Experimental study of the development mode of gas-cap edge-water reservoir: A case study of Khasib reservoir of Halfaya oilfield in Iraq

Based on the oil, gas and water distribution characteristics of Khasib reservoir in Halfaya oilfield, Iraq, a core displacement experiment is designed to evaluate the influence of different displacement methods and displacement parameters on oil displacement efficiency. The research shows that, in the displacement method with water injected from the edge of the reservoir, early depletion production is conducive to the elastic expansion of the gas cap, forming the three-dimensional displacement of “upper pressure and lower pushing”, and the oil displacement effect is good. When gas injection at the top and water injection at the edge are used for synergistic displacement, the injection timing has different influences on the oil displacement effects of high and low parts. Considering the overall oil displacement efficiency, the injection pressure should be greater than the bubble point pressure of crude oil. Two displacement methods are recommended with the reasonable injection time at 20–25 MPa. The injection speed has the same influence on different injection media. Appropriately reducing the injection speed is conducive to the stability of the displacement front, delaying the breakthrough of injection media and improving the oil displacement effect. The reasonable injection rate of water flooding is 0.075 mL/min, the reasonable injection rates of water and gas are 0.15 mL/min and 0.10 mL/min, respectively in gas-water synergistic displacement. Gas-water synergistic displacement is conducive to the production of crude oil at high position, and has crude oil recovery 5.0%–14.8% higher than water flooding from the edge, so it is recommended as the development mode of Khasib reservoir at the middle and late stages.