The Impact of Dissolved Species on the Reservoir Fluids and Rock Interactions in Carbonates
Authors: Alotaibi M. B. , Yousef A. A. 
Published: June 2015

Adjusting the injection water chemistry during waterflooding for both carbonate and sandstone reservoirs show significant impact on oil recovery. In carbonates, however, the role of ions plays a key role in rock-fluid interaction, and eventually impact 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 using conventional methods such as coreflooding, contact angle, and imbibition/drainage procedures. Potential mechanisms for rock wettability alteration were proposed, including sulfate adsorption, mineral dissolution, ionic exchange, and improving fluid diffusion among different pore systems. Further research studies have pointed out 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 using unique preparation procedures and sophisticated advanced techniques. Individual and combined dissolved cations and anions were studied at fixed salinity. The ultrasonic homogenizer bath was utilized 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 the different ions on the 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 salts, in specific, significantly influenced the surface potential at the calcite/water interfaces. The hard ions as calcium and magnesium, on the other hand, shifted the zeta (ζ) potential of calcites toward the positive sides. 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, cations and anions ratio, as in SmartWater and key ions solutions. In addition, the dissolved divalent cations can play a role in the interactions at the stern layer boundary and eventually it can impact the surface charges at oil/water interfaces. The study results show the importance of ions in controlling the surface charges at different interfaces and how that can affect the SmartWater flooding and oil recovery processes.

Key words: carbonates, oil-in-water emulsion, electrokinetics, ionic strength