Boundary Tension and Wettability

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Boundary Tension and Wettability
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Immiscible Phases

• Earlier discussions have considered only a single
  fluid in the pores
• porosity
• permeability
• Saturation fraction of pore space occupied by a
  particular fluid (immiscible phases)
• SwSoSg1
• When more than a single phase is present, the
  fluids interact with the rock, and with each other

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DEFINITION OF INTERFACIAL TENSION

• Interfacial (boundary) tension is the energy per
  unit area (force per unit distance) at the
  surface between phases
• Commonly expressed in milli-Newtons/meter (also,
  dynes/cm)

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BOUNDARY (INTERFACIAL) TENSION
Modified from PETE 311 Notes
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DEFINITION OF WETTABILITY

• Wettability is the tendency of one fluid to
  spread on or adhere to a solid surface in the
  presence of other immiscible fluids.
• Wettability refers to interaction between fluid
  and solid phases.

• Reservoir rocks (sandstone, limestone, dolomite,
  etc.) are the solid surfaces
• Oil, water, and/or gas are the fluids

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WHY STUDY WETTABILITY?

• Understand physical and chemical interactions
  between
• Individual fluids and reservoir rocks
• Different fluids with in a reservoir
• Individual fluids and reservoir rocks when
  multiple fluids are present
• Petroleum reservoirs commonly have 2 3 fluids
  (multiphase systems)
• When 2 or more fluids are present, there are at
  least 3 sets of forces acting on the fluids and
  affecting HC recovery

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DEFINITION OF ADHESION TENSION

• Adhesion tension is expressed as the difference
  between two solid-fluid interfacial tensions.

• A negative adhesion tension indicates that the
  denser phase (water) preferentially wets the
  solid surface (and vice versa).
• An adhesion tension of 0 indicates that both
  phases have equal affinity for the solid surface

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CONTACT ANGLE
The contact angle, q, measured through the denser
liquid phase, defines which fluid wets the solid
surface.
AT adhesion tension, milli-Newtons/m or
dynes/cm) ? contact angle between the
oil/water/solid interface measured through the
water, degrees ?os interfacial energy between
the oil and solid, milli-Newtons/m or dynes/cm
?ws interfacial energy between the water and
solid, milli-Newtons/m or dynes/cm ?ow
interfacial energy (interfacial tension) between
the oil and water, milli-Newtons/m or dynes/cm
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WETTING PHASE FLUID

• Wetting phase fluid preferentially wets the solid
  rock surface.
• Attractive forces between rock and fluid draw the
  wetting phase into small pores.
• Wetting phase fluid often has low mobile.
• Attractive forces limit reduction in wetting
  phase saturation to an irreducible value
  (irreducible wetting phase saturation).
• Many hydrocarbon reservoirs are either totally or
  partially water-wet.

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NONWETTING PHASE FLUID

• Nonwetting phase does not preferentially wet the
  solid rock surface
• Repulsive forces between rock and fluid cause
  nonwetting phase to occupy largest pores
• Nonwetting phase fluid is often the most mobile
  fluid, especially at large nonwetting phase
  saturations
• Natural gas is never the wetting phase in
  hydrocarbon reservoirs

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WATER-WET RESERVOIR ROCK

• Reservoir rock is water - wet if water
  preferentially wets the rock surfaces
• The rock is water- wet under the following
  conditions
• ?ws gt ?os
• AT lt 0 (i.e., the adhesion tension is negative)
• 0? lt ? lt 90?
• If ? is close to 0?, the rock is considered
• to be strongly water-wet

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WATER-WET ROCK

• 0? lt q lt 90?

• Adhesive tension between water and the rock
  surface exceeds that between oil and the rock
  surface.

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OIL-WET RESERVOIR ROCK

• Reservoir rock is oil-wet if oil preferentially
  wets the rock surfaces.
• The rock is oil-wet under the following
  conditions
• ?os gt ?ws
• AT gt 0 (i.e., the adhesion tension is positive)
• 90? lt ? lt 180?
• If ? is close to 180?, the rock is considered to
  be strongly oil-wet

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OIL-WET ROCK

• 90? lt q lt 180?

• The adhesion tension between water and the rock
  surface is less than that between oil and the
  rock surface.

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INTERFACIAL CONTACT ANGLES, VARIOUS ORGANIC
LIQUID IN CONTACT WITH SILICA AND CALCITE
From Amyx Bass and Whiting, 1960 modified from
Benner and Bartel, 1941
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• GENERALLY,
• Silicate minerals have acidic surfaces
• Repel acidic fluids such as major polar
• organic compounds present in some crude oils
• Attract basic compounds
• Neutral to oil-wet surfaces

• Carbonate minerals have basic surfaces
• Attract acidic compounds of crude oils
• Neutral to oil-wet surfaces

Tiab and Donaldson, 1996
Caution these are very general statements and
relations that are debated and disputed by
petrophysicists.
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WATER-WET
OIL-WET
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OIL-WET
WATER-WET
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Brown, G.E., 2001, Science, v. 294, p. 67-69
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WETTABILITY IS AFFECTED BY

• Composition of pore-lining minerals
• Composition of the fluids
• Saturation history

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WETTABILITY CLASSIFICATION

• Strongly oil- or water-wetting
• Neutral wettability no preferential
  wettability
• to either water or oil in the pores
• Fractional wettability reservoir that has
  local
• areas that are strongly oil-wet, whereas most
• of the reservoir is strongly water-wet
• - Occurs where reservoir rock have variable
  
• mineral composition and surface
  chemistry
• Mixed wettability smaller pores area water-wet
• are filled with water, whereas larger pores
  are
• oil-wet and filled with oil
• - Residual oil saturation is low
• - Occurs where oil with polar organic
  compounds
• invades a water-wet rock saturated with
  brine

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IMBIBITION

• Imbibition is a fluid flow process in which the
  saturation of the wetting phase increases and the
  nonwetting phase saturation decreases. (e.g.,
  waterflood of an oil reservoir that is
  water-wet).
• Mobility of wetting phase increases as wetting
  phase saturation increases
• mobility is the fraction of total flow capacity
  for a particular phase

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WATER-WET RESERVOIR, IMBIBITION

• Water will occupy the smallest pores
• Water will wet the circumference of most larger
  pores
• In pores having high oil saturation, oil rests
  on a water film
• Imbibition - If a water-wet rock saturated with
  oil is
• placed in water, it will imbibe water into the
  smallest
• pores, displacing oil

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OIL-WET RESERVOIR, IMBIBITION

• Oil will occupy the smallest pores
• Oil will wet the circumference of most larger
  pores
• In pores having high water saturation, water
  rests on an
• oil film
• Imbibition - If an oil-wet rock saturated with
  water is
• placed in oil, it will imbibe oil into the
  smallest
• pores, displacing water
• e.g., Oil-wet reservoir accumulation of oil in
  trap

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DRAINAGE

• Fluid flow process in which the saturation of the
  nonwetting phase increases
• Mobility of nonwetting fluid phase increases as
  nonwetting phase saturation increases
• e.g., waterflood of an oil reservoir that is
  oil-wet
• Gas injection in an oil- or water-wet reservoir
• Pressure maintenance or gas cycling by gas
  injection
• in a retrograde condensate reservoir
• Water-wet reservoir accumulation of oil or gas
  in trap

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IMPLICATIONS OF WETTABILITY

• Primary oil recovery is affected by the
  wettability of the system.
• A water-wet system will exhibit greater primary
  oil recovery.

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WATER-WET
OIL-WET
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IMPLICATIONS OF WETTABILITY

• Oil recovery under waterflooding is affected by
  the wettability of the system.
• A water-wet system will exhibit greater oil
  recovery under waterflooding.

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Water-Wet System
Oil-Wet System
Effect on waterflood of an oil reservoir?
From Levorsen, 1967
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IMPLICATIONS OF WETTABILITY

• Wettability affects the shape of the relative
  permeability curves.
• Oil moves easier in water-wet rocks than oil-wet
  rocks.

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IMPLICATIONS OF WETTABILITY
? p. 274
Modified from Tiab and Donaldson, 1996
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IMPLICATIONS OF WETTABILITY
Water injection, pore volumes
Modified from NExT, 1999
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WETTABILITY AFFECTS

• Capillary Pressure
• Irreducible water saturation
• Residual oil and water saturations
• Relative permeability
• Electrical properties

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LABORATORY MEASUREMENT OF WETTABILITY

• Most common measurement techniques
• Contact angle measurement method
• Amott method
• United States Bureau of Mines (USBM) Method

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NOMENCLATURE
AT adhesion tension, milli-Newtons/m or
dynes/cm) ? contact angle between the
oil/water/solid interface measured through the
water (more dense phase), degrees ?os
interfacial tension between the oil and solid,
milli-Newtons/m or dynes/cm ?ws interfacial
tension between the water and solid,
milli-Newtons/m or dynes/cm ?ow interfacial
tension between the oil and water,
milli-Newtons/m or dynes/cm
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References

• 1. Amyx, J.W., Bass, D.M., and Whiting, R.L.
  Petroleum Reservoir Engineering, McGrow-Hill Book
  Company New York, 1960.
• 2. Tiab, D. and Donaldson, E.C. Petrophysics,
  Gulf Publishing Company, Houston, TX. 1996.
• 3. Core Laboratories, Inc. A course in the
  fundamentals of Core analysis, 1982.
• Donaldson, E.C., Thomas, R.D., and Lorenz, P.B.
  Wettability Determination and Its Effect
• on Recovery Efficiency, SPEJ (March
  1969) 13-20.