The feasibility of CO2 and N2 injection for the Tahe fracture-cavity carbonate extra-heavy oil reservoir: An experimental study

The Tahe Ordovician carbonate reservoir is special and unique, with its fracture and cavity serving as both pathway and crude oil storage space. Additionally, its extra-heavy crude oil is highly viscous and dense. The combination of all these characteristics leads to considerable challenges in the development of the Tahe extra-heavy oil reservoir. Due to the inelasticity of various enhanced oil recovery methods, in the study presented in this paper, CO2 and N2 were chosen as the displacing medium for the investigation of gas flooding feasibility for the Tahe carbonate extra-heavy oil reservoir. The results of physical modeling experiments showed that for Tahe crude oil, the CO2 possessed excellent solubility and viscosity reduction rate under high pressure and temperature conditions. The N2 had less influence on the extra-heavy oil than the CO2. For CO2, dissolution and extraction were the important displacement mechanisms. However, in the fracture-cavity core, high pressure caused gas breakthrough channels to form easily. The produced oil possessed better quality due to the asphaltene and resin deposition, and the CO2 was able to continue extracting light hydrocarbon along with the gas breakthrough channel. For N2, pressure maintenance was the major displacement mechanism. Due to its low solubility and extraction ability, the N2 caused gas breakthrough more easily than the CO2. The serious overlap effect reduced the gas sweep efficiency and led to the lower cumulative recovery.