Composition & PVT (Fluid properties

as a function of Pressure, Volume

and Temperature)

Statoil module – Field development

Magnus Nordsveen

Status: Draft
Content
Gas condensate
•  Compositions field
•  Phase transfer, phase envelops and reservoir types Comp Mole%
N2 0.95
•  Water and Hydrates CO2 0.6
H20 0.35
C1 95
C2 2.86
C3 0.15
iC4 0.22
nC4 0.04
iC5 0.1
nC5 0.03
C6 0.07
C7 0.1
C8 0.08
C9 0.03
C10+ 0.13

Status: Draft
Compositions of gas and oil
Gas condensate
field
C1 - methane: Comp Mole%
N2 0.95
CO2 0.6
H20 0.35
C1 95
C2 - ethane: C2 2.86
C3 0.15
iC4 0.22
nC4 0.04
iC5 0.1
iC4 - isobutane nC5 0.03
C6 0.07
nC4 – n-butane C   C7 0.1
C8 0.08
C   C   C9 0.03
C10+ 0.13
C  

Status: Draft
Compositions of gas and oil

•  Isomers: Different structure configurations of same carbon numbers

•  75 isomers of decane C10H22 (single bounds)
•  366319 isomers of C20H42 (single bounds)

•  Complexity further increased by double bounds, triple bounds, rings, other atoms

H   H  
Ethylene: C   = C  
H   H  

Status: Draft
 Gas chromatography
Fingerprint analysis

’Normal’, paraffinic oil

Status: Draft
Characterisation of fluids based on
composition

•  Thousands of components from methane to large

polycyclic compounds
•  Carbon numbers from 1 to at least 100 (for heavy oils Comp Mole%
N2 0.95
probably about 200) CO2 0.6
•  Molecular weights range from 16 g/mole to several H20
C1
0.35
95
thousands g/mole C2 2.86
C3 0.15
iC4 0.22
nC4 0.04
iC5 0.1
nC5 0.03
C6 0.07
C7 0.1
C8 0.08
C9 0.03
C10+ 0.13

Status: Draft
Characterization challenge
Comp Mole%
N2 0.95
•  Low carbon number components: CO2 0.6
H20 0.35
– Possible to measure with reasonable accuracy C1 95
C2 2.86
– Known properties C3 0.15
iC4 0.22
•  Higher carbon number components: nC4 0.04
iC5 0.1
–  consists of many variations with different properties nC5 0.03
C6 0.07
–  cannot measure individual components C7 0.1
C8 0.08
•  Characterization: Lump C10 and higher into C10+ C9
C10+
0.03
0.13

Status: Draft
Compositions and PVT important for:

• Value and market

• Field development solution
–  Reservoir (gas, oil, heavy oil)
–  Wells and flowlines
–  Processing (subsea, platform, onshore plant)
–  Pipeline transport to shore (gas, condensate, oil)
–  Offloading to ship (condensate and oil)

Status: Draft
Compositions and PVT important for:

• Wells and flowlines

– Pressure and temperature drop
•  Phase transfer (gas/oil split)
•  Densities
•  Viscosities
•  Surface tension
•  Conductivities
•  Heat capacity
–  Wax, hydrates, Asphaltenes

Status: Draft
Content

•  Compositions
•  Phase transfer, phase envelops and reservoir types
•  Water and Hydrates

Status: Draft
Phase diagram for a single component

Dense phase
P
Critical point

Critical point Water:

Tc=374 C
Solid Liquid Pc=218 bar

Gas

Trippel point

Status: Draft
Phase diagram for C3 (99%) and nC5 (1%)

Liquid

Gas & Liquid Gas

Status: Draft
Phase diagram for C3 (50%) and nC5 (50%)

Bubble point line

Liquid

Dew point line

Gas &
Liquid
Gas

Status: Draft
Phase envelope of an oil reservoir

2 phase
mixture

Status: Draft
Phase envelope of a gas condensate reservoir
Tres, Pres

Liquid Gas

2 phase
mixture

Status: Draft
Phase envelops for 3 reservoir types
Gas Condensate
C

C
Oil
Pressure

Heavy oil

C = Critical point

Temperature

Status: Draft
Phase envelope and P, T conditions from
reservoir to platform (oil field)

2 phase
mixture

Status: Draft
Pressure drop from reservoir to platform

•  Holdup: β – liquid volume fraction in the cross section

•  Oil density: ρo
•  Gas density: ρg
•  Effective density: ρeff = βρo + (1-β) ρg

•  Gravitational pressure drop: dPgrav = ρeffgH

(g: gravity, H: Height)
•  Total pressure drop: dP = dPgrav + dPfric

Status: Draft
Pressure drop from reservoir to platform

Holdup   Effective Height dPgrav dPfric* dP*

density [m]   [bar]   [bar]   [bar]  
[kg/m3]  
0   80   2000   16   ?   ?  
0.5   440   2000   86   ?   ?  
1   800   2000   157   ?   ?  
*need more detailed calculations (will be addressed later in course)

Status: Draft
Equations of state (EOS) & Phase envelope

•  An equation correlating P (pressure), V (volume) and T (temperature) is called an

equation of state

RT
•  Ideal gas law: PV = nRT <=> P= (good approx. for P < 4 bar)
–  n: moles, R: gas constant, ν : molar vvolume

RT a
•  Van der Waals cubic EOS: P= − 2
v−b v
•  a: is a measure for the attraction between the particles
•  b: is the volume excluded from ν by the particles

Status: Draft
Equations of state (EOS) & Phase envelope

Measured Model prediction

Family of PV isotherms for a pure component Family of PV isotherms for a cubic EOS

Status: Draft
PVTSim

•  In the oil industry we typically use software packages to characterize the fluid
based on a measured composition
•  In Statoil we use PVTSim from Calsep

•  Ref: Phase Behavior of Petroleum Reservoir Fluids (Book),

Karen Schou Pedersen and Peter L. Christensen, 2006.

Status: Draft
Content

•  Compositions
•  Phase transfer, phase envelops and reservoir types
•  Water and Hydrates

Status: Draft
Water in hydrocarbon reservoirs - flowlines

In reservoir:
–  Separate liquid water layer
–  Water vapour in gas layer
In wells/flowlines:
–  Condensed water in gas condensate flowlines
–  Produced water from oil reservoirs
•  Liquid water and hydrocarbons are essentially immiscible in each other
–  However, liquid water and oil can form emulsions/dispersions
•  With water, oil and gas present in flowlines, there are generally
–  2 liquid fields and 1 gas field

Status: Draft
Gas hydrates (Burning “snow”)

•  Ice/snow crystals of water and gas

molecules
•  Can cause pipeline blockage

Status: Draft
Gas hydrates
Hydrate formation requires:

Access to small molecules Gas molecules stabilise cages made of

C1, C2, C3, I-C4, CO2, H2S, N2 water molecules.

Access to free water

Condensed water is good enough

High enough pressure

Hydrates can be stable at 10-15 bar

Low enough temperature

But still good summer temperature

Status: Draft
Gas hydrates

Gas molecules stabilise cages made of

water molecules.

Status: Draft
Safety Hazards of Moving Hydrate Plugs
(From Chevron Canada Resources, 1992)

A hydrate plug moves

down a flowline at very
high velocites.

Closed Valve

If the velocity is high enough, the Closed Valve

momentum of the plug can cause pressures
large enough to rupture the flowline.

Status: Draft
Status: Draft
End of Lecture - Composition & PVT
Content: Gas condensate
field
•  Compositions Comp Mole%
•  Phase transfer, phase envelops and reservoir types N2
CO2
0.95
0.6
•  Water and Hydrates H20
C1
0.35
95
C2 2.86
C3 0.15
iC4 0.22
nC4 0.04
iC5 0.1
nC5 0.03
C6 0.07
C7 0.1
C8 0.08
C9 0.03
C10+ 0.13

Status: Draft
 Thank you

Status: Draft