Molecular simulation studies on enhanced gas recovery mechanisms in tight sandstone reservoirs

The complex interplay between fluid molecules and pore surfaces within the nanoscale spaces of tight sandstone formations critically governs the in-situ state and ultimate extractability of natural gas. The inherently low permeability, coupled with intricate gas-water distributions, poses significant challenges for efficient gas production from such reservoirs, typically resulting in suboptimal recovery factors. Consequently, the injection of gases such as CO₂ and N₂ to enhance recovery has emerged as a prominent research focus. This study employs molecular simulation to investigate the underlying dynamics. First, the mechanisms of methane displacement by CO₂ and N₂ are analyzed, and a minimum energy configuration for adsorbed methane is established using Grand Canonical Monte Carlo (GCMC) simulations. Subsequently, the displacement efficiency of CH₄ under varying temperature and pressure conditions is evaluated. Key findings include: (a) CH₄ adsorption capacity in the reservoir rock increases with pressure but declines as temperature rises. (b) The mean square displacement and diffusion coefficients for the three gases follow the order CH₄ > N₂ > CO₂ under identical conditions. (c) Elevated temperatures accelerate molecular diffusion, leading to higher diffusion coefficients. Due to its rapid diffusion, N₂ primarily displaces methane through volumetric sweeping. In contrast, CO₂ exerts a stronger influence via competitive adsorption, proving more effective than N₂ in promoting methane desorption. (d) Quantitative interaction energy analysis shows that CO₂ has a stronger adsorption affinity (average interaction energy of −8.2 kcal/mol) compared to CH₄ (−5.6 kcal/mol) and N₂ (−3.1 kcal/mol), confirming its superior displacement capability. • Two-dimensional NMR technology enables the characterization of fluids within shale formations across varying scales. • A quantitative framework has been developed to assess the wettability of pore systems in multi-scale shale reservoirs. • Interfacial tension behaviors between shale oil and diverse injection media have been systematically determined.