Download Equation of State

Advanced Modeling
PC-SAFT
Next Generation
Equation of State
PC-SAFT
Next Generation Equation of State
For decades cubic equations of state (EoS) have
been an industrial standard for phase equilibrium
calculations in the oil industry. The industry has
learned to take the most out of those equations and
get away with their known weaknesses.
So why look for alternatives?
Segments
PVT simulations are finding their way into complex
production systems including online systems for
surveying and planning production and for evaluating
the impact of production changes. In an attempt
to match all phase properties it is often seen that a
patchwork of different thermodynamic models are
used. This is thermodynamically inconsistent and it
would be preferable if all phases and phase properties
could be handled by a single model. The PC-SAFT EoS
has those basic qualities.
Chains
PC-SAFT stands for Perturbed Chain Statistical
Association Fluid Theory and was developed from
molecular theory.
Historical Background
Interactions
PVT simulations have been a Calsep trademark since the
company was founded in 1982. Several publications on PVT
algorithms and models carry Calsep’s name . Calsep is the
company that developed the PVT simulator, PVTsim.
PVTsim is used by more than 250 oil and engineering
companies throughout the world. A close contact with the
oil industry is secured by the many consulting projects within
EoS modeling and flow assurance carried out by Calsep’s
specialist team of consultants.
1982
1988
1990 - 2000
2013
Calsep was
founded in 1982
to offer simulation
services to the oil
and gas industry.
Calsep released the
1st version of the PVT
simulator, PVTsim.
PVTsim extended with tuning options, common EoS, and flow
assurance capabilities (aqueous mixtures, hydrate, wax and
asphaltenes). Interface options to reservoir, process and flow
simulators.
PC-SAFT
an alternative to
cubic EoS.
A Part of
Calsep wants to set the standard for
the next generation EoS. The ambition is to make
the PC-SAFT model available in all calculation
options in PVTsim.
PC-SAFT will provide users of Calsep’s software
with a thermodynamic modeling tool, which until
now has been reserved for a limited number of
scientists.
The compressibility factor
with association contribution
and association strength
Advantages over Cubic EoS
While cubic equations can model phase equilibria of hydrocarbon mixtures quite well, they are inaccurate
for liquid densities unless a volume correction is used. Pressure derivatives and gas phase Z-factors are
almost unaffected by the volume correction and continue being inaccurate.
Although the PC-SAFT model is based on advanced molecular theory, only 3 component specific
parameters (m, σ and ε) are required to model both phase equilibria and bulk properties of hydrocarbon
mixtures. The results for compressibility, speed of sound and gas Z-factors are superior to what is seen
with a cubic equation. This makes the PC-SAFT equation well suited to model phase equilibria for systems
with more than one liquid phase as for example oil-asphaltene mixtures.
For more information visit
www.calsep.com
Density, H2O-EtOH
Speed of Sound, MEG
Experimental
Cubic EoS
Cubic EoS
PC-SAFT
PC-SAFT
Density
Speed of Sound
Experimental
Temperature
Mole fraction of ethanol
PC-SAFT and Asphaltenes
Compressibility and Gas Z-Factors
Capturing the correct volumetric changes
with pressure is important when simulating if
asphaltenes precipitate. PC-SAFT has shown
good results when simulating asphaltenes
onset points and modeling the effect of gas
injection on the precipitation of asphaltenes.
Z-factor predictions with PC-SAFT are superior
to cubic EoS simulations over a wide range of
temperatures and pressures. The same is seen when
simulating liquid compressibilities over large pressure
ranges.
C7+ Characterization
Calsep has developed its own PC-SAFT C7+
characterization procedure and implemented
it in PVTsim. The starting point for using the
PC-SAFT model in PVTsim is a compositional
analysis of the reservoir fluid. Compared
to using a cubic EoS no additional input is
necessary. The characterization procedure
implemented in PVTsim will give good
predictions for the phase behavior of a wide
range of reservoir fluids, including both oils
and condensates.
Association Term
For polar mixtures with strong molecular interactions
(for example water-alcohol mixtures) it is possible to
use the PC-SAFT model with an association term which
will specifically account for association between polar
components. The association term requires only two
extra parameters (associating energy and volume).
This accurate description of the polar-polar molecular
interactions makes it possible to obtain a very good
description of aqueous mixture properties. Results
show that aqueous mixture densities and speed of
sound are simulated much better with PC-SAFT with
association than with a cubic EoS.
Advantages of PC-SAFT
GOM Reservoir Fluid
Experimental
Cubic EoS
Compressibility
PC-SAFT
Pressure
Good prediction of pure component vapor pressures and liquid densities
Better derivatives of density than cubic EoS (also better compressibility and
speed of sound)
The model has its foundation in statistical mechanics, and it specifically
accounts for size and shape of the molecules
PC-SAFT shows good results for modeling asphaltene precipitation
An association term accounting for hydrogen bonding can be included
The association term improves predicted properties of aqueous mixtures
compared to the cubic EoS
Copenhagen Denmark
Houston USA
Calsep A/S
Parallelvej 12
DK-2800 Kgs. Lyngby
Calsep, Inc.,
10370 Richmond Avenue, Suite 1375
Houston, Tx 77042
+45 45 87 66 46
Phone
+45 45 87 62 72
Fax
[email protected]
+1 281 759 0844
Phone
+1 281 759 0845
Fax
[email protected]
Dubai UAE
Kuala Lumpur Malaysia
Calsep FZ LLC, Dubai Internet City
Building 14, Office 210
P.O. Box 500534
Calsep Asia Pacific Sdn Bhd,
Suite 19-05, G-Tower,
199 Jalan Tun Razak, 50400
+971 4 391 3667
Phone
+971 4 390 8208
Fax
[email protected]
+60 3 2162 6551
Phone
+60 3 2162 1553
Fax
[email protected]
www.calsep.com