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Properties of Reservoir FluidsPhase Behavior- Single Component System
Fall 2010 Shahab Gerami 1
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Thermodynamic studies are generally focused on arbitrarily chosen systems
while the rest of the universe is assumed as surroundings.
System Boundary: the surface of the system - real or imaginary - is called a
boundary.
A system is a region of space or quantity of matter we want to study.
System & Surrounding
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A system is called a closed system if it does not exchange matter with thesurroundings, in opposite to an open system which exchanges matter with
the surroundings. Both systems may exchange energy with the surroundings.
Open system (or control volume)Closed system (or control mass)
Closed & Open Systems
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Closed System with Moving Boundary
Open System (Control Volume)Open System (Control Volume)
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The concept of a closed system is of major interest in applied hydrocarbon
thermodynamics.
It is called a homogeneous closed system if it contains a single phase,
e.g. a natural gas phase or an oil phase.
A heterogeneous closed system contains more than one phase.
Phase: a phase is defined as a physically homogeneous portion of matter.
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Phase
The phases of a heterogeneous system are separated by interfaces and are
optically distinguishable.
It is not obligatory that a phase is chemically homogeneous. It may consists of
several compounds, e.g. of a large number of various hydrocarbons.
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Property
A property is a characteristic of a system to which numerical values can be
assigned to describe the system (Mass, Temperature, Pressure ,Density,...)
Property:
1. Extensive Property: Extensive properties are properties which can be
counted and their value for the whole system is the sum of the value for
subdivisions of the system.
They depend on the extent of the system.
Examples: Volume, Mass
2. Intensive Property: Intensive properties are independent of the size (mass or
volume) of the system. Examples: Density, Temperature
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Property
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The state of a system is defined by the values of its properties.
A system is in equilibrium if its properties are not changing at any givenlocation in the system.
This is also known as thermodynamic equilibrium or total equilibrium.
Equilibrium implies balance--no unbalanced potentials (driving forces) in the
system.
We will distinguish fourdifferent types of equilibrium
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State
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State and Equilibrium
Thermodynamics deals with equilibrium states
A system is in thermodynamic equilibrium if it maintains thermal, mechanical,
phase, and chemical equilibrium.
1. Thermal equilibrium -- the temperature does not change with time
2. Mechanical equilibrium -- Pressure does not change with time3. Chemical equilibrium -- molecular structure does not change with time4. Phase equilibrium mass and composition of each phase is unchanging
with time (i.e., same liquid/gas or liquid/solid composition)
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State functions or state variables are those properties for which the
change in state only depends on the respective initial and final state.
It is this path-independent characteristic of the state functions that makes it
possible to quantify any change of a system.
1
ath 1
ath
ath
State Functions
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Equilibrium has been defined as a state of rest.
In an equilibrium state, no further change or - more precisely - no net-flux will
take place unless one or more properties of the system are altered.
On the other side, a system changes until it reaches its equilibrium state.
Any change of a system is called a thermodynamic interest in thethermodynamic study of the system:
adiabatic (no heat added to or removed from the system),
isothermal (constant temperature),
isobaric (constant pressure),
isochoric (constant volume).
A process is called reversible if it proceeds through a series of equilibrium
states in such a way that the work done by forward change along the path is
identical to the work attained from the backward change along the same path.
However, all real processes are irreversible with varying degrees of departure
from a reversible one.11
Equilibrium
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Temperature ScalesTemperature Scales
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Atmospheric, Absolute, Gage, and
Vacuum Pressures
Atmospheric, Absolute, Gage, and
Vacuum Pressures
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Phase BehaviorHydrocarbon reservoirs = Rock + Fluids
Reservoir Fluid: Water in brine form and a gaseous and/or liquid hydrocarbon
phase are regarded as reservoir fluids.
The phase behavior of the actual hydrocarbon mixture in the reservoir can be
described as a function of the state of the system.
A system in thermodynamic equilibrium possesses an accurately defined
relationship between the state variables. These are united in the so-called
equation of state:
By specification of two variables, the third will be stipulated.
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A phase diagram is a concise graphical method of representing phase
behavior of fluids. It provides an effective tool for communicating a large
amount of information about how fluids behave at different conditions.
Phase Diagram
Two Classes of Fluids
1. Pure-component systems: the composition is not a variable and therefore
cannot influence behavior.
2. Mixtures: the behavior of a mixture is strongly controlled by composition. In
fact, as the number of components in the system increases, the complexity of
the phase diagram increases as well.
Two components
Multi components
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Single or Pure-Component System
The curve in Figure 1 is called the vapor pressure curve or boiling point curve.
The line also represents the dew point curve and the bubble point curve; one on
top of the other.
This curve represents the transition between the vapor and liquid states.
Figure 1
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Vapor Pressure: The pressure that the vapor phase of a fluid exerts over its
own liquid at equilibrium at a given temperature.
Dew Point: The pressure and temperature condition at which an infinitesimal
quantity of liquid (a droplet) exists in equilibrium with vapor. It represents the
condition of incipient liquid formation in an initially gaseous system. Notice that it
can be also visualized as a liquid system where all but an infinitesimal quantity
of liquid has been vaporized.
Bubble Point: The pressure and temperature condition at which the system is
all liquid, and in equilibrium with an infinitesimal quantity (a bubble) of gas. This
situation is, in essence, the opposite of that of the dew point.
Definition of Basic Terms
Figure 1
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Definition of Basic TermsNOTE: For single-component systems, one single curve represents all three of
these conditions (vapor pressure, dew point and bubble point conditions) simply
because Vapor Pressure = Dew Point = Bubble Point for unary systems.
In Figure 1, once a saturation pressure has been selected, there is one (and
only one) saturation temperature associated with it. This is only true for a single
component system. In other words, this is the only temperature (at the given
pressure), at which liquid and gas phase will co-exist in equilibrium. The rule
that governs the uniqueness of this point, for a single-component system, is
called the Gibbs Phase Rule.
Figure 1
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Complete P-T Diagram for Pure-component
Systems
Two very important thermodynamic points bound the vapor pressure curve:
the Critical Point at its upper end and
the
Triple Point at its lower end. 19
Figure 2
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The Triple Point is the meeting point of the vapor pressure, solidification
and sublimation curves; hence, it represents the only condition at which allthree phases of a pure substance (solid, liquid and gas) can co-exist in
equilibrium.
Figure 2
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Triple Point
For water, the triple point is at 273.16 K (0.01 rC or 32.018 rF) and
0.6113 kPa (0.0887 psia).
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At the Critical Point, gas and liquid are in equilibrium without any interface to
differentiate them; they are no longer distinguishable in terms of their properties.
As we recall, the only location on the P-T diagram where liquid and gas can befound together in equilibrium is along the vapor pressure curve. Hence, the critical
point is clearly the maximum value of temperature and pressure at which liquid
and vapor can be at equilibrium. This maximum temperature is called the critical
temperature (Tc); the corresponding maximum pressure is called the critical
pressure (Pc).
Figure 3
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Critical Point
The critical point is the point at
which all intensive properties of the
gas and liquid phases are equal.
An intensive property is a property
independent of the quantity of the
system. Pressure, temperature,
density, composition, and viscosity
are examples of intensive
properties.
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Properties OfThe Critical Point (Tc,Pc)
(For Pure Substances)
Temperature and pressure for which liquid and vapor are no longer
distinguishable.
For T > Tc, liquid and vapor will not co-exist, no matter what the pressure is.
For P > Pc, liquid and vapor will not co-exist, no matter what the temperature is
The vapor-liquid critical point in a pressure-
temperature phase diagram is at the high-temperature extreme of the liquid-gas phase
boundary. The dotted green line gives the
anomalous behavior of water.
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isobaric heating
Sensible Heat: Its main purpose is to
cause an increase in temperature of thesystem.
Latent Heat: It serves only one purpose:
to convert the liquid into vapor. It does
not cause a temperature increase.
the phase boundary is crossed
Two Thermodynamic Path to Go from A to B
Path AD: Isothermal compression
Path DE: Isobaric heating
Path EB: Isothermal expansion
thephase boundary is NOT crossedat all
We went from an all-liquid condition
(point A) to an all-vapor condition
(point B) without any sharp phase
transition.
The phase boundary represents
a sharp discontinuity in density(and other physical properties)
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PV Diagram for Pure Systems
The temperature is being held constant; Isothermal compression process
path F-G: two-phase condition, the liquid (L) and vapor (V) co-exist in
equilibriumE: all-vapor condition
F: saturated vapor condition (vapor in equilibrium with an infinitesimal amount
of liquid; dew pint
G: saturated liquid condition (liquid in equilibrium with an infinitesimal amount of
vapor; bubble point
L+V
Where is path F-G in this figure?
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Family of P-V Isotherms for a Pure Component
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P-V Phase Diagram for a Pure Component
The critical point has a point of inflexion(change of curvature).
The critical point also represents the
maximum point (apex) of the P-v envelope
The criticality conditions:
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Three-dimensional Diagram of
Single-Component System
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