Laboratory Investigation of the Impact of Injection-Water Salinity and Ionic Content on Oil Recovery From Carbonate Reservoirs

The impact of brine salinity and ion composition on oil recovery has been an area of research in recent years. Evidence from laboratory studies, supported by some field tests targeting mainly sandstones, has distinctly shown that injecting low-salinity water has a significant impact on oil recovery. Although the potential for carbonates has not been thoroughly investigated, some reported studies have excluded carbonates from this effect. The main objective of this paper is to investigate the potential of increased oil recovery by altering the salinity and ionic composition of the injection water for carbonate reservoirs, define the recovery mechanisms, and eventually transform the emerged trend to full-fledged reservoir technology. This paper presents the results of different laboratory studies to investigate the impact of salinity and ionic composition on oil/brine/rock interactions and draws conclusions on potential recovery mechanisms. Also, it provides a laboratory coreflooding study conducted using composite rock samples from a carbonate reservoir to investigate the impact of salinity and ionic composition on oil recovery. The experimental parameters and procedures were well designed to reflect the reservoir conditions and current field injection practices, including reservoir pressure, reservoir temperature, and salinity and ionic content of initial formation water and current types of injected water. The experimental results revealed that substantial tertiary oil recovery beyond conventional waterflooding can be achieved by altering the salinity and ionic content of field injected water. The new emerged trend is distinct from what has been addressed in previous reported studies on topics of low-salinity waterflooding for sandstones or seawater injection into high-temperature chalk reservoirs. On the subject of recovery mechanisms, the results showed that altering the salinity and ionic composition of the injected water has a significant impact on the wettability of the rock surface. Also, nuclear-magnetic-resonance (NMR) measurements indicated that dilution of seawater can cause a significant alteration in the surface relaxation of the carbonate rock and also can enhance connectivity among pore systems because of rock dissolution. The results, observations, and interpretations addressed in this study provided compelling evidence to suggest that the key mechanism for the emerged trend is wettability alteration.