Tight oil plays have emerged as dominant producers, contributing 65% of domestic crude oil production in 2021, with the Bakken play being one of the top three oil-producing plays in the United States. Previous lab work has demonstrated that rich gas and CO2 will permeate into Middle Bakken core samples under reservoir conditions and mobilize upward of 90% of the hydrocarbon present. For conventional oil plays, microbubble CO2 injection has been shown to reduce viscous fingering and improve sweep efficiency in both lab and field injection tests. This research applies the technique in rock samples from unconventional oil plays. This study aims to evaluate and compare the recovery performance of microbubble gas injection and continuous gas injection in tight formations in North Dakota.
Core flooding tests were conducted on rock plugs saturated with dead Bakken oil to evaluate the recovery process and compare recovery factors (RF). Core samples were collected from the Red River and Bakken Formations, with permeabilities of 9 mD and 246 nD, respectively. Two gases were used for flooding: CO2 and a rich gas mixture. Both nonmicrobubble and microbubble gas flooding was performed on the Red River plug. For the Bakken plug, continuous gas flooding was tested. Additionally, a study was performed to evaluate the recovery performance between two types of CO2 flooding processes (conventional and microbubble) on the Red River plug by using X-ray computed tomography to visualize the displacement history during the tests.
For the Red River plug, results showed that CO2 injection greatly outperformed rich gas regardless of flooding process. Both RF and visualization results suggest that microbubble CO2 has better recovery performance than continuous CO2. A higher differential pressure (ΔP) was generated during the microbubble CO2 process, which contributed to the additional recovery. While testing with rich gas, both processes produced similar amounts of oil. The ΔP profiles from both processes were very similar, with no major variation. For the Middle Bakken plug, conventional CO2 flooding produced 14% more oil than the rich gas injection. Because of the ultralow matrix permeability and limited pore volume, further testing of the microbubble process in fractured Bakken plugs is recommended.