Introduction
Recent studies have shown that water properties can be modified based on salt composition and concentration, resulting in increased oil recovery in comparison to conventional water flooding in which formation brine is injected.
However, chemical surfactants present important challenges that need to be dealt with before using them. Hence, there has been an increased focus on studying biological surfactants as good substitutes for chemical surfactants in EOR processes.
The objective of this study was to investigate the potential application of environment-friendly bio-surfactants, rhamnolipid and GreenZyme®, combined with controlled salinity water in EOR processes.
This investigation included: solubility tests of the bio-surfactants in varied salinity to check compatibility with salinity in oil bearing reservoirs, emulsification and IFT test on oil-brine interactions under varied salinitiy, and wettability effects was tested through spontaneous inhibition.
The combination of these bio-surfactants with controlled salinity water injection was assessed through comprehensive core flooding experiments using outcrop carbonate rock samples, dead crude oil and synthetic brine solutions.
Materials and sample preparation:
1) all brine solutions were prepared with reagent grade NaCl, CaCl2, 2HCl2, MgCl2, 6H2O and Na2SO4 salts of >95% purity;
2) the crude used is a light dead oil from the North Sea (Density 0.906 g/cc, Viscosity 60.422 cp, °API 24.75);
3) bio-surfactant concentration 1 wt.%; 4) core plugs from outcrop of the Estaillades limestone from France.
Results
Bio-surfactant Solubility: GreenZyme® was soluble in all the electrolyte solutions at every tested temperature.
Emulsification Activity: GreenZyme® activity was found to be monotonic with time and a wider spread was observed with range of salinity. Initially, a direct correlation was observed between emulsification activity and brine salinity, but at equilibrium, similar effect was observed with the brine solutions and the highest emulsion index of 50.00±4% with controlled salinity water.
IFT: reduction of oil-brine interfacial tension was observed with the use of GreenZyme®. As concentration was increased from 0 to 5%, IFT was reduced from 15.56 mN/m to 2 mN/m (a reduction of 87%).
Wettability: imbibition tests with GreenZyme® solution showed wettability alterations towards increased water wetness while proving that adding GreenZyme® to the controlled salinity water, improved efficiency.
Core Displacement Experiments:
Controlled Salinity Flooding in 3 Stages: core plugs were continuously displaced first with high salinity water, followed by controlled salinity water displacement and finally displaced with controlled salinity water mixed with GreenZyme®. The first displacement accounted for 71.45% oil recovery, the second one added 12.07% and the third added another 1.86%, for a total of 85.38%.
Controlled Salinity Flooding in 2 Stages: core plugs were continuously displaced first controlled salinity water, followed by displacement with controlled salinity water mixed with GreenZyme®. The oil recovery for the first displacement was 77.73%, while the second added another 3.87%, for a total of 81.6%.
Conclusion
The study demonstrated the potential of the application of controlled salinity water injection mixed with GreenZyme® to improve oil recovery through a strategic injection. The results of this study will also help improve understanding of the underlying mechanisms of this type of application in EOR processes.
Aforementioned results showed that GreenZyme® works in any salinity environment, it can emulsify efficiently crude oil, it effectively reduces IFT significantly, it alters rock wettability towards a water-enzyme wet state and, last but not least, core displacement experiments proved that GreenZyme® improves oil recovery.
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