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