Geo–Storage Across Scales: From Nanopore Theory to Reservoir Practice for CO2–Driven Methane Displacement in Ultra–Deep Illite–Kerogen Shales

Ultra–deep shale geo–storage that couples secure CO₂ sequestration with enhanced CH₄ recovery demands explicit links from nanopore physics to reservoir–scale practice, yet how pore size and hybrid mineral-organic interfaces jointly govern CO₂–CH₄ competitive adsorption, mobility, and CH₄ displacement under the Lower Cambrian Qiongzhusi temperature–pressure window remains insufficiently quantified; here we address this by molecular–dynamics simulations in composite illite–TypeI kerogen slit nanopores spanning 2–10nm and five reservoir state points—330.15K and 30MPa, 360.15K and 45MPa, 390.15K and 60MPa, 420.15K and 75MPa, 450.15K and 90MPa—and by extracting gas-surface interaction energies, cohesive–energy density, a dimensionless competitive–adsorption indicator, self–diffusion coefficients, near–wall density integration, and CH₄ displacement efficiency during CO₂ injection into CH₄–saturated pores as constitutive inputs for dual–porosity⁄dual–permeability upscaling. Confinement amplifies selectivity: CO₂ consistently outcompetes CH₄ on both illite and kerogen, creating CO₂–rich adsorption layers that nearly exclude CH₄ from 2 nm surfaces; the competitive–adsorption indicator is ‹1 at 2nm (surface–dominated regime), rises to ≈1.3 at 4nm, and reaches ≈2.4–2.5 at 10nm at the highest temperature and pressure (mixed–fluid regime), while diffusion analysis shows CO₂ remaining surface–bound and slower than CH₄, which—once dislodged—resides in the central mixed fluid and is more mobile. Displacement metrics reveal a clear pore–width control: CH₄ displacement efficiency increases from ≈70–77% (2nm) to ≈85-89% (10nm), peaks near 390.15K and 60MPa, and declines slightly at 450.15K and 90MPa as elevated temperature weakens adsorption; near–wall integration confirms persistent CO₂ occupancy with a marked preference for illite across all conditions. Collectively, these pore–scale relations deliver a physically grounded design map for field deployment: (1) prioritize illite–rich, small–pore intervals to maximize durable CO₂ trapping and storage security; (2) leverage larger nanopores (≥4–10 nm) as the mobility corridor for liberated CH₄ to enhance deliverability; (3) schedule injections around moderate temperature and high pressure to balance displacement and retention; and (4) port the measured selectivity–width trends, cohesive–energy densities, diffusivity contrasts, near–wall occupancy fractions, and displacement curves directly into continuum simulators to forecast CO₂–EGR performance and monitoring signatures in Qiongzhusi–type ultra–deep reservoirs by connecting atomistic mechanisms to engineering–relevant operating windows and upscaling parameters.