Comparison of CO₂ and nitrogen injection at low temperatures in high temperature reservoirs for improving oil recovery

Gas flooding methods have been used as successful tertiary recovery agents for decades. Recent studies suggested the effectiveness of using low temperature CO₂ injection in high temperature reservoirs. With respect to current economic limitations, nitrogen and CO₂ seems to be one of the most conventional ways to improve the oil recovery. The high temperature reservoirs may not be suitable for isothermal gas injections as they would suffer from gravity override of the gas, while low temperature injection could minimize the effect. The main goal of this study is to compare the low temperature injection of these two fluids in high temperature reservoirs aiming to improve the oil recovery. To obtain a proper comparison between CO₂ and nitrogen, several simulations have been constructed and performed. Thermal compositional runs seem to be the perfect candidate to achieve the goals of this study. The results suggest that with equal injection amount and duration of both fluids, liquid CO₂ is a better candidate for improving oil recovery. Ultimate recoveries for CO₂ and nitrogen injection at same initial conditions were recorded as 73% and 55%, respectively. The effectiveness of liquid CO₂ for oil recovery was recorded as 1.52 barrels of oil for each barrel of injection, whereas a barrel of nitrogen recovered only an average of 0.65 barrel of oil. The viscosity reduction was more severe in liquid CO₂, which suggests better mobility for the oil than in nitrogen injection. The CO₂ breakthrough happened at an earlier time than nitrogen but due to much higher ultimate recovery, it can be overlooked. The overall conclusion suggests that low temperature CO₂ injection can be considered a beneficial and promising method for enhancing oil recovery in high temperature reservoirs.