When two immiscible fluids such as oil and water are present in rock pores, interfacial tension arises at the boundary between the fluids due to imbalanced molecular forces. Wettability refers to whether the rock preferentially interacts with and spreads one fluid over the other. It is determined by measuring the contact angle between the fluids and solid surface. Wettability affects fluid distributions and flow properties in the reservoir, with water-wet rocks typically yielding more oil during waterflooding recovery than oil-wet rocks.

1. BOUNDARY TENSION
AND WETTABILITY

2. 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

3. 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)

4. INTERFACIAL TENSION
Immiscible fluids: when you bring them into contact they do
not mix
Two fluids are separated by an interface
The molecules are attracted more to their own kind
Oil
Rock
water

5. INTERFACIAL TENSION
Interfacial tension: is the work required to create a
unit area of new surface
s = F / L ( N/m or dynes/cm )
F = 2 σ L

6. BOUNDARY (INTERFACIAL) TENSION
GAS
LIQUID
Modified from PETE
311 Notes
• Imbalanced molecular forces at phase boundaries
• Boundary contracts to minimize size
• Cohesive vs. adhesion forces
LIQUID
(dense phase)
Molecular
Interface
(imbalance
of forces)
GAS
SOLID

SOLID
Cohesive force
Adhesion force

7. 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

8. sow

water
WETTABILITY
Oil
grain surface
sSo
sSw
sSo  sSw = sow cos 
s 
s
cos S o S w
= Young-Laplace equation
ow

s
Glass is water-wet in the
presence of air but air-wet in
the presence of mercury.
The angle made by the
interface with the solid is called
the contact angle (measured
through the wetting phase).
The contact angle will increase
if the wetting phase is
increasing (imagine a pool of
water spreading over the glass
plate) and decrease if the
wetting phase saturation is
decreasing (the pool of water
being sucked away through a
straw leaves a film of water
behind).

9. WETTABILITY

water
Oil
grain surface

water
Oil
grain surface
Water wet Oil wet

10. TEFLON - OIL WET (WATER REPELLANT)
SURFACE

11. QUARTZ - WATER WET - ATTRACTIVE -SURFACE

12. STAINLESS STEEL - NEUTRAL WETTING

13. 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

14. DEFINITION OF ADHESION TENSION
Adhesion tension is expressed as the
difference between two solid-fluid
interfacial tensions.
s s s cos T os ws ow A =  = 
• 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

15. CONTACT ANGLE
Oil
Water
sow

Oil Oil
sos
The contact angle,  , measured through
the denser liquid phase,
defines which fluid wets the solid
sos sws
Solid
A surface. T = adhesion tension, milli-Newtons/m or dynes/cm)
 = contact angle between the oil/water/solid interface measured through the water, degrees
sos = interfacial energy between the oil and solid, milli-Newtons/m or dynes/cm
sws = interfacial energy between the water and solid, milli-Newtons/m or dynes/cm
sow = interfacial energy (interfacial tension) between the oil and water, milli-Newtons/m or dynes/cm

16. 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.

17. 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

18. 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:
sws > sos
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”

19. WATER-WET ROCK

sos sws
• 0 <  < 90
sow
Solid
Water
Oil
sos
• Adhesive tension between water and the
rock surface exceeds that between oil and
the rock surface.

20. 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:
sos > sws
AT > 0 (i.e., the adhesion tension is positive)
90 <  < 180
If  is close to 180, the rock is considered to be
“strongly oil-wet”

21. OIL-WET ROCK
Water

sos sws
• 90 <  < 180
Oil
Solid
sow
sos
• The adhesion tension between water and the
rock surface is less than that between oil and the
rock surface.

22. INTERFACIAL CONTACT ANGLES,
VARIOUS ORGANIC LIQUID IN
CONTACT WITH SILICA AND CALCITE
SILICA SURFACE
CALCITE SURFACE
WATER
WATER
From Amyx Bass and Whiting, 1960; modified from Benner and Bartel, 1941

23. 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

24. WATER-WET OIL-WET
Ayers, 2001
GRAIN
FREE WATER
OIL
WATER

SOLID (ROCK)
WATER
SOLID (ROCK)
OIL

GRAIN
BOUND WATER
FREE WATER
Air


OIL
OIL
RIM
 < 90
Oil
 > 90 WATER
WATER

25. WORLDWIDE RESERVOIR WETTABILITY SURVEYS
(LIQUID HYDROCARBON BEARING RESERVOIRS)
Clastic Formations (Sandstones)
15% 55% 30%
OW Nw/mw WW

26. WORLDWIDE RESERVOIR WETTABILITY SURVEYS
(LIQUID HYDROCARBON BEARING RESERVOIRS)
Carbonate Formations
40% 50% 10%
OW Nw/mw WW

27. Factors affecting wettability
 Composition of the fluids
 No correlation with crude composition
 Water-wet reservoirs have low asphaltene
 Some low-asphaltene crudes have mixed wettability
 Rock mineralogy
 Composition of pore-lining minerals
 Saturation history
 Formation brine and pH
 Pressure and temperature
 Thickness of water layer

28. 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

29. 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

30. 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

31. 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

32. 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

33. IMPLICATIONS OF WETTABILITY
Primary oil recovery is affected by the
wettability of the system.
A water-wet system will exhibit greater
primary oil recovery.

34. WATER-WET OIL-WET
Ayers, 2001
GRAIN
FREE WATER
OIL
WATER

SOLID (ROCK)
WATER
SOLID (ROCK)
OIL

GRAIN
BOUND WATER
FREE WATER
Air


OIL
OIL
RIM
 < 90
Oil
 > 90 WATER
WATER

35. 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.

36. IMPLICATIONS OF WETTABILITY
Wettability affects the shape of the
relative permeability curves.
Oil moves easier in water-wet rocks than
oil-wet rocks.

37. IMPLICATIONS OF WETTABILITY
Core
no
1 2 3 4 5 6 7 8 9 10 11 12
80
60
40
20
0
123
4
5
Percent
silicone Wettability
0.00
0.649
0.020
0.176
0.200
- 0.222
2.00
- 0.250
1.00
- 0.333
Curves cut off at Fwd •100
1 2
3
4
5
Water injected, pore volumes
Recovery efficiency, percent, Soi
Modified from Tiab and Donaldson, 1996
?
p. 274

38. IMPLICATIONS OF WETTABILITY
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%)
Water injection, pore volumes
80
60
40
20
0
1 2 3 4 5 6 7 8 9 10
Recovery efficiency, percent Spi
Modified from NExT, 1999

39. WETTABILITY AFFECTS:
• Capillary Pressure
• Irreducible water saturation
• Residual oil and water saturations
• Relative permeability
• Electrical properties

40. LABORATORY MEASUREMENT OF WETTABILITY
Most common measurement techniques
Contact angle measurement method
Amott method
United States Bureau of Mines (USBM)
Method
Note that wettability measurements using core samples yield an indication of
wetting preference of the rock in the lab…. Not necessarily in the reservoir.
Knowing the wettability does not allow us to predict multi-phase flow properties.
We still need to know capillary pressure and relative permeability in order to predict
the multi-phase properties. However, knowing the wettability helps us understand
the reservoir and anticipate or explain its behavior.

41. 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
sos = interfacial tension between the oil and solid, milli-Newtons/m or
dynes/cm
sws = interfacial tension between the water and solid, milli-Newtons/m or
dynes/cm
sow = interfacial tension between the oil and water, milli-Newtons/m or
dynes/cm