A study was carried out to investigate corrosion at the cement/casing interface for 13Cr casing steel. Cement simulated pore solution (CSPS) was developed by exposing cement pieces to 5% NaCl at 100 °C and 10 MPa in equilibrium with CO2. Pore solution was extracted from the cement pieces using a die press. Chemical analysis of the pore solution extract was performed and used for preparing CSPS for 13Cr corrosion testing. Corrosion tests were performed in CSPS at equilibrium with CO2 or CO2+H2S at 10 MPa at 4, 85, and 200 °C. Corrosion rates were determined using linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and mass loss coupons. Addition of H2S at 4 °C increased the corrosion rate, while at 85 °C it slightly decreased the corrosion rate. Only a small effect from H2S addition was observed at 200 °C. Cyclic voltammetry (CV) results showed passivation/depassivation of 13Cr in CSPS at 4 and 85 °C, while a transition to active corrosion was observed at 200 °C. Surface analysis using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) found that both the coverage and complexity of the corrosion products formed increased dramatically with increasing temperature.
Corrosion of a casing steel at the cement-casing interface in oil and natural gas wells can be described as a competition between two forces. On the one hand is the force of corrosion, promoted by a formation water that can be rich in chlorides and acid gases, including CO2 and H2S. On the other hand, the cement serves as a protective force which provides a physical barrier slowing the diffusion of formation water to the casing and provides Ca(OH)2 to maintain a relatively high pH. As long as the solution inside the cement pores remains saturated with Ca(OH)2, which has low solubility, steel at the interface often remains in a passive state due to the alkaline pH. Over time, however, saturation of the cement pores with formation water will convert the Ca(OH)2 to CaCO3 if carbonic acid, or dissolved CO2, is present, and pH of the pore water will decrease. Chlorides will also attack the passive film, opening the casing to pitting and localized corrosion. Cement defects, such as cracks and annular spaces, can speed up the process and allow aggressive environments to form at the casing interface.