Abstract
Safe completion of High Pressure High Temperature (HPHT) ultra-deep oil or gas wells is often carried out using high density (2.22 g/ml, 18.5 ppg) clear brines composed of Zn and Ca bromides. The corrosion rate of such brines with carbon steel at temperatures of 175 to 230°C (350 to 450 °F) has been reported in the literature as relatively small and inconsequential given the short residence time of these fluids in the hole during completion and/or workover operations. Recently, however, significant corrosion damage has been observed on a carbon steel coiled tubing string during well intervention operations, and on other incidental downhole high strength carbon steel workover tools. In order to resolve these apparent discrepancies a series of corrosion tests were performed using rotating cylinder probes cut from actual coiled tubing stock. Very high corrosion rates (up to 100 mm/y or 4000 mpy) were observed in the beginning of exposure. Over time the corrosion rates decreased 20 to 50 fold depending on the length of observation. This decrease was attributed to the formation of a surface film consisting of Zn-Fe bromide. While iron ions are quite soluble in the brine, the Zn-Fe-Br complex on the metal surface was not, and continued to grow with time. The film had a lacquer like appearance and appeared to be continuous. The kinetics were parabolic in nature revealing the protective nature of the film. Activation energies for corrosion rates under flowing as well as stagnant conditions were determined at approximately 8.5 kcal/mol.°K. This suggested mixed corrosion rate control by the electrochemical charge transfer and diffusion through the corrosion product film. Mass transfer into the solution did not seem to be rate determining because of observed minor dependence of the corrosion rate on flow rate (rotational speed).
