Carbonation of silica cement at high-temperature well conditions

Cements for well environments with temperatures above 110 °C are typically designed with silica additions. This is the case for many of the wells in the North Sea, which is a region promising for large-scale geological storage of CO₂ from European sources. Wells are probable leakage paths in carbon capture and storage (CCS) projects, and it is therefore important to understand how CO₂ interacts with cement under downhole conditions. In this study, microstructural changes associated with carbonation of cement with and without silica were followed using micro-computed tomography, X-ray diffraction and scanning electron microscopy. The rims of the cement cores exposed to CO₂-saturated brine consisted of a carbonated region and a bicarbonated region. In the silica cement sample, the carbonated region consisted of two distinct layers with a rough interface region containing wormhole-like features. The formation of these two layers in the silica cement is proposed to be due to calcium carbonate dissolution and re-precipitation during exposure to CO₂-saturated brine. The results illustrate the importance of the effect of additives for offshore CO₂-storage well integrity.