Annular corrosion risk analysis of gas injection in CO2 flooding and development of oil-based annulus protection fluid

To investigate the corrosion behavior of gas-injection well of CO2 flooding and then to take measures to inhibit the corrosion of casing, firstly an HTHP autoclave and surface characterization technology are applied to analyzing the corrosion behavior of casing and then a new type of oil-based annulus protection fluid has been developed to avoid the casing corrosion issue. The results show that the casing in the simulated supercritical CO2-saturated aqueous brine phase (SC CO2-saturated aqueous phase) has severe uniform corrosion, while that in the simulated water-saturated supercritical CO2 (SC CO2 phase) has severe pitting corrosion. The temperature and CO2 partial pressure can promote the water film adsorbed on the steel surface, further inducing the corrosion of casing. In order to prevent casing corrosion, an oil-based annulus protection fluid suitable for gas-injection well in CO2 flooding is developed, and the preparation process of oil-based annulus protection fluid is proposed. The oil-based annulus protection fluid is composed of white oil and 1000 mg/L imidazoline inhibitor. The flash point of the oil-based annulus protection fluid is over 160 °C, the freezing point −23 °C, and the density 0.85 g/cm3. The corrosion inhibition rate is more than 80% when P110 steel is in SC CO2 phase with oil-based annulus protection fluid, and it is more than 95% when P110 steel is in SC CO2-saturated aqueous phase. It proves that the newly developed oil-based annulus fluid can effectively solve the corrosion problem of P110 steel as casing in gas-injection well of CO2 flooding.