Armouring of well cement in H 2 S–CO 2 saturated brine by calcite coating – Experiments and numerical modelling Nicolas Jacquemet a,⇑ , Jacques Pironon a , Vincent Lagneau b , Jérémie Saint-Marc c a Université de Lorraine, CNRS-CREGU, Faculté des Sciences, BP 239, 54506 Vandoeuvre-lès-Nancy Cedex, France b Mines Paristech, 35 rue Saint Honoré, 77305 Fontainebleau Cedex, France c TOTAL S.A., Centre Scientifique et Technique Jean Feger, Avenue Larribau, 64018 Pau Cedex, France article info Article history: Received 17 December 2010 Accepted 9 December 2011 Available online 20 December 2011 Editorial handling by R. Fuge abstract The active acid gas (H 2 S–CO 2 mixture) injection operations in North America provide practical experience for the operators in charge of industrial scale CO 2 geological storage sites. Potential leakage via wells and their environmental impacts make well construction durability an issue for efficiency/safety of gas geolog- ical storage. In such operations, the well cement is in contact with reservoir brines and the injected gas, meaning that gas–water–solid chemical reactions may change the physical properties of the cement and its ability to confine the gas downhole. The cement-forming Calcium silicate hydrates carbonation (by CO 2 ) and ferrite sulfidation (by H 2 S) reactions are expected. The main objective of this study is to determine their consequences on cement mineralogy and transfer ability. Fifteen and 60 days duration batch experi- ments were performed in which well cement bars were immersed in brine itself caped by a H 2 S–CO 2 phase at 500 bar–120 °C. Scanning electron microscopy including observations/analyses and elemental mapping, mineralogical mapping by micro-Raman spectroscopy, X-ray diffraction and water porosimetry were used to characterize the aged cement. Speciation by micro-Raman spectroscopy of brine trapped within syn- thetic fluid inclusions were also performed. The expected calcium silicate hydrates carbonation and ferrite sulfidation reactions were evidenced. Furthermore, armouring of the cement through the fast creation of a non-porous calcite coating, global porosity decrease of the cement (clogging) and mineral assemblage con- servation were demonstrated. The low W/R ratio of the experimental system (allowing the cement to buffer the interstitial and external solution pH at basic values) and mixed species diffusion and chemical reactions are proposed to explain these features. This interpretation is confirmed by reactive transport modelling per- formed with the HYTEC code. The observed cement armouring, clogging and mineral assemblage conserva- tion suggest that the tested cement has improved transfer properties in the experimental conditions. This work suggests that in both acid gas and CO 2 geological storage, clogging of cement or at least mineral assem- blage conservation and slowing of carbonation progress could occur in near-well zones where slight water flow occurs e.g. in the vicinity of caprock shales. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction More than 60 acid gas (mixture of CO 2 and H 2 S, also referred to as ‘‘sour’’ gas) injection operations into geological reservoirs have been active in North America (Bachu and Gunter, 2005; Bennion and Bachu, 2008). As sour hydrocarbon reservoirs (naturally rich in H 2 S and CO 2 ) are being produced in the Arabian Gulf and in cen- tral Asia, hydrocarbon producers from these areas are also turning to this practice to dispose of these undesirable components (Bennion and Bachu, 2008). Furthermore, acid gas injection opera- tions provide practical experience for the operators in charge of industrial scale CO 2 geological storage sites (albeit with different gas compositions), a promising option for mitigating the anthropo- genic release of CO 2 into the atmosphere (IPCC, 2005). The Rousse pilot in France is a good example of a gas field from an acid gas province converted into CO 2 storage. The CO 2 -rich gas mixtures coming from pre-combustion capture processes and considered for Carbon Capture and Storage (CCS) operations contain small pro- portions of H 2 S(Jacquemet et al., 2009). Depicting the H 2 S effect in the context of acid gas injection operations could help to predict the behaviour of such gas mixtures in CCS operations. Leakage via wells is an issue for the long-term safety of acid gas and CO 2 geological storage (Machel, 2005; Damen et al., 2006). A steel pipe (casing) surrounded by a sheath of cement, itself sur- rounded by the geological medium, is a typical well architecture (see Fig. 1 in Gasda et al., 2004). The reactivity of these materials 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.12.004 ⇑ Corresponding author. Present address: BRGM, 3 Avenue Claude Guillemin, BP 6009, 45060 Orléans Cedex 2, France. Tel.: +33 (0)2 38 64 34 65; fax: +33 (0)2 38 64 37 19. E-mail addresses: n.jacquemet@brgm.fr, nicolas_jacquemet@yahoo.fr (N. Jacquemet). Applied Geochemistry 27 (2012) 782–795 Contents lists available at SciVerse ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem