A Molecular Simulation Model for Kerogen Reconstruction and Methane Adsorption under Slickwater Exposure

Abstract

This study investigates the effect of polyacrylamide (PAM), a key additive in fracturing fluids, on methane (CH₄) adsorption in kerogen. An average molecular model of kerogen was constructed based on outcrop shale from the Longmaxi reservoirs in the Sichuan Basin, China. The model was established using organic elemental analysis, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and carbon-13 nuclear magnetic resonance (¹³C NMR). Molecular dynamics (MD) simulations and grand canonical Monte Carlo (GCMC) methods were employed to systematically evaluate the influence of PAM on CH₄ adsorption in kerogen nanopores by analyzing isothermal adsorption curves, relative concentration distributions, and adsorption heat. The reconstructed two-dimensional kerogen model has a molecular formula of C₂₄₅H₃₀₀N₆O₁₆S₅, with a three-dimensional unit cell density of 1.106 g/cm³. Results indicate that at low pressures (<25 MPa), PAM enhances CH₄ adsorption by increasing adsorption affinity, whereas at high pressures (>25 MPa), it inhibits CH₄ adsorption by occupying pore volume and blocking adsorption sites. These findings reveal the dual role of PAM in modulating CH₄ adsorption across varying pressure conditions. This study provides fundamental insights into the microstructure of Longmaxi shale and offers a theoretical basis for optimizing slickwater formulations to enhance shale gas recovery.