Experimental study on the corrosion behavior of wellbore cement with a leaking channel under different acidic environments

In the scenario of geologic CO₂ storage, the injection of CO₂ can create a carbonic acid-rich environment in the reservoir, and Cl^-- and SO₄^2--rich low-pH environments may form in the reservoir if the reservoir brine contains high concentrations of Cl^- and SO₄^2-. The analysis of morphological changes in wellbore cement containing leakage channels before and after the reaction in different acidic environments is crucial for assessing the risk of CO₂ leakage along the internal cracks of wellbore cement. This study characterizes the morphological and structural changes of wellbore cement with a leaking channel before and after the flow of CO₂-saturated brine and then compares the results with the structural changes of channels after exposure to HCl and H₂SO₄ solutions. The results indicate that the cement around the leaking channel dissolves, and the channel volume increases when exposed to CO₂-saturated brine. The reaction is more intense at the inlet end than at the outlet end, and some cracks form around the channel. As the HCl solution flows through the channel, a hydrate precipitate that contains calcium and aluminum forms from the inlet to the middle of the channel. This is due to the aqueous phase cations (Ca²⁺ and Al³⁺ released from the hydrated cement phases) mixing with the high pH pore fluid ahead of the acid front. Upon flow of the H₂SO₄ solution through the channel, a thin layer of precipitation forms on both the inlet and outlet ends of the channel. XRD analysis indicates that the precipitation comprises gypsum (CaSO₄·2H₂O), which forms due to the reaction between SO₄^2- in the H₂SO₄ solution and Ca²⁺ in the cement hydration product. The volume of the channel decreased after exposure to HCl and H₂SO₄ solutions, indicating that secondary precipitation resulting from the reaction between the cement and acid exceeded the cement dissolution, and the hydrochloric and sulfuric acidic environments had a limited effect on the expansion of the wellbore cement's internal channel. The experimental results of this study also indicate that in an acidic environment with the same pH, the CO₂-saturated brine is the most corrosive to wellbore cement, followed by hydrochloric acid, and sulfuric acid is the least corrosive.