Characteristics and geological implications of carbon isotope fractionation during deep shale gas production:A case study of the Ordovician Wufeng Formation-Silurian Longmaxi Formation in the Luzhou Block of Southern Sichuan Basin
The medium-shallow shale gas in the southern part of the Sichuan Basin(hereinafter referred to as Southern Sichuan Basin)has been efficiently developed in a large scale.Deep shale gas is an important field for reserves and production increase at present and in the future.In order to explore the influential factors and geological implications of the carbon isotope fractionation in the deep shale gas in the Southern Sichuan Basin,this paper systematically conducts an on-site desorption test of 12 cores and continuous monitoring of 2 production wells with the deep Ordovician Wufeng Formation-Silurian Longmaxi Formation shale in the Luzhou Block of Southern Sichuan Basin as the research object.Then,the carbon isotope(δ13C1 and δ13C2)fractionation mechanisms of methane and ethane are analyzed,and the isotope fractionation models are established.In addition,the mathematical model and method to calculate adsorbed/free gas ratio and EUR on the basis of isotope fractionation parameters are established.Finally,it is recommended to support the analysis of the occurrence state,enrichment characteristics and flow and production patterns of shale gas.And the following research results are obtained.First,in the desorption process of deep shale gas in the Southern Sichuan Basin.the δ13C1 values increase by 10.3‰ to 22.7‰,and the δ13C2 values increase by a maximum of only 4.2‰.In the process of shale gas well production,both methane and ethane exhibit a carbon isotope fractionation amplitude of approximately 8‰.Second,the complete process of shale gas production universally exhibits a four-stage isotope fractionation pattern characterized by"stable-lighter-heavier-fighter again".The desorbea gas in a canister only shows a heavier stage corresponding to matrix dominated gas production.The wellhead gas of production well shows a lighter stage corresponding the combined gas production from fractures and matrix.Changes in either ambient teasmeperature or pressure can modify the original isotope fractionation pattern of natural gas,with the degree of influence being closely related to the specific stage of the process.Third,compared with the desorbed gas samples from the cores and the wellhead gas samples in the middle/late production period,wellhead gas samples in the early production period can reflect the original carbon isotopic compositions of natural gas in the reservoir more accurately.Fourth,the multi-scale and multi-mechanism coupled carbon isotope fractionation model can accurately calculate the key parameters such as in-situ gas content and adsorbed/free gas ratio of deep shale,predict the EUR of gas well,and effectively evaluate the adsorbed/free gas ratio in the production process.In conclusion analyzing the influential factors and mechanisms of carbon isotope fractionation in the process of shale gas production can provide a novel perspective and technological means for exploring the occurrence state,enrichment characteristics and flow production patterns of deep shale gas,which is conductive to the benefit development of deep shale gas.
Sichuan BasinDeep shale gasIsotope fractionationIn-situ gas contentAdsorbed/free gas ratioEUR prediction
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