Carbonation of borehole seals: Comparing evidence from short-term laboratory experiments and long-term natural analogues

• Published in: Applied geochemistry Vol. 30; pp. 161 - 177
• Main Authors: Rochelle, Christopher A., Milodowski, Antoni E.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.03.2013
• Subjects: Boreholes; calcium; Calcium carbonate; Carbon dioxide; Carbonation; cement; Cements; cracking; groundwater; laboratory experimentation; long term experiments; Marine; Phases; Porosity; Seals; shrinkage; silica; Northern Ireland; British Isles, Northern Ireland
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2012.09.007

► Carbonation proceeds via a dissolution–precipitation mechanism. ► Carbonated cement has heterogeneous porosity at a micro-scale. ► Initially-formed rinds of carbonate minerals may hinder complete cement carbonation. It is crucial that the engineered seals of boreholes in the vicinity of a deep storage facility remain effective for considerable timescales if the long-term geological containment of stored CO2 is to be effective. These timescales extend beyond those achievable by laboratory experiments or industrial experience. Study of the carbonation of natural Ca silicate hydrate (CSH) phases provides a useful insight into the alteration processes and evolution of cement phases over long-timescales more comparable with those considered in performance assessments. Samples from two such natural analogues in Northern Ireland have been compared with samples from laboratory experiments on the carbonation of Portland cement. Samples showed similar carbonation reaction processes even though the natural and experimental samples underwent carbonation under very different conditions and timescales. These included conversion of the CSH phases to CaCO3 and SiO2, and the formation of a well-defined reaction front. In laboratory experiments the reaction front is associated with localised Ca migration, localised matrix porosity increase, and localised shrinkage of the cement matrix with concomitant cracking. Behind the reaction front is a zone of CaCO3 precipitation that partly seals porosity. A broader and more porous/permeable reaction zone was created in the laboratory experiments compared to the natural samples, and it is possible that short-term experiments might not fully replicate slower, longer-term processes. That the natural samples had only undergone limited carbonation, even though they had been exposed to atmospheric CO2 or dissolved HCO3- in groundwater for several thousands of years, may indicate that the limited amounts of carbonate mineral formation may have protected the CSH phases from further reaction.