This document summarizes key concepts related to reservoir phase behavior and interfacial phenomena. It includes: - A typical pressure-temperature diagram showing the critical point, bubble point curve, and dew point curve used to classify reservoirs as oil or gas based on temperature. - Definitions of surface tension, interfacial tension, and surface free energy as forces that exist at boundaries between phases. - Explanations of liquid, solid, and liquid-liquid interfaces with examples. Wettability is also introduced as the preferential wetting of solids by liquids. - Figures illustrating fluid distributions and interfacial energies in water-wet and oil-wet systems. Young's equation relates

1. Suez University
Faculty of Petroleum & Mining Engineering
Reservoir Mechanics
Student
Belal Farouk El-saied Ibrahim
Class / III
Section / Engineering Geology and Geophysics
The Reference / Geology of Petroleum
(A.J.Leversen)
Presented to
Prof. Dr. / Shouhdi E. Shalaby

2. Main Topics
 Phase Relationships.
 Interface Phenomena.
Surface Tension.
Interfacial Tension.
Surface Free energy.

3. Phase Relationships.
Pressure-Temperature Diagram
Figure 1 shows a typical pressure-temperature diagram of a
multicomponent system with a specific overall composition.
Although a different hydrocarbon system would have a
different phase diagram, the general configuration is similar.
These multicomponent pressure-temperature diagrams are
essentially
used to:
• Classify reservoirs
• Classify the naturally occurring hydrocarbon systems
• Describe the phase behavior of the reservoir fluid

5. Pressure-Temperature Diagram
• Critical point—The critical point for a
multicomponent mixture is referred to as
the state of pressure and temperature at
which all intensive properties of the gas
and liquid phases are equal (point C). At
the critical point, the corresponding
pressure and temperature are called the
critical pressure pc and critical temperature
Tc of the mixture.

6. Pressure-Temperature Diagram
• Bubble-point curve—The bubble-point
curve (line BC) is defined as the line
separating the liquid-phase region from the
two-phase region.
• Dew-point curve—The dew-point curve
(line AC) is defined as the line separating the
vapor-phase region from the two-phase
region.

7. Pressure-Temperature Diagram
• Oil reservoirs—If the reservoir temperature
T is less than the critical temperature Tc of
the reservoir fluid, the reservoir is classified
as an oil reservoir.
• Gas reservoirs—If the reservoir
temperature is greater than the critical
temperature of the hydrocarbon fluid, the
reservoir is considered a gas reservoir.

8. Interfacial Phenomena
 When phases exist together, the
boundary between two of them is
termed an interface.
 The properties of the molecules
forming the interface are often
sufficiently from those in the bulk of
each phase that they are referred to
as forming an interfacial phase.

9. Interfacial Phenomena
Several types of interface can exist, depending
on whether the two adjacent phases are in the
solid, liquid or gaseous state.
For convenience, we shall divide these various
combinations into two groups, namely liquid
interfaces and solid interfaces.

10. Interfacial Phenomena
Classification of Interfaces
Interfacial Types & Examples of Interface
Tension
Phase
- No interface possible Gas - gas
Gas - solid ySV Solid surface, table top
Liquid - liquid yLL Liquid-liquid interface, emulsion
Liquid - solid yLS Liquid-solid interface, suspension
Solid-solid interface, powder particles
in contact.
Solid - solid ySS
Liquid surface, body of water exposed
to atmosphere
Gas - liquid уLV

11. Liquid Interfaces
Surface and Interfacial Tension
Surface
 The term surface is customarily used when
referring to either a gas-solid or a gas-liquid
interface.
 “Every surface is an interface.”

12. Liquid Interfaces
 Surface tension-a
force pulling
the molecules of
the interface
together resulting
in a contracted
surface.
- Force per unit
area applied
parallel to the
surface.Unit in
dynes/cm or N/m

13. Liquid Interfaces
Interfacial
tension
 Is the force per
unit length
existing at the
interface
between two
immiscible liquid
phases and like
surface tension,
has the units of
dyne/cm..

14. Liquid Interfaces
 Surface Free
energy – increase
in energy of the
liquid and the
surface of the
liquid increase.
-work must be done
to increase liquid
surface.
γ – surface tension or
surface free energy
per unit surface.

15. Liquid Interface
 Surface Free energy
W = γ Δ A
where W is work done or surface free energy increase
express in ergs(dyne.cm); γ is surface tension in
dynes/cm and Δ A is increase in area in cm sq.
What in the work required to increase area of a liquid
droplet by 10 cm sq if the surface tension is 49
dynes/cm?
W = 49 dynes/cm x 10 cm sq = 490 ergs

16. Liquid Interfaces
 When oleic acid is
placed on the
surface of a water ,
a film will be
formed if the force
of adhesion b/n
oleic accid
molecules and
water molecules is
greater than the
cohesive forces b/n
the oleic acid
molecules
themselves.

17. Liquid Interfaces
 Work of adhesion(Wa), which is the energy
required to break the attraction between the unlike
molecules.(water to oil)
 Work of cohesion(Wc), required to separate the
molecules of the spreading liquid so that it can flow
over the sublayer.(oil to oil and water to water)
Spreading of oil to water occurs if the work of adhesion
is greater than the work of cohesion.
Spreading coefficient(S) – difference between Wa
and Wc.
Positive S – if oil spreads over a water surface.

18. Liquid Interfaces
Surface and Interfacial Tension
 When a drop of oil is added on the surface of
water, three things may happen:
1. The drop may spread as a thin film on the
surface of water.(positve S)
2. It may form a liquid lens if the oil cannot
spread on the surface of water.(negative S)
3. The drop may spread as a monolayer film
with areas that are identified as lenses.

19. Liquid Interfaces
Substance S (dynes/cm)
50.4
45.8
45.5
45.2
42.4
32 (250)
24.6
13
8.9
3.4
0.22
-3.19
-13.4
Ethyl alcohol
Propionic acid
Ethyl ether
Acetic acid
Acetone
Undecyclenic acid
Oleic acid
Chloroform
Benzene
Hexane
Octane
Ethylene dibromide
Liquid petrolatum
Initial Spreading Coefficients, S, at 20◦ C

20. Wettability
• Water and oil (or gas) in reservoirs coexist in an
immiscible state (i.e., the water phase does not
mix miscibly with the hydrocarbon phase). There
is a natural and strong interfacial tension between
the two fluids that keeps them separate, regardless
of how small the individual droplets may be. A
common example of this immiscible nature is a
household salad dressing made of oil and vinegar.

21. • In all reservoirs connate water is immiscible with
the oil or gas, but chemicals can be injected into
the reservoir to reduce interfacial tension and make
the water phase miscible with the oil. There are
advantages in doing this, and it is a form of
enhanced oil recovery.
• The oil and gas phases in reservoirs also generally
behave immiscibly. However, at certain pressures,
temperatures, and compositions, they may become
miscible.

22. • Wettability can be defined as the ability of a fluid
phase to preferentially wet a solid surface in the
presence of a second immiscible phase. In the
reservoir context, it refers to the state of the rock
and fluid system; i.e., whether the reservoir is
water or oil wet. Three possible states of
wettability in oil reservoirs exist as shown in
Figure 2. The arrows represent the tangent to the
angle between the water droplet and the rock
surface. The water droplet is surrounded by the oil
phase.

23. • Wettability is generally classified into three
categories: (1) The reservoir is said to be
water wet; that is, water preferentially wets
the reservoir rock, when the contact angle 
between the rock and water is less than 90,
(2) neutral wettability case would exist at a
contact angle of 90, and (3) oil wet occurs
at a contact angle greater than 90.

24. Figure 2 Three possible states of wettability in oil reservoirs.
• Other lesser known types of wettability are:
• Neutral or intermediate wettability – no
preference is shown by the rock to either fluid;
i.e., equally wet.

25. • Fractional wettability – heterogeneous wetting; i.e.,
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 oil-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.
• Mixed wettability – refers to 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.

26. • Figures 3and 4 represent microscopic views of
water-wet and oil-wet systems, respectively.
Figure 3 Microscopic fluid saturation distribution in a water-wet rock [Pirson, 1963]

27. Figure 4 Microscopic fluid saturation distribution in a oil-wet rock [Pirson, 1963]

28. • The contact angle is a measure of the wettability of the
rock-fluid system, and is related to the interfacial
energies by Young’s equation,
• os - ws = ow cos (1)
• where:
• os = interfacial energy between oil and solid, dyne/cm;
• ws = interfacial energy between water and solid,
dyne/cm;
• ow = interfacial energy, or interfacial tension, between
oil and water, dyne/cm;
• contact angle at oil-water-solid interface measured
through the water phase,
• deg.

29. • Figure 5 identifies the variables in Equation (1)
Figure 5 Relationship of oil-water-solid interfacial tensions and contact angle