Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres Effect of nano-silica and curing conditions on the reaction rate of class G well cement exposed to geological CO 2 -sequestration conditions Yeon Jong Jeong a , Kwang Soo Youm b , Tae Sup Yun a, ⁎ a Department of Civil and Environmental Engineering, Yonsei University, Yonsei-ro 50, Seodaemun-gu, Seoul, 03722, Republic of Korea b Technical Division, GS E&C, Grand Seoul, Jong-ro 33, Jongno-gu, Seoul, 03159, Republic of Korea ARTICLE INFO Keywords: Oil well cement Degradation Carbonation Nano-silica CO 2 sequestration ABSTRACT Chemical reactions of class G oil well cement submerged in CO 2 -saturated brine were experimentally in- vestigated to evaluate the effects of nano-silica and curing conditions. The progression of depths of reaction zones (depletion, carbonation, and degradation zone) with reaction time up to 62 days were quantitatively measured based on 3D X-ray CT in conjunction with the chemical analysis. The results show that there were no significant difference in the progress of carbonation zone regardless of the curing conditions and nano-silica. The measure of reaction depth for tested specimens concludes that the curing at the high pressure and temperature is more effective to prevent the depletion zone from progressing into the cement interior than the addition of the nano-silica, supported by XRD and NMR results. Image-based measurements, coupled with chemical analyses, quantified the evolution of the reactive zones in the oil well cement, providing insights into the underlying reaction mechanism. 1. Introduction Resilient carbon sequestration relies on the interface between the CO 2 injections well and the storage formation being sealed by oil well cement (OWC) [1]. The cement must form a strong, durable barrier to prevent the stored CO 2 from leaking into deep saline aquifers (< 1000 m): the leakage rate should be below 0.1% per year over 1000 years [2]. However, injected CO 2 can dissolve into the brine within the storage formation at the high pressure and temperature (hereafter P-T) condition found in deep wells. The CO 2 dissolved brine can then form an acidic environment, which can chemically react with the OWC, altering its composition, substantially weakening its struc- ture, and increasing its permeability [1–6]. This will cause CO 2 leakage by flowing through degraded and damaged wells and finally lead to the special safety concerns for the geological storage of CO 2 such as mi- gration to other formations and contamination of potable groundwater [7,8]. Regarding microstructural and compositional changes in the chemically reacting OWC exposed to the CO 2 -brine solution, it has been known that three distinct zones of calcium hydroxide (Portlandite, Ca (OH) 2 ) depletion, carbonation, and degradation are formed [2,4,9–11]. The dissolution of CO 2 into a brine solution forms H + and HCO 3- , followed by the formation of CO 3 2– (Eq. (1)). Calcium hydroxide in cement matrix becomes depleted by the reaction with CO 3 2– and leads to ionized Ca 2+ and the dissolution of HCO 3 - . It subsequently forms carbonate (Calcite, CaCO 3 ) (Eq. (2)). + ↔ + ↔ + + − + − CO HO H HCO 2H CO 2 2 3 3 2 (1) + + → + + → + + − + − − CaCO HO Ca(OH) 2H CO Ca HCO OH 2 2(s) 3 2 2 (aq) 3 (aq) (aq) 3(s) 2 (aq) (2) The continuous supply of CO 2 solution, depletion of Ca(OH) 2 , and the lower pH environment not only dissolve the pre-formed CaCO 3 (Eq. (2)) but also cause the formation of additional bicarbonate (HCO 3 - ) again. Therefore, the depletion of Ca(OH) 2 and the formation of CaCO 3 concurrently take place, which is often called ‘calcium carbonate leaching’ (Eq. (3)). + + → + + − CaCO CO HO Ca HCO (aq) 3(s) 2 2 2 (aq) 3 (3) The decomposition of calcium silicate hydrates (C–S–H) yields amorphous silicate gel by decalcifying Ca 2+ and reacting with CO 2 dissolved solution (Eq. (4)), which increases the porosity and reduces the strength of the OWC [2]. + − − → + − + + + H C S H Ca am SiO HO s () 2 2(s) 2 (4) However, the casting of OWC slurry in a deep well would be cured under the high P-T conditions, which reduces the permeability and hardens the C–S–H, improving its resistance against diffusion-driven chemical reactions compared with curing under ambient pressure and https://doi.org/10.1016/j.cemconres.2018.05.001 Received 19 September 2017; Received in revised form 1 April 2018; Accepted 2 May 2018 ⁎ Corresponding author. E-mail addresses: jyjrafael@yonsei.ac.kr (Y.J. Jeong), ksyoum@gsenc.com (K.S. Youm), taesup@yonsei.ac.kr (T.S. Yun). Cement and Concrete Research 109 (2018) 208–216 0008-8846/ © 2018 Elsevier Ltd. All rights reserved. T