An experimental investigation of wettability alteration during CO 2 immiscible flooding Saad M. Al-Mutairi, Sidqi A. Abu-Khamsin, Taha M. Okasha, Saudi Aramco, M. Enamul Hossain n Department of Petroleum Engineering, College of Engineering Science, King Fahd University of Petroleum & Minerals, Dhahran 31261, KFUPM Box: 2020, Saudi Arabia article info Article history: Received 6 June 2012 Accepted 17 May 2014 Available online 2 June 2014 Keywords: oil-wet reservoir exposure time contact angle reservoir modeling enhanced oil recovery abstract Wettability has been recognized as one of the key parameters controlling the remaining oil-in-place. The knowledge of reservoir wettability is essential to understand the fluid displacement mechanisms, and to develop strategies for achieving higher recovery factors. One of the effective approaches to enhance oil recovery significantly is wettability alteration which has been investigated extensively in the literature. This paper investigates wettability alteration experimentally, on continuous basis, during CO 2 immiscible flooding. Measurements of the contact angle between oil, carbonated brine and a slice of rock cut from a carbonate core plug were conducted. The results indicate that the rock wettability is altered from oil-wet to intermediate-wet when the oil/rock system is exposed to dissolved CO 2 . The extent of wettability alteration is controlled by CO 2 exposure time; as such time is increased, alteration of wettability progresses towards an apparent limit. It is also found that as the CO 2 concentration increases in the brine, wettability alteration increases. Based on the experimental finding, an empirical model is developed to describe such continuous wettability alteration. The findings of this study can be applied to the cases where CO 2 is injected in a watered out, oil-wet reservoir at a pressure below the miscibility pressure. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Wettability alteration is an effective approach to enhance oil recovery significantly. Buckley et al. (1998) summarized four main factors affecting wettability alteration which are oil composition, brine chemistry, rock surface mineralogy and the system temperature, pressure and saturation history. The adsorption and the deposition of organic polar components in the crude oil can alter most of the rock surface chemistry. Brine salinity and pH strongly affect the charge of the rock surface where the rock surface becomes positively charged when the pH is decreased and the rock surface becomes negatively charged when the pH is increased. The solubility of wettability- altering compounds tends to increase when temperature and pressure are elevated. Wettability alteration during CO 2 flooding has been investigated extensively in the literature through core experiments (Stalkup, 1970; Tiffin and Yellig, 1983; Maini et al., 1986; Potter, 1987; Lin and Huang, 1990; Rao et al., 1992; Vives et al., 1999; Zekri et al., 2007; Fjelde and Asen, 2010). Investigators have proved experimentally that the wettability can be altered because of CO 2 injection. Also, the laboratory experiments showed that the interfacial tension reduction may contribute to wettability alteration (Chalbaud et al., 2006; Shariat et al., 2012). On the other hand, a limited number of modeling studies on wettability alteration during CO 2 displacement is docu- mented in the literature (Tehrani et al., 2001; van Dijke and Sorbie, 2002; Farhadinia and Delshad, 2010; Kalaei et al., 2012; Mutairi et al., 2012). Most of the documented experiments or modeling works conducted on wettability alteration were devoted to CO 2 miscible displacement where a super-critical CO 2 phase comes into contact with the reservoir fluids. Work on displacement by CO 2 under low pressures in a watered out reservoir has not been reported. This paper reports the results of a study on wettability alteration on a continuous basis under such conditions. The study involved experiments to measure the oil/brine contact angle on a core slice cut from a carbonate core in the presence of CO 2 . The contact angle was measured at different times until equilibrium is attained. The results were used to develop a simple model that predicts wettability alteration with time. 2. Development of model on wettability alteration Core flooding experiments showed that the maximum oil recovery apparently occurs in neutral or slightly oil-wet cores (Morrow and Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/petrol Journal of Petroleum Science and Engineering http://dx.doi.org/10.1016/j.petrol.2014.05.008 0920-4105/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ966 38602305 (O); fax: þ966 38604447. E-mail addresses: menamul@kfupm.edu.sa, dr.mehossain@gmail.com (M.E. Hossain). Journal of Petroleum Science and Engineering 120 (2014) 73–77