Interplay Between Ca2+, Mg2+, and SO42- Ions and Their Influence on the Zeta-Potential of Limestone during Controlled-Salinity Waterflooding

Substantially enhanced oil recoveries from carbonate reservoirs have been accredited to the modification of ionic content and salinity of the injection water and subsequent alteration of reservoir rock wettability toward more water-wet. Modification of surface charges at rock/oil/water interfaces is believed to be the main mechanism that controls the process of wettability alteration. This paper presents a systematic zeta-potential measurement study that investigates the individual and relative impacts of Ca2+, Mg2+, and SO42- ions on a limestone rock from the Middle East. The main target of the study is to understand the impact of each potential determining ion individually and in the presence of another ion for insightful explanations of the overall impact of injection water during ionically controlled waterflooding in carbonate reservoirs. The novelty of the current study is the comprehensive investigation of the interplay between potential determining ions and the resultant impact on limestone surface charges. Different modified brines were prepared based on the ionic composition of Arabian Gulf seawater, which is the main source of injection water in the region. Model oil, containing carboxylic acid, was used to represent the oil phase in aging procedures. Interactions at the rock/oil/water interfaces were assessed using zeta-potential measurements of unaged and aged limestone particles. The results show that Ca2+, Mg2+, and SO42- are potential determining ions that can alter surface charges of unaged and aged limestone. SO42- ions were found to have a profound impact on the generation of negative surface charges on limestone as well as in suppressing the preferential ability of Ca2+ and Mg2+ ions to generate positive surface charges. Results also showed that limestone possesses negative surface charges when conditioned in unmodified seawater and twice diluted seawater. Comparing the obtained results from this study with the literature suggests that similarities in the mineralogical composition of carbonates rocks do not necessarily reflect in similar electrokinetic behavior, even under the same experimental conditions. Moreover, this study shows that the zeta-potential of limestone is not linearly correlated with decreased salinity. Furthermore, it is shown that conditioning limestone in sequentially diluted seawater will result in an increase in the magnitude of the negative charges up to twice diluted seawater, after which the magnitude of the negative surface charges will start to decrease. This finding suggests that a different mechanism is responsible for the modification of carbonate surface charges during late stages of sequential low-salinity waterflooding than the mechanism that controls the modification of surface charges during early stages of the process.

Automated summary

The study reveals that modifying the ionic composition of injection water significantly impacts oil recovery from carbonate reservoirs, with SO42- ions having a profound influence on surface charges of limestone.