Boundary Tension and Wettability
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)
– Sw+So+Sg=1
• When more than a single phase is present, the fluids interact with the rock, and with each other
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)
BOUNDARY (INTERFACIAL) TENSION
Modified from PETE311 Notes
• Imbalanced molecular forces at phase boundaries• Boundary contracts to minimize size• Cohesive vs. adhesion forces
LIQUID(dense phase)
MolecularInterface
(imbalanceof forces)
GASSOLID
LIQUID
GAS
SOLID
Cohesive forceAdhesion force
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
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
DEFINITION OF ADHESION TENSION
• Adhesion tension is expressed as the
difference between two solid-fluid
interfacial tensions.
cosowwsosTA
• 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
CONTACT ANGLE
The contact angle, , 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
Solid
Water
Oil
Oil Oil
os ws
ow
os
• 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.
WETTING 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
NONWETTING PHASE FLUID
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 > os
• AT < 0 (i.e., the adhesion tension is negative)
• 0 < < 90
If is close to 0, the rock is considered to be “strongly water-wet”
WATER-WET ROCK
• Adhesive tension between water and the rock surface exceeds that between oil and the rock surface.
• 0 < < 90
Solid
Water
Oil
os ws
ow
os
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 > ws
• AT > 0 (i.e., the adhesion tension is positive)
• 90 < < 180If is close to 180, the rock is considered to be “strongly oil-wet”
OIL-WET ROCK
• 90 < < 180
• The adhesion tension between water and the rock surface is less than that between oil and the rock surface.
Solid
Water
Oil
os ws
ow
os
From Amyx Bass and Whiting, 1960; modified from Benner and Bartel, 1941
INTERFACIAL CONTACT ANGLES,VARIOUS ORGANIC LIQUID IN
CONTACT WITH SILICA AND CALCITE
OR
GA
NIC
LIQ
UID
S
SILICA SURFACE
CALCITE SURFACE
WATER
WATER
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 relationsthat are debated and disputed by petrophysicists.
WATER-WET OIL-WET
Ayers, 2001
FREE WATER
GRAIN
SOLID (ROCK)
WATER
OIL
SOLID (ROCK)
WATER
OIL
GRAIN
BOUND WATER
FR
EE
WA
TE
R
OIL
OILRIM
< 90 > 90WATER
OilAir
WATER
OIL-WETWATER-WET
WATER
WATERWATER
Air Oil
From Levorsen, 1967
Brown, G.E., 2001, Science, v. 294, p. 67-69
From Tiab and Donaldson, 1996
n = 161 ls., dol.
CONTACT ANGLE: Triber et al.-Water-wet = 0 – 75 degrees -Intermediate-wet = 75 – 105 degrees-Oil-wet = 105 – 180 degrees
n = 30 silicate and 25 carbonates
CONTACT ANGLE:-Water-wet = 0 – 80 degrees -Intermediate-wet = 80 – 100 degrees-Oil-wet = 100 – 180 degrees
WETTABILITY IS AFFECTED BY:
• Composition of pore-lining minerals
• Composition of the fluids
• Saturation history
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
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
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
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
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
IMPLICATIONS OF WETTABILITY
• Primary oil recovery is affected by the
wettability of the system.
– A water-wet system will exhibit
greater primary oil recovery.
WATER-WET OIL-WET
Ayers, 2001
FREE WATER
GRAIN
SOLID (ROCK)
WATER
OIL
SOLID (ROCK)
WATER
OIL
GRAIN
BOUND WATER
FR
EE
WA
TE
R
OIL
OILRIM
< 90 > 90WATER
OilAir
WATER
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.
From Levorsen, 1967
Effect on waterflood of an oil reservoir?
Water-Wet System
Oil-Wet System
IMPLICATIONS OF WETTABILITY
• Wettability affects the shape of the
relative permeability curves.
– Oil moves easier in water-wet rocks
than oil-wet rocks.
IMPLICATIONS OF WETTABILITY
1 2 3 4 5 6 7 8 9 10 11 120
20
40
60
8012345
Coreno
Percentsilicone Wettability0.000.0200.2002.001.00
0.6490.176
- 0.222- 0.250- 0.333
Curves cut off at Fwd •100
1 23
45
Water injected, pore volumes
Rec
ove
ry e
ffic
ien
cy, p
erce
nt,
So
i
Modified from Tiab and Donaldson, 1996
?p. 274
IMPLICATIONS OF WETTABILITY
Water injection, pore volumes
0
20
40
60
80
1 2 3 4 5 6 7 8 9 10
Squirrel oil - 0.10 N NaCl - Torpedo core ( • 33 O W • 663, K • 0945, Swi • 21.20%)
Squirrel oil - 0.10 N NaCl • Torpedo Sandstone core, after remaining in oil for 84 days ( • 33.0 W • 663, K • 0.925, Swi • 23.28%)
Rec
ove
ry e
ffic
ien
cy, p
erce
nt
Sp
i
Modified from NExT, 1999
WETTABILITY AFFECTS:
• Capillary Pressure
• Irreducible water saturation
• Residual oil and water saturations
• Relative permeability
• Electrical properties
LABORATORY MEASUREMENT OF WETTABILITY
Most common measurement techniques
– Contact angle measurement method
– Amott method
– United States Bureau of Mines (USBM) Method
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
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.
4. Donaldson, E.C., Thomas, R.D., and Lorenz, P.B.: “Wettability Determination and Its Effect
on Recovery Efficiency,” SPEJ (March 1969) 13-20.
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