SummaryThe 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.

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 ion 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 a laboratory coreflooding study conducted using composite rock samples from a carbonate reservoir to investigate the impact of salinity and ionic composition on oil/brine/rock interactions, and eventually 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, salinity and ionic content of initial formation water and current types of injected water. Also, this study provides detailed discussion and interpretation for potential mechanisms.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. This was also confirmed by nuclear magnetic resonance (NMR) measurements. 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.

Improved/enhanced oil recovery by tuning the ionic composition of the injection water is currently deemed as new emerging recovery method. In the recent years, extensive research on oil/brine/rock systems has shown that injecting low salinity brines has a significant impact on oil recovery from sandstone reservoirs. Although, the potential for carbonates has not been carefully investigated, some reported studies have excluded carbonates from this effect. In view of research results for the last three years, we demonstrated in previous reports (SPE 137634; SPEREE Journal, vol. 14(5), pp. 578-593, SPE 143550, SPE 141082) that substantial oil recovery beyond conventional waterflooding from carbonates can be achieved by optimizing ionic composition of field injection brine. Also, research confirmed that the driving mechanism is wettability alteration. In this paper, we present the results of new reservoir condition laboratory coreflooding studies, conducted using composite rock samples from different carbonate reservoir, to investigate the impact of ionic content on oil recovery at both secondary and tertiary recovery modes. Also, we report a broad range of laboratory studies addressing role of water ions. In addition, we briefly disclose the results of first-ever field application conducted in a carbonate reservoir to demonstrate the SmartWater Flood potential. The experimental results revealed that substantial tertiary oil recovery beyond conventional waterflooding can be achieved by altering the ionic content of field injection water. Similar potential has been confirmed also in the secondary recovery mode. For recovery mechanisms, the new results confirmed that wettability alteration is the main cause for the substantial increase in oil recovery. Compared to previous reported work, the variation in oil recovery from two different carbonate reservoirs is attributed to the variations in reservoir temperature and also the chemistry of initial formation water. For field application, two field trials confirmed that injection of SmartWater achieved ∼7 saturation units reductions in residual oil beyond conventional seawater flooding. Considering these field trials are the first-ever applications in carbonate reservoirs, they further provided another confirmation that SmartWater Flood is emerging recovery method targeting carbonate reservoirs.

Water flooding is the most widely applied method for recovering oil from reservoirs. The salinity and ion composition of the injection water in the past have not been considered as a key parameter in oil recovery from water-flooded reservoirs. In recent years, evidence — from laboratory studies verified by some field tests mainly targeting sandstones — has shown that injecting low salinity water has a significant impact on oil recovery. The potential for carbonates has not been thoroughly investigated where some of the reported studies have excluded carbonates from this effect. Saudi Aramco, through its upstream research arm (the Advanced Research Center) has initiated a research program tagged "Smart WaterFlood" to explore the potential of increasing oil recovery by tuning the injection water properties. Based on the research work for the last three years, we present in this paper the results of reservoir condition laboratory coreflooding studies, conducted using composite rock samples from a carbonate reservoir, to investigate the impact of salinity and ionic composition on oil recovery. Also, we report a broad range of laboratory studies addressing oil recovery mechanisms. The experimental results revealed that substantial tertiary oil recovery beyond conventional waterflooding can be achieved by altering the salinity and ionic content of field injection water. The new trend is distinct from what have been addressed in previous reported studies on topics of low salinity waterflooding for sandstones, or seawater injection into high temperature chalk reservoirs. Laboratory studies have also shown that altering the salinity and ionic content of the injection water has a significant impact on the wettability of the rock surface. The results, observations, and interpretations addressed in this study provided compelling evidence to suggest that the key mechanisms for substantial oil recovery by altering salinity of field injection brine is wettability alteration.

Improved/enhanced oil recovery by tuning the ionic composition of the injection water is currently deemed as new emerging recovery method. In the recent years, extensive research on oil/brine/rock systems has shown that injection low salinity brines in sandstone reservoirs has a significant impact on the amount of oil recovered. Although, the potential for carbonates has not been thoroughly investigated, some reported studies have excluded carbonates from this effect. Based on the research work in the last years, we demonstrated in a previous reports (137634; SPEREE Journal, vol. 14(5), pp. 578-593, SPE 143550, SPE 141082) that substantial oil recovery beyond conventional waterflooding from carbonates can be achieved by optimizing the salinity and ionic composition of field injection brine. Also, research confirmed that the main cause for the substantial increase in oil recovery is wettability alteration of carbonate rock surface. This paper highlights extensive and a broad range of laboratory studies including wettability, surface chemistry, and zeta potential studies to define the role of water ions in the induced wettability alteration, which is crucial in determining the optimum composition of injection water for future field applications. The rock surface chemistry studies identified the potential mechanisms for wettability alteration triggered by injecting chemistry-optimized field injection water. The contact angle results confirmed that a sufficient reduction in the ionic strength of field injection water is required to trigger the effect of wettability alteration. Zeta potential measurements successfully addressed the impact of multivalent ions on rock surface charges. All evidence gathered during this research work indicates that interplay of water ions is important factor in this new recovery method.