Differences in Geochemical Compositions between Canister Desorption-Derived and Hydraulic Fracturing-Produced Shale Gas: Implications for Understanding Partial Carbon Isotopic Reversal in Shale Gas Field

Abstract

Gas content and geochemical composition of shale gas are routinely employed for resource potential assessment and production optimization strategies. Shale gas is conventionally collected from horizontal wells after hydraulic fracturing. In the early exploration stage, gas can also be collected by canister desorption of freshly drilled shale cores in the field. However, for the same shale layer, little attention has been paid to the geochemical compositional differences between hydraulic fracturing-produced shale gas (HFS gas) and canister desorption-derived shale gas (CDS gas), and the influence on gas production evaluation. In this study, the CDS gas and HFS gas from the Wufeng-Longmaxi shale of the Yiye 2 well, Middle Yangtze region (South China) were analyzed to evaluate the genetic origin and the influence of desorption on gas compositional and isotopic compositional shifts. The HFS gas is an over-mature dry gas, dominated by methane (94.06% to 96.44%) with a small amount of ethane and nitrogen, averaging 1.17% and 2.38%, respectively. The isotopic composition indicates thermogenic, oil-prone cracked gases. The CDS gas has relatively low hydrocarbon content and high N₂ content. Carbon isotopic patterns of HFS gas are characterized by partial isotopic reversal, i.e., δ¹³C₁ > δ¹³C₃ > δ¹³C₂. In contrast, the CDS gas displays larger isotopic variation and different isotopic reversal patterns, i.e., δ¹³C₃ > δ¹³C₁ > δ¹³C₂. The differences in shale gas components and isotopic compositions between HFS gas and CDS gas are probably caused by successive gas diffusion. Gas diffusion of the CDS gas caused decreased concentrations of light carbon molecules in shale reservoirs and increased concentrations of heavy molecules. Isotopic fractionation during degassing results from molecular mobility differences, which are influenced by molecular adsorption/diffusion. The amplitude of δ¹³C variation during desorption is related to shale pore structure. Isotopic fractionation is weak in mesopores, macropores and fractures with good interconnectivity after hydraulic fracturing, resulting in a slightly restricted diffusion for the HFS gas. The ¹³C-depleted CH₄ in the CDS gas resulted from preferential diffusion of light molecules in matrix-related nanopores resulting in more efficient “molecular sieves” and enhanced isotopic fractionation. Secondary alteration, including gas diffusion, can be identified using the cross plots of ln (C₁/C₂) value versus δ¹³C₁-δ¹³C₂. Considering the variation in geochemical compositions of the two different collecting gases, any deviations in molecular composition will alter the isotopic fractionation.

Keywords

shale gas / Wufeng-Longmaxi formations / gas diffusion / canister desorption / isotopic reversal / middle Yangtze region / petroleum geology

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