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It was previously documented that seawater was able to change the wettability of oil-saturated chalk toward more water-wet conditions and that enhanced spontaneous imbibition of water was observed. The efficiency of the imbibition process was improved by increasing the sulfate concentration in seawater. Both calcium and sulfate present in seawater are potential determining ions toward the chalk surface; it is therefore expected that both of the ions are involved in the wettability modifying process, and the symbiotic effects between the ions are studied in this paper with the aim of improving oil recovery from moderate water-wet chalk. Outcrop chalk samples were aged at 90 °C in an acidic crude oil for at least 4 weeks. The concentration of Ca2+ was varied both in the imbibing seawater and in the initial brine. Chromatographic wettability tests showed that the initial wetting condition of the chalk was not significantly affected by changing the concentration of Ca2+ in the initial brine. Increased oil recovery by spontaneous imbibition was observed with increasing concentration of Ca2+ in the imbibing seawater or increasing concentration of Ca2+ in the initial brine. The imbibition increased as the temperature increased, but care must be taken to avoid precipitation of CaSO4(s) at high temperatures. The spontaneous imbibition tests confirmed that SO4 2- and Ca2+ played an important role in the wettability modifying process. A chemical mechanism for the wettability alteration was suggested to involve the coadsorption of SO4 2- and Ca2+ onto the chalk surface, which resulted in desorption of carboxylic material.

This paper is one among others on IOR studies in carbonates regarding the effects of potential determining ions on the wettability alteration, and the subsequent effect on oil recovery by spontaneous imbibition. Besides the oil properties, water-composition and rock properties are also important to wettability. It has been documented in previous related work that SO42− and Ca2+ are strong potential determining ions and greatly influence the surface charge of carbonate rocks. This work concentrates on evaluating the effect of various ratios of the two ions on wettability alteration and spontaneous imbibition at various temperatures. The ionic concentrations were varied both in the initial and imbibing brines. Imbibition tests were performed at 40, 70, 100 and 130°C using out-crop chalk samples, which were aged for 4–6 weeks at 90°C in a modified crude oil with an acid number of 2.0 mgKOH/g. Additional tests were carried out to evaluate SO42-as a wettability modifier without the presence of Ca2+ and/or Mg2+. The results show that the three divalent ions Ca2+, Mg2+, and SO42-, which are naturally present in seawater, are important in changing the surface charge of chalk and are more effective when they are all present in solution in favourable ratios. They seem to play different roles in the two processes: wettability alteration and spontaneous imbibition, both of which are temperature-dependant. Sulfate adsorption is the key factor in the wettability alteration, but Ca2+ and Mg2+ seem to play a promotional role apparently by lowering the characteristic temperature for the wettability shift from neutral to moderate water-wet. Wettability alteration and improved oil recovery have also been linked to the stability of the wetting film, attraction forces and activation energy of the process in relation to the [Ca2+]/[SO42-] ratio, and temperature through measurement of contact angles on calcite crystals. Imbibing brines with higher ratios seem to require lower activation energy to establish a liquid-rock equilibrium interface to initiate spontaneous imbibition. The brines also have stronger attraction forces to calcite and require less work of adhesion than brines with low ratios, or completely depleted in Ca2+ and Mg2+. Brines with higher [SO42-] seem to possess more wetting effects and increased potential to spread as the temperature increases. Introduction Until recently, most of the wettability alteration and spontaneous imbibition studies have concentrated on sandstones with limited literature on chalk based on empirical deductions. One of the main parameters in wettability and imbibition studies is the water-composition. The presence of monovalent and divalent ions (Na+ and Ca2+) has been reported not to have significant effects on the spontaneous displacement of oil from sandstone cores.[1] However, such detailed studies on chalk have been scanty. The other key factors in imbibition are the temperature and pH. Thomas et al.[2] and Dangerfield and Brown[3] observed that oil recovery from strongly water-wet chalk and sometimes intermediate-wet chalk increases with increasing temperature. Other researchers like Kyte et al.[4] and Cuiec et al.[5] reported similar behaviour in carbonates. These observations show that better comparison of spontaneous imbibition results is achieved only when the same temperature is considered. In a recent related study, Zhang et al.[6] suggested that Ca2+ as a potential determining ion enhances wettability alteration of chalk and promotes imbibition by playing more or less a catalytic role. It has also been observed that the divalent ions (Ca2+ and Mg2+), like SO42-, can adsorb on the chalk surface. The catalytic role was physically supported by the abrupt change in the zeta potential after only small amounts of Ca2+ were added to an aqueous chalk suspension. Related work to this effect has also been documented by Pierre et al.[7]

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