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Effect of Very Low Ph on Degradation of Class G Cement: Implications for Co2 Geological Storage

Identification and mitigation of CO2 leakage through well cement, particularly for abandoned wells, is currently considered one of the major technical challenges associated with ensuring security of CO2 storage. Any potential leakage pathways in a typical abandoned well would likely be along the well cement, and would arise due to the interactions between CO2, brine and the well cement that affect mineral stability and lead to carbonation [1-3]. Any resultant degradation, together with a general increase of porosity and permeability, could lead to the loss of cement structural integrity. Despite the importance of pH in identifying the rates of chemical reactions, very few studies have been performed in brine-CO2 mixtures at high pressure [4], and more specifically the effect of low pH on cement wellbore has received little attention. Therefore, the aim of this work is to understand the effect of low pH on wellbore integrity under geological CO2 storage conditions and to assess whether or not the interactions between CO2, brine and the well cement can cause a noticeable change in the cement’s microstructure. This paper presents the results of a series of laboratory experiments using Class G cement, reacted at pH 1.3 in batch reactors for periods of up to 9 months at specific pressure and temperature conditions (PCO2=150-250-265 bar and T=50-80-90°C), which are representative of different depths along a typical wellbore. The prepared brine composition mimics that of North Sea seawater, but sodium hydrogen sulphate (NaHSO4) was used instead of a neutral salt like sodium sulphate (Na2SO4) in order to decrease the pH of the brine to a very low value, thereby stress testing the system in extremis. As a result, the brine used for this work has a pH of 1.3, while typical North Sea seawater has a pH of 8.1 [5]. Following the hydrothermal batch reactor studies, it was clear that the cement samples had undergone chemical reactions, as they showed significant colour change. In the case of the samples that were under PCO2=150 bar and T=50°C conditions, the outermost region