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.
Ren et al. (Mon,) studied this question.