Summary
The chemistry of injection water affects oil recovery from carbonate reservoirs by smartwater flooding. It is widely believed that the ions present in the smartwater alter the wettability of carbonate rocks, depending on their type and the amounts present. Although some effort has been made to understand the effects of salinity and water-ion compositions on wettability in carbonates, the prior research studies were mostly limited to contact angle, spontaneous imbibition, and coreflooding.
In the current study, adhesion forces between a carbonate substrate and a crude-oil droplet in the brines of varying ionic compositions were measured directly by using a custom-designed integrated-thin-film drainage apparatus (ITFDA) equipped with a bimorph force sensor. In addition, the liberation kinetics of crude oil from carbonate rocks were determined using an optical microscope-based liberation cell at both ambient and elevated temperatures. These measurements were complemented with thermogravimetric analysis (TGA) and standard macroscopic data such as water-contact angles and ζ-potentials. The effect of individual cations [calcium (Ca2+); magnesium (Mg2+)] and anions [sulfate (SO42−)] on wettability, adhesion, and oil liberation in carbonates was studied by using reservoir rock surfaces, reservoir crude oil, and different brines composed of a single type of salt at a fixed low salinity. Both deionized (DI) water and low-salinity brine composed of sufficient amounts of the three key ions (SO42−, Ca2+, and Mg2+) were also used as the baseline for these experiments.
The results showed a significant increase in water wettability (or decrease in contact angles) with low-salinity brines compared with DI water, depending on the types of ions present in these brines. The presence of SO42− increased the water wettability the most, followed by the Ca2+ and Mg2+ ions. The ζ-potential data of carbonate rock minerals in DI water/brines showed similar trends on surface charges to correlate well with contact angles. Increasing the water wettability of brines on carbonate surfaces decreased the adhesion force between the oil and the rock in the corresponding brines. The adhesion forces on the carbonate surface were found to be in the following order: DI water > Mg2+ brine > Ca2+ brine > low-salinity brine with SO42−, Ca2+, and Mg2+ ions > SO42− brine. Such favorable changes in adhesion forces in turn led to more efficient crude-oil liberation from carbonates at a microscopic scale when exposed to different low-salinity brines than in DI water. The dynamic oil-liberation data from carbonates at both ambient and elevated temperatures demonstrated the significant advantage of low-salinity brine containing SO42− ions compared with DI water, but showed only its slight effectiveness over the low-salinity brine composed of three key ions. The TGA further confirmed the efficiency of both the low-salinity brines, composed of SO42− and the three key ions, to liberate more crude oil from carbonates.
The findings from different microscopic- to macroscopic-scale measurements reported in this work clearly indicate the importance of both lower salinity and the major role of certain ions in the smartwater to effectively release crude oil from carbonates. It can also be concluded that low-salinity water containing sufficient amounts of three key ions can become a practical smartwater for waterflooding operations, considering the adverse effect of SO42− ions on the interactions at the crude-oil/water interface as well as the reservoir damage resulting from scaling and souring issues.
