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 CO2-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.