WettAbility

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 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

LIQUID

GAS

SOLID

Cohesive force

Adhesion 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

ow

• 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 < < 180

If 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

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 relations

that 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

OIL

RIM

< 90 > 90

WATER

Oil

Air

WATER

OIL-WET WATER-WET

WATER

WATER WATER

Air Oil

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

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

OIL

RIM

< 90 > 90

WATER

Oil

Air

WATER


• 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


• Wettability affects the shape of the

relative permeability curves.

– Oil moves easier in water-wet rocks

than oil-wet rocks.


1 2 3 4 5 6 7 8 9 10 11 12 0

20

40

60

80 1 2 3 4 5

Core no

Percent silicone Wettability

0.00 0.020 0.200 2.00 1.00

0.649 0.176

- 0.222 - 0.250 - 0.333

Curves cut off at Fwd •100

1 2 3

4

5

Water injected, pore volumes

Re

co

very

eff

icie

nc

y,

pe

rce

nt,

So

i

Lab work4 shows that a strongly water-wet system will have breakthrough of water after

most of the production of oil has taken place, and very little production of oil will occur

after water breakthrough.

For oil-wet systems, water breakthrough occurs earlier in the flood and production

continues for a long period after water breakthrough at a fairly constant water/oil

production ratio.


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%)

Re

co

very

eff

icie

nc

y,

pe

rce

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.

WettAbility

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