Spontaneous imbibition of seawater into preferentially oil-wet chalk cores — Experiments and simulations L. Yu a, ⁎ , 1 , S. Evje b , H. Kleppe a , T. Kårstad a , I. Fjelde b , S.M. Skjaeveland a a Faculty of Science and Technology, Department of Petroleum Engineering, University of Stavanger, N-4036, Norway b International Research Institute of Stavanger (IRIS), Prof. Olav Hanssensvei 15, N-4068, Norway abstract article info Article history: Received 24 October 2007 Accepted 22 February 2009 Keywords: Spontaneous imbibition Wettability alteration Modelling/simulation Spontaneous imbibition is an important IOR process, especially for fractured carbonate reservoirs with low permeability matrix blocks. If the chalk is oil-wet, the process will not take place. Previous studies have shown that seawater may increase the water-wetness. The sulphate ions in seawater may alter the wetting conditions of the chalk surface, especially at high temperatures. One-dimensional imbibition tests of water into vertically placed, preferentially oil-wet chalk cores were performed, with non-sulphate formation water as a reference. The cores were sealed and only open to flow at one or both end faces. For core plugs with both ends open, a delay period was observed if the core initially was 100% oil saturated, and the difference in oil recovery from top and bottom was about 2–4% of OOIP. For core plugs with initial water saturation, the difference was increased to 14% of OOIP with higher oil production from the top. For cores with only the upper end face open to flow, only countercurrent imbibition takes place. Higher oil recovery was observed with seawater than with formation water as imbibing brine. Cleaned core plugs can be more easily rendered partially oil-wet. A numerical model was developed to describe the seawater imbibition process, including the effect of wettability alteration, and used to simulate the experiments. The model includes molecular diffusion and adsorption of salts (sulphate), and gravitational and capillary forces. The salts in the seawater diffuse into the formation water initially present in the core, absorb onto the rock surface and induce wettability alteration. Two measured capillary pressure curves are used in the simulation. The curve with seawater is taken as the water-wet extreme, and the curve with non-sulphate formation water is taken as the oil-wet extreme. The capillary pressure curve is dynamically shifted from oil-wet to water-wet conditions proportionally to the absorb amount of salt. The simulation results match the experimental data well. The inclusion of the dynamic shift of the wettability condition controlled by molecular diffusion results in delayed oil recovery, in line with the experimental results. The model can easily be extended to include different types of ion concentration, e.g. of magnesium and calcium, to include more of the chemical reactions taking place. © 2009 Elsevier B.V. All rights reserved. 1. Introduction About half the world's discovered oil reserves are in carbonate reservoirs and many of them are naturally fractured (Roehl and Choquette, 1985). Spontaneous imbibition of water from the fractures into the matrix takes place if the reservoir is water-wet. However, up to 65% of carbonate rocks are oil-wet and 12% are intermediate-wet (Chilingar and Yen, 1983). Wettability affects fluid distributions and flow in the reservoir during production (Anderson 1986b), and it affects almost all types of core analyses (Anderson, 1986a). And for fractured carbonate reservoirs, wettability has widely been described as an important factor to consider for waterflooding to increase oil recovery (Zhou et al., 2000; Morrow and Mason, 2001; Tong et al., 2002; Hirasaki and Zhang, 2004). There are many published papers on wettability alteration by surfactants (Spinler and Baldwin, 2000; Seethepalli et al., 2004), but for practical application the cost may be prohibitive. However, seawater has been injected into the naturally fractured Ekofisk chalk reservoir in the North Sea for nearly 20 years with great success (Sylte et al., 1988). And recent laboratory studies indicate that seawater helps to improve oil recovery from moderately water-wet fields such as the Ekofisk field (Austad et al., 2005; Høgnesen et al., 2005; Zhang Journal of Petroleum Science and Engineering 66 (2009) 171–179 Abbreviations: EF, Ekofisk Formation Water; SSW, Sea Water; WA, Wettability Alteration. ⁎ Corresponding author. Tel.: +47 47712058. E-mail address: LIPY@statoilhydro.com (L. Yu). 1 Started working in StatoilHydro, Norway after August 2008. 0920-4105/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2009.02.008 Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol