Results – pH and Dissolved Fe
The long term nature of environmental evolution under nominally constant conditions was reflected in the results (see Figure 3). The pH was seen to stabilise after approximately 30 days, whereas the iron concentrations that developed to peak values within the first few days continued to reduce beyond the 30 day mark. Indeed, the effect of temperature and pressure was more pronounced in the absolute Fe concentrations than for the pH values, although trends between them were similar. Water chemistry models agreed well with the pH measurements when the actual Fe2+ concentrations were inputted.
Over the range investigated, increasing the CO2 partial pressure increased [Fe2+] and reduced the pH (see Figure 4). The effect of temperature on pH was not consistent at all pressures but overall, an increase in temperature related to a reduced pH. The effect of temperature was more evident in the [Fe2+], with higher concentrations at lower temperatures.
For a given test temperature and CO2 partial pressure, the pH increased with the concentration of dissolved iron concentration. The test solution remained supersaturated with iron throughout the duration of the experiments. The variation of [Fe2+] could be visualised in the aliquots removed (see Figure 5).
The dissolved iron concentration is associated with the CO2 corrosion mechanism of steel (Nešić, 2007, Nyborg, 2002). Dissolution of CO2 acidifies the solution and accelerates the anodic dissolution of steel. The solubility of CO2 increases with pressure, explaining the more rapid acidification of solution and dissolution of Fe at 40barg CO2 partial pressure. However, the pH values appeared to substantially converge towards the end of the tests, whereas differences in dissolved iron concentration vs. temperature persisted. The stabilisation in pH values for all test temperatures at 40barg CO2 and 20barg CO2 over the long term can be attributed to the stability, structure and thickness of the scales formed.
Results – Corrosion Rate
It was shown that the corrosion rate decreased as CO2 pressure and temperature increased (see Figure 6). This is in keeping with trends reported for carbon steels at partial pressures between 15-80barg CO2, 50-65°C (Choi, 2011; Bai, 2018). However, in contrast to these studies, where corrosion rates were reported to vary between 1 and 20mm/year, the annular corrosion rates in the present study were found to be an order of magnitude lower, at circa 0.1mm/year. The low corrosion rates are associated with the V/A, i.e., the degree of occlusion of the annulus and the supersaturated conditions.
Results – Corrosion Product
Low corrosion rates were associated with protective iron carbonate scale formed on the surface of the test specimens. At 60°C, the scale was thickest. Low V/A ratios, and higher partial pressures both seemed to promote scale (Mitzithra, 2020). At 30°C, more through thickness discontinuities were observed, and these may be implicated in the slightly higher corrosion rates observed. Formation of scales at such low temperatures are not often observed in more open systems, and may be somewhat related to the fluid flow, and increased solubility of siderite at lower temperatures (Anderko, 2000).
This project has demonstrated that corrosion in confined annular environments is relatively reduced compared to those in more open systems. Although effects of temperature and pressure were evident, they are relatively inconsequential on an engineering level. However, disruption of the protective scale through upset conditions and fatigue warrants further investigation.
The [Fe2+] and pH were shown to vary significantly over time and this could have a consequence for cracking mechanisms. In particular, the reduction of these values are below those reported in some other short term studies. This highlights a potential issue, for example on the prediction of sulphide stress cracking (SSC) in mild sour annular environments, as it is dependent on both the H2S partial pressure and the pH.
The CO2 stress corrosion cracking cited in the literature remains a concern and should be studied further. However, incidences of this issue in the field seem to be rare and no incidents of cracking were identified in this study.
We would like to thank Petrobras for their guidance and support of the work, and the TWI laboratory technicians and engineers involved in carrying out this study.
For a list of references, please email: email@example.com.
This project was funded by TWI’s Core Research Programme.