A fractal model for predicting multicomponent effective diffusion coefficients with application to CCUS in tight reservoir

For tight reservoirs dominated by micro- and nanopores, the confined phase behavior of CO₂ and in-situ fluids significantly impacts multicomponent diffusion and underground multiphase flow. To address the challenge of measuring effective multicomponent diffusion coefficients under high-temperature and high-pressure conditions in tight porous media, this study proposes a fractal-based theoretical model. The model is integrated into compositional simulations to predict multiphase flow and analyze the impact of multicomponent diffusion on CO₂ flooding and storage. The Volume Translated Peng-Robinson Equation of State (VTPR-EOS) is modified to include criticality shifts and capillary forces, accurately capturing CO₂ and in-situ fluid phase behavior under tight reservoir conditions. Multicomponent diffusion is described using Fick’s and Maxwell-Stefan’s laws, while the effective diffusion coefficients are derived based on fractal theory. A heterogeneous 2D model (50×20 grid) is constructed with porosity distribution generated by a stochastic Gaussian method, and the effective diffusion coefficient correlation terms are validated against empirical models. Simulation results show that the inclusion of confined phase behavior enhances molecular diffusion, increasing CO₂ mole fractions by up to 46.8% within the sweep area. Multicomponent diffusion expands the CO₂ sweep area and improves concentration uniformity along the displacement direction for both miscible and immiscible simulations, with minimal impact on pressure distribution. In immiscible simulations, CO₂ injection extracts lighter components, leading to higher residual fluid density in the sweep area. This fractal-based model reduces uncertainties associated with empirical models by incorporating reservoir-specific pore structures and properties. It can be integrated into compositional simulation codes for large-scale and long-term reservoir simulations, providing valuable insights into CO₂ utilization and storage in tight and shale reservoirs.