Abstract: The depleted development vertical wells in the Mahu sag area have high initial production and rapid decline. Conventional water injection is not suitable. Through the conversion of hydrocarbon gas injection can greatly improve the recovery rate. The minimum miscibility pressure in the gas injection test area determined by the slim tube method is 41.71 MPa. Miscible flooding can generally be achieved under the current formation pressure in the Mahu area. According to the reservoir characteristics and geomechanical parameters of the Mahu area, natural fractures and artificial descriptions are also included in the simulation, and optimization of the fracture distribution shape; and a three-dimensional fracture network model is established. Based on the post-fracturing fracture network model, researches on well layout mode, gas injection rate and gas drive reservoir production law are carried out. The results show that the average fracture network of hydraulic fracturing has a major axis of 173.90 m, a longest axis of 846.90 m, and a minor axis of 6.44 m; the deployment well trajectory of oil production wells is 2/3 thicker from the top, and the gas spread is larger; the gas injection rate is optimized with the goal of maintaining miscibility In the first 10 years of the design, 55 000 cubic meters of gas per day were injected into horizontal wells, and 40 000 cubic meters per day in the next 10 years. The recovery rate of the test area can reach 22.5%; hydrocarbon gas will be preferentially displaced along the long pressure fractures of the production well after pressure, give priority to the use of crude oil in this area, resulting in uneven gas injection spread; with the injection of hydrocarbon gas, the viscosity of crude oil along the displacement front is greatly reduced. At the same time, because the injected hydrocarbon gas and crude oil are miscible, the gas is injected 5 years later. The viscosity of the remaining crude oil within the SRV range of the oil well increases significantly. Based on this research, it can effectively guide the formulation of technical policies for field development of the Mahu hydrocarbon gas injection enhanced oil recovery test.

Key words: Mahu Sag, hydrocarbon gas, miscible flooding, numerical simulation, injection-production parameters