Design of water-based annulus protection fluid for CO2 flooding injection well

To solve the issue that casing and tubing can be easily corroded in the injection well of CO2 flooding, a novel water-based annulus protection fluid suitable for injection well in CO2 flooding was developed. Firstly, the appropriate additives, such as the deoxidizer, inhibitor, bactericide, pour point depressant and density regulators, were selected according to the service environment of wellbore. On this basis, the synergism among the additives was evaluated. Then, the ratio of additives was optimized to form the formulation of water-based annulus protection fluid. Finally, the performance evaluation of the water-based annulus protection liquid was evaluated. The results showed the composition of the water-based annulus protection fluid was commercial corrosion inhibitor (XCN20) (1500 ppm), CT4-42 bactericide (1 g/L), D-isoascorbic acid sodium deoxidizer (3 g/ L), sodium formate (100 g/L), glycol 20%. The density of the protection fluid was 1.01-1.17 g/cm(3), and its freezing point was - 2-20 degrees C. The dissolved oxygen content in the injected water was controlled within 0.3 ppm by using the protection liquid. The corrosion inhibition rate of the water-based annulus protection liquid was higher than 98% in aqueous phase, and the corrosion rate of steel reached the corrosion control index of oil field (0.076mm/a) in gas-aqueous phase.