International Journal of Greenhouse Gas Control 48 (2016) 155–170 Contents lists available at ScienceDirect International Journal of Greenhouse Gas Control j ourna l h o mepage: www.elsevier.com/locate/ijggc Characterization and modeling of the alteration of fractured class-G Portland cement during flow of CO 2 -rich brine H. Abdoulghafour a,∗ , P. Gouze a , L. Luquot b,c , R. Leprovost a a Géosciences Montpellier, Université de Montpellier, UMR 5243 CNRS, Montpellier, France b Institute of Environmental Assessment and Water Research (IDAEA), CSIC, c/ Jordi Girona 18, 08034 Barcelona, Spain c Associated Unit: Hydrogeology Group (UPC-CSIC), Spain a r t i c l e i n f o Article history: Received 22 July 2015 Received in revised form 16 December 2015 Accepted 18 January 2016 Available online 11 February 2016 Keywords: CO2 leakage Cement alteration Fracture permeability Experimental study Diffusion-reaction modeling a b s t r a c t We investigate experimentally the alteration of fractured class-G cement flowed by CO 2 -rich brine. The experiment mimics a mechanically damaged rough-walled fractured cement annulus at temperature 60 ◦ C and pressure 10 MPa. The experiment consists of flowing a reservoir-equilibrated brine mixed with CO 2 (partial pressure of 2.3 MPa) through the fracture of average aperture 14 m at constant flow rate (100 L min -1 ). This flow rate corresponds to pressure gradient representative of an average in situ hydrodynamic condition. Results indicate an intense alteration of the cement with a large removal of mass at the scale of the sample. However, the fracture alteration patterns are triggered by the initial heterogeneity of the fracture aperture; the aperture of the low aperture zones tends to decrease due to calcite precipitation whereas preferential paths develop in the zones of higher aperture associated. Nevertheless, the expected large permeability increase triggered by the mass removal is mitigated by the precipitation of a low density Si-rich amorphous material. The alteration rate will decrease with time because of the increasing distance of diffusion between the fracture where the reactants are actively renewed by advection and the portlandite and C-S-H dissolution fronts. The different zones of reaction can be adequately modeled by a simple 1D diffusion-reaction model using published kinetics coefficients and extrapolation to larger times than the experiment time can be drawn. Altogether, and in addition to the previous studies of the alteration of fractured well cement annulus, this study shows that the leakage potential is strongly controlled by the initial distribution of the aperture along the fracture: low aperture zones will tend to self-heal while localized flow in connected high aperture paths will be perennial. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction In many porous media with low matrix permeability, such as indurated carbonate, claystones and cement paste, predomi- nant fluid flow takes place in the fracture network (Zimmerman and Bodvarsson, 1996; Detwiller et al., 2000). Accordingly, frac- tures control the mass transfers and the mechanical properties in many environments of significant importance including petroleum, geothermal, water supply, waste disposal and engineered struc- tures. Numerous studies have been carried out to investigate the physics of (reactive) flow through fractures (e.g., Adler and Thovert, 2000; Xu and Pruess, 2001; Gouze et al., 2003; Detwiller et al., 2003; Szymczak and Ladd, 2004; Singurindy and Berkowitz, 2005; Garcia-Rios et al., 2015; Davila et al., 2016) showing that disso- lution and precipitation processes may strongly and irreversibly ∗ Corresponding author. E-mail address: Halidi@gm.univ-montp2.fr (H. Abdoulghafour). affect the hydrodynamic properties of the fracture. For instance, fractures in the caprock and the well cement annulus are recog- nized as the main potential source of confinement failure of CO 2 geological storage reservoirs (Shipton et al., 2005; Oldenburg et al., 2009; Davila et al., 2016). Cement is usually used to seal cased wellbore in order to isolate the targeted reservoir from the over- laying geological layers. Technologies for installing cement annulus at depth (usually ranging from 1 to 2 km in the case of CO 2 stor- age) benefit from decades of experiences in oil industry, but it still a challenging task due to the large variability of the natural environments and the few possibilities of verification. Structural defects, such as micro-cracks and fractures may develop during the cement emplacement or may be induced by tectonic, ther- mal stresses and over pressure events that may occur during the CO 2 injection (Ravi et al., 2002; Watson and Bachu, 2007, 2009). If flow of reservoir fluid occurs through fractured cement annulus, triggered by the pressure difference between the storage reservoir and the overlaying permeable layers, one may anticipate strong, far from equilibrium reactions, because Portland cements are strongly http://dx.doi.org/10.1016/j.ijggc.2016.01.032 1750-5836/© 2016 Elsevier Ltd. All rights reserved.