KFUPM ENGINEERING

Department of Petroleum Engineering

PETE-525: Advanced Petrophysics

Wettability

By
Dr. Mohamed Mahmoud
241
Core analysis directed on reservoir properties—overview

2
 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. In the petroleum context, wettability is the tendency of
a reservoir rock surface to preferentially contact a particular
fluid in a multiphase or two-phase fluid system.
OilfieldWiki

3
• Wettability is the ability of
1. a liquid to maintain contact
2. with a solid surface
3. in the presence of another fluid; liquid or gas.
• Wettability is
– a three phase (W for water, O for Oil, & S for solid)
phenomena.
– controlled by the balance between the intermolecular
forces between
• adhesive (liquid to surface) and
• cohesive (liquid to fluid) forces.
4
Wettability

• The term “wetting” means liquid spreads or coats the solid

surface and “non-wetting” means the liquid balls up and runs
off the surface
• Wettability controls the distribution of the fluids in the
reservoir and therefore the oil recovered.

Rock Oil Brine

Water-wet Mixed-wet Oil-wet

Wettability types: oil displacement in water and oil-wet reservoirs
during waterflooding

6
 Cohesion and Adhesion
• The force of cohesion is defined as the force of attraction
between molecules of the same substance.

• The force of adhesion is defined as the force of attraction

between different substances, such as a solid and a liquid.

• Attractive forces between unlike molecules.

7
 Occurrence of Wettability
• Wettability is a result of intermolecular
interactions when the liquid(s) and solid are
brought together.

• The degree of wetting (wettability) is

determined by a force balance between
adhesive and cohesive forces.

8
 Importance of Wettability

Wettability affects reservoir performance

• Relative permeability

• Capillary pressure

• Electrical parameters

• Oil Recovery

9
 Concept of Wettability

Surface tension:
• The work required to increase the size of the
surface of a phase is referred to as the surface
tension.
• As a measure of work per unit area or force
per wetted length.
• The liquid surface is in contact with gas (air)
or its vapor.
• Surface tension has the unit N/m or
dynes/cm.
10
 Concept of Wettability

Interfacial tension:

• Interfacial tension is defined as the work expended

to increase the size of the interface between two
adjacent phases (liquids) which are immiscible.
• As a measure of work per unit area or force per
wetted length.
• Interfacial tension has the units N/m or dynes/cm.

11
Wettability

• Measurement of Wettability:
Direct Indirect
Contact angle Relative Perm.
USBM Capillary Press.
Amott index
 Wettability

Wettability:
(a)definition of the angle q
and interfacial tension
terms;

(a)water-wet rock (water-oil

system);

(c) oil-wet rock (water-oil

system).
16
Wettability

• Is strongly water-wet if the contact angle is from 0

to70°

• Has intermediate wettability if the contact angle is

between 70 and 110°

• Is strongly oil-wet if the contact angle is between 110

and 180°

17
 Measuring Methods of Surface and
Interfacial Tensions
Ring method: (Du Noüy)
The force acting on an optimally wettable ring as a result
of the tension of the withdrawn liquid lamella when
removing the ring is measured in this method.
Plate method: (Wilhelmy)
The force acting on an optimally wettable plate which is
immersed vertically in the liquid is measured in this
method.
Rod method
As the plate method, wherein a cylindrical rod with a
smaller wetted length is used for measurement with a
smaller liquid volume. 18
Ring and Plate Methods

19
 Measuring Methods of Surface and
Interfacial Tensions
Bubble pressure method:
The maximum internal pressure of a gas bubble which is
formed in a liquid by means of a capillary is measured.

Drop volume method:

The volume of a drop of liquid produced at a vertical
capillary is measured at the moment of its detachment.
This method is mainly used for measuring the interfacial
tension.

20
Bubble Pressure & Drop Volume
Tensiometers

21
 Measuring Methods of Surface and
Interfacial Tensions
Pendant drop method:
The shape of a drop suspended from a needle is
determined from the surface/interfacial tension and the
weight of the drop.
Can control pressure and temperature.

Spinning drop method:

The shape of a drop spinning in a denser phase is used to
determine the interfacial tension.
Can control temperature.

22
Pendant Drop Tensiometer

23
Spinning Drop Tensiometer

24
Contact Angle Measurement

25
 Young’s Equation
Wettability represents a balance of forces that occur at the
interface between three phases, one of which is a solid. The
equation describing this balance was first developed by
Young. For an oil, water, and solid system, the equation
would be:
 os −  ws +  ow cos(qC ) = 0
 ws −  os
cos(qC ) =
 ow
σos = interfacial energy between oil and solid
σws = interfacial energy between water and solid
σow = interfacial tension between oil and water
θC = contact angle
26
Young’s Equation

27
Contact Angle and Wettability

28
 Classification of Petroleum
Reservoirs Wettability
• Water wet
• Oil wet
• Neutral or Intermediate
• Fractional (heterogeneous wetting)
• Mixed wettability

29
 Classification of Petroleum
Reservoirs Wettability
• Water wet
Water preferentially wets the reservoir rock, when the
contact angle qo between the rock and water is less than
90o .

31
 Classification of Petroleum
Reservoirs Wettability
• Oil wet
Oil preferentially wets the reservoir rock, when
the contact angle is greater than 90o.

32
Classification of Petroleum Reservoirs
Wettability
• Water wet and Oil wet

33
Classification of Petroleum Reservoirs
Wettability
• Neutral or Intermediate
No preference is shown by the rock to either fluid; i.e.,
equally wet.
Neutral wettability case would exist at a contact angle of 90o.

34
 Fractional Wettability
(Heterogeneous Wetting)
• Portions of the rock are strongly oil wet, whereas other
portions are strongly water wet.
• Occurs due to variation in minerals with different surface
chemical properties.
• Silicate water interface is acidic, therefore basic constituents
in oils will readily be absorbed resulting in an water-wet
surface.
• In contrast, the carbonate water interface is basic and will
attract and absorb acid compounds.
• Since crude oils generally contain acidic polar compounds,
there is a tendency for silicate rocks to be neutral to water-
wet and carbonates to be neutral to oil-wet.

35
 Mixed Wettability

• Small pores occupied by water and are water-wet,

while larger pores are oil-wet and continuous.

• Subsequently, oil displacement occurs at very low

oil saturations resulting in unusually low residual
oil saturation.

36
Classification of Petroleum
Reservoirs Wettability

37
 Factors affecting reservoir
wettability
1. Rock matrix

Mixed-Wet Oil •
•

•
Distribution of minerals
Pore shape or surface
curvature
Pore surface roughness

Reservoirs 2. Formation water

•
•
•
Amount of Swir
Distribution of Swir
Salinity
• pH
3. Formation live crude oil
• Heavier asphaltene
• Lighter but polar compounds
4. Formation T and P

Oil wet neutral/intermediate wet water wet

-1 0 1
The majority of oil reservoirs are mixed-wet
With pore surfaces covered by Swir remain their original water wetness
While pore surfaces exposed to Soi may change to certain degrees of oil wetness
 Factors Affecting Wettability

• Rock mineralogy

• Oil composition
• Brine composition
• Temperature
39
 Factors Affecting Wettability
• Rock mineralogy
- Sandstone: Quartz, Silica: (SiO2)
- Limestone: Calcite (CaCO3)
- Dolomite: Calcite and Magnesium
[CaMg(CO3)2]
- Clays
- Anhydrite
- Other minerals

40
 Factors Affecting Wettability

Oil composition
• Asphaltenes and resins may be present in crude oil.
• They are made up of relatively high molecular
weight, polar, polycyclic, aromatic ring compounds.
• Asphaltenes do not dissolve in crude oil but exist as
a colloidal suspension.
• Resins, on the other hand, are readily soluble in oil.

41
 Factors Affecting Wettability

• Oil composition
– Silica (sandstone) and calcite (limestone) are
naturally water-wet.
– The adsorption of asphaltene particles on
the rock surface alter the wettability from
water-wet to oil-wet.

42
 Factors Affecting Wettability

• Brine composition
Potential determining ions like Ca2+ , Mg2+ and
SO42− have influence on the surface charge of the
carbonate rock and are thereby linked to its wetting
properties.

Add Fe to these ions.

43
 Factors Affecting Wettability

• Temperature
– The interfacial tension and force of adhesion is a
function of temperature.

– As a result the wettability changes with temperature.

– As the temperature increases, calcite surface becomes

more water-wet.

44
 Summary

Reservoir rock wettability

• For conventional reservoirs formed by oil
migration, reservoirs were originally water wet
• This originally water wet reservoir may become
mixed wet during geological time of crude oil
migration and aging, resulting in
– higher w/f oil recovery comparing to water wet due
to film flow
– different log responses in the near wellbore flush
zone and flooded intervals
 Methods to Determine Wettability
• Contact Angle
– Contact angle is one of the oldest and most common
techniques to measure wettability.
– The device used is called contact angle goniometer.
The surface and fluids used to measure the angle can
be placed in a cell to control the temperature and
pressure.
– Live oil can be used with this technique.

46
Methods to Determine Wettability
• Contact Angle

47
Methods to Determine Wettability
• Contact Angle
Factors that must be taken into account measuring contact
angles:
- Surface roughness
• Usually polished surfaces such as quartz and calcite
are used.
• These crystals may not be representative of grain
surfaces in porous media.
• It has also been observed that contact angle is
affected by the surface roughness which requires
extra care when using polished reservoir rocks.

48
Methods to Determine Wettability
• Contact Angle
Effect of surface roughness on contact angle.

49
 Methods to Determine Wettability

• Contact Angle
- History of which fluid first contacts the surface will affect
the measured value of the contact angle.
- Advancing contact angle: when water comes into
equilibrium with a surface previously in contact with oil.
- Receding contact angle: when oil comes into equilibrium
with a surface previously in contact with water.

50
Methods to Determine Wettability
Advancing and Receding Contact Angles

51
Methods to Determine Wettability
Contact Angle
- Time to reach equilibrium (when the contact
angle is independent of time) may vary from
seconds to days or years.

- Consequently, the contact angle measured in

the laboratory may not represent the natural
wettability of the system under examination.

52
 Surface Wettability- Contact Angle Based Classification

Contact Angle Based Example: Contact angle tests for a

drop of water on a smooth mica
Wettability Definitions surface in bulk solutions of oil +
are kind of arbitrary surfactant

Wettability Classification Range of Static Contact Angle at Equilibrium []

Strongly Water-Wet 0 – 30

Water-Wet 30 – 60

Weakly Water-Wet 60 – 80

Neutrally Wet 80 – 100

Weakly Oil-Wet 100 – 120

Oil-Wet 120 – 150

Strongly Oil-Wet 150 – 180

https://pubs.acs.org/doi/abs/10.1021/acs.energyfuels.0c01335
 Surface Wettability Measurement- Summary

• Contact angle is a nice concept to understand/explain wettability

• But it is difficult to measure for rocks due to the requirements of
o Surfaces being flat and
o Surfaces being smooth
• And it does not show effect of Swir, and it is a measurement of
current (t=0+) rock surface properties.
Also note that contact angle displays hysteresis behaviors, i.e., how
the measurement is conducted
o receding angle < advancing angle
Thus, when using contact angle in petrophysical and reservoir
engineering calculations, be cautious!
 Methods to Determine Wettability

• Amott
Amott wettability index is obtained by measuring the volumes of fluids
produced from a core sample during spontaneous imbibition and forced
displacement cycles.

• Saturate the core with oil at irreducible water (brine) saturation and age to
restore the wettability if needed.

• After aging, immerse the core in brine and measure the volume of oil
produced by spontaneous imbibition of brine (Vo1).

55
 Methods to Determine Wettability

• Centrifuge the core under brine and measure the additional volume of oil
displaced (Vo2). Forced displacement by injection of brine may be used instead
of a centrifuge. Also oil-wet membrane can be used to displace

• After centrifuging, immerse the core in oil and measure the volume of brine
produced by spontaneous imbibition of oil (Vw1).

• Centrifuge the core under oil and measure the additional volume of brine
displaced (Vw2). Forced displacement by injection of oil may be used instead of
a centrifuge. Also water-wet porous plate can be used to displace the brine.

56
Methods to Determine Wettability

• Amott

57
Methods to Determine Wettability

• Amott
The wettability indices are calculated as follows:
Wettability index of water
WIw = Vo1/(Vo1+Vo2)
Wettability index of oil
WIo = Vw1/(Vw1+Vw2)
The above two indices range from 0 to 1.
WIAmott-Harvey = WIw - WIo
WIAmot ranges between -1 to 1.
An index of -1 indicates strongly oil-wet system.
An index of +1 indicates strongly water-wet system.
An index close to zero indicates neutral or intermediate wettability.

58
Methods to Determine Wettability
• USBM (United States Bureau of Mines)
The USBM wettability index is obtained by obtaining two capillary pressure
curves using the centrifuge or porous plate and membranes.

• Saturate the core with oil at irreducible water (brine) saturation and age to
restore the wettability if needed.

• Obtain the capillary pressure curve by displacing oil by brine at several

capillary pressure values and measure the brine saturation from the volume
of oil produced until residual oil is reached.

62
Methods to Determine Wettability
• Obtain the capillary pressure curve by displacing brine by oil at several
capillary pressure values and measure the brine saturation from the volume
of brine produced.

• Calculate the area under the oil displacing brine cycle (A1) and under the
brine displacing oil cycle (A2).

• Calculate the USBM wettability index (Iw).

Iw= log10 (A1/A2)

63
Methods to Determine Wettability
• USBM (United States Bureau of Mines)

64
Water Wet System
Oil Wet System
 Summary: Methods of Wettability Measurements
Contact Angle Amott USBM
Advantages Disadvantage Advantages Disadvantage Advantages Disadvantage
s & Concerns s & Concerns s & Concerns
Easy to understand Requires a smooth, Suitable for rock End points only; i.e., Suitable for rock Spontaneous
flat, and wettability systems reach the wettability imbibition is not
homogenous solid measurement same recovery are measurement considered, a
surface characterized as concerns for strongly
having the same wetted systems
wettability,
regardless how
fast/slow to reach
the end points
Easier to restore for Difficult to relate to More sensitive to Difficult to reach Good for weakly Average saturation is
repeat tests rock surface strongly wetted recovery plateau, wetted systems that usually used for the
wettability systems that especially for spontaneous calculation of the
spontaneous spontaneous imbibition is weak area under the Pc
imbibition dominate imbibition curve
Useful for Difficult to test at Measurement Fast Is a qualitative
systematical studies, reservoir conditions uncertainty of indicator, can not
such as oils, brines, (fluids, T, and P) produced fluids by used quantitatively
solids, aging, etc spontaneous
imbibition
Usable quantitatively Is a qualitative
indicator, can not
used quantitatively
Best practice: Combined Amott-USBM Method
 Summary: Rock Wettability

A fixed time (72 hrs) for

spontaneous
imbibition may not sufficient
for:
– weakly wetted rocks
– low permeability rocks
– viscous oils
– systems with low IFTs, and
– samples with small open surface/volume Zhou et al., SPEJ, June 2000
https://www.onepetro.org/conf
erence-paper/SPE-35436-MS
The best practice is to wait until production by
spontaneous imbibition reaches a plateau.

Muthana and Ma, PEDMD 007/2018, 1 March 2018

Methods to Determine Wettability
• Scanning Electron Microscope (SEM)

76
 Methods to Determine Wettability
• Scanning Electron Microscope (SEM)

77
Methods to Determine Wettability
Effect of pore wall mineralogy on contact angle

78
Methods to Determine Wettability
Effect of pore wall mineralogy on contact angle

79
 Methods to Determine Wettability
• NMR
Changes in longitudinal relaxation time.
The distribution of grains at water/oil or air/water interfaces.

80
 NMR Wettability
• Freedman et al. presented how NMR T2 measurements could
evaluate wettability qualitatively.
• NMR measurements on oil/water saturated pore system have
sensitivity to wettability due to the enhancement of relaxation rate
(shorter T2 time) when wetting fluid contacts pore surfaces.
• The dominant relaxation mechanism for the wetting phase is the
surface relaxation while the non-wetting fluid is not significantly
influenced by surface relaxation since it does not coat or contact
pore surfaces
• In this case, the non-wetting fluid inside the pores shows bulk and
diffusion relaxations only, and consequently it tends to behave like
bulk fluid
81
 NMR Wettability
• Looyestijn et al. introduced a quantitative wettability index
from NMR.
• When compared with USBM, the proposed NMR approach
could provide reasonable prediction in carbonates of relatively
low permeability (few mD) when using 20 cp oil viscosity.
• Nevertheless, the accuracy of the approach decreases with
increasing oil viscosity and increasing pore-sizes .
• Additionally, such approach requires a pore-size dependent
fluid saturation distribution, which can be challenging to obtain
in rocks with complex pore geometry

82
 NMR Wettability
• Al-Mahrooqi et al. proposed a simple pore-scale model to
evaluate wettability based on T2 measurements.
• The model consists of a bundle of capillary tubes with a
triangular cross section, and it was used to investigate the
relationship between wettability and NMR relaxation times.
• Based on the experimental and modelling results, the authors
observed that T2 values at residual and irreducible saturation are
sensitive to the same amounts used to compute the Amott–
Harvey index.
• The model was tested against known wettability synthetic and
real sandstone samples characterized by various wettability

83
 NMR Wettability

• Branco et al. used Al-Mahrooqi model to calculate wettability

index for carbonate rocks. Results obtained from Al-Mahrooqi
index did not agree well with that obtained from Amott-Harvey.
• This was attributed to the complex pore geometry of carbonate
rocks where Al-Mahrooqi model does not seem to work well.
• T1/T2 ratio has also been used to evaluate wettability.
• However, are limited to the laboratory since they are time
consuming and require separation of oil and water signals which
is impractical for field application.

84
 NMR Wettability

Sample Density (g/cc) Viscosity (cp)

Brine 1.052 1
Oil 1 0.889 41
Oil 2 0.891 43

85
 NMR Wettability

T2 distributions of Berea Sandstone 100% brine saturated (Sw), after primary drainage (Swi)
and after forced imbibition (Sor) using oil 1(a) and oil 2 (b). The black and red dotted lines
reflect the modal T2 times for bulk phase brine and oil, respectively

86
 NMR Wettability

T2 distributions of Indiana limestone 100% brine saturated (Sw), after primary drainage (Swi)
and after forced imbibition (Sor) using oil 1(a) and oil 2 (b). The black and red dotted lines
reflect the modal T2 times for bulk phase brine and oil, respectively

87
 NMR Wettability

Sample T2,swi (s) T2,sor (s) Iw Io Inmr IAmott-Harvey

1S 0.07 0.15 0.95 0.17 0.78 0.79

2S 0.07 0.15 0.95 0.17 0.78 0.79
1H 0.06 0.93 0.67 0.31 0.36 0.32
2H 0.04 1.12 0.6 0.52 0.08 0.02

88
 NMR Wettability

T2w,Sor = 0.149 s (LWD in the invaded zone)

T2,Bw = 2.78 s (benchtop) ,T2,Bo = 0.086 s (benchtop) T2o,Swi = 0.072 s (Logging or Geosteering in the Uninvaded zone)

89
 NMR Wettability
Relying on the non-wetting fluid response

90
 Dielectric Wettability
• In porous medium, 2 types of electrochemical
interactions:
1. Fluid-Fluid Interactions (between 2 immiscible fluids).
2. Fluid-Solid Interaction (between the rocks grains and the
fluids forming the Electrical Double Layer (EDL)).
(after Garcia and Heidari, 2018)
• EDL controls the wettability condition and can be
evaluated using zeta potential.

(after Fatehah et al., 2014)

(after Toumelin et al., 2008)

91
• Dielectric Polarization (Dielectric Theory)

Conduction Current Displacement Current

Inversely proportional to Proportional to the permittivity
the resistivity of the (capacitance) and the rate of applying
object the electrical field (frequency)

5
•
Introduction
Dielectric Polarization
• The electrical field is applied with varying
frequency (time delay) which permits the
material under investigation to polarize and
reflect the stored energy.
• The reflected signal will have a reduction in the
amplitude and phase shift.

7
8
 (after Garcia and Heidari, 2018)

21-Dec-2023 | MS Thesis Dissertation (g202101670) 9

• Dielectric Measurements at Different Water Saturation (Berea)

35 200
BS-1 @ Sw=1 BS-1 @ Sw=1

Imaginary Permittivity (Dimensionless)

Real Permittivity (Dimensionless)

30 BS-2 @ Sw=1 BS-2 @ Sw=1

25 150 BS-1 @ Swirr1
BS-1 @ Swirr1
20 BS-2 @ Swirr1 BS-2 @ Swirr1
BS-1 @ Swirr2 100 BS-1 @ Swirr2
15
10 50
5
0 0
1.E+07 1.E+08
Frequency (Hz)
1.E+09 1.E+07 1.E+08
Frequency (Hz)
1.E+09
8 0.40 BS-1 @ Sw=1
BS-1 @ Sw=1
Loss Tangent (Dimensionless)

BS-2 @ Sw=1 BS-2 @ Sw=1

6 0.30 BS-1 @ Swirr1
BS-1 @ Swirr1

Conductivity (S/m)
BS-2 @ Swirr1 BS-2 @ Swirr1
4 BS-1 @ Swirr2 0.20 BS-1 @ Swirr2

2 0.10

0 0.00
1.E+07 1.E+08 (Hz)
Frequency 1.E+09 1.E+07 1.E+08 (Hz)
Frequency 1.E+09
• The Relationship between the Dielectric and Wettability
Index

1
USBM and NMR Wettability

0.5
Indices

0
0 50 100 150 200

-0.5 USBM
y = 6.4813E-03x - 3.5537E-01
NMR R² = 0.8694
-1 The difference in Imaginary Permittivity
between Sw=1 and Swirr
5
2
Dielectric Wettability Index
 1 1
BS-1
0.8

Dielectric Wettability Index

Sample
0.6 IL-1

(Dimensionless)
0.4
(%) (%) Dimensionless Dimensionless Dimensionless Dimensionless IL-2 BS-2
0.2
18.82%
BS-1 23.06 151.630 7.553 144.077 0.73 y=x 0 FB-1
BS-2 25.74 7.82% 76.845 4.031 72.814 0.16 R² -1
= 0.9736 -0.5 FB-2
-0.2 0 0.5 1
FB-1 13.94 6.63% 70.708 1.823 68.885 0.16
FB-2 13.42 2.31% 32.734 2.121 30.613 -0.17 -0.4
IL-1 34.27 14.58% 163.458 4.969 158.489 0.63 -0.6
IL-2 30.35 11.92% 111.968 3.294 108.674 0.14
-0.8
-1 -1
USBM Wettability Index
(Dimensionless)
• The Relationship between the Dielectric and Wettability
Differences

0.8
• The imaginary permittivity and other

The Difference in
Berea
measurements relying on it (loss tangent and 0.6 Indiana

USBM
conductivity) were found to be the best 0.4 FB
y = 2.250E-03x + 2.431E-01
correlating with the wettability changes.
R² = 0.9619
0.2

• A linear relationship was observed between the 0

difference in the USBM wettability index (Master 0 50
The difference in100 150
Imag. Permittivity…200
vs Treated) and the following: 0.8

The Difference in
1. The drop in the imaginary permittivity due to the Berea
0.6 Indiana
wettability alteration @ 10 MHz.

USBM
0.4 FB
2. The drop in the conductivity due to the
wettability alteration @ 10 MHz. y = 3.663x + 2.431E-01
0.2 R² = 0.9619

0
0 0.02difference
The 0.04 in Conductivity
0.06 0.08 … 0.1

4
9
 Methods to Determine Wettability
• Relative Permeability Curves
• The end point krwe at Sor and the water saturation at the intersection
point of kro and krw curves.
• krwe at Sor less than 0.5 indicates water-wet and greater than 0.5
indicates oil-wet.
• Water saturation at the intersection point greater than 0.5 indicates
water-wet and value less than 0.5 indicated oil-wet.

102
Methods to Determine Wettability
•Relative Permeability Data

103
Hydrogen Wettability

104
Hydrogen Wettability

Water contact angles in

hydrogen atmosphere on
various rock and
mineral systems as a
function of pressure at
different temperatures
(a: 293-308K, b: 323K,
c: 343-353K).

105
Hydrogen Wettability

106
Hydrogen Wettability

107
Hydrogen Wettability

108
Hydrogen Wettability

109
Hydrogen Wettability

110
Hydrogen Wettability

111
Hydrogen Wettability

112
Hydrogen Wettability

113
Effect of Rock/Fluid Interactions on
Wettability and Recovery

114
 Wettability Alteration

• Chemicals

• Adjust brine composition and salinity

• CO2

115
 Effect of Rock/Fluid Interactions on
Wettability and Recovery
• The Double Layer refers to two parallel layers of
charge surrounding the object. The first layer, either
positive or negative, comprises ions adsorbed onto the
object due to chemical interactions.
• The second layer is loosely associated with the object.
It is made of free ions that move in the fluid under the
influence of electrical attraction or thermal motion
rather than being firmly anchored. It is thus called the
"diffuse layer".

116
Effect of Rock/Fluid Interactions on
Wettability and Recovery

117
Effect of Rock/Fluid Interactions on
Wettability and Recovery

118
 Effect of Rock/Fluid Interactions on
Wettability and Recovery

• The zeta potential is the electric potential in the

interfacial double layer (DL) at the location of
the slipping plane relative to a point in the bulk fluid
away from the interface.

In other words, zeta potential is the potential

difference between the dispersion medium and the
stationary layer of fluid attached to the dispersed
particle.
 Effect of Rock/Fluid Interactions on
Wettability and Recovery
The zeta potential is a key indicator of the stability of colloidal dispersions. The
magnitude of the zeta potential indicates the degree of electrostatic repulsion
between adjacent, similarly charged particles in a dispersion. For molecules and
particles that are small enough, a high zeta potential will confer stability, i.e., the
solution or dispersion will resist aggregation. When the potential is small, attractive
forces may exceed this repulsion and the dispersion may break and flocculate. So,
colloids with high zeta potential (negative or positive) are electrically stabilized
while colloids with low zeta potentials tend to coagulate or flocculate
 Effect of Rock/Fluid Interactions on
Wettability and Recovery
• The total acid number (TAN) is a measurement of acidity that
is determined by the amount of potassium hydroxide in
milligrams needed to neutralize the acids in one gram of oil.
• Acid crude oil means that the acid number is > 0.5 mg KOH/g,
and high TAN crude means that the acid number is > 1.0 mg
KOH/g.
• The crude oil acidity is cause by the presence of Sulphur and
carboxylic and naphthenic acids.
• Oil acid and base numbers influence wetting through their
effect on electrostatic interactions with the mineral surface.

121
 Effect of Rock/Fluid Interactions on
Wettability and Recovery

• The pH of a solution is a measure of the molar concentration

of hydrogen ions in the solution and as such is a measure of
the acidity or basicity of the solution.

• In water we find an actual range of pH from about 0 to 14. It

is convenient to classify aqueous solutions according to their
pH. If the pH of a solution is less than 7, the solution is
called acidic; if the pH is about 7, the solution is neutral; if the
pH is greater than 7, the solution is called basic.

122
 Effect of Rock/Fluid Interactions on
Wettability and Recovery

• In an acidic solution, the concentration of

hydrogen ions is greater than the
concentration of hydroxide ions.
• In a neutral solution, the concentrations of
hydrogen ions and hydroxide ions are roughly
equal.
• In a basic solution, the concentration of
hydroxide ions is greater than the
concentration of hydrogen ions.
123
 Effect of Rock/Fluid Interactions on
Wettability and Recovery
The sand SiO2 molecule can react with hot water, and
water containing salts. To form silanol groups, which
are Bronsted acids, weak acids capable of freeing a
proton:
SiO2 +2H2O Si(OH)2
Si – OH SiO- + H+
In general:
- Silicate minerals have acidic surfaces
- Repel acidic fluids such as major polar organic
compounds present in some crude oils
- Attract basic compounds
- Neutral to water-wet surfaces Tiab and Donaldson, 2015
124
 Effect of Rock/Fluid Interactions on
Wettability and Recovery

The surface of carbonate rocks are basic and

consequently they react with the acid compounds in
crude oils and exhibit neutral to oil wet characteristics.

• Carbonate minerals have basic surfaces

• Attract acidic compounds of crude oils
• Neutral to oil-wet surfaces

Tiab and Donaldson, 2015

125
 Effect of Rock/Fluid Interactions on
Wettability and Recovery
• The limestone and dolomite are also more influenced
by both pH and the presence of multivalent cations.

• The magnitude of the surface charges varied

considerably and could adopt positive or negative
surface charges depending on the solution properties.

Tiab and Donaldson, 2015

126
 Effect of Rock/Fluid Interactions on
Wettability and Recovery
• Limestone and dolomite rocks have both negative and positive surface
charges because of random exposure of oxygen (with negative orbital
clouds) and calcium/magnesium atoms with positive charges.

• If the surface is in contact with water, hydronium ions are attached to the
negative charges and the hydroxyl ion to the positive metallic ions
producing a water-wet surface.

• When an acetic asphaltene compound arrives at the site, its chemical

potential for coulombic reaction is greater than the associated water
molecules and displacement takes place.

• The van der Waals and electrostatic forces are both negative, therefore the
approach of a basic polar compound to the surface is increasingly
attractive without an energy barrier to overcome for a reaction to take
place.
127
 Effect of Rock/Fluid Interactions on
Wettability and Recovery
• The Lewis acid/base interactions, which are pH dependent, are the
principle reactions occurring between a water film and the oil-water- rock
interfaces.

• As long as a water film remains between the oil and rock, the rock surface
will remain water-wet, but if the film collapses (because of diffusion of
polar compounds from the oil-water interface through the water film to the
rock surface, or the influence of DLVO forces) polar compounds can
adhere to the rock by physical adsorption and/or coulombic reactions
leading to a change of wettability from water-wet to oil-wet.

• The presence of multivalent cations in the water film can promote oil-
wetting by partial reaction with polar compounds at the oil-water interface.

• This reaction and diffusion is time dependent and thus responsible for the
gradual rate of attaining a stable wetting condition after a rock sample is
saturated with water and oil (aging time). 128
Effect of Rock/Fluid Interactions
on Wettability and Recovery

129
Effect of Rock/Fluid Interactions
on Wettability and Recovery

From130
Oilfield Review
Effect of Rock/Fluid Interactions
on Wettability and Recovery

131
Donaldson and Alam, 2008
Effect of Wettability on Relative Permeability

Packings of powdered dolomite as the porous medium. Water and

refined oil was used as displacing fluids. Octanoic acid in the oil was
used to control wettability.
Fig. 38: Al_Sayari-SS-2009-PhD-Thesis From
132 Morrow, SPE Journal, 1973
 Effect of Wettability on Oil Recovery

• It has long been known that wettability is a primary

determinant of waterflood recovery efficiency.
• Strongly oil-wet reservoirs give the least oil recovery and the
best recovery appears to be the mixed-wet reservoirs,
particularly where surface film drainage mechanism is also
observed.
• While most researchers are in agreement on the least
waterflood oil recovery for oil-wet reservoirs, there is a lack of
consensus as to the wetting condition for maximum oil
recovery.
• The only consensus seems to be that the best oil recovery is
achieved when the reservoir is at some intermediate-wetting
state – not strongly oil-wet and not strongly water-wet.

133
Effect of Wettability on Residual Oil Saturation

134
IMPLICATIONS OF WETTABILITY
 Wettability Alteration

• Chemicals
- Surfactants
- Caustics
- Nanoparticles
• Adjust brine composition and
salinity
• CO2
136
 Effect of Wettability Alteration by Nanoparticles

Lipophobic and hydrophilic polysilicon nanoparticles (LHP) can be

adsorbed on pore walls and can give rise to blockage of pore throats,
leading to change in wettability of sand rock surfaces. The
mechanism for enhanced oil recovery consists of change of
wettability of reservoir rock from hydrophobic to hydrophilic under
the influence of adsorbed lipophobic and hydrophilic polysilicon.

From Int Nano Lett, Nov. 2015

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