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

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- in-creased, 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 signif-icant. 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.

Automated summary

Recent studies have shown that adjusting the concentration of SO42- can greatly enhance low-salinity waterflooding in carbonate reservoirs. This study investigated the effects of adding Cr2O72- and C6H5COO- to low-salinity water on enhanced oil recovery, compared with SO42-. The results support the use of Cr2O72- in low-salinity waterflooding, as it helps improve the parameters that are effective in enhanced oil recovery.