In this paper, optimization of miscible CO 2 water-alternating-gas (CO 2 -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 CO 2 at the actual reservoir temperature. Third, a total of nine coreflood tests are conducted through respective waterflooding, continuous miscible CO 2 flooding, and miscible CO 2 -WAG injection. In the miscible CO 2 -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 CO 2 gas-alternating-water (CO 2 -GAW) injection is also tested as an opposite fluid injection sequence of the miscible CO 2 -WAG injection. It is found that, in general, the CO 2 enhanced oil recovery method is capable of mobilizing the light crude oil in the Bakken tight core plugs under the miscible conditions. The miscible CO 2 -WAG injection has the highest oil RF (78.8% in test 3), in comparison with waterflooding (43.2% in test 1), continuous miscible CO 2 flooding (63.4% in test 2), and miscible CO 2 -GAW injection (66.2% in test 8). Furthermore, using a smaller WAG slug size for CO 2 -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 CO 2 -WAG injection. The CO 2 consumption in the optimum miscible CO 2 -WAG injection is much less than that in the continuous miscible CO 2 flooding.

In this paper, miscible CO 2 water-alternating-gas (CO 2 -WAG) injection in the Bakken formation is experimentally studied and optimized. First, tight sandstone reservoir rock samples from the Bakken formation are characterized. Second, the saturation pressure, oil-swelling factor, CO 2 solubility, CO 2saturated Bakken light crude oil density and viscosity are measured. Third, the vanishing interfacial tension (VIT) technique is applied to determine the minimum miscibility pressure (MMP) of the Bakken light crude oil and CO 2 at the actual reservoir temperature. Last, a total of nine coreflood tests are conducted through respective waterflooding, continuous miscible CO 2 flooding, and miscible CO 2 -WAG injection. In the miscible CO 2 -WAG injection, different WAG slug sizes of 0.125, 0.250, and 0.500 pore volume (PV) 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 CO 2 gas-alternating-water (CO 2 -GAW) injection is also tested as an opposite fluid injection sequence of the miscible CO 2 -WAG injection. It is found that in general, CO 2 enhanced oil recovery (CO 2 -EOR) method is capable of mobilizing the light crude oil in the Bakken tight core plugs under the miscible condition. The miscible CO 2 -WAG injection has the highest oil RF (78.8% in Test #3), in comparison with waterflooding (43.2% in Test #1), continuous miscible CO 2 flooding (63.4% in Test #2), and miscible CO 2 -GAW injection (66.2% in Test #8). Furthermore, using a smaller WAG slug size of CO 2 -WAG injection leads to a higher oil RF. The optimum WAG slug ratio is approximately 1:1 for the Bakken tight oil formation. More than 60% of the light crude oil is produced in the first two cycles of the miscible CO 2 -WAG injection. The CO 2 consumption in the optimum miscible CO 2 -WAG injection is much less than that in the continuous miscible CO 2 flooding. was 304,206 barrels per day (BOPD) (Kuuskraa, 2012). Several major CO 2 -EOR methods have been developed and applied, such as continuous CO 2 flooding, CO 2 huff-n-puff process, and CO 2 wateralternating-gas (CO 2 -WAG) injection under the immiscible or miscible condition. Continuous CO 2 flooding has been proven to be a highly effective EOR method in the petroleum industry (Alquriaishi and Shokir, 2011). The major CO 2 -EOR mechanisms include CO 2 miscible or immiscible displacement, CO 2 -induced interfacial tension (IFT) reduction, oil viscosity reduction, oil-swelling effect, light and intermediate hydrocarbons extraction (Blunt et al., 1993;Cao and Gu, 2013a). However, continuous CO 2 flooding also has some obvious limitations. Technically, a low volumetric sweep efficiency and an early CO 2 breakthrough (BT) are often caused by both viscous fingering and gravity overriding (Dellinger et al., 1984). Economically, an extremely large amount of CO 2 is required in the continuous CO 2 flooding. Relatively high capital and operating costs of CO 2 acquisition, transportation, s...