The Role of Individual and Combined Ions in Waterflooding Carbonate Reservoirs: Electrokinetic Study
Authors: Alotaibi M. B. , Yousef A. A. 
Published: June 2016

Adjusting the injection-water chemistry during waterflooding for both carbonate and clastic reservoirs shows a significant effect on oil recovery. In carbonates, however, the role of ions plays a key role in rock/fluid interaction, and eventually affects the rock wettability. Research studies for carbonate-rock systems have been continuously conducted to identify the reaction mechanisms that modify the rock wettability toward water-wet. Most of these studies are conducted at macroscopic scales by use of conventional methods such as coreflooding, contact-angle, and imbibition/drainage procedures. Potential mechanisms for rock-wettability alteration were proposed including sulfate (SO4) adsorption, mineral dissolution, ionic exchange, and improving fluid diffusion among different pore systems. Further research studies have noted that certain ions have a significant role in the proposed mechanisms. Moreover, the main interactions are expected to take place at rock/fluid and/or fluid/fluid interfaces.

In this paper, more attention is given to indirect measurements of carbonate- and crude-oil-surface charges at different ionic composition and temperatures by use of unique preparation procedures and advanced techniques. Individual and combined dissolved cations and anions were studied at fixed salinity. An ultrasonic homogenizer bath was used to create oil-in-water emulsions and carbonate suspensions in different brines at high temperatures. To determine the interactions between immiscible fluids with carbonates, oil-in-water emulsions were prepared in the presence of carbonate particles. To mimic the reservoir condition, the aging effect on the chemical interactions of emulsions and carbonate suspensions was investigated.

The findings in this study bring new insights on the effect of different ions on crude-oil components and carbonate-rock interactions at fixed salinity. Individual ions including cations and anions altered carbonate-surface charges and interacted differently at interfaces, although all water recipes have the same salinity. Individual sodium (Na) salts, in particular, significantly influenced the surface potential at the calcite/water interfaces. The hard ions as calcium (Ca) and magnesium (Mg), on the other hand, shifted the ζ-potential of calcites toward the positive side. These divalent ions can either adsorb directly on the negative sites or penetrate the adsorbed hydrolysis layer of water on the calcite surface. The electrical properties of calcites are also affected by the ionic content and the cation/anion ratio, as in SmartWater (a mixture of dissolved salt species such as cations and anions) (Yousef et al. 2012) and key ion solutions. In addition, the dissolved divalent cations can play a role in the interactions at the Stern-layer boundary and eventually they can affect the surface charges at oil/water interfaces. In view of the ζ-potential results, only SmartWater, sodium chloride (NaCl), and sodium sulfate (Na2SO4) solutions were able to create electrical repulsions between oil/water and calcite/water interfaces. As a result, wettability of the rock will be altered to water-wet, thus enhancing oil recovery.