Water chemistry has a significant effect on oil recovery. It was found that low salinity water has a good potential for improving oil recovery in sandstone reservoirs. However, the mechanisms by which low salinity water injection works are still uncertain. This means the optimum conditions for improving oil recovery are unknown. In this paper, coreflood experiments were conducted to understand how cation type in injected water may affect oil recovery. In addition, testing multiple salinities helped to identify the optimum salinity.

Several waterflood experiments at high pressure-high temperature (HPHT) were conducted on Berea sandstone cores, 6 in. length. Single cation water solutions were prepared to be injected in cores at different concentrations of NaCl, CaCl2, and MgCl2. Effect of oil composition and characteristics on improvement of oil recovery by waterflooding was investigated by using two different types of crude oils. In addition, core effluent fluid was collected and analyzed using ICP-OES and Spectrophotometer to study interactions between rock and injected fluid.

This work contributes to the understandings of impact of cation exchange on oil recovery by low salinity water flooding. Experiments showed that exchanging certain cations is the primary reason of higher oil recovery. Absence of Ca2+ in injected water allowed water to leach Ca2+ from the rock surface which, caused surface instability and allowed higher oil volume to be released from the rock surface and be recovered. Results demonstrate that the existing cations in the injected water solution have more dominant effect on oil recovery than the concentration of salts in water. In addition, oil composition has a significant effect on the efficiency of waterflooding, and hence oil recovery. These findings help in introducing the optimum water salinity.

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