International Journal of Greenhouse Gas Control 12 (2013) 18–25 Contents lists available at SciVerse ScienceDirect International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc Wettability behaviour of CO 2 at storage conditions Raheleh Farokhpoor a,∗ , Bård J.A. Bjørkvik b , Erik Lindeberg b , Ole Torsæter a a NTNU Petroleum Engineering and Applied Geophysics, NO-7491 Trondheim, Norway b SINTEF Petroleum Research, S.P. Andersens veg. 15A, NO-7031 Trondheim, Norway article info Article history: Received 4 July 2012 Received in revised form 30 October 2012 Accepted 1 November 2012 Available online 20 December 2012 Keywords: CO2 wettability behaviour in geological storage condition Measuring contact angle in CO2–brine–substrate system Effect of pressure, temperature and salinity Temporal effect on contact angle Mineral representative of reservoir and caprock abstract The capillary-sealing efficiency of the caprock is one of the major factors that control the safety of geolog- ical CO 2 storage. Possible changes in wettability due to physical–geochemical processes could possibly decrease the capillary entrance pressure and reduce the sealing integrity of the caprock. Changes in wet- tability have therefore been investigated by measuring the CO 2 contact angle on some selected minerals typical for reservoir rocks in the presence of brine at reservoir conditions. In this paper, a set of CO 2 contact angle data are reported for quartz, feldspar, calcite and muscovite mica, representing reservoir and seal rock, at pressure, temperature and salinity conditions representative of a CO 2 storage operation. Among these minerals, quartz, feldspar and calcite are strongly water wet with non-significant change in contact angle versus pressure while the water wettability of muscovite mica changed from strongly water-wet to intermediate water-wet with increasing pressure. Also wettability alterations of CO 2 as function of time for calcite and muscovite mica at constant pressure, temperature and salinity were investigated. The water wettability of the calcite mineral did not change significantly at the time scale of the laboratory experiment while there was a marked decrease in the water wettability of mica. A maximum in contact angle near the critical pressure was observed at 36 ◦ C for feldspar, calcite, and quartz and was maybe masked in the case of muscovite mica by the significant increase in contact angle with increasing pressure. A relatively strong change in CO 2 compressibility is observed at 36 ◦ C, less so at 66 ◦ C, indicating that the observed maximum may be related to the near-critical behaviour of the CO 2 phase. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction The first task for carbon capture and storage is to find suitable locations to store CO 2 safely and permanently. The concerns regard- ing underground CO 2 storage are very similar to those identified for the gas storage industry by Dr. Donald Katz in the 1960s. Over the past 90 years, 10 leakage issues have been observed across USA, Canada and Europe. These were due to wellbore leakages, caprock leakages and selection of too shallow reservoir (Perry, 2005). CO 2 invasion in the caprock or seal layer may occur according to different physical processes: (i) CO 2 diffusion in brine which saturates the caprock or seal layer, (ii) capillary breakthrough of the CO 2 phase into the seal by exceeding the threshold pressure and (iii) existence of fracture in the cap rock (Chiquet and Broseta, 2005). One of the major factors that control the safety of geological CO 2 storage is the capillary-sealing efficiency of the caprock. When the pressure in the CO 2 phase (P CO 2 ) is high enough to displace the ∗ Corresponding author. Tel.: +47 96833676. E-mail address: raheleh.farokhpoor@ntnu.no (R. Farokhpoor). water in the caprock, capillary leakage occurs. This takes place at the threshold capillary entry pressure (P ce ) for the CO 2 phase which is defined by Eq. (1) (Thomas et al., 1968): P ce = P CO 2 - P water ≈ 2 w-CO 2 cos() R . (1) Here, R is the radius of the largest pore throats in the caprock,  w-CO 2 , the brine/CO 2 interfacial tension and  is the contact angle of the mineral/brine/CO 2 system which is measured in the water phase. P water is the pressure in water (brine) saturating the seal layer. Since during CO 2 leakage, the water phase is displaced by the CO 2 phase, the relevant angle is the CO 2 advancing contact angle (Shah et al., 2008). The effect of CO 2 on the wetting behaviour of typical shale min- erals, coal and carbonate rock have been studied before. Broseta et al. (2012) presented changes of mica and quartz from strongly water-wet systems at low pressures (gaseous CO 2 ) to intermediate- wet systems at higher pressures (dense CO 2 ). According to the authors, decrease in pH at high CO 2 pressure decreases the (repul- sive) electrostatic interactions between the interfaces that favour water-wettability. Chiquet et al. (2007), Broseta et al. (2012) and Shah et al. (2008) reported that the wettability alteration of 1750-5836/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2012.11.003