Carbonation of silica cement at high-temperature well conditions

Abstract

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.