1. Low salinity water flooding Experimental experience and challenges
Force workshop
Ingebret Fjelde

2. Outline
Questions
Experiences
Challenges
Concluding remarks

3. Questions
Are the mechanisms in low salinity water flooding understood?
Can low salinity water flooding be used in all sandstone fields?
If not, when can it be used?
Will low salinity water flooding improve the oil recovery in all sandstone reservoirs?
Can oil recovery be estimated by existing reservoir simulators?

4. Low salinity water flooding can not be understood by classic EOR
No miscible process
No reduction in IFT
Mobility control?
Favorable mobility control if M<1
Reducing krw or mo
Increasing kro or mw
Non of these seems to be dominating
When all other explanations have failed put the blame on wettability

5. Literature Reported to improve oil recovery in sandstones
Core experiments and reservoirs
Reported to depend on, e.g.
Multicomponent ion exchange important
Clay content (sometimes fines produced)
Composition formation water (Ca2+, Mg2+)
Oil composition (crude oil, no effect with white oils)
Initial water saturation required
pH increase (≈ alkaline flood, but no correlation with AN)

6. Tulsa SPEIOR 2008 SPE 113480, Endicott field
Improved oil recovery shown in core floods and single well tracer test
Kaoline content important
SPE
Mechanisms
Multiple-component ionic exchange (MIE) between adsorbed crude oil components, cations in the in situ brine and clay mineral surfaces
Single well tracer test Alaskian reservoir
High salinity water and produced water Sor=0.300.02
Non optimised low salinity brine Sor=0.280.02
Optimised low salinity brine Sor=0.200.02
Composition of optimised brine not given

7. Irreversible?
SCA
Re-aged cores higher oil recovery at high salinity, but no oil production in tertiary low salinity water flooding
Not likely due to multiple-component ionic exchange

8. Spontaneous imbibition limestone Formation water vs low salinity water
Low salinity water can also increase oil recovery in limestone
Probably not only clastic clays that are important for low salinity water flooding of sandstone

9. Adsorption surface active components onto surfaces
Surface charge
pH, salinity and brine composition
Adsorption density depends on salinity and brine composition
Desorption
Depend on the same parameters as adsorption
By decreasing salinity adsorption density decreases
Change of brine composition can also change surface charge
Electrical double-layer expands with decreasing brine salinity

10. Adsorption of acidic crude oil components onto chalk
Acid number in effluent during injection of crude oil to chalk

11. Surfactant adsorption Decreases with decreasing salinity
Adsorption equilibrium
Usually reversible
Reduction of salinity will give desorption
Similar expected for surface active components in crude oils
Asphaltenes may be an exception
Somasundaran, P. and Hanna, H.S., ”Physico-chemical aspects of adsorption at solid/liquid interfaces,”
in Improved Oil Recovery by Surfactant and Polymer flooding, ed. D.O. Shah and R.S. Schechter,
Academic Press, New York, 1977, pp

12. Drilling fluids
Water in drilling fluids can give swelling of clay and shale and reduction of permeability
Solved by using inhibitive drilling fluids
Water based drilling fluid with high salinity
Emulsified mud with high salt concentrations
Swelling clay: Bulk study
Low salinity water flood may not be carried out in all oil reservoirs

13. Mechanisms Alteration of flow saturation functions
Description of mechanisms required

14. Challenge Time effects
Analyses of effluents necessary to confirm that adsorption equilibrium has been established
Preparation of initial state at high salinity
Preparation of final state at low salinity
Studies of mechanisms
Time effects can be important in the laboratory, but not in the fields
But laboratory results will be used as inputs to simulators for estimations of oil recovery potential

15. Time effects 1 Adsorption can be slow Oil components
Aging of core plugs
Chemicals
Surfactants
Polymers
I.Fjelde, T. Austad and J. Milter, ”Adsorption VII. Dynamic adsorption of a dual surfactant system onto reservoir cores at sea water salinity,” J. Petr. Science & Eng., 13 (1995),

16. Time effects 2 Desorption surface active components can be slow
Interfacial tensions (IFT) between effluent samples and formation water during back-production of mud filtrate with crude oil
Fjelde, I., “Slow retention and release processes during drilling with emulsified drilling fluids,” 18th International Oil Field Chemicals Symposium, Geilo, Norway, March 2007.
IFT (crude oil – brine) ≈ 30 mN/m
Mud filtrates

17. Challenge Crude oils Complex mixtures of surface active oil components
Many types of surface active components
Different crude oils different types and concentrations
Interactions between crude oil components
Interaction between resins and asphaltenes important for asphaltene solubility / dispersion
Simplified systems may not give good enough description
Concentrations of surface active oil components and their solvency different in stock tank oil and live oil
Some oil components soluble in water

18. Challenge Crude oils cont.
Mixing of crude oils with low aromatic synthetic oil
Will reduce concentrations of surface active oil components
Can reduce solvency of some of the surface active components, e.g. will increase aggregation of asphaltene molecules in oil phase and on surfaces

19. Challenge Crude oils / Oxidation
Oxidation of crude oil will increase concentrations of acidic components
Oxidation products can have other properties than original polar components
Oxidation products can be less soluble, e.g. oxidation asphaltenes
Important to compare concentration of surface active components in used crude oil vs in original reservoir oil
Isolation of polar components from crude oil especially critical
After short term storage, often difficult to dissolve all of the polar components because of oxidation

20. Challenge Correct sampling in laboratory
Proposed mechanisms are known to depend on
Conditions
Temperature, pressure and pH
Different properties can be affected, e.g.
Solubility (ions and oil components)
Interactions (ions, oil components and rock surfaces)
Sampling should be carried out at test conditions
Alternative, have to verify that sampling can be carried out at other conditions, e.g. room temperature and 1 atm

21. Challenge Reservoir conditions
High temperature and high pressure
Characterisation of mechanisms can be difficult at reservoir conditions, e.g.
Interactions between surfaces and oil components and ions
Zeta potential
Contact angles

22. Challenge Potential estimates
Parametic study Identify mechanisms
Modelling experiments Simple but good enough description
Preliminary studies in simplified fluid and rock systems, but final validation at reservoir conditions
Extension larger scale Behaviour larger scale
Potential estimate Optimisation reservoir process, e.g. inj. strategy

23. Concluding remarks
Low salinity water flooding an environmentally friendly EOR method
Open the route for alkaline flooding and alkaline/surfactant flooding
Need best practice for low salinity water flooding potential evaluation
Description of mechanisms and determination of recovery potential should be confirmed using reservoir fluids and reservoir rocks at reservoir conditions