Miscible CO2 Water-Alternating-Gas (CO2-WAG) Injection in a Tight Oil Formation

Abstract

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...

Three different periods of CO2 dissolution into a light crude oil

“…The molecular weight of the Bakken light crude oil was measured to be 168.0 g/mol by using an automatic high‐sensitivity wide‐range cryoscopy (Model 5009, Precision Systems Inc., USA). The compositional analysis result of the cleaned light crude oil was obtained by using the standard ASTM D86 method and can be found elsewhere . The molar percentages of C 1–10 , C 11–20 , C 21–30 , and C 31+ were found to be 64.19, 26.94, 6.26, and 2.61 mol%, respectively.…”

“…The oil‐swelling factor of the Bakken light crude oil‐CO 2 system at a given P eq and T res = 56.0 °C was measured by using a DBR PVT system (PVT‐0150‐100‐200‐316‐155, DBR, Canada). The detailed experimental setup and procedure for the oil‐swelling factor measurements were described in the previous study . Here, the Bakken light crude oil‐swelling factor f s is defined as the ratio of the CO 2 ‐saturated light crude oil volume at P eq and T res to the dead light crude oil volume at P a = 1 atm and T res .…”

“…The compositional analysis result of the cleaned light crude oil was obtained by using the standard ASTM D86 method and can be found elsewhere. [27] The molar percentages of C 1-10 , C 11-20 , C [21][22][23][24][25][26][27][28][29][30] , and C 31þ were found to be 64. 19 PVT Tests CO 2 solubility in the Bakken light crude oil was measured by using a flash method.…”

“…The detailed experimental setup and procedure for the oil-swelling factor measurements were described in the previous study. [27] Here, the Bakken light crude oil-swelling factor f s is defined as the ratio of the CO 2 -saturated light crude oil volume at P eq and T res to the dead light crude oil volume at P a ¼ 1 atm and T res .…”

Comprehensive analysis of influencing factors and mechanism of nano emulsion spontaneous imbibition oil displacement in tight sandstone reservoir

“…Domestic and foreign scholars have found that the primary recovery rate of tight oil reservoirs obtained by hydraulic fracturing is generally low, and the oil well production decreases rapidly after 1 year (Baihly et al, 2010;Zhang et al, 2015;Rodriguez and Maldonado, 2019). Most reservoirs need to supplement formation energy by water injection or gas injection, but tight reservoirs often encounter problems such as injection difficulties or insufficient surrounding gas sources (Chen and Reynolds, 2015;Han and Gu, 2015). Therefore, how to effectively replenish formation energy is of great significance for improving recovery efficiency of tight oil reservoirs.…”