reservoir mechanics
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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
Main Topics
Phase Relationships.
Interface Phenomena.
Surface Tension.
Interfacial Tension.
Surface Free energy.
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
Phase Relationships.
• 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.
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.
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.
Pressure-Temperature Diagram
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.
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.
Interfacial Phenomena
Interfacial Phenomena
Classification of Interfaces
Solid-solid interface, powder particles in contact.
ySSSolid - solid
Liquid-solid interface, suspensionyLSLiquid - solid
Liquid-liquid interface, emulsionyLLLiquid - liquid
Solid surface, table topySVGas - solid
Liquid surface, body of water exposed to atmosphere
уLVGas - liquid
No interface possible-Gas - gas
Types & Examples of InterfaceInterfacial Tension
Phase
Liquid InterfacesSurface 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.”
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
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..
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.
Liquid Interface
Surface Free energy
W = γ ∆ A
where W is work done or surface free energy increase
express in ergs(dyne.cm); γ is surface tension in
dynes/cm and ∆ 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
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.
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.
Liquid InterfacesSurface 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.
Liquid Interfaces
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
S (dynes/cm)Substance
Initial Spreading Coefficients, S, at 20◦C
• 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.
Wettability
• 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.
• 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.
• 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.
• 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.
Figure 2 Three possible states of wettability in oil reservoirs.
• 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.
• 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]
Figure 4 Microscopic fluid saturation distribution in a oil-wet rock [Pirson, 1963]
• 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.
• Figure 5 identifies the variables in Equation (1)
Figure 5 Relationship of oil-water-solid interfacial tensions and contact angle
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