Under certain conditions, relying on the gravity differentiation of the oil, the nitrogen gas enters the oil wet crack that cannot be entered by the injected water, and the residual oil is displaced. It relies on the gravity separation of the oil field to replace the residual oil trapped in the fracture hole by gravity. The injected nitrogen is dissolved in the crude oil to expand the volume of the crude oil, and the oil drainage effect reduces the residual oil saturation of the formation under the ground separation condition. It is to change the direction of fluid flow and displace the residual oil in the fracture channel. It is based on the differentiation of oil and gas gravity, and to push out the remaining oil that the injected water in the upper part of the mining structure fails to reach.