6745825-wettability
TRANSCRIPT
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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
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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)
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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
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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
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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
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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”
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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
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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
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OIL-WETWATER-WET
WATER
WATERWATER
Air Oil
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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
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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
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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