Tuned Low-Salinity Waterflooding in Carbonate Reservoirs: Impact of Cr2O72-, C6H5COO-, and SO42-

Abstract

Summary Recent studies have revealed that tuning SO42− concentration can significantly boost low-salinity waterflooding (LSWF) in carbonate reservoirs. In this study, for the first time, Cr2O72− and C6H5COO− were added to low-salinity water (LSW), and their effect on enhanced oil recovery (EOR) was investigated and compared with SO42−. This study aimed to primarily determine whether adding anions other than SO42− to LSW could further reduce interfacial tension (IFT) and make the carbonate surface water-wet thereby improve oil recovery. To this end, we added Cr2O72−, C6H5COO−, and SO42− to LSW at different concentrations and performed IFT, contact angle, and zeta potential experiments. Then, the flooding test was carried out in a calcite-coated micromodel for each anion at the optimal concentration. The results showed that the carbonate surface became water-wet and intermediate-wet as the concentrations of Cr2O72− and SO42− increased, respectively. However, the presence of C6H5COO− did not alter the carbonate surface wettability (i.e., it remained oil-wet). The calcite dissolution, ion pair formation, and salting-in effect mechanisms might alter the carbonate surface wettability. Also, IFT declined with increasing each anion concentration in LSW (with the highest reduction associated with Cr2O72−), but this reduction was not significant. Several mechanisms might lead to the IFT reduction, including the salting-in effect, H+ effect, and surface excess concentration. Regarding zeta potential tests, although raising the concentrations of Cr2O72− and SO42− could make the charge of the carbonate surface more negative, increasing the concentration of C6H5COO− had little effect on altering this charge. Additionally, Cr2O72− made the oil/brine zeta potential positive, although the oil/brine surface charge became negative in the presence of C6H5COO− and SO42−. As for anions’ affinity, Cr2O72− and C6H5COO− had the highest and the lowest affinity for the interface, respectively. Besides, the amount of oil recovery for LSW was 18.7±1.6%. Compared with LSW, LSW2S and LSW1Cr raised oil recovery by 3.6 and 14.3%, respectively. The results of this study support that Cr2O72− can be used in LSWF. Indeed, this anion (compared with SO42−) helps produce more oil from carbonate reservoirs by improving the parameters that are effective in EOR.