Low salinity waterflooding has recently gained more attention in carbonate formations mainly due to the cost effectiveness of this improved oil recovery technique. Despite all the recent work in this area, the underlying mechanism is still controversial, making field implementation very limited.
In this work, we explore the underlying mechanism behind the low salinity waterflooding effect (LSWE) in limestone rocks by investigating the role of divalent ions. This was achieved by implementing techniques such as contact angle and IFT measurements, zeta potential measurement and coreflooding experiments tailored towards a thorough understanding of the LSWE. Seawater and low salinity brines deprived of one divalent ion were prepared while maintaining theirionic strength.
Our data shows that reducing brine ionic strength results in wettability alteration which led to improved oil recovery in limestone rocks. This observation was attributed to surface charge reduction as measured using zeta potential. The absence of Mg2+ ion in both seawater and low salinity brine resulted in surface charge polarity change from positive to negative for pure limestone/brine interface, which resulted in a significant wettability alteration as measured on contact angle from oil wet to intermediate wet for limestone surfaces. The absence of Ca2+ ions had affected both zeta potential and contact angle resulting in the least alteration in rock surface wettability. Due to the low concentration of SO42− ions in the seawater and low salinity brines, SO42- had an insignificant effect on zeta potential and coreflooding. The surface charge of aged limestone/brine systems were determined, showing the influence of oil/brine interface over limestone/brine interface in the overall crude oil/brine/rock (COBR) system. This was due to the presence of carboxylic acid present in the crude oil and indicated proper aging procedure. Divalent ions varied the interfacial tension of seawater brine/oil system potentially resulting in better mobility control due to suppressed crude oil snap-off. At low salinity, the divalents ions present in the brine was insufficient to influence the interfacial tension.
The results from the various seawater and low salinity brines concludes that low salinity waterflooding effect occurs in limestones rocks partially due to the expansion of the electrical double layer as indicated by debye length. This results in detachment of carboxylic materials from the rock surface.