Corrosion Behavior of 13Cr Casing in Cement Synthetic Pore Solution

A test procedure was designed and employed for exposure and degradation testing of well cements in a simulated downhole environment of offshore deep drilling. A Class H cement sample was crushed to coarse grains and was exposed to 5 % mass NaCl(aq) solution in contact with CO₂(g) at 100 ˚C. The partial pressure of CO₂ was 10 MPa. After the 200-hour exposure, the liquid phase from the testing, or the ‘original pore solution’, was diluted and extracted for qualitative and quantitative analyses of the key ionic species. Significant variation was observed between samples drawn in situ and ex situ from the original pore solution. Ca²⁺(aq), Na⁺(aq), and some K⁺(aq) were determined to be the primary cationic species. Cl^-(aq) and SO₄^2-(aq) were found to be major anionic species, though the concentration of Cl^-(aq) was lower than that originally in the NaCl(aq) solution. The compounds used to achieve the equilibrium speciation corresponding to the “original pore solution” were modeled using commercial software OLI Analyzer Studio 9.0, and were used to develop the cement synthetic pore solution (CSPS) recipe. The CSPS was then used for corrosion characterization of high chromium well casing steel, type 13Cr, grade L-80. The steel samples were exposed to CSPS at 100 ˚C and 10 MPa pressure in contact with CO₂(g) with a Pt mesh counter electrode and a custom made Ag/AgCl electrode. The corrosion behavior of the steel was investigated using a set of electrochemical methods consisting of the open circuit potential, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) at 3-hour intervals to detect steady state conditions. The CO₂-rich CSPS was stirred during these experiments, which lasted 60 hours. At the end of 60-hour exposure, the stirring was stopped and the same set of electrochemical tests was performed without stirring to detect any effect of convection or diffusion. The corrosion potential in the measurements was similar, between -0.4 and ‑0.5 V vs. standard hydrogen electrode. The average corrosion rate of the steel samples was on the order of 0.1 mm/year. This was considered high for an environment in which steels expected to be passivated. Polarization resistance obtained from EIS data was similar to that determined through LPR. No significant change in the corrosion behavior was observed between stirred and unstirred conditions, and the EIS data did not suggest that corrosion process was controlled by diffusion. The high corrosion rate was believed to be due to neutralization of the calcium hydroxide by the dissolved CO₂, which then moved the corrosion potential of the steel from the passive region to the active region. This would provide a highly corrosive environment when combined with the high chloride content and high temperature of the CSPS. While it was noted that the condition studied here represented a worst-case scenario for degradation of the cement material, the magnitude of the corrosion rate shows that care must be taken to prevent damage to the cement coating, as the resulting environment could result in severe corrosion and premature failure of critical steel components like a well casing.