An experimental investigation is presented that shows oil recovery and wettability changes accompanying alteration of injection water salinity. The study aims to provide a better understanding of the impact of salinity leading to greater oil recovery and the mechanisms that take place for carbonate rocks.
A series of forced imbibition experiments was conducted using crude-oil at different total salinity and ionic composition. In the first test set, three brines of different salinity were injected sequentially into reservoir cores with realistic initial oil and water saturation. Additional incremental oil recovery of 4.4-6.4% of the original oil in place (OOIP) was observed, during the tertiary stage. The injection of synthetic seawater, that has a salinity of 55 kppm, was replaced by a new brine (MgSO4) of similar total salinity (45 kppm) and rich in Mg2+ and SO42- ions. Injection of twice-diluted seawater of 27 kppm, during the tertiary stage, yielded an incremental oil recovery of 0.9%. The effect of reducing the total salinity was evaluated in a second test set using outcrop limestone cores and another crude-oil. An incremental oil recovery increase of 6.5% was observed when twice-diluted seawater (29 kppm) was injected during the tertiary stage following seawater injection.
Wettability alteration was assessed using classic crude-oil contact-angle tests on smooth calcite surfaces. Contact-angle measurements suggest that the release of oil is caused by a wettability shift toward water wetness. The static water contact-angle was reduced in first test set from 92.7° to 55.4° when the brine was switched from seawater to the magnesium-rich brine. Similar reduction was observed in the second test set. The static water contact-angle was reduced from 70.1° to 58.9° when the brine was switched from seawater to twice-diluted seawater.
The contribution of the rock/brine/oil interactions to the wettability was evaluated by measuring zeta potential of water/oil and water/solid interfaces. DLVO (Derjaguin, Landau, Verwey and Overbeek) theory of surface forces, using the measured zeta potentials rationalized observations of oil recovery in the case of low water salinity.
The finding of this work provides a new approach for utilizing salinity alteration to achieve greater oil recovery. The impact of water salinity is usually correlated with low water salinity or with increasing the concentration of the key divalent ions. This study combines both salinity modification schemes to increase tertiary oil recovery.