Water-Alternating-Gas (WAG) injection is an established Enhanced Oil Recovery (EOR) technique developed to improve sweep efficiency an‎d mobility control during gas injection. This research presents a comprehensive simulation study to investigate the displacement mechanisms an‎d performance of a CO2-WAG process in a mature reservoir. The methodology involved two key phases utilizing the CMG (Computer Modelling Group) software suite. First, a detailed phase behavior study was conducted using PVTsim Nova to model Pressure-Volume-Temperature (PVT) data from the XX field. This analysis quantified the significant influence of CO2 dissolution on reservoir fluid properties, confirming that CO2 injection increases the oilʹs bubble point pressure, causes significant volumetric swelling, an‎d critically reduces oil viscosity. Second, a 3D heterogeneous reservoir model (24x24x9 grid) was developed in the GEM compositional simulator using an inverted 5-spot pattern. Four scenarios were compared under equivalent hydraulic conditions: (1) Base Case (natural depletion), (2) Water Flood (WF), (3) Continuous CO2 Flood (C-CO2), an‎d (4) CO2-WAG. The simulation results clearly demonstrate the superior performance of the CO2-WAG strategy. The CO2-WAG scenario achieved the highest ultimate Oil Recovery Factor (RF) of nearly 48%, compared to 44.5% for C-CO2, 42.5% for WF, an‎d 36% for the Base Case. While the C-CO2 case provided the highest initial peak oil rate, it suffered from poor mobility control, evidenced by a rapid an‎d sharp increase in the Gas-Oil Ratio (GOR). The CO2-WAG process, by contrast, provided a more sustained production rate an‎d a significantly lower, flatter GOR curve, indicating superior mobility control. Final oil saturation maps confirmed that CO2-WAG achieves a more uniform an‎d widespread reduction in residual oil, mitigating the gravity override an‎d channeling seen in the continuous injection case. This study concludes that the CO2-WAG process provides the optimal balance of high microscopic displacement an‎d stable macroscopic sweep, making it the most favorable EOR option for maximizing recovery in this reservoir.

محسن محمدي

حميدرضا شاه وردي , روح اله هاشمي