2 89a Phase Behavior Single Component System

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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.

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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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2 89a Phase Behavior Single Component System

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