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Boundary Tension & Wettability
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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