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.