Kinetic isotope fractionation mechanisms during shale gas displacement under nanoconfinement and implications for geochemical evaluation

The physical triggers for carbon isotope reversal (e.g., δ 13 C 1 > δ 13 C 2 ) in deep shale gas have long been obscured by free gas and remain controversial. To decouple the physical isotope fractionation mechanisms within extremely confined spaces and quantify their impact on gas reservoir evaluation, this study establishes a cross-scale framework integrating dynamic displacement experiments, a Continuous Stirred-Tank Reactor (CSTR) model, and Zero-Point Energy (ZPE) modified Molecular Dynamics (MD) simulations, using 13X zeolite with uniform channels as the physical end-member. The results show that after stripping the free gas background interference via the CSTR model, the intrinsic desorption fractionation extremum of methane reaches −90.3‰. Microscopic simulations further confirm that this fractionation process is governed by the kinetic differentiation of Knudsen diffusion and the thermodynamic retention effect driven by ZPE. Possessing a lower zero-point energy, 13 C faces a higher desorption energy barrier (ΔE a , 13 C > ΔE a , 12 C ) leading to its delayed release during the late stage of displacement, which directly drives the “V-shaped” rollover of the isotopic trajectory. This study clarifies for the first time that the deep coupling of strong pore confinement and gas composition drying is the underlying physical trigger for isotope reversal, avoiding the severe overestimation of reservoir thermal maturity caused by the blind application of conventional geochemical empirical formulas. Meanwhile, this paper proposes using the “V-shaped” rollover point of produced gas isotopes as a novel, non-intrusive engineering indicator to accurately identify the reservoir depletion state (matrix residual gas <20%) and define the optimal time window for CO 2 -Enhanced Gas Recovery.