample after continuous desorption of 0.9 h, and its recovery rate was about 24.8%, which was still in the initial stage of drainage production.