The cemented annulus behind casings and liners is a critical barrier element not only for oil and gas wells, but also for geothermal wells and underground CO2 storage. Corroded casing has been identified as one possible leakage pathway along wellbores. The presence of CO2 and H2O at the cement-casing interface promote and accelerate casing corrosion and cement carbonation. Two different sets of experimental studies were conducted and compared to understand different mechanisms of corrosion to reflect potential well conditions and their effects on leakage.

The first study involved the analysis of a sandwich section, composed of 13 3/8-in and 9 5/8-in casing with wellbore cement in between recovered from the upper part of a well in the Valhall field in the North Sea. A slice of the recovered cross section was exposed to environmental corrosion for about thirty days at ambient conditions. Analytical measurements were undertaken to evaluate the porosity and corrosion states together with the influence of contaminants on the corrosion rate.The second study involved laboratory investigations on assemblies of steel plates and wellbore cement (API class G). The steel in each assembly is corroded with different mechanisms (environmental and electrochemical), producing different corrosion rates. Single-phase gas flow testing was carried out on the sample assemblies to investigate the effects of corrosion on the permeability of the corrosion product and consequently the leakage potential. Additional analytical measurements were used (SEM-EDS, XPS) to evaluate the porosity, chemical composition and oxidation states.

For the sandwich slim cross section evaluated in the first study, results of chemical characterization using spectroscopic analysis (XRF, FTIR) and post processing using principal component analysis, show that the major components of corrosion are iron oxides, however the corrosion is highly correlated to the presence of BaSO4 (Barium sulphate).The results suggest that the presence of slurry contaminants, such as drilling mud and spacers, may have an important influence in the oxidation rate acceleration. The gas flow tests from the second study indicate that corroded casing can have a significant effective permeability and that corroded casing can serve as a significant leakage path along the axis of a wellbore. The major components of corrosion found in the samples were different oxidation states of iron, but there was no discernable difference in the composition of corrosion products from specimens corroded by different mechanisms. Differences in cement porosity were observed in both experiments.

We have shown that corroded casing is substantially permeable and a potential wellbore leakage pathway. Further, our results suggest that corrosion at the casing-cement interface may be affected by contamination in the cement slurry.A better understanding of corrosion mechanisms is essential for the remediation of leaky wells.

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