SCA2009-43 1/6 IS WETTABILITY ALTERATION OF CARBONATES BY SEAWATER CAUSED BY ROCK DISSOLUTION? Tor Austad, Skule Strand and Tina Puntervold University of Stavanger, Norway This paper was prepared for presentation at the International Symposium of the Society of Core Analysts held in Noordwijk, The Netherlands 27-30 September, 2009 INTRODUCTION In a series of studies, Austad and co-workers [1-5] have documented that seawater at high temperatures is able to modify the wetting conditions of chalk towards more water- wet conditions, and in that way seawater can act as a “Smart” EOR fluid. Through systematic experimental studies, it was verified that Ca 2+ , Mg 2+ , and SO 4 2- were the active ions in the wettability alteration process. Studies by BP at complete reservoir conditions for the Valhall field have confirmed the results [6]. It was documented by Zeta potential measurements that these ions acted as potential determining ions towards chalk, i.e. they were able to adsorb onto the chalk surface and modify the surface charge. The mutual interaction between SO 4 2- and Ca 2+ at various temperatures is clearly illustrated by Fig. 1a and b. Thus, as long as both anionic and cationic potential determining ions are present in solution, the Zeta potential will not change very much as the temperature is increased, but the reactivity of the actual ions increases [7]. Fig. 1a Sulfate adsorption in chalk at increasing temperatures. [Ca 2+ ] and [SO 4 2- ] constant and equal to SW concentration [7]. Fig. 1b The concentration of Ca 2+ in solution decreases as the retention of SO 4 2- increases with increasing T. [Ca 2+ [/SO 4 2- ] ratio was constant and equal to SW concentration.[7] At high temperature, Mg 2+ became much more reactive and even displaced Ca 2+ from the chalk surface lattice. At 130 °C, the decrease in [Mg 2+ ] was equal to the increase in [Ca 2+ ] by flooding SW slowly (1 PV/D) through a carbonate core, Fig. 2 [8]. The increase in [Ca 2+ ] of SW appeared to correlate linearly with the temperature, Fig 3. No change in surface charge is taking place by this substitution reaction as illustrated by Eq. 1. CaCO 3 (s) + Mg 2+ = MgCO 3 (s) + Ca 2+ (1) 0.0 0.5 1.0 0.5 1.0 1.5 2.0 2.5 P V C/C o C/Co SCN Test #7/1 SW at 23°C A=0.174 C/Co SO4 Test #7/1 SW at 23°C C/Co SCN Test #7/2 SW at 40°C A=0.199 C/Co SO4 Test #7/2 SW at 40°C C/Co SCN Test #7/3 SW at 70°C A=0.297 C/Co SO4 Test #7/3 SW at 70°C C/Co SCN Test #7/4 SW at 100°C A=0.402 C/Co SO4 Test #7/4 SW at 100°C C/Co SCN Test #7/5 SW at 130°C A=0.547 C/Co SO4 Test #7/5 SW at 130°C 0.0 0.5 1.0 0.5 1.0 1.5 2.0 2.5 PV C/Co C/Co Ca2+ Test #7/1 SW at 23°C C/Co Ca2+ Test #7/2 SW at 40°C C/Co Ca2+ Test #7/3 SW at 70°C C/Co Ca2+ Test #7/4 SW at 100°C C/Co Ca2+ Test #7/5 SW at 130°C