PROPERTIES OF

PETROLEUM RESERVOIR

FLUIDS

E.J. Burcik. 1957. Properties of petroleum reservoirs fluids. John Wiley & sons, inc. London

References

Hydrocarbons

Hydrocarbons are compounds formed by two elements only, hydrogen and carbon. Because of their different molecular structures, they have different chemical and physical properties.

One of the main characteristic of hydrocarbons consists in their ability to oxidize quickly releasing a large quantity of thermal

energy.

Petroleum oil and natural gas are themselves often referred to as "hydrocarbons“. However they often contain substantial amounts of nitrogen, sulfur, oxygen, trace metals, and other elements.

From point of view of chemistry

For petroleum engineers

DEFINITIONS

C Dry gas (gaseous @ any p,T

conditions)

C3-C4 LPG: Liquefied Petroleum Gas

C5-C8 Intermediate components or

condensates

C13+ Heavy fractions

Qualitative phase behavior

of hydrocarbon systems

Single-Component Systems

Consider a single, pure fluid at a constant temperature, in a cylinder fitted with a frictionless piston.

p is the pressure applied on the piston

p > pv only liquid

pv is the vapor pressure of the liquid

if @ equilibrium

p < pv only vapor

p = pv Vapor and liquid

At a given temperature, the pressure determines the kind and number of phases that are present

p

T

Vapor

Liquid Solid

B

C

A

Tc

pc

G

System behavior as a function of pressure and temperature, studied through PVT laboratory analysis.

Single-Component Systems

Pressure-Temperature diagram – phase diagram

O

AO: vapor pressure

Lines

OB: sublimation pressure

OC: melting points (slope positive for hydrocarbons, negative for water)

Points

O: triple point. Pressure and temperature at which solid, liquid and vapor coexist under equilibrium conditions.

Critical Temperature (Tc): temperature above which a vapor cannot be liquefied, regardless of the applied pressure.

Single-Component Systems

Intensive property (or bulk property): does not depend on the system size or the amount of material in the system. Example: density, viscosity, etc..

Extensive property: depends on the system size or the amount of material in the system. Example: mass, volume, etc..

Critical Pressure (pc): minimum pressure necessary for liquefaction of vapor at the critical temperature

A: critical point (pc, Tc). The intensive properties of the liquid and the vapor phases become identical and they are no longer distinguishable

Some definitions…..

Single-Component Systems

Pressure-volume diagram

p

volume

D

C B

A

pv

AB: vapor

Phases

BC: vapor + liquid (liquid and gas coexist at the vapor pressure)

CD: liquid

T = const < Tc

C: bubble point. The system is all liquid except for an infinitesimal amount of vapor

B: dew point. Liquid begins to condense.

Points

Vapor pressure = dew point pressure = bubble point pressure

In a single component system…

Two-Component Systems

Pressure-volume diagram

p

volume

D

C

B

A

AB: vapor

Phases

BC: vapor + liquid (liquid and gas coexist)

CD: liquid

T = const < Tc

Pb: bubble point.

PD: dew point.

Points

pb

pD

dew point pressure ≠ bubble point pressure

In a two/multi-component system…

Two-Component Systems

Pressure-volume diagram

p

volume

T1

T2

T3

critical point

Critical point: is the point where the bubble point line and the dew point line meet

UNDERSATURATED

OIL RESERVOIRS

GAS- CONDENSATE

RESERVOIRS

SINGLE PHASE

GAS RESERVOIRS

Multi-Component Systems

Pressure-Temperature diagram – phase diagram

Bubble Point: pressure at which the oil releases the first gas bubble

Dew Point: pressure at which the gas releases the first oil drop

Cricondentherm: highest temperature at which liquid can exist

Critical Point: The intensive properties of the liquid and the gas phases become identical and they are no longer distinguishable

Multi-Component Systems

Definitions…..

Hydrocarbons at Reservoir Conditions

E: Dry gas reservoirs

D: Wet gas reservoirs

Tr > Tcricondentherm

Surface/transport conditions are outside the two phase envelope

Tr > Tcricondentherm

Surface/transport conditions are inside the two phase envelope

Hydrocarbons at Reservoir Conditions

Tc<Tr < Tcricondentherm C: Gas-condensate reservoirs

A: Under-saturated oil reservoirs

B: Saturated oil reservoirs Tr<Tc

C.H. Whitson. M. R. Brule. 2000. Phase behavior. SPE Monograph Series. Richardson, Texas.

Hydrocarbons at Reservoir Conditions

As the pressure decreases, liquid or “condensate” is formed. This happens until a limiting value of the pressure, after which further pressure reduction results in re-vaporization.

The region in which this phenomenon takes place is called the “retrograde condensation” region, and reservoirs with this kind of behaviour are called “retrograde condensate reservoirs”.

Gas-condensate reservoirs

Dpp

Dpp 1 phase in reservoir

Hydrocarbons at Reservoir Conditions

OIL SATURED

UNDERSATURED

VOLATILE OIL

GAS DRY GAS

CONDENSED GAS

BLACK OIL

Phase Envelopes

Component Intermediate

oil

Volatile

oil

Gas-

condensate Wet gas Dry gas

CH4 48.83 64.36 87.07 95.85 86.67

C2H6 2.75 7.52 4.39 2.67 7.77

C3H8 1.93 4.74 2.29 0.34 2.95

C4H10 1.60 4.12 1.74 0.52 1.73

C5H12 1.15 2.97 0.83 0.08 0.88

C6H14 1.59 1.38 0.60 0.12 …

C7H16 + 42.15 14.91 3.80 0.42 …

Mol. Wt. C7H16 + 225 181 120 157 …

GOR, SCF/bbl 625 2000 18,200 105,000 Inf.

°API gravity 34.3 50.1 60.8 … …

Liquid colour Greenish

black

Medium

orange

Light straw Water

white

…

1.

Mole composition & other properties of typical Single-Phase Reservoir Fluids

Single-Phase Reservoir Fluid

Volumetric Relations

Oil Oil Oil

Liberated Gas

Liberated gas

Expansion of

previously

liberated gas

Liberated gas

Pre

ssu

re

Temperature

Oil Oil Oil

Liberated Gas

Liberated gas

Expans ion of

previous ly

liberated gas

Liberated gas

Pre

ssu

re

Temperature

Termodynamic Conditions

RESERVOIR CONDITIONS

•Pressure

•Temperature

NORMAL CONDITIONS:

•p=1 atm (14.69 psi)

•T=0°C (273.15 K)

STANDARD or STOCK TANK CONDITIONS:

•p=1 atm (14.69 psi)

•T=15°C (288.15 K)

Viscosity

definition

Viscosity is friction within a fluid that results from the strength of molecule to molecule attractions. In other words it is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress.

References

C.R. Fitts. 2002. Groundwater science. Academic Press. London, UK

Newton's Theory

z

Vx = 0

Vx = V

Consider two flat plates separated by a thin film of fluid

slide one plate laterally, the fluid resist shearing

the faster you slide the plate, the grater the resistance

V = velocity of the plate zd

vdAF A = area of the fluid film

A

F = resisting force

= is known as the coefficient of viscosity, the viscosity, the dynamic

viscosity, or the Newtonian viscosity

Dynamic viscosity

zd

vdAF

m

sm

mN 2

Units

sPa

SI Pa s (Pascal-second)

Cgs - OF P (poise) or cP (centipoise)

Water at 20 °C has a viscosity of 1.0020 cP.

1 P = 1 g·cm−1·s−1

Conversions:

1 cP = 10-3 Pa·s = 1 mPa·s

1 P = 10-1 kg·m−1·s−1 = 10-1 Pa·s

Related parameter

= dynamic viscosity

r = fluid density

kinematic viscosity

…rarely used in petroleum engineering…

SI m2/s

Cgs - OF St (stokes) or cSt (centistoke)

1 stokes = 100 centistokes = 1 cm2 s−1 = 0.0001 m2 s−1.

1 centistokes = 1 mm2 s-1 = 10-6m2 s−1

Conversions:

s

m

m

kg

sPa 2

2

r