Abstract
For successful CO2 storage in underground reservoirs, the potential problem of CO2 leakage needs to be addressed. A profoundly improved understanding of the behavior of fractured cement under realistic subsurface conditions including elevated temperature, high pressure and the presence of CO2 saturated brine is required. Here, we report in-situ X-ray micro computed tomography (µ-CT) studies visualizing the micro-structural changes upon exposure of cured Portland cement with a leak path to CO2 saturated brine at high pressure. Carbonation of the bulk cement, self-healing of an artificially engineered leakage path in the cement specimen, and leaching of CaCO3 were thus directly observed. The precipitation of CaCO3, which is of key importance as a possible healing mechanism of fractured cement, was found to be enhanced in confined regions having limited access to CO2. For the first time, we are thus able to estimate quantitatively the growth kinetics of CaCO3 under high pressure. Combining the µ-CT observations with scanning electron microscopy resulted in a detailed understanding of the processes involved in the carbonation of cement.