Hydrocarbon Phase Behaviour
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Transcript of Hydrocarbon Phase Behaviour
Hydrocarbon Phase Behaviour
What is a phase?
• A phase is any homogeneous and physically distinct region that is separated from another such region by a distinct boundary
• For example a glass of H2O with some ice in it contains one component (the water) exhibiting three phases; liquid, solid, and gaseous (the water vapour).
• The most relevant phases in the oil industry are liquids (water & oil), gases (or vapours), and to a lesser extent, solid.
General Hydrocarbon phase Behaviour
• As the conditions of pressure and temperature vary, the phases in which hydrocarbons exist, and the composition of the phases may change.
• It is important to understand the initial condition of fluids to be able to calculate surface volumes represented by subsurface hydrocarbons.
• It is also necessary to be able to predict phase changes as the temperature and pressure vary both in the reservoir and as the fluids pass through the surface facilities, so that the appropriate subsurface and surface developments plans can be made.
• Phase behaviour describes the phase or phases in which a mass of fluid exists at given conditions of pressure, volume and temperature (PVT)
PHASE BEHAVIOUR OF HYDROCARBONS
When fluids are produced from a subsurface reservoir to the surface both temperature and pressure are reduced.
The P-T changes result in two kinds of phase change in the produced fluids:
1. Liquid may condense from the produced gas.2. Gas may evolve from the produced liquid.
Similar changes take place in the subsurface reservoir as a result of the isothermal (constant temperature) pressure change generated by fluid production:
3. Condensate (liquid) may be produced in the reservoir from the gasphase.
4. Solution gas may be evolved in the reservoir from the liquid phase.
Concept of Equilibrium• Two phases reach the state of equilibrium when no changes-variation of
composition-occur with time if the system is left at the prevailing constant pressure and temperature.
• Any multiphase system reaches equilibrium when it attains its minimum energy level.
• The properties exhibited by any substance depend upon its phase, namely, whether it is in solid, liquid, or gaseous phase.
• Substances can be classified into two types – pure or single component, and multicomponent.
• Phase behaviour relationship can be determined from laboratory pressure-volume-temperature (PVT) studies, or using theoretical / empirical methods such as the use of equations of state ( PV = nRT)
• These relationships are frequently shown graphically as phase diagrams.
Liquid
Liquid
GasGa
s
Phase behaviour of a pure componentHow isothermal pressure change affects fluid phase?
Flui
d Pr
essu
re
Fluid Volume
Note: Isothermal pressure change is a common assumption for hydrocarbon production.
A
B
C
DLiquidLiquid
Gas
Gas
Phase behaviour of a pure componentHow temperature affects fluid phase (in a constant pressure)
Tem
pera
ture
Fluid Volume
Phase behaviour of fluids
Liquid Phase
Solid Phase
Temperature
Pres
sure
Supercritical Phase
Gas Phase
Phase behaviour of a pure fluid (single component)
Liquid Phase
Temperature
Pres
sure
Supercritical Phase
Gas Phase
Critical Point (CP)
Tcrit
Pcrit
The vapour pressure line separates the P-T stability field for liquid from that for gas.CP represents the critical point for the phase corresponding to the critical pressure Pcrit and critical temperature Tcrit. Above this point the behaviour of the two fluid phases are indistinguishable.
Vapour Pressu
re Line
TWO-COMPONENT SYSTEM
P
T
PHASE 1&2LIQUID
PHASE 1&2GAS
PHASE 1GAS
+PHASE 2LIQUID
For two separate individual phases, the vapour pressure lines are different. Phase 1 represents a lower molecular weight alkane (e.g., ethane).
Phase 2 represents a higher molecular weight alkane (e.g., heptane).
LIQUID
GAS
LIQUID+
GASP
T
CB CP
CT
Phase Behaviour of
a Multi-component System
For a multi-component system, the bubble-point line divides the liquid stability field from the liquid + gas field. The dew-point line divides the liquid + gas field from the gas stability field.
The bubble-point (BPL) and dew-point (DPL) lines meet at the critical point (CP).
CB = cricondenbar (max. P). CT = cricondentherm (max. T)
oil condensate gas
Bubble-point Line
Dew-point Line
PHASE BEHAVIOUR
The various lines between the BPL and DPL are labeled with the percentage of liquid in the liquid + gas stability field. On the BPL the fluid is 100% liquid and on the DPL the fluid is 0% liquid (i.e., 100% gas.)
At the point X, the fluid consists of 70% liquid and 30% gas.
CP
LIQUID
GAS
P
T
20%40%60%80%
X
ISOTHERMAL OIL PRODUCTIONA
B
LIQUID
GAS
P
T
CP
20%40%60%80%
In an oil reservoir, when an isothermal pressure drop occurs from A to B, gas begins to be exsolved from solution in the liquid at the BPL.Gas exsolution increases the compressibility of the reservoir and makes liquid recovery less efficient. Pressure maintenance is sometimes used to prevent liquid oil reservoirs from crossing the BPL.
D
E
CP
LIQUID
GAS
P
T
20%40%60%80%
AC
B
ISOTHERMAL GAS PRODUCTION
Consider an isothermal reduction in pressure from point A to point B. The fluid in the reservoir is dry gas throughout the P-T path.
Now consider an isothermal pressure drop from point C. At point D, the P-T path crosses the DPL and liquid condenses from the gas. At E, the P-T path recrosses the DPL and the condensate is revapourized.
LIQUID
CP
GAS
P
T
“CONDENSATE” FLUIDS
Retrograde gas “condensate” fluids preferentially deposit the valuable, heavier alkane fraction in the pore space when the P-T path crosses the DPL. This loss of liquid moves the phase envelope down and to the right in P-T space.
The result is that liquid is not revapourized and may not be recoverable. Dry gas injection can prevent such losses by maintaining the reservoir pressure above the DPL.
Phase enevelopes for natural gas reservoir fluids
From Ayala, 2006
Initial conidtion within the reservoir
Surface conidtion
conidtion within the reservoir after delpletion
Phase behavior of a Reservoir’s hydrocarbon during isothermal production
Oil and Gas:Two phase oil and/or gas below CP.(Retrograde Gas) Condensate: Single phase wet gas between the CP and CT.Gas:Single phase dry gas above CT.
Condensate
Schematic Pressure- Temperature Diagram of a Binary Mixture
The phase rule indicates that in a binary vapour- liquid system, both the temperature and the pressure are independent variables
The phase envelope, inside which the two phase coexist, is bounded by the bubble point and dew point curve
The two curves meet at the critical (C), where all differences between and two phases vanish and the phases become indistinguishable
Two phase can coexist at some conditions above critical point
The highest pressure (B) and the highest temperature (D) on the phase envelope are called the cricondenbar and cricondentherm, respectively
Pre
ssu
re
Temperature
Critical Point
C
Dew
- Poi
nt
Cur
ve
Bub
ble-
Poi
nt C
urve
Two Phase Region
B
D
Vapor Pressure Curve for pure Component A
Critical Point
Cricondenbar
Two phase envelope for mixture A+B
Cricondentherm
Dew point
Temperature
Pre
ssu
rePressure- Temperature Diagram for a
Binary System
Bubble point
Vapor Pressure Curve for pure Component B
Bubble Point0% vapour,100% liquid
A2
Critical Point
Cricondenbar
Two phase region
Cricondentherm
Dew point
100% vapour, 0% liquid
50% vapour, 50% liq
uid
85% vapour, 15% liq
uid
A1
Temperature
Pre
ssu
rePressure- Temperature Diagram for a
Binary System
As the pressure drops the compositions of both the liquid and the gas phases change continuously: at the bubble point the first gas appears and at the dew point vapour alone remains
One consequence of this behavior is that the pressure- temperature plot is no longer a simple curve as for the single component; instead, it is an envelope
The maximum pressure defined by this envelope is known as the cricondenbar; above it, the liquid and gas phases cannot coexist
The maximum temperature defined by the envelope (the cricondentherm) is one above which the two phase cannot coexist
The critical point is the point on the envelope at which the properties of the gaseous and liquid phases become identical- it is not related in any simple way to the cricondenbar or the cricondenthermAlso shown are lines of various volume percentage of the liquid phase
Pressure- Volume Diagram of Binary Mixture
Pre
ssu
re
Bu
bb
le-
Po
int
Cu
rve
Two Phase Region
C, Critical Point
Dew- Point Curve
Vapour
Liquid
T <Tc
T3
Volume
T >Tc
T =Tc
A series of expansions can be performed at various constant temperatures and a pressure volume diagram built up and the locus of the bubble point and dew point values gives the bubble point and dew point lines which meet at the critical pointConditions under the bubble point and dew point lines represent the conditions where two phase coexist whereas those above these curves represent the conditions where only one phase existAt the critical temperature the P, T curve goes through the critical point
Variation of Saturated Fluid Density With Temperature
The densities of vapour and liquid phases approach each other as the temperature increases
They become equal at conditions known as the critical point
All the differences between the phases are reduced as the system approaches the critical point
The phases become the same and indistinguishable at the critical pointTemperature
Critical Point
Saturated Liquid
Saturated Vapour
Den
sity