Study on Non-Equilibrium Diffusion Characteristics of Supercritical Condensate Gas and Dry Gas

Condensate gas reservoirs development usually adopts the dry gas injection to enhance condensate recovery. Conventional understanding suggests complete miscibility between condensate gas and injected dry gas without biphasic interface formation. However, field observations from the Yaha condensate gas reservoir challenge this assumption; during the injection process, the interface between the two gases evolves into a transition zone. At present, there is little research to describe this phenomenon mathematically. The study first designed experimental procedures for condensate gas and dry gas diffusion in PVT cells based on the pressure-temperature conditions, fluid composition of the Yaha condensate gas reservoir. These experiments were conducted to investigate interfacial characteristics during the development process. Subsequently, through systematic observation of phase behavior features including interface variations, a theoretical diffusion model was established to quantitatively characterize the dynamic evolution of three distinct zones: the injected dry gas slug, transition zone, and original condensate gas region. This model effectively captures the transient interactions and mass transfer processes between the injected gas and reservoir fluids. The visual PVT experiment demonstrates the vertical diffusion process between condensate gas and dry gas at a non-equilibrium state, providing the experimental basis for the theoretical model. The non-equilibrium diffusion model could calculate the diffusion coefficient of each component and fluid density variation at different positions in the PVT cell during the diffusion process. The results indicate that the density of condensate gas is much higher than that of dry gas and the molecular diffusion coefficient of each component in condensate gas is lower than that in dry gasby roughly 1–2 orders of magnitude. Besides, the diffusion coefficients of components are inversely proportional to their molecular weights and tend to decrease more rapidly at the initial stage. During the diffusion process, the density difference between dry gas and condensate gas gradually decreases along with the thickening of the transition zone and reaches a minimum value at the equilibrium state. In dynamic processes of hydrocarbon reservoir development, non-equilibrium states should be recognized as universally prevalent, while complete equilibrium conditions prove fundamentally inadequate. This finding necessitates re-evaluation of phase behavior models for gas injection processes in condensate reservoirs, provides another solution for accurately characterizing gas injection phase behavior and displacement features, thereby enabling more realistic evaluation of enhanced recovery mechanisms.