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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) S w +S o +S g =1

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immiscible phases
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
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)
slide4

Imbalanced molecular forces at phase boundaries

  • Boundary contracts to minimize size
  • Cohesive vs. adhesion forces

GAS

LIQUID

GAS

SOLID

Cohesive force

Adhesion force

Molecular

Interface

(imbalance

of forces)

LIQUID

(dense phase)

SOLID

BOUNDARY (INTERFACIAL) TENSION

Modified from PETE

311 Notes

definition of wettability
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
slide6

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
DEFINITION OF ADHESION TENSION
  • Adhesion tension is expressed as the difference between two solid-fluid interfacial tensions.
  • 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

Oil

sow

Water

Oil

Oil

sos

sws

sos

Solid

CONTACT ANGLE

The contact angle, q, 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

wetting phase fluid
WETTING PHASE FLUID
  • 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 fluid
NONWETTING 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
water wet reservoir rock
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

Oil

sow

Water

sos

sos

sws

Solid

WATER-WET ROCK
  • 0 <q< 90
  • Adhesive tension between water and the rock surface exceeds that between oil and the rock surface.
oil wet reservoir rock
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

sow

Water

Oil

sws

sos

sos

Solid

OIL-WET ROCK
  • 90 <q< 180
  • The adhesion tension between water and the rock surface is less than that between oil and the rock surface.
slide15

WATER

SILICA SURFACE

ORGANIC

LIQUIDS

WATER

CALCITE SURFACE

INTERFACIAL CONTACT ANGLES,

VARIOUS ORGANIC LIQUID IN

CONTACT WITH SILICA AND CALCITE

From Amyx Bass and Whiting, 1960; modified from Benner and Bartel, 1941

slide16

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.

slide17

WATER-WET

OIL-WET

Air

OIL

OIL

Oil

WATER

WATER

SOLID (ROCK)

 < 90

SOLID (ROCK)

 > 90

WATER

WATER

FREE WATER

OIL

GRAIN

GRAIN

OIL

RIM

BOUND WATER

FREE WATER

Ayers, 2001

slide18

Oil

Air

WATER

WATER

WATER

WATER-WET

OIL-WET

slide21

n = 161 ls., dol.

n = 30 silicate and 25 carbonates

From Tiab and Donaldson, 1996

  • CONTACT ANGLE: Triber et al.
  • Water-wet = 0 – 75 degrees
  • Intermediate-wet = 75 – 105 degrees
  • Oil-wet = 105 – 180 degrees
  • CONTACT ANGLE:
  • Water-wet = 0 – 80 degrees
  • Intermediate-wet = 80 – 100 degrees
  • Oil-wet = 100 – 180 degrees
slide22

WETTABILITY IS AFFECTED BY:

  • Composition of pore-lining minerals
  • Composition of the fluids
  • Saturation history
slide23

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
  • 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
slide25

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
slide26

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
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
IMPLICATIONS OF WETTABILITY
  • Primary oil recovery is affected by the wettability of the system.
    • A water-wet system will exhibit greater primary oil recovery.
slide29

WATER-WET

OIL-WET

Air

OIL

OIL

Oil

WATER

WATER

SOLID (ROCK)

 < 90

SOLID (ROCK)

 > 90

WATER

WATER

FREE WATER

OIL

GRAIN

GRAIN

OIL

RIM

BOUND WATER

FREE WATER

Ayers, 2001

implications of wettability1
IMPLICATIONS OF WETTABILITY
  • Oil recovery under waterflooding is affected by the wettability of the system.
    • A water-wet system will exhibit greater oil recovery under waterflooding.
slide31

Water-Wet System

Oil-Wet System

Effect on waterflood of an oil reservoir?

From Levorsen, 1967

implications of wettability2
IMPLICATIONS OF WETTABILITY
  • Wettability affects the shape of the relative permeability curves.
    • Oil moves easier in water-wet rocks than oil-wet rocks.
implications of wettability3

Core

no

Percent

silicone

Wettability

1

0.00

0.649

2

0.020

0.176

80

3

0.200

- 0.222

1

2

4

2.00

- 0.250

3

5

- 0.333

1.00

60

Curves cut off at Fwd •100

4

Recovery efficiency, percent, Soi

40

5

20

0

1

2

3

4

5

6

7

8

9

10

11

12

Water injected, pore volumes

IMPLICATIONS OF WETTABILITY

?

p. 274

Modified from Tiab and Donaldson, 1996

implications of wettability4

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%)

80

Recovery efficiency, percent Spi

60

40

20

0

1

2

3

4

5

6

7

8

9

10

IMPLICATIONS OF WETTABILITY

Water injection, pore volumes

Modified from NExT, 1999

slide35

WETTABILITY AFFECTS:

  • Capillary Pressure
  • Irreducible water saturation
  • Residual oil and water saturations
  • Relative permeability
  • Electrical properties
laboratory measurement of wettability
LABORATORY MEASUREMENT OF WETTABILITY

Most common measurement techniques

  • Contact angle measurement method
  • Amott method
  • United States Bureau of Mines (USBM) Method
nomenclature
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
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.
  • Donaldson, E.C., Thomas, R.D., and Lorenz, P.B.: “Wettability Determination and Its Effect
  • on Recovery Efficiency,” SPEJ (March 1969) 13-20.