Perpustakaan Digital ITB

CO2-enhanced gas recovery (CO2-EGR) couples incremental methane production with geological CO2 storage, but field pilots such as K12-B have repeatedly shown CO2 reaching producers earlier than forecast models predict, a discrepancy linked to the crude treatment of molecular diffusion relative to gravity segregation. This study isolates and quantifies the individual and combined contributions of these two mechanisms in a high-permeability gas reservoir, using an ECLIPSE 300 radial sub-model to decouple the physics under idealised geometry and a tNavigator full-field model built on the Volve field framework with Ty Formation gas-condensate fluid to evaluate diffusivity sensitivity under realistic heterogeneity and a sealing fault. CO2 injection raises the CH4 recovery factor from 55.6% under depletion alone to 75.5% with injection, an uplift of about 20 percentage points that is insensitive to the diffusion setting. The moleculardiffusion coefficient exerts a clear, monotone effect on the produced CO2 mole fraction at the producer through roughly the first 11 years of the forecast, consistent with classical mixing zone theory. The front profiles show that signature directly in both models, the mixing zone widening and the 50% contour retreating towards the injector as the coefficient rises, so that diffusion spreads the front rather than translating it. At the twelve year snapshot the produced stream carries 18.5 mol% CO2 with diffusion switched off against 24.5 mol% at a constant coefficient of 1.5 m2 /day. Beyond year 11 this ordering inverts. By the end of the 20-year forecast the no-diffusion case yields a higher produced CO2 mole fraction (60.85 mol%) than the 1.5 m2 /day case (56.6 mol%), the reverse of the early-time ranking. This reversal, absent from the idealised radial model, is attributed to a sealing fault positioned between the injector and producer that diverts the CO2 front and interacts with a laterally constrained flow path near the producer, though confirmation through flux or streamline diagnostics is left for future work. The results indicate that in a faulted, geometrically constrained field the produced CO2 mole fraction need not increase monotonically with the diffusion coefficient over the full field life, and that the diffusion setting governs the timing and concentration of produced CO2 far more than it governs ultimate recovery