The Study of Timing of Cyclic Injections in Miscible CO2 WAG

Abstract When water alternating gas flooding utilizing carbon dioxide (CO2 WAG) is injected at different stages after secondary recovery, the difference in oil/water/gas saturations influences the micro and macro displacement efficiencies due to multiple mechanisms, thus the performance of WAG could be different and an optimum timing could exist. In this study, the timing of cyclic injections in miscible CO2 WAG is experimentally investigated. WAG performance is studied in core flood experiments performed in Berea sandstone cores. Synthetic brines and South Slattery crude oil from Wyoming are used to restore the core to its reservoir conditions, and ultra high purity CO2 is used as the miscible gas. The experiments are conducted at 57°C and at a pressure 20% above the minimum miscibility pressure (MMP) to ensure miscible condition; slim tube experiments to obtain the MMP of the system are performed prior to the core floods. The experiments utilize a WAG ratio of 1:1, an optimized half cycle slug size of 0.1 PV, and a total slug size of 2 PV. The timing of cyclic injections is studied by introducing WAG at different stages between secondary and tertiary mode. The WAG performance is determined by analyzing the percent oil recovery, CO2 breakthrough time, and tertiary recovery factor (TRF) The earlier WAG is injected, the longer CO2 stays in the core before its breakthrough. However, the results show that the best timing of cyclic injections for miscible CO2 WAG is when water flooding produces half of the oil that can be flooded by secondary recovery alone, i.e., when the flood front passes through the middle of the core. Injecting WAG too early or too late will result in either low macro sweep efficiency or low micro displacement efficiency. The analysis also shows that the phenomena are the results of multiple mechanisms. Similar studies using numerical simulations have been reported but never been experimentally investigated. The study of timing of cyclic injections is essential in understanding the mechanisms, as well as optimizing the performance of miscible CO2 WAG.