An Introduction to

Wettability of Oil Reservoirs

Petrophysics and Surface Chemistry Group

Petroleum Recovery Research Center

New Mexico Tech

Outline

• The big picture

• Microscopic view of oil reservoirs

• Capillary phenomena

• Surface chemistry in oil reservoirs

Outline

• The big picture

• Microscopic view of oil reservoirs

• Capillary phenomena

• Surface chemistry in oil reservoirs

Geologic scale

Structural trap

Outline

• The big picture

• Microscopic view of oil reservoirs

• Capillary phenomena

• Surface chemistry in oil reservoirs

Microscopic view of an oil reservoir

Blue color is epoxy in interconnected pore spaces.

Typical sandstone.

Less porous or less permeable?

More complicated sandstone

Early diagenetic spheroids in a fine-

to medium-grained subarkose,

showing variations in internal

patterns and typical radial-fibrous

texture.

St Bees Sandstone Formation

(Triassic), Cumbria, UK.

White - quartz; yellow - stained K-

feldspar; brown - iron oxide;

blue - stained porosity.

Scale bar: 100 microns.

from: STRONG, G. E. & PEARCE, J. M. 1995. Carbonate spheroids in Permo-Triassic sandstones of

the Sellafield area, Cumbria. Proceedings of the Yorkshire Geological Society Vol. 50, Pt 3, 209-211.

Kaolinite (clay)

Kugler, R.L. and Pashin, J.C., 1994,

Reservoir heterogeneity in Carter sandstone,

North Blowhorn Creek oil unit and vicinity,

Black Warrior basin, Alabama: Geological

Survey of Alabama Circular 159, 91 p.

Authigenic kaolinite, Carter

sandstone, Black Warrior

basin, Alabama

Illite (clay)

http://www.creationresearch.org/vacrc/sem02.html

"Hairy" illite clay found in the Coconino sandstone - 2000X — The

fine hair-like structure is actually crystalline mineral and is a

diagenetic alteration product of other minerals in the subsurface.

Outline

• The big picture

• Microscopic view of oil reservoirs

• Capillary phenomena

• Surface chemistry in oil reservoirs

Common characteristics of oil-bearing rocks

• Pores in which oil is found are small

d < 100µ

• Surface area is high

>> 1 m2/g

• Fluid-fluid interfaces coexist.

• Capillary forces hold oil in place.

Capillary phenomena

• Capillary rise

• Capillary pressure

• Interfacial tension

• Contact angles

Capillary rise

0 1,000 2,000 3,0000.001

0.01

0.1

1

10

100

tube radius (microns)

heig

ht o

f ris

e (m

ete

rs)

Capillary pressure

h

Pc = ∆ρ∆ρ g h

Pc = Pnw - Pw

Pw

Pnw

Pc = 2γγ / rr

Interfacial tension, γ

Liquid 1

Liquid 2

γγ11

γγ22

γγ11d γγ22

d

γγ11d γγ22

d

γγ11d γγ22

dγγ11γγ1212 γγ22= + − 2

F.M. Fowkes, “Attractive Forces at Interfaces,”

Ind. Eng. Chem. (Dec 1964) 56, 40-52.

Why is this interface curved?

Pw

Pnw

θθ

Water-wet (oil is non-wetting)

water

oil

θθ = 0°

Oil-wet (oil is spreading)

water

θθ = 180°

Partial wetting: 0° < θ < 180°

γγos = γγws + γγow cos θθ

water

oilθ

γγos

γγow

γγws

Crude oil contacting a clean surface

mica

brine

crude oil

A fresh drop of crude

oil under water does

not wet the surface.

After it contacts the

surface, the area under

the drop can become

oil-wet.

If so, a drop of crude

oil remains adhering

to the surface.

Adhesion of crude oil under brine

AFM shows adsorption

clean mica A-93 treated mica

PB brine + A-93, 3 weeks, 80°C, wash with cyclohexane

Contact angle on treated surface

decane

mica

water

Brine mediates crude oil/solid interactions

A-93 crude oil aged at 80°C

0 10 20 30 40 500

30

60

90

120

150

180

Aging time in oil (days)

Decane/w

ate

r conta

ct angle

(deg)

0.01

0.1

1.0

pH 4

pH 6

pH 8

NaCl (M)

Wettability affects oil recovery

0 2 4 6 8 10 12 140

20

40

60

80

100

Pore Volumes Injected

Oil

Recovery

(%

OO

IP)

Wettability of an oil reservoir?

• There is not one simple answer.

• Measure properties of reservoir cores

– preserved in original condition

– restored to original condition

• Consider the underlying surface chemistry.

Outline

• The big picture

• Microscopic view of oil reservoirs

• Capillary phenomena

• Surface chemistry in oil reservoirs

Crude oil fractions

Crude oil

Asphaltenes

Maltenes

Saturates Aromatics Resins

dilute with n-alkane

solution precipitate

adsorb on silica

elute with:

(2) toluene (3) toluene/methanol(1) alkane

no direct effect on wetting can adsorb -> alter wetting

Mechanisms of interaction

• Polar (water absent)

• Ionic (water present)

– acid/base

– ion binding

• Surface precipitation

Mechanisms of interaction

• Polar (water absent)

• Ionic (water present)

– acid/base

– ion binding

• Surface precipitation

Basic oil components

low pH high pH

N

R

H+++N

R

H

oil

Base

- - - - - - - - -glass / mica

water4

6

8

10

pH0.01

M

0.10

M1.0

M

2.0

M[NaCl]

Acidic oil components

low pH high pH

H+

+C

OO

R

-COHO

R

oil

Acid

- - - - - - - - -glass / mica

water4

6

8

10

pH0.01

M

0.10

M1.0

M

2.0

M[NaCl]

Examples of acid/base interactions

A-93 crude oil aged at 80°C

0 10 20 30 40 500

30

60

90

120

150

180

Aging time in oil (days)

Decane/w

ate

r conta

ct angle

(deg)

0.01

0.1

1.0

pH 4

pH 6

pH 8

NaCl (M)

Mechanisms of interaction

• Polar (water absent)

• Ionic (water present)

– acid/base

– ion binding

• Surface precipitation

Ion binding

H+

+C

OO

R

-COHO

R

oil

water

- - - - - - - - -Ca2+

Ca2+

Ca2+

glass / mica

Acid:

slow interactions

can be very strong

not very predictable

Mechanisms of interaction

• Polar (water absent)

• Ionic (water present)

– acid/base

– ion binding

• Surface precipitation

One asphaltene molecule?

SS

S

S

S

N

N

N

O

O

S

S

S

S

S

S

S

S

= O

N

VO

N

N

S

C: 81.0%

H: 8.0%

N: 1.4%

O: 1.0%

S: 7.3%

V: 0.8%

Mol. Wt.: 6191

H/C: 1.18

Strausz et al., 1991

Asphaltene aggregation

PRI

volume fraction of oil

RI oil

heptane

colloids

precipitate

macromolecules

Effects of asphaltenes on wetting

unstable

0 0.2 0.6 0.8 10

30

60

90

120

150

180

Oil Volume Fraction

Ad

van

cin

g C

on

tact

An

gle

(d

eg

)

Prudhoe Bay

(A-93)

plus n-heptane

0.4

Al-Maamari and Buckley, SPE 59292

pH 4

pH 8

More examples

180

0 0.2 0.4 0.6 0.80

30

60

90

120

150

Ad

van

cin

g a

ng

le (

deg

)

Oil volume fraction (f )

A-93

Lagrave

Mars-Pink

Mars-Yellow

Tensleep

v,oil

High contact angles at onset

Lagrave

180

0

30

60

90

120

150

Ad

van

cin

g a

ng

le (

deg

)

A-93

Mars-Pink

Mars-Yellow

Tensleep

-0.04 0 0.04 0.08 0.12RI = RI - P

RI∆∆

Summary

• Oil is found in the small pore spaces in rocks

where it coexists with water (and possibly a gas

phase).

• Capillary forces can hold oil in place.

• Magnitude of capillary forces depends on

– pore size

– IFT

– wettability (contact angles)

• Wettability is determined by surface chemistry