Effect of Pressure, Initial Water Saturation, Rock Type and Injection Rate on Oil Recovery and Displacement Efficiency of CO2 Injection in Carbonate Reservoirs

Abstract

Abstract Miscible gas injection is the most widely applied enhanced oil recovery (EOR) method in light oil carbonate reservoirs, both as tertiary and secondary method. Miscible gas has high displacement efficiency and usually results in a low residual oil saturation (Sorm) in the part of the reservoirs that is contacted with gas. Accurate determination of Sorm and understanding the parameters that affect displacement efficiency are crucial for successful miscible gas EOR projects. This paper presents a comprehensive experimental program designed to investigate the effect of experimental pressure, pore volume injected, injection rate, rock type and initial water saturation on the displacement efficiency, Sorm and recovery factor of miscible/near-miscible CO2 injection. The CO2 injection experiments were performed using live crude oil at pressures above the minimum miscibility pressure (MMP) using reservoir core samples of up to 1 ft long and 2 in. diameter. All CO2 injection experiments were performed using vertically oriented cores, with gas injection from the top unless stated otherwise. The experimental results show that: 1-Oil recovery decreases as pressure decreases with Sorm increases by more than 20 saturation units as the pressure decrease from 4250 psi to 2700 psi; 2-CO2 breakthrough was much earlier at lower pressure which leads to more CO2 recycling, 3-The recovery factor is strongly affected by pore volume injected, however, the effect of the CO2 injection volume is much more significant at lower pressure; 4-The injection rate has insignificant effect on oil recovery and Sorm for miscible or near miscible CO2, due to the low IFT between oil and CO2, 5-Rock heterogeneity has a strong effect on oil recovery and CO2 breakthrough and hence on CO2 recycling and economy of the projects., and 6-The presence of mobile water at the beginning of CO2 injection resulted in lower displacement efficiency and increased Sorm. However, this water blocking effect should be determined experimentally for a given reservoir rock/fluid system. The results of this study cannot be generalized for other reservoirs. The results of this study have important implications for the design and performance predictions of CO2 gas injection. Starting CO2 injection at higher reservoir pressure is recommended due to its superior displacement efficiency and less CO2 recycling due to later breakthrough. However, higher pressure may negatively impact the required CO2 volume, the compression cost and project economics.