Performance evaluation and mechanism with different CO2 flooding modes in tight oil reservoir with fractures

Due to the abundant reserves in tight reservoirs and the large amounts of residual oil after implementing common enhanced oil recovery methods, profitable methods with a high oil recovery such as CO2 flooding are urgently needed. Although CO2 flooding has attracted much attention and obtained good results in China and abroad, it has the critical problem of a low sweep efficiency, especially in tight reservoirs that develop with hydraulic fractures. In this study, the minimum miscible pressure of oil and CO2 was measured as 17.8 MPa, higher than the original formation pressure. Subsequently, the minimum miscible pressure was matched by means of numerical simulation, and the method of adding intermediate components to the injection CO2 was studied to reduce the miscible pressure in the study area. Afterwards, fractures were added to the geological model to study the mechanism of gas channeling under different CO2 injection modes. The continuous CO2 injection (CGI) mode requires the largest gas injection volume, which is easily affected by fractures and production pressure differences to form irregular gas channeling. Soaking alternating gas injection (SAG) can effectively supplement the formation pressure through periodic well switching, gas channeling delay, and radial gas flooding pattern formation. Water alternating CO2 (WAG) flooding can delay gas channeling, and the rhombus or square gas flooding pattern is formed due to the influence of the well pattern and well spacing. Foam flooding can better mitigate gas channeling but has a poor displacement effect. Through comparison of the various gas injection modes, it is concluded that WAG is the best CO2 flooding mode in the research area, and the cumulative increase in oil over 20 years can reach 390,466 t. The outcome of this work is important to CO2 flooding in tight reservoirs with fractures. The CO2 flooding mechanism in tight reservoir with fractures was examined.