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)

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

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

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”

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

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-WETWATER-WET

WATER

WATERWATER

Air Oil

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

• 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 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