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During the process of geological CO2 storage, the injection of CO2 can lead to the creation of a reservoir environment enriched with carbonic acid. If the reservoir brine contains high concentrations of chloride (Cl-) and sulfate (SO42-) ions, low-pH environments rich in these ions may also develop. Assessing the risk of CO2 leakage through internal cracks in wellbore cement necessitates a detailed analysis of morphological changes in cement containing leakage channels before and after exposure to various acidic conditions. This study examines the morphological and structural alterations in wellbore cement with leakage channels before and after the following of CO2-saturated brine, and compares these changes to those observed after exposure to hydrochloric acid (HCl) and sulfuric acid (H2SO4) solutions. The findings reveal that cement surrounding the leakage channel undergoes dissolution, resulting in an increase in channel volume when exposed to CO2-saturated brine. This reaction is more pronounced at the inlet end compared to the outlet end, accompanied by the formation of cracks in the vicinity of the channel. As HCl solution flows through the channel, a hydrate precipitate containing calcium and aluminum forms from the inlet to the middle, attributed to the mixing of aqueous phase cations (Ca2+ and Al3+ released from hydrated cement phases) with high-pH pore fluid ahead of the acid front. Upon exposure to H2SO4 solution, a thin precipitation layer forms at both the inlet and outlet ends of the channel, composed of gypsum (CaSO4·2H2O) as indicated by X-ray diffraction (XRD) analysis, resulting from the reaction between SO42- in the acid solution and Ca2+ in the cement hydration product. Following exposure to HCl and H2SO4 solutions, the channel volume decreased, suggesting that secondary precipitation from the cement-acid reaction surpassed cement dissolution, limiting the expansion of the wellbore cement’s internal channel in hydrochloric and sulfuric acidic environments. The experimental results further indicate that, within acidic environments of equivalent pH, CO2-saturated brine exhibits the highest corrosivity towards wellbore cement, followed by hydrochloric acid, with sulfuric acid demonstrating the least corrosive.
| Country | China |
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