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

摘要

In this paper, miscible CO, simultaneous water-and-gas (CO2-SWAG) injection in the tight Bakken formation is experimentally studied. The effective viscosities of high-salinity water and supercritical CO, 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 CO2-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, 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 CO2-SWAG injection with an injected water-gas ratio (WGR) of 1:3 in volume has the highest oil recovery factor (RE). The miscible CO, water-alternating-gas (CO2-WAG) injection achieves a slightly higher oil RE than that of the miscible CO, flooding, whereas the waterflooding followed by the miscible CO, flooding has the lowest oil RE. In addition, the WGR shows strong effects on the fluid production trends of the miscible CO2-SWAG injection. A water bank might be formed ahead of the water CO, mixture in the miscible CO2-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, alone, the water CO, mixture has a lower mobility in the tight reservoir core plugs. Hence, the highest oil RE of the optimum CO2-SWAG injection with the lowest injected WGR of 1:3 is attributed to a well-controlled water CO, mobility and a substantially weakened waterblocking effect.