Optimization of Miscible CO2 Water-Alternating-Gas Injection in the Bakken Formation

In this paper, optimization of miscible CO2 water-alternating-gas (CO2-WAG) injection in the Bakken formation is experimentally studied. First, tight sandstone reservoir rock samples from the Bakken formation are characterized. Second, the vanishing interfacial tension technique is applied to determine the minimum miscibility pressure of the Bakken light crude oil and CO2 at the actual reservoir temperature. Third, a total of nine coreflood tests are conducted through respective waterflooding, continuous miscible CO2 flooding, and miscible CO2-WAG injection. In the miscible CO2-WAG injection, different WAG slug sizes of 0.125, 0.250, and 0.500 pore volume and different WAG slug ratios of 2:1, 1:1, and 1:2 are used to study their specific effects on the oil recovery factor (RF) in the Bakken formation. In addition, miscible CO2 gas-alternating-water (CO2-GAW) injection is also tested as an opposite fluid injection sequence of the miscible CO2-WAG injection. It is found that, in general, the CO2 enhanced oil recovery method is capable of mobilizing the light crude oil in the Bakken tight core plugs under the miscible conditions. The miscible CO2-WAG injection has the highest oil RF (78.8% in test 3), in comparison with waterflooding (43.2% in test 1), continuous miscible CO2 flooding (63.4% in test 2), and miscible CO2-GAW injection (66.2% in test 8). Furthermore, using a smaller WAG slug size for CO2-WAG injection leads to a higher oil RF. The optimum WAG slug ratio is approximately 1:1 for the Bakken tight formation. More than 60% of the light crude oil is produced in the first two cycles of the miscible CO2-WAG injection. The CO2 consumption in the optimum miscible CO2-WAG injection is much less than that in the continuous miscible CO2 flooding.