Carbonation of wellbore cement by CO2 diffusion from caprock

To evaluate the risk of corrosion of cement by geosequestered CO2, samples are being retrieved from wells placed in natural CO2 deposits [e.g., Crow et al., 2009]. If the cement passing through the cap rock is carbonated, it may indicate that annular gaps or cracks have allowed carbonic acid to come into contact with the cement. However, it must be recognized that the pore water in the cap rock has become saturated with CO2 over geological time. After the well is placed, the CO2 will diffuse toward the cement and react with it. A simple analysis of the diffusion kinetics demonstrates that carbonation depths of millimeters to centimeters can be expected from this reaction within the lifetime of a well, in the absence of any cracks or gaps. Therefore, the occurrence of carbonation in cement sealing natural CO2 deposits must be interpreted with caution.