Numerical Simulation of CO2-N2 Gas Injection with Polymer and Low-Salinity Postflush for Enhanced Oil Recovery in a Moderately Pressured Heavy Oil Reservoir

Abstract This study evaluates the synergy of CO2-N2 co-injection with polymer flooding and low-salinity postflush for enhanced oil recovery (EOR) in a mature, moderate-pressure heavy oil reservoir. A CMG-based numerical model, calibrated to field-derived fluid properties and production history, simulated four scenarios: (1) base case (no EOR), (2) CO2-N2 co-injection with polymer/postflush, (3) N2-polymer/postflush, and (4) CO2-polymer/postflush. Injections included 2 MMscf CO2, 2 MMscf N2, 2,000 bbl polymer slug, and 1,000 bbl low-salinity postflush, tailored per scenario. Results indicate the N2-polymer/postflush case achieved optimal performance: 9.38 MMSTB cumulative oil (5.4% gain over the base case's 8.9 MMSTB) and minimal water production (1,270 MSTB), reflecting improved sweep efficiency. CO2-N2-polymer/postflush yielded 9.08 MMSTB (1,460 MSTB water), while CO2-polymer/postflush produced 9.00 MMSTB. Gas production peaked at 3.38 MMSCF in N2-driven cases. The findings demonstrate N2-assisted polymer flooding with low-salinity postflush as a viable strategy to boost recovery and reduce water cut in mature heavy oil systems.