FRACTAL MODEL FOR HETEROGENEOUS EFFECTIVE DIFFUSION AND ITS APPLICATION IN COMPOSITIONAL SIMULATIONS OF CO₂ FLOODING IN TIGHT RESERVOIRS

The extremely low porosity and permeability in tight reservoirs make diffusion effects significant and non-negligible. Accurately estimating diffusion coefficients is critical for predicting CO 2 -enhanced oil recovery and CO 2 storage (CCUS–EOR) in such reservoirs. However, predicting effective vapor–liquid phase diffusion in porous media and incorporating it into large-scale compositional simulations remains a major challenge. This study combines fractal theory with Fick and Maxwell–Stefan diffusion frameworks, proposing a simplified fractal diffusion model that incorporates fractal parameters and porosity to better characterize CO 2 and in situ oil diffusion in heterogeneous tight reservoirs. Numerical models were developed using MATLAB to simulate both immiscible and miscible CO 2 flooding within multiphase, multicomponent flow equations while considering diffusion effects. The results reveal that diffusion significantly increases the area influenced by CO 2 and improves the uniformity of CO 2 concentration along the flow path. Confined phase behavior in micro- and nanopores enhances diffusion, increasing CO 2 mole fractions in displacement zones. In immiscible flooding, CO 2 extraction of lighter oil components results in higher residual oil density compared to models that neglect diffusion. This fractal-based model for heterogeneous effective diffusion, combined with insights into the role of diffusion in CO 2 flooding in tight reservoirs, offers important implications for developing more effective CCUS–EOR strategies.