Research On The Composition And Isotope Fractionation Characteristics During The Degassing Process Of Shale Gas

The exploitation of shale gas is a dynamic process of gas under different pressure conditions.When the gas reservoir pressure is reduced to the critical desorption pressure,the adsorbed gas undergoes desorption,and then diffuses together with the free gas in the shale matrix and micropores,and flow to the wellbore through fracture network finally.All the dynamic processes of shale gas in the original stratum have an important influence on the gas composition and isotope composition.Therefore,the research on the composition and isotope fractionation characteristics during the degassing process of shale gas have significant effect in the identification of shale gas enrichment intervals,the assessment of resource potential and recoverable reserves,the prediction of shale gas exploitation degree and the monitoring of fracturing effect.In this study,the organic geochemical characteristics,mineral composition,and physical properties of shale in the Wufeng-Longmaxi Formation from Well Shengye 1 were analyzed by TOC,rock pyrolysis,X-ray diffraction,porosity and permeability,and nitrogen adsorption experiments at low temperature.In addition,core degassing and methane isothermal adsorption experiments show that the Wufeng-Longmaxi shale has good gas-bearing properties,and the original gas content of the shale was recovered by the USBM method.The calculated results show that the loss of gas in the coring process exceeds 40%.This makes the gas content recovery error larger.Fractionation of carbon isotopes in the adsorption process of methane indicates that the process of adsorption will cause isotope fractionation,and the higher the TOC content of shale(the greater the amount of adsorbed gas),the greater the isotope fractionation.Therefore,the desorption-diffusion fractionation mechanism was used to reasonably explain the components and the methane carbon isotope fractionation in the shale core analysis of Well Shengye 1,and the gas composition and isotope fractionation at the wellhead during the early stage of production in Well Longye 1.At the same time,the kinetic model was used to calibrate the gas volume and the methane carbon isotope value in the degassing process of the Longmaxi Formation shale.The calibration results were almost the same as the experimental data,indicating that the kinetic model can be used for the fitting of degassing process and carbon isotope fractionation of shale gas with high accuracy.The calibration activation energy results show that the shale gas degassing process is mainly a physical process,and the activation energy is far less than the energy required for the chemical reaction.At the same time,because the activation energy is higher than that required for the methane adsorption/desorption process,we proved that the degassing of shale gas should include both desorption and diffusion process from the energy point of view.The carbon isotope fractionation of methane is mainly derived from the difference in activation energy between light and heavy carbon isotons.The accurate calibration of the analytical gas volume and carbon isotope value of the shale core by the kinetic model also shows that the isotope fractionation has great potential in the gas recovery of shale gas.