Miscible CO2 Simultaneous Water-and-Gas (CO2-SWAG) Injection in the Bakken Formation

In this paper, miscible CO 2 simultaneous water-and-gas (CO 2 -SWAG) injection in the tight Bakken formation is experimentally studied. The effective viscosities of high-salinity water and supercritical CO 2 mixtures with 12 different water volume fractions are measured at the actual reservoir conditions by using a capillary viscometer. A total of six coreflood tests with four different miscible CO 2 -EOR schemes are conducted in the tight reservoir core plugs collected from the Bakken formation (Canada). It is found that the measured effective viscosity of the saline water–CO 2 mixture increases with the water volume fraction and can be reasonably modeled by using the Arrhenius equation. The coreflood test results indicate that the miscible CO 2 -SWAG injection with an injected water–gas ratio (WGR) of 1:3 in volume has the highest oil recovery factor (RF). The miscible CO 2 water-alternating-gas (CO 2 -WAG) injection achieves a slightly higher oil RF than that of the miscible CO 2 flooding, whereas the waterflooding followed by the miscible CO 2 flooding has the lowest oil RF. In addition, the WGR shows strong effects on the fluid production trends of the miscible CO 2 -SWAG injection. A water bank might be formed ahead of the water–CO 2 mixture in the miscible CO 2 -SWAG injection with a higher injected WGR of 3:1 or 1:1. Furthermore, the mobility ratio of the injected fluid(s) to light crude oil is calculated based on the measured steady-state flow rate and pressure gradient in each coreflood test. In comparison with water or CO 2 alone, the water–CO 2 mixture has a lower mobility in the tight reservoir core plugs. Hence, the highest oil RF of the optimum CO 2 -SWAG injection with the lowest injected WGR of 1:3 is attributed to a well-controlled water–CO 2 mobility and a substantially weakened waterblocking effect.