Investigation on Hydrocarbon Gas Miscible Flooding in Deep-Tight Reservoir

Abstract

Tight reservoirs in China’s Bohai Bay Basin has abundant oil and gas resources. Depletion drive can only achieve an oil recovery lower than 10% OOIP, and water flooding is not suitable for this kind of reservoir. Hydrocarbon gas flooding is considered as one of the most promising technologies for developing tight reservoirs. In this paper, the influences of hydrocarbon gas injected on oil properties at reservoir condition, the minimum miscibility pressure and the enhanced oil recovery performance of hydrocarbon gas flooding are investigated through PVT test experiment, slim tube experiment, core flooding experiment and reservoir numerical simulation method, the hydrocarbon gas injection pressure is predicted and a simple economic evaluation is conducted, using tight reservoir X Block in Dagang Oilfield as the research object. The experimental results indicate that hydrocarbon gas can increase the expansion coefficient of oil and decrease density and viscosity of oil. The minimum miscibility pressure (MMP) between oil from X Block and hydrocarbon gas from Y gas reservoir is about 45 MPa. High displacement pressure and low displacement rate are conducive to delaying gas breakthrough and improving sweep efficiency and oil recovery. Reservoir numerical simulation results demonstrate that gravity assisted hydrocarbon gas flooding can obtain the best development performance, with a cumulative oil production of 8.42 × 104 t and a final oil recovery of 33.48%, which is 21.96% points higher than that of depletion drive. In order to maintain the formation pressure to be above MMP, the hydrocarbon gas injection pressure (well-head pressure) should be higher than 37 MPa, thus hydrocarbon gas compressor, injection pipeline and wellhead and safety control system of 50 MPa grade are prerequisite for the hydrocarbon gas miscible flooding project considering formation pollution and other unfavorable factors.