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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