Artigos-Congressos

Resumo: RIP2025-00099: It is common sense that all CCS wells must be designed for potential CO2 exposure. Cement integrity is essential to prevent migration of stored CO2 out of the storage zones. In this context, electrochemical techniques can give valuable information on electrolyte migration from outside through the cement barrier and eventually reaching the well casing. The challenge is to design an experimental setup to perform electrochemical measurements that mimic CCS wells conditions. Herein we present the results of electrochemical impedance measures performed at 65 oC and 10 MPa of CO2 and 0.02MPa of H2S/CO2 (balance) with 80,000 ppm of NaCl with setup specially developed to follow the changes on casing/cement interface. All tests were performed from the cement curing up to 30 days exposure to field conditions reproduced in laboratory. Two distinct cement paste were evaluated, C1 and C2, and carbon steel as casing metal. By the impedance diagrams, recorded periodically over the entire test, it was possible to follow the interface changes and correlate the results with cement paste performance and carbon steel corrosion resistance at CCS wells conditions. The analyses of electrochemical measures associated with Raman spectra interpretation of compounds formed on carbon steel after exposure to high pressure environments (10 MPa) gave valuable insights on cement paste role on casing corrosion performance. During the immersion period at high CO2 or H2S/CO2 pressure it was easy to identify any cracks on cement due to changes on electrochemical impedance response of carbon steel/cement interface. To both, C1 and C2, exposed to supercritical CO2 for 30 days the following compounds were identified on carbon steel surface by Raman analyses: Fe3O4, g-Fe2O3 e b-FeOOH(Cl). FeCO3 was present only when cement was physically damaged. These findings support the electrochemical results: open arcs on Nyquist diagrams, clear indication of resistive interface. Higher impedance values were observed to casing/cement interface with C2 compared to C1 in agreement with designed cement composition. On test carried out with H2S/CO2 the interface processes might be similar but happens faster once to the same frequencies the impedance components were lower. Within the test time CO2 and H2S only reach the casing surface when cement was physically damaged. The designed experimental setup was effective to evaluate casing/cement interface at CCS well condition and allowed to investigate the influence of cement paste and environment composition on casing corrosion, a valuable contribution to study the performance of new cement paste.

Resumo: RIP2025-00148: Corrosion models are crucial for estimating uniform corrosion. The understanding of CO2 corrosion mechanism has been extensively developed in the literature over the last 50 years. It is well known that CO2 enhanced corrosion of pipelines steel. The hydrogen evolution reaction increases in the presence of CO2 compared to a strong acid solution at the same pH. After years of discussions in the literature it was understood that CO2 acts as a buffering agent, where the dissociation of carbonic acid produces an additional H+ reservoir, which is the precursor of the cathodic reaction1-2. On the other hand, Almeida et al.3, using electrochemical impedance at OCP and calculation, demonstrated that, if CO2 was able to act on the free iron surface, producing (FeCO2)ads as reported in the literature4 its relaxations would appear as a capacitive loop. Thus, two consecutive capacitive loops should be seen. This diagram has never been observed since CO2 is not able to act on free iron surface. Nonetheless, if CO2are able to play any role in the corrosion mechanism, this can only be seen by polarizing the system.

Considering the above, electrochemical impedance was used to investigate the effect of CO2 on X65 carbon steel corrosion in different pH values. An experimental methodology was performed to polarize anodically and also cathodically the system, with small and fixed quantities of chloride, in the presence and absence of CO2.

The preliminary results at pH 3 and pH 4, show that in general, the impedance has two faradaic loops. The inductive loop is associated with relaxation of (FeOH)ads5. Hering, the diagrams clearly showed that CO2 does not act directly on the free iron surface, showing that the mechanism is not changed. However, a change in the characteristic frequency of the inductive loop in the medium containing CO2 was observed and highlight the role of CO2 in this corrosion mechanism. The mechanism of corrosion by CO2 will be discussed in this work, and a new reaction mechanism will be presented. These results and discussions bring new study perspectives to understand the effects of contaminants, such as SO2 and NO2, present in CO2 injection systems, and can elucidate the role of each contaminant in the carbon steel corrosion mechanism in CCS systems.