Study on the Non-Equilibrium Diffusion Mechanism of CO2–Natural Gas Multi-System

Injecting CO2 into gas reservoirs is a crucial approach for enhancing natural gas recovery and achieving CO2 geological storage, where the gas–gas diffusion behavior between CO2 and CH4 directly influences gas mixing efficiency. Direct observation of the spatiotemporal evolution of concentration fields during diffusion remains insufficient. In this study, a gas–gas diffusion experimental system capable of multi-time and multi-space stratified sampling within a high-temperature high-pressure PVT cell was established based on real reservoir fluid compositions. Non-equilibrium diffusion experiments were conducted under different pressures, different initial CO2 mole fractions, and different diffusion times. A diffusion model was developed according to Fick’s second law. The results suggest that the gas column can be divided into a natural gas zone, a transition zone, and a CO2 zone by the dimensionless concentration gradient threshold. At 5 MPa, the transition zone width expands rapidly within the first 4 h (dimensionless width increases from 0 to 0.6902), after which growth slows. Increasing pressure significantly inhibits diffusion, reducing transition zone width and prolonging equilibration time. Rising initial CO2 concentration also suppresses diffusion mixing, particularly in the later stage. Component profile analysis confirms that, under high pressures and high CO2 concentrations, the diffusion flux across the interface is weakened. Compared to CH4, the diffusion equilibration time of CO2 is shorter and more sensitive to pressure changes. The obtained diffusion coefficients (CH4: 2.92 × 10−8 to 4.79 × 10−8 m2/s; CO2: 3.91 × 10−8 to 6.08 × 10−8 m2/s) are on the order of 10−8 m2/s, consistent with bulk-phase PVT literature data, validating the reliability of the experimental method and inversion model. This study lays an experimental foundation for predicting multi-component gas mass transfer under conditions of CO2-enhanced gas recovery and CO2 geological storage.