# Investigation of the Synergistic Effects of Different Salts in Smart Water Injection Fluids on Oil–Brine Interfacial Tension

## TL;DR

This study examines how different salts in smart water injection fluids affect oil-brine interfacial tension and oil recovery in carbonate rocks.

## Contribution

The paper identifies synergistic effects of specific salt combinations on interfacial tension under varying salinity conditions.

## Key findings

- High salinity increases IFT with oil due to the bilayer effect of CaCl2 and organic acids.
- Low salinity shows NaCl significantly reduces IFT through the salting-in phenomenon.
- Synergistic effects between CaCl2 and NaHCO3 in high salinity and NaCl and NaHCO3 in low salinity improve oil recovery.

## Abstract

Although the need for an energy transition
is increasingly
evident, fossil fuels will remain essential for humanity, highlighting
the necessity for responsible migration strategies. In this context,
low-salinity injection methods enable smart management of oil production
while addressing the United Nations Sustainable Development Goals
(SDG 7) in the 2030 Agenda. These methods reduce the environmental
impact of oil production while ensuring the continuation of fossil
fuel extraction necessary for social well-being in the coming decades.
Despite the absence of consensus on the optimal saline composition
and concentrations in smart water, its effectiveness is largely linked
to changes in wettability and reductions in interfacial tension (IFT)
within the oil–brine–rock system. This study explores
the synergistic effects of sodium chloride, calcium chloride, and
sodium bicarbonate across two total dissolved solids ranges (0–70,295
mg/L and 2200–24,813 mg/L) on IFT with Brazilian presalt oil,
comparing the results with oil recovery factors obtained from carbonate
rock tests. The findings indicate that salinity significantly influences
the individual effects of each salt on IFT. In high-salinity scenarios,
the presence of CaCl2 notably increases IFT with oil, suggesting
that the bilayer effect between Ca2+ ions and organic acids
predominates. Conversely, at low salinity, NaCl demonstrated the most
significant impact in reducing the IFT, indicating that the salting-in
phenomenon prevails. The most pronounced synergistic effects occurred
between CaCl2 and NaHCO3 in high-salinity environments
and between NaCl and NaHCO3 in low-salinity situations.
In both cases, the interference was positive, suggesting that the
deprotonation of organic acids facilitated by the basic salt does
not aid in lowering the IFT when combined with other salts. However,
in high-salinity environments, NaHCO3 exhibited negative
interference but contributed to a reduction in IFT. A correlation
was observed between the decrease in the IFT and an increase in the
oil recovery factor, particularly under conditions with high NaCl
concentrations and low to medium levels of CaCl2 and NaHCO3.