Many offshore reservoirs have been seawater-flooded, which could affect the low salinity EOR potential. This paper evaluates the low salinity EOR potential after seawater flooding for a high temperature (148 °C), high salinity (~170000 ppm) sandstone reservoir. Secondary oil recovery by low salinity injection is compared to secondary seawater and tertiary low salinity water injection.
Oil recovery experiments were performed at reservoir temperature using preserved reservoir cores. To mimic initial wetting conditions of the original oil reservoir, the cores were mildly cleaned and restored with the formation water and reservoir crude oil. Secondary seawater injection was performed, as well as secondary and tertiary low salinity water injection. Solid-brine surface reactivity tests were performed to evaluate chemical interactions, initial wetting, and potential for wettability alteration in the reservoir system.
The pH of the first produced water in the oil recovery tests indicated favorable initial wetting conditions, i.e. mixed wettability, which is necessary for obtaining low salinity EOR effects by wettability alteration. However, the oil recovery tests showed no tertiary low salinity EOR effects after seawater flooding. Secondary low salinity injection resulted in 6-10% higher oil recovery compared to that obtained by secondary seawater injection. Low salinity injection normally generates cation exchange on the pore mineral surface promoting a pH increase and good conditions for observing wettability alteration. The produced water sample pH show that unfavorable Crude Oil-Brine-Rock interactions reduced the pH increment and the low salinity EOR potential after seawater injection. The surface reactivity tests also confirmed higher EOR potential by secondary low salinity injection, seen by a higher pH increase, which during low salinity injection triggers wettability alteration of the rock surface towards more water-wet, thereby recovering more oil.
The results show that low salinity injection into a seawater-flooded reservoir is likely to be less efficient due to unfavorable chemical interactions and reduced pH increase, and therefore there is a reduced potential for wettability alteration. However, for this high temperature and high salinity reservoir secondary low salinity water injection could be an efficient EOR method.