Mechanism and influencing factors of EOR by N2 injection in fractured-vuggy carbonate reservoirs

Multi-scale fractures and karst caves are the main reservoir bodies in fractured-vuggy carbonate reservoirs, and there are still various types of remaining oil after water flooding. Oilfield development practices demonstrate that the method of N2 injection can enhance oil recovery effectively. However, the development effect of a single well with gas injection is highly different, and the mechanism and influencing factors of gas injection for enhanced oil recovery are not clear. To reveal the mechanism and influencing factors of enhanced oil recovery through physical experiments and numerical simulation, we have produced a series of physical models of fracture and cave combinations based on the characteristics of this type of reservoir, established a visual physical simulation experimental apparatus and process, and analyzed the main factors affecting the production efficiency of gas injection wells in combination with the actual development of oilfields. The research results demonstrate that the primary types of remaining oil after water flooding in fractured-vuggy carbonate reservoirs are oil at the top of karst caves, oil shielded from high flow channels and oil not swept by the wells. The mechanism of gas injection-enhanced oil recovery is primarily composed of three aspects. First, the injected gas forms a secondary gas cap because of the effect of gravitational differentiation, and the gas cap can displace the remaining oil on top of the karst cave that the injected water cannot drive; Then, injected gas can change the pressure field distribution and the direction of fluid flow after water injection, driving the remaining oil that is shielded by high flow diversion channels; in addition, the injected gas has a certain effect on the reservoir energy supply. The geological conditions, such as the remaining reserves of injecting gas wells, filling level, well-reservoir position relationship and bottom water energy, have great influence on the development effect. The starting time of gas injection has an optimal point, and injecting too early or late will affect the oil recovery. The cyclic gas injection quantity and gas injection velocity are the primary injection production parameters that influence the production effect of a gas injection well. Finally, the principle of selecting a well for gas injection according to the research results is discussed, which can provide reference for selecting gas injection wells and injection methods of fractured-vuggy reservoirs in Tahe Oilfield.