Res. Char. and PVT Analysis in VMGSim
Reservoir Fluid Characterization and PVT
Analysis in VMGSim
Herbert Loria - VMG Calgary
Introduction
Petroleum reservoir fluids are naturally occurring mixtures of gas and oil that
exist at elevated pressures and temperatures. Reservoir fluid compositions
typically include hundreds or thousands of hydrocarbons and a few nonhydrocarbons, like nitrogen, carbon dioxide and hydrogen sulphide [1]. The
physical properties of these mixtures depend primarily on composition, pressure,
temperature and volume (PVT) conditions. Accurate data for the phase
behaviour of these hydrocarbon mixtures is needed to improve oil recovery.
Sometimes, experimental PVT data is available, but not many measurements are
carried out for a given mixture; furthermore, it is expensive to investigate the full
range of phase behaviour that can occur during a recovery process or a
separation chain.
In the absence of PVT laboratory data, predictions of the reservoir fluid behaviour
can be obtained by using a well-established equation of state to compute the
phase behaviour and a correct representation of the composition of the reservoir
fluid [2]. Therefore, reservoir engineers must rely on characterization schemes
and thermodynamic models to calculate the missing data.
The objective of this communication is to show how reservoir characterizations
and PVT analyses can be carried out using VMGSim. A description of the new
PVT Analysis unit operation, new to VMGSim 9.0, is included in this document;
this unit operation is able to calculate the most common properties measured in
PVT experiments and can be used to adjust characterization or thermodynamic
model parameters to fit calculations to experimental data.
Reservoir Fluid Representation
The nature and composition of a reservoir fluid depends on the depositional
environment of the formation from which the fluid is produced. Crude oil and
natural gas are composed of many compounds with a wide range of molecular
weights. Some estimates suggest that perhaps 3,000 organic compounds exist in
a single reservoir fluid [1]. The lighter and simpler compounds are produced as
natural gas after surface separation, whereas the heavier and more complex
compounds are obtained from crude oil at stock tank conditions.
The heavier components are lumped into a “plus” fraction instead of being
identified individually. The petroleum fractions lumped together and labeled as
the “plus” fraction are known as undefined petroleum fractions. Nearly all
naturally occurring hydrocarbon systems contain a quantity of undefined
fractions, and methods for characterizing them are essential. VMGSim can
characterize these undefined fractions in different ways using the Oil
Characterization environment or the recently integrated PIONA Characterization.
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
1
Res. Char. and PVT Analysis in VMGSim
Reservoir PVT Calculations
Reservoir PVT Calculations are mostly volumetric balances that can be obtained
using a stepwise computational procedure using an equation of sate. Equations
of state have found widespread acceptance as tools that permit the convenient
and flexible calculation of complex phase behaviour of reservoir fluids [2]. Some
of these applications include the determination of equilibrium ratios, dew point,
bubble point, vapour pressure, and PVT properties. For hydrocarbon systems, a
good default selection of an equation of sate is VMGSim’s Advanced PengRobinson (APR). The calculation of the most common PVT properties and
experiments has been integrated into the new PVT Analysis unit operation in
VMGSim 9.0.
PVT Analysis Unit Operation
To use the new PVT Analysis unit operation in VMGSim the following steps
have to be followed:
1) Set up the thermodynamic model based on a cubic equation of state (APR is
a good default for hydrocarbon systems).
2) Characterize the reservoir fluid. The reservoir can be characterized by means
of the Oil Characterization environment or a PIONA Slate in the Oil Source
unit operation.
3) Add a PVT Analysis unit operation to the flowsheet and connect it to a
Material Stream containing the reservoir fluid.
4) Perform the PVT calculations; there are six PVT Experiments available in this
calculator:
i.
Constant Mass Expansion (CME)
ii.
Constant Volume Depletion (CVD)
iii.
Differential Liberation (DL)
iv.
Separator Test (Sep)
v.
Volumetric (Vol)
vi.
Viscosity (Vis)
For each experiment the reservoir and standard conditions (Pressure,
Temperature and Z Factor) have to be given as well as the pressure and
temperature data. All reservoir calculations are made assuming vapour-liquid
phases. If the Differential Liberation ad Separator experiments are calculated the
Differential Liberation results can be adjusted to Separator data by checking the
Adjust PVT Parameters box.
Note:
The PVT Analysis jargon is very vast. In some PVT experiments, depending on
the author or laboratory, the same property name is used to represent different
things, whereas different names can represent the same property.
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
2
Res. Char. and PVT Analysis in VMGSim
Fluid Identification
The reservoir fluids used in the PVT Analysis unit operation are automatically
identified before calculations for the PVT experiments are performed; this is done
to avoid the calculation of experiments that are not recommended for certain
types of fluids. The reservoir fluids are classified as “Oils” or “Gas Condensates”.
“Oil” means that the saturation pressure of the fluid (black and volatile oil) at the
reservoir temperature is at its bubble point and, “Gas Condensate” means that
the fluid has a retrograde behaviour, thus the largest dew point is reported as the
saturated pressure.
The message box of the unit operation shows the following messages once the
calculation has identified the reservoir fluid type.
Reservoir Characterization and PVT Analysis Example
The following example will be used to show how a black oil reservoir can be
characterized using the PIONA Characterization scheme. The Oil Source unit
operation will be used to match the crude physical properties and saturation
pressure; then the crude will be connected to a PVT Analysis unit operation and
its results will be compared to experimental data. The experimental data for this
example is taken from McCain [3].
Reservoir Fluid Characterization
To start, open a new VMGSim case, select the Advanced Peng-Robinson
thermodynamic model as the active property package in Thermo Model form
and, add the following pure components that are part of the crude oil analysis:
Nitrogen, Carbon Dioxide, Methane, Ethane, Propane, iso-Butane, n-Butane, isoPentane and n-Pentane.
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
3
Res. Char. and PVT Analysis in VMGSim
Click on the PIONA Slate button at the bottom of the Thermo Model form to
open the PIONA Characterization environment and enter the following
parameters:
Observe that the Olefin family was not selected since naturally occurring crude
oils contains negligible quantities of this family. Click on the Create Slate button
to create and install the PIONA components. Now, go to the flowsheet
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
4
Res. Char. and PVT Analysis in VMGSim
environment and add an Oil Source unit operation, check the Carbon Number
(Cn) Compositional Analysis box and select Cn as the Analysis type.
Go to the Cn Analysis tab and enter the following analysis data (in weight or
mass %) from McCain [3]:
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
5
Res. Char. and PVT Analysis in VMGSim
In addition to the hydrocarbon analysis add the following crude physical
properties in the Summary tab of the Oil Source:
Bulk MW = 93.66
Std Liquid Density @ 60 F = 70.17 API (43.76 lb/ft3)
Reservoir Saturation Pressure @ 220 F = 2634.7 psia
Note that the temperature for the desired saturation pressure can be set in the
Settings tab.
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
6
Res. Char. and PVT Analysis in VMGSim
In the Settings tab set the following Regression parameters and clear the
Olefins check box from the PIONA Family Inclusion since no Olefins were
created in the PIONA Slate.
Click in the Regress Parameters button to find the best combination of PIONA
components fractions that match the regressed physical properties. Observe the
close match between experimental and calculated values, specially the
Saturation Pressure.
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
7
Res. Char. and PVT Analysis in VMGSim
PVT Analysis
Set the Temperature, Pressure and Mass Flow of the Oil Source to 60 F, 14.7
psia and 100 lb/h respectively. Connect a Material Stream to the outlet of the Oil
Source and then connect a PVT Analysis unit operation to the stream.
S1
P VT
Analysis
PVT1
OilFeed1
Constant Mass Expansion (CME) Experiment Open the unit operation, check the Constant Mass Expansion (CME) box in the
Summary tab and add the Reservoir Temperature (220 F). Go to the CME tab
and add the following pressure values (first column, note that the data is given
psig but it was entered in psia) [3] for a Constant Mass Expansion analysis.
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
8
Res. Char. and PVT Analysis in VMGSim
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
9
Res. Char. and PVT Analysis in VMGSim
Compare the calculated Relative Volume and Y-Function values to the
experimental data, the average absolute error respect to the experimental data is
only 0.11 and 1.21 % respectively, as seen in the next plots.
Differential Liberation (DL) Experiment Now, add the Differential Liberation experiment by checking the Differential
Liberation (DL) box from the Summary tab and provide the Standard Conditions
for the calculations. These conditions change from place to place; therefore, a
default of 60 F, 14.7 psi and Z = 1 was selected. The standard Z factor can be
used as ideal (Z= 1) or calculated from the property package.
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
10
Res. Char. and PVT Analysis in VMGSim
The experimental Differential Liberation data [3] in the following table is reported
at 60 F and 14.65 psia. Add the 12 stage pressures reported in the following
table to the PVT Data form.
Compare the calculated Relative Volume, Oil and Gas Formation Factors
values to the experimental data, the average absolute error respect to the
experimental data is only 3.28, 2.18 and 3.69 % respectively, as seen in the next
plots.
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
11
Res. Char. and PVT Analysis in VMGSim
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
12
Res. Char. and PVT Analysis in VMGSim
Separator (SEP) Experiment The following table [3] shows the experimental results of four one-stage
Separator experiments, they are used to find the separator pressure that
produces the higher API gravity and lower gas to oil ratio and oil formation
volume factor.
The Separator experiment from The PVT Analysis unit operation can calculate one set of
Separator experiments at a time, to add the first set of separator conditions at 100 psig
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
13
Res. Char. and PVT Analysis in VMGSim
(114.7 psia) check the Separator (Sep) box from the Summary tab and enter the
separator condition from the previous table at 100 psig.
In order to get the results of the four sets of separator experiments in an efficient way, a
Case Study can be applied using the initial Separator Pressure as the independent
variable and the Stock Tank density, Total Std GOR and Saturated Oil Formation
Volume Factor as the dependent variables. The Case Study can be accessed from the
Tool Menu and then selecting Case Study…
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
14
Res. Char. and PVT Analysis in VMGSim
The following table show the results of the Case Study compared to the experimental
separator data, observe the close agreement between experimental and calculated
results.
Separator Experiment 1
Stock Tank Oil API Gravity (60/60 F)
Total Std Gas to Oil Ratio [SCF/bbl]
Saturated Oil Formation Volume Factor
Experimental
40.5
778
1.481
VMGSim
43.12
765.43
1.46
Error%
6.47
1.62
1.64
Separator Experiment 2
Stock Tank Oil API Gravity (60/60 F)
Total Std Gas to Oil Ratio [SCF/bbl]
Saturated Oil Formation Volume Factor
Experimental
40.7
768
1.474
VMGSim
43.32
756.22
1.45
Error%
6.44
1.53
1.62
Separator Experiment 3
Stock Tank Oil API Gravity (60/60 F)
Total Std Gas to Oil Ratio [SCF/bbl]
Saturated Oil Formation Volume Factor
Experimental
40.4
780
1.483
VMGSim
43.14
764.42
1.46
Error%
6.78
2.00
1.82
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
15
Res. Char. and PVT Analysis in VMGSim
Separator Experiment 4
Stock Tank Oil API Gravity (60/60 F)
Total Std Gas to Oil Ratio [SCF/bbl]
Saturated Oil Formation Volume Factor
Experimental
40.1
795
1.495
VMGSim
42.89
775.73
1.46
Error%
6.97
2.42
2.07
Separator Gas Analysis The gas composition and properties at every Separator stage can also be calculated, the
following table shows the gas composition for the first stage of Separator Experiment at
100 psig.
To obtain these values check the Incl. Separator Gas Analysis box from the Sep tab
and observe the results in the Sep Gas Analysis tab.
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
16
Res. Char. and PVT Analysis in VMGSim
Note that different Cn+ values can be defined in the Option frame as well as a different
definition for the NGL Content.
The following plot and table compare the predicted and experimental gas compositions
and physical properties for the first stage of the Separator Experiment at 100 psig.
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
17
Res. Char. and PVT Analysis in VMGSim
SG
NGL Cont. (GPM)
GHV
Experimental
0.786
7.516
1321
VMGSim
0.796
7.929
1338.76
AAE %
1.31
5.50
1.34
This example showed how the PIONA Characterization, the Oil Source and PVT
Analysis unit operations can be combined to match PVT experimental data. This
combination is very helpful especially when dealing with more than one reservoir fluid in
a flowsheet since the same PIONA Slate can be used in different Oil Sources to
represent all the participant reservoir fluids.
Another important property that can be calculated in the PVT Analysis unit operation is
the Viscosity of the oil and gas at different stage pressures. An upcoming newsletter will
explore into the details of this particular experiment and what can be done in order to
tune the viscosity model results to the available experimental data.
References
[1] Whitson, C. H. and Brule, M. R. Phase Behavior. Richardson, TX: Society of
Petroleum Engineers, 2000
[2] Ahmed, T. Equations of State and PVT Analysis, Gulf Publishing Company,
2007
[3] MacCain Jr., W. D. The Properties of Petroleum Fluids 2nd Ed. Tulsa, OK:
PenWell Publishing Company, 1989
© 2015, Virtual Materials Group Inc. Do not copy unless authorized in writing by Virtual Materials Group.
18