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

Abstract

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 CO 2 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 CaCO 3 and SiO 2 , 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 CaCO 3 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 CO 2 or dissolved HCO 3 -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. IntroductionAcceptable performance of the geological storage of CO 2 will need to be demonstrated in order to satisfy operational, regulatory and public acceptance criteria, and a key element of this will be the effectiveness of the seals preventing CO 2 migration. At a generic level, the occurrence of natural CO 2 accumulations demonstrates the containment potential of caprock materials that have not been penetrated by human activity over time periods measurable in 10

Experimental assessment of well integrity for CO2 geological storage: Batch experimental results on geochemical interactions between a CO2–brine mixture and a sandstone–cement–steel sample

“…Rochelle and Milodowski (2013) also observed cement hydration during an N 2 -pressurized experiment. Under N 2 -saturated brine conditions, the alteration depth was approximately 0.6 mm and remained almost constant over the reaction time, while the alteration depth ranged from 0.5 to 2.0 mm under wet N 2 conditions (Fig.…”

“…The sidewall samples from CO 2 wells have demonstrated that if appropriately constructed, then well cements can prevent significant migration of CO 2 from reservoirs for decades. Laboratory experiments using composite samples of the casing and cement have shown that casing does not corrode if the bonding between the cement and casing is tight (Bachu and Bennion, 2009;Carey et al, 2010;Rochelle and Milodowski, 2013). On the other hand, from laboratory experiment results, CO 2 can cause both selfhealing and degradation of simple cement (reviewed by Zhang and Bachu, 2011).…”

Experimental Study of Well Cement Carbonation under Geological Storage Conditions

“…In contrast, the carbonation zone has the lowest Cl content. Enrichment of Cl in unaltered cement was observed previously, and was attributed to its association with CSH gel containing very little Al [7]. It is evident that the Na, K, and Cl profiles can be used to describe the extent of carbonation.…”

Sensitivity of chemical cement alteration – modeling the effect of parameter uncertainty and varying subsurface conditions

“…During leaching, calcium ions are removed from the C‐S‐H structural layers and to counteract the resulting charge imbalance, siloxane bonds form between the different layers (i.e., polymerization of the silicate chains) making the C‐S‐H more closely packed . The volume decrease has been shown to cause cracking as a result of shrinkage in samples of C‐S‐H of natural cement analogue systems . However, the porosity decrease will be compensated by calcite precipitation during carbonation, in contrast to pure leaching decalcification.…”