Modeling of gravity drainage and molecular diffusion mechanisms during miscible gas injection in naturally fractured reservoirs

Gas injection in naturally fractured reservoirs helps maintain reservoir pressure and improve oil recovery. While gravity drainage has been extensively researched, particularly in immiscible gas injection scenarios, the combined effects of gravity drainage and molecular diffusion during miscible gas injection with CO₂ have not been as widely studied. This lack of a comprehensive understanding of these coupled mechanisms represents a significant research gap, which is critical for optimizing Enhanced Oil Recovery (EOR) strategies in these complex reservoirs. This study aims to address this gap by implementing a hybrid simulation and modeling approach using Eclipse-300 and COMSOL to accurately capture fluid flow and mass transfer phenomena during miscible CO₂ injection. We utilize single-block and multi-block models to investigate the mechanisms of gravity drainage and molecular diffusion. The simulation results, which were validated against available experimental data, show good agreement. Our findings highlight that molecular diffusion plays a significant role in oil recovery, especially for matrix blocks with small heights and very low permeability. For instance, a representative simulation showed a total methane recovery of 35.99 % after 15 h. The results also underscore the importance of using appropriate oil-gas saturation functions for accurate recovery predictions.