Boundary Tension & Wettability

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

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)

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

F / L ( N/m or dynes/cm )

Interfacial tension: is the work required to create a unit area of new surface

F = 2 σ L

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

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

water

Oil

grain surface

oS

wS

ow

cosowwSoS

cos S o S w

ow

Young-Laplace equation

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).

water

Oil

grain surface

water

Oil

grain surface

Water wet Oil wet

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

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

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.

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

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”

• 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

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”

• 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

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

OW WWNw/mw

Clastic Formations (Sandstones)

15% 30%55%

OW WWNw/mw

Carbonate Formations

40% 10%50%

Factors affecting wettability

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 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

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

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

Oil recovery under waterflooding is

affected by the wettability of the

system. A water-wet system will exhibit

greater oil recovery under

waterflooding.

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 11 120

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

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

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.

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