Modified sea water has been shown to affect the oil recovery fraction considerably during secondary and tertiary waterfloods. Available soluble potential ions (i.e. Ca2+, Mg2+ & SO42-) in the interacting waterflood (ITW) are suggested to play a key role in increasing the displacement efficiency of oil. In previous studies, compositions of injected waterfloods (IJW) have been correlated to the observed oil recovery. This study highlights differences between IJW and ITW for different studies reported in literature.

To calculate the composition of ITW, the speciation calculation of IJW in contact with reservoir rock was conducted at the stated pressure and temperature conditions. The Extended UNIQUAC model was used because of its high accuracy and optimized parameters. The amounts of soluble ions in the ITW and the changes taking place due to dissolution/precipitation was calculated for 16 SmW-EOR experiments reported in literature.

The calculations showed that there can be significant amounts of fines formation in ITW during the use of different smart waterfloods. Fines formations were attributed to three primary causes. (1) Cation substitution on mineral surfaces. (2) Interaction between new IJW and existing brine in the pore space. (3) Variation in temperature and pressure conditions. Precipitation / dissolution of calcite, dolomite, and anhydrite created a significant difference between the IJW and ITW. The reservoir composition and NaCl concentration in IJW also play a significant role in possible fines formations. It was observed that the composition of soluble potential ions in ITW was significantly different from that in IJW. The amounts of soluble potential ions in ITW and the amounts of fines formation taking place was correlated to the observed oil recovery. The increased oil recovery observed in various experiments repeatedly correlated well with the amount of fines formed in the pore space. In several cases, the amount of soluble potential ions in the ITW did not correlate with the observed oil recovery. Experiments involving Caspian carbonate were also analyzed, and significant correlation between fines formation and oil recovery was observed. These results together suggest that fines formation can be used to explain both low salinity and smart water flooding experiments, and the two recovery processes may not necessarily have distinct mechanisms.

This study quantitatively explains a series of previous experiments reported in the literature and shows how fines formation can lead to major differences between the IJW and ITW. It also shows that fines formation previously unaccounted for can be precisely calculated at exact reservoir conditions using the Extended UNIQUAC model.

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