Low salinity water flooding Experimental experience and challenges

1
Low salinity water floodingExperimental
experience and challenges

• Force workshop
• Ingebret Fjelde

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Outline

• Questions
• Experiences
• Challenges
• Concluding remarks

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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?

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Low salinity water flooding can not be understood
by classic EOR

• No miscible process
• No reduction in IFT
• Mobility control?
• Favorable mobility control if Mlt1
• 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

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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)

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Tulsa SPEIOR 2008

• SPE 113480, Endicott field
• Improved oil recovery shown in core floods and
  single well tracer test
• Kaoline content important
• SPE 113976
• 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
  Sor0.30?0.02
• Non optimised low salinity brine Sor0.28?0.02
• Optimised low salinity brine Sor0.20?0.02
• Composition of optimised brine not given

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Irreversible?

• SCA2006-36
• 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

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Spontaneous imbibition limestoneFormation 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
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Adsorption surface active components onto surfaces

• Adsorption
• 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

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Adsorption of acidic crude oil components onto
chalk

• Acid number in effluent during injection of crude
  oil to chalk

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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. 2005-74
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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
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MechanismsAlteration of flow saturation functions

• Description of mechanisms required

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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

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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),
1993-201.
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Time effects 2Desorption 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, 25-28 March 2007.

IFT (crude oil brine) 30 mN/m
Mud filtrates
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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

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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

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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

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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

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Challenge Reservoir conditions

• 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

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Challenge Potential estimates
Parametic studyIdentify mechanisms
Modelling experimentsSimple but good enough
description
Preliminary studies insimplified fluid and
rocksystems, but final validationat reservoir
conditions
Extension larger scaleBehaviour larger scale
Potential estimateOptimisation reservoir
process, e.g. inj. strategy
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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