Source Attribution of Produced Methane During Shale Gas Recovery Under Stepwise Depressurization: A Molecular Dynamics Study

During depressurization-driven shale gas production, methane migration and state transformation in nanopores affect the source composition of produced gas. However, the relative contributions of initially free and initially adsorbed methane remain difficult to quantify at the molecular scale. In this study, we develop a Frame-0-based source-tracing framework for methane recovery in an idealized graphene square nanopore using molecular dynamics simulations under a stepwise depressurization protocol. Radical Voronoi local density and a two-component Gaussian mixture model are used to assign one-time initial labels to methane molecules at Frame 0. PID–preserving cross-frame tracking is then used to quantify the stage-wise and cumulative source contributions from the two initial populations. For the representative case of R = 10 nm and T = 353.15 K, the stage-wise fraction from the initially free population decreases from 79.5% to 62.2% as pressure decreases, while that from the initially adsorbed population increases from 20.5% to 37.8%. Increasing pore width mainly enhances total recovery through the contribution of initially free methane. Increasing temperature improves the contributions from both populations, with a stronger effect on initially free methane. The present results provide a molecular-scale quantitative characterization of methane initial-source attribution under the current stepwise depressurization protocol and establish a source-tracing framework that can be further extended to more realistic pore models.