composition & pvt (fluid properties as a function of ... · pdf filecomposition & pvt...
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Composition & PVT (Fluid properties
as a function of Pressure, Volume
and Temperature)
Statoil module – Field development
Magnus Nordsveen
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)
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
Content
• Compositions
• Phase transfer, phase envelops and reservoir types
• Water, Hydrates and Ice
Comp Mole%
N2 0.95
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
Compositions of gas and oil
Comp Mole%
N2 0.95
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
C
C
C C C
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
C C
H
H
H
H
’Normal’, paraffinic oil
Lab analysis of samples
• Gas Chromatography and distillation
• Mass spectrometry (not standard)
• Viscosity measurements
• Boiling point
• Wax appearance temperature, wax deposition, etc.
• Hydrate equilibrium temperature (HET)
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
probably about 200)
• Molecular weights range from 16 g/mole to several
thousands g/mole
Comp Mole%
N2 0.95
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
• Low carbon number components:
–Possible to measure with reasonable accuracy
–Known properties
• Higher carbon number components:
– consists of many variations with different properties
– cannot measure individual components
• Characterization: Lump C10 and higher into C10+
Comp Mole%
N2 0.95
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
Content
• Compositions
• Phase transfer, phase envelops and reservoir types
• Water, Hydrates and Ice
Phase diagram for a single component
Critical point
Trippel point
P
T
Solid Liquid
Gas
Dense phase
Phase diagram for C3 (99%) and nC5 (1%)
Phase diagram for C3 (50%) and nC5 (50%)
Phase envelope of a gas condensate reservoir
C
C
C
Gas Condensate
OilHeavy oil
C = Critical point
• Holdup: b – liquid volume fraction in the cross section
• Oil density: r
• Gas density: r
• Effective density: r br b r
• Gravitational pressure drop: dPgrav = r
(g: gravity, H: Height)
• Total pressure drop: dP = dP + dP
Holdup Effective
density
[kg/m3]
Height
[m]
dPgrav
[bar]
dPfric*
[bar]
dP*
[bar]
0 80 2000 16 ? ?
0.5 440 2000 86 ? ?
1 800 2000 157 ? ?
*need more detailed calculations (will be addressed later in course)
Equations of state (EOS) & Phase envelope
• An equation correlating P (pressure), V (volume) and T (temperature) is called an
equation of state
• Ideal gas law: PV = nRT <=> (good approx. for P < 4 bar)
– n: moles, R: gas constant, : molar volume
• Van der Waals cubic EOS:
• a: is a measure for the attraction between the particles
• b: is the volume excluded from by the particles
2v
a
bv
RTP
v
RTP
Equations of state (EOS) & Phase envelope
• 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.
Content
• Compositions
• Phase transfer, phase envelops and reservoir types
• Water, Hydrates and Ice
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
Gas hydrates (Burning “snow”)
• Ice/snow crystals of water and gas
molecules
• Can cause pipeline blockage
Gas hydrates
Hydrate formation requires:
High enough pressure Hydrates can be stable at 10-15 bar
Low enough temperature But still good summer temperature
Access to small molecules C1, C2, C3, I-C4, CO2, H2S, N2
Access to free water Condensed water is good enough
Gas molecules stabilise cages made of water molecules.
Gas hydrates
Gas molecules stabilise cages made of water molecules.
Hydrate formation domain
0
50
100
150
200
250
300
350
400
0 5 10 15 20 25 30
Temperatur (°C)
Try
kk (
bara
)
Hydrate domain
Temperature (°C)
Pressu
re (
bar)
No hydrates
Normal operational
domain
Chemicals move
the hydrate curve
Hydrate formation curves Mono Ethylene Glycol (MEG) as inhibitor “defroster”
Chemicals move
the hydrate curve
No hydrates
Normal operational
domain
Safety Hazards of Moving Hydrate Plugs (From Chevron Canada Resources, 1992)
A hydrate plug moves
down a flowline at very
high velocites.
Closed Valve
Closed ValveIf the velocity is high enough, the
momentum of the plug can cause pressures
large enough to rupture the flowline.
Ice
• In deep waters the sea bed temperature can be lower than 0 C
– Ormen Lange: -1 C at sea bed
• Large pressure drop can give large temperature drop due to the Joule Thompson
effect
– Over chokes
– In long gas condensate flowlines
Ice formation temperature as function of pressure
-2.5
-2
-1.5
-1
-0.5
0
0 50 100 150 200 250 300
Pressure [bar]
Tem
pera
ture
[oC
]
Condensed water
Ice formation temperature as function of MEG
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0 1 2 3 4 5 6 7 8 9 10
wt% MEG in water+MEG
oC
Tem
p
MEG wt%
Ice
Water
Ice
• Normally hydrates are formed before ice
• Inhibition to avoid hydrates will also hinders ice
• However, in depressurized flowlines (hydrates will not form) ice may form
• Statoil has not experienced ice formation in flowlines
Thank you