Evaluation of water-alternative-gas flooding in enhancing oil recovery for deep reservoir

The water flooding potential of the target reservoir in Nanpu 3–2 zone has been distinctly declining year by year, making it difficult to achieve effective development. There is an urgent need to explore alternative displacement methods to improve oil recovery. Based on the petrophysical characteristics of the reservoir, artificial cores were made. The oil-saturated cores were prepared according to actual reservoir forming. After fully displacing oil-saturated cores with brine, the cores were displaced by N2, CO2, and hydrocarbon gas, respectively, and the displacement effectiveness of the three gases was evaluated. To explain the differences in the displacement effects, minimum miscibility pressure and nuclear magnetic resonance experiments were conducted. This study finds that gas flooding significantly modifies the flow channels, reducing the resistance caused by the flow channels during displacement, but it has a minimal impact on the overall porosity of the rock. Compared to water flooding, gas flooding can enhance oil recovery, but N2 struggles to reach the miscible state, leading to only a 5% improvement. Both CO2 and hydrocarbon gas can reach a miscibility state, allowing to displace oil from smaller pores, with oil recovery increasing by over 30%. During the displacement process, CO2 encounters greater resistance from water, resulting in a larger pressure differential, which enables it to penetrate tiny pores and displace oil from them. An alternating method of water flooding and hydrocarbon gas flooding is recommended to enhance the reservoir recovery. This research would provide insights for water-alternative-gas flooding with similar reservoirs.