Nanoconfinement Drives Non-Monotonic Methane Density in Shale Nanopores: Direct Observation of Competitive Water–Gas Exchange

The efficiency of unconventional gas recovery is fundamentally governed by the nanoscale partitioning and competitive exchange between methane and water in shale reservoirs, yet direct experimental insight into their coupled behavior under in situ conditions (pressure and temperature) remains elusive. Here, we integrate a first-of-its-kind approach, combining contrast-matching ultra- and small-angle neutron scattering (USANS/SANS) at 50 MPa with vapor sorption, cryo-microscopy, and molecular simulations, to achieve unprecedented direct visualization of methane–water distribution across the multiscale pore–fracture network of the Longmaxi Shale. We report a fundamental revision to classical models, discovering that methane density exhibits a non-monotonic dependence on pore size, increasing with nanoconfinement but peaking sharply at ≈2 nm before declining in sub-2 nm pores. Mechanistically, water forms continuous films on hydrophilic quartz surfaces but aggregates into distinct clusters on organic kerogen. The stronger affinity of water for quartz and its increasing interaction with kerogen at higher saturations enable it to displace adsorbed methane competitively, thereby directly controlling local gas saturation. Furthermore, we reveal that microfractures are the dominant pathways for early water flowback and subsequent gas migration, with pore–fracture connectivity dictating fluid accessibility and phase exchange efficiency. These results establish a mechanistic link between interfacial nanophysics and macro-scale reservoir performance, providing a scientific foundation for optimizing hydraulic fracturing and maximizing resource recovery in deep sedimentary systems.