Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 1/19
Rubis Guided Session #7:
Compositional Isothermal model
A01 • Introduction
The compositional isothermal model detailed here allows simulations with an arbitrary number
of components where the Peng-Robinson (with volume translation) equation of state is used
for the hydrocarbon phases. The water phase is then treated separately by mean of
correlations; it is considered immiscible, and no gas dissolution in it is allowed. The boundary
conditions must be limited to ‘no flow’ or ‘constant pressure’ kind. Note that this formulation is
not simply an extension / reduction of the compositional thermal model: beyond the obvious
fact that less variables are needed (there is no temperature equation anymore…), the problem
equations are now such that massive parallelization of the flash calculations can be achieved
when the simulation is performed, leading to a modeling much faster than thermal modeling
cases.
In this example we will set a producer to deplete a closed reservoir filled with gas condensate
and oil. Lean gas is being injected in the underlying oil zone leading to a large pressure
support preventing the gas gap to dive below the dew point. In a second run the injector is not
started and comparisons of the runs show that recycling of the lean gas allow for a much
larger oil recovery.
B01 • Document Creation, PVT Initialization
Create a new Rubis document by clicking and accept all defaults in the Reservoir – field infos
dialog:
Fig. B01.1 • ‘Field Infos’ dialog
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Click ‘OK’ to validate.
Click on the PVT icon in the control panel simulation page to obtain the following display:
Fig. B01.2 • ‘PVT definition’ dialog
We will select ‘EOS (Peng-Robinson)’ and change the reservoir temperature to 350° F.
Click now on to access the compositional dialog:
Fig. B01.3 • ‘Compositional PVT definition’ dialog
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We will consider in this example a mixture composed of C2 (ethane), C10 (N-decane), and C12
(N-dodecane). Start by selecting from the left pane these components and add them to the
current composition - The current composition pane will show four components. Select
‘Methane’ and click on ‘Delete’ to obtain the following composition:
Fig. B01.4 • ‘Compositional PVT definition’ dialog
In order to visualize the properties of the fluid sample, we need to set a reference mixture
composition. To achieve so, click on ‘Edit mixture composition’ and input the following molar
composition, then click ‘OK’ to validate:
Fig. B01.5 • Edition of reference mixture molar composition
From the compositional PVT definition dialog, click now on ‘Edit component properties’ to
access the pure component properties dialog:
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 4/19
Fig. B01.6 • Edition of component properties
We will keep here all parameters to their default values and just click on ‘OK’ to get back to
the compositional PVT definition dialog. We can navigate through the various tabs to edit the
corresponding evaluated properties. Click finally on the ‘P. bubble’ tab:
Fig. B01.7 • Reference composition phase envelope
The table on the right hand side displays the bubble point pressure as a function of
temperature for the reference composition while the plot shows the complete phase envelope
composed of the bubble points (yellow), direct due points (green) and retrograde due points
(red) for the reference composition. Note for later use that by looking up at the plot, we see
that for a temperature of 350° F this composition gives a bubble point pressure of
approximately 1310 psia.
Click on ‘OK’ to validate this PVT definition.
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 5/19
C01 • Reservoir Geometry
Let us proceed with the definition of the reservoir geometry by setting the reservoir shape, the
number of layers and their respective thicknesses. Start by double-clicking on the contour in
the 2DMap, set the reservoir as a rectangle and set it to the following dimensions:
Fig. C01.1 • ‘Field contour’ dialog
Click on ‘OK’ to validate and click next on the ‘Geometry’ button available in the simulation
control panel tab: In the ‘Reservoir-Geometry’ dialog, we will keep the number of layers to 1
and its default thickness of 30 ft, but we will slightly tilt the reservoir. To achieve so, change
the top layer 1 type to ‘Data Set’, and click on the button to define the top horizon as
follows:
Fig. C01.2 • Layer 1 top horizon definition
Click on ‘OK’ to validate the changes.
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 6/19
D01 • Reservoir Properties
Click on the ‘Properties’ button to access the reservoir properties dialog.
Fig. D01.1 • Reservoir Properties Dialog
For the sake of simplicity we are going to keep a uniform description of the rock petrophysical
properties, and only change the permeability to 100 md while we will keep default values for
all other data.
We need to change to KrPc curves by specifying a residual oil saturation equal to 0. To proceed
click on the KrPc definition button ; the following dialog will appear:
Fig. D01.2 • KrPc definition Dialog
Set the residual oil saturation (Sorg) to zero in the first tab, then move to the ‘Pc Data’ tab and
set as well the residual oil saturation and the ‘PcMax’ value to zero. Click ‘OK’ to validate.
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 7/19
We need now to initialize the fluid column: click on the initial state button , the initial state
definition dialog will appear:
Fig. D01.3 • Initial state definition dialog
In this multiphase context we will specify the depth of the Gas-Oil Contact (GOC) as well as a
‘Reference Initial Pressure’ at a ‘Reference Depth’ in the initial fluid column. In this case we
want to initialize the fluid as a saturated gas cap overlying a condensate zone: to achieve so,
specify a reference initial pressure of 1300 psia at a reference depth of 6100 ft, and set also
the GOC at the same 6100 ft depth. Click on ‘OK’ to validate the initial state setup.
A few more explanations may be useful at this stage to precise what we actually did in this
quick initialization input: if a GOC is defined with its reference pressure point, Rubis will derive
the mixture composition of the corresponding fluid such that the gas-cap and the underlying oil
are in equilibrium at the contact. From the GOC input, phase compositions are adjusted to
ensure thermodynamic equilibrium in the initial fluid column:
- If the saturation pressure is not specified in the ‘Saturation pressure’ tab the
saturation pressure is considered constant below the GOC while the fluid is
considered at saturation in the gas cap. This result in a constant composition in the
oil zone and in a varying one in the saturated gas cap. Figure D01.4 (left pane)
illustrates the corresponding pressure profiles in a column in a general three phase
context – this situation corresponds to the input we just made.
- If the saturation profile is defined in the ‘Saturation pressure’ tab, the compositions
are adjusted in consequence and the saturation pressure is ‘forced’ at the GOC. If
the prescribed saturation pressure is larger than the reference pressure at the GOC,
it is lowered to the reference pressure and recomputed downward by adjusting the
oil composition. The resulting oil zone will hence be saturated and its composition
computed with an iterative process in the column. Figure D01.4 (right pane)
illustrates the corresponding pressure profiles in that case.
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 8/19
Fig. D01.4 • Initial phases and saturation pressure profiles corresponding to
a typical under-saturated oil zone (left) and to a saturated oil zone (right).
In order for the equilibrium condition at the GOC to be possibly met, one must ensure that the
chosen ‘Reference Initial Pressure’ at the corresponding reservoir temperature lies within the
critical point loci of all possible mixture of the chosen components.
In the present example, it occurs that Rubis will internally compute the following molar
compositions at the GOC:
%mol C2 %mol C10 %mol C12
Oil 0.5962 0.3028 0.1010
Gas 0.9390 0.0507 0.0103
In order to illustrate the consistency of the compositions computed at the GOC, one could go
back to the compositional PVT definition dialog and set in turns these two compositions as the
reference composition. Plotting the phase envelopes for these two compositions on top of each
other would lead to the plot presented in Figure D01.5. We can see from there that the
envelopes intersect at the specified GOC conditions. Care must then be taken in setting the
‘Reference Initial Pressure’ so that this condition can be met – in practice, Rubis will refuse to
proceed with the simulation step and issue a warning when the prescribed equilibrium occurs
to be impossible.
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 9/19
Fig. D01.5 • Phase envelopes for the two compositions at the GOC
E01 • Well Definition
In our example we are going to define one vertical producer and one vertical gas injector. In
the 2D Map tab , click on the button to create a vertical well and interactively place a
well in the western part of the reservoir (the exact position of the well will be set in the
following), place a second well in the eastern part.
Click next on the ‘Wells’ button in the simulation control panel page to access the
‘Reservoir – Wells’ dialog:
Fig. E01.1 • Reservoir – Wells dialog
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 10/19
Rename first the wells to ‘Producer’ and ‘Injector’ as shown on figure 4.1. Then, change the
position of the producer to X=-1250 ft and Y=0 ft, and set the injector’s location to X=1000 ft
and Y = 0 ft as shown below - In each case, you will need to visit the ‘Cross-section view’ tab
and to click on the icon to adjust the trajectory to fully penetrating:
Fig. E01.2 • Well Geometry dialog
Check that the producer well is carrying a unique, fully penetrating perforation in the Producer
– Perforations dialog – the unique perforation should run from MD=6000 ft to MD=6030 ft. In
turn, we will also define a unique perforation for the injector, but this will be a limited entry
one as we want to restrain it in the [6218 ft – 6230 ft] range as shown below:
Fig. E01.3 • Well Perforations dialogs
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 11/19
Even though we will not spend time defining each model in details and keep all defaults
instead, we will set the wellbore model to ‘All trajectory’ for the producer well, and to ‘Only
hydrostatic gradient’ for the injector, as shown below:
Producer Injector
Fig. E01.4 • Setting wellbore models for the different wells
We will now define a unique well control for each of these wells, and set that the producer is
following a surface gas rate target of 200 Mscf/D, whereas the injector is re-injecting the same
amount (we can keep the default constraint pressure values). In essence, you should end-up
with the display shown in Fig. E01.5 and E01.6:
Fig. E01.5 • Well control dialog for the producer
Fig. E01.6 • Well control dialog for the injector
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 12/19
Last but not least, we will now precise the composition of the gas phase being injected: to
achieve so, click on the ‘Injection Hydrocarbon Composition’ button in the Injector – controls
dialog and set the injected fluid molar composition as in Figure E01.7:
Fig. E01.7 • Injected fluid composition dialog
By doing so, we just specified that the injector well will actually not re-inject the same fluid as
the one produced, but only the light component…
Before to proceed with the simulation of the problem, create a cross-section going through
both wells in the reservoir, as shown below:
Fig. E01.8 • Cross-section path
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F01 • Building the Grid
Click on the ‘Grid’ button in the Simulation tab to access the grid dialog:
Fig. F01.1 • Simulation-Grid dialog
Accept all defaults and click on OK to obtain the problem geometrical grid – which will contain
about 6150 cells:
Fig. F01.2 • Unstructured Voronoi Grid
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G01 • Run Settings and Initialization Step
We now need to specify the run settings for the run as well as the results to be output. In
order to do so, click on the run settings icon to access the ‘Simulation-run settings’ dialog:
Fig. G01.1 • Simulation-run settings dialog
In the ‘Time settings’ tab, set the starting date to January 1st, 2013 and set the end date of
the simulation to December 31st, 2015.
Click next on the Results tab, and add results logs as additional output for the producer with a
time period of 2000 hr and a ‘MDmin’ of 0 - in practice, we will output production logs from
bottomhole to surface:
Fig. G01.2 • Result settings
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We will not store restarts in our case: visit the Restart settings tab, and uncheck the ‘Store
restarts’ checkbox.
Exit the Simulation – run settings with OK, and click on ‘Initialize’ the problem to build the
simulation grid and the initial fluid distribution. Maximizing the cross-section and selecting the
saturations as the property being shown should lead you to the following display:
Fig. G01.3 • Initial fluid saturation profile along the West-East reservoir cross-section
H01 • Numerical Simulation
Click now on Simulate to start the numerical simulation of the defined problem – the
simulation should be completed in approximately 370 time steps. Once the simulation is
completed, maximize the 3D plot and animate the saturation fields with time to get the
following display:
Initial saturation field (t=0):
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Saturation field after 3500 hr of
production:
Saturation field after 7000 hr of
production:
As can be seen, the pressure support from the gas injector allows maintaining the pressure
above the dew point in the gas cap, prohibiting any condensate to form in the upper region of
the reservoir. That being said, a (small) condensate dropout actually shows up along the
producer wellbore itself as pressure decreases with depth:
Fig. H01.1 • Oil and pressure logs output at the producer
This ‘temporary’ dropout is due to the fact that the pressure in the wellbore gets below the
mixture saturation pressure (more and more oil appears), crosses the two-phase P-T region
and becomes small enough to converge towards the dew point (the oil turns into gas again).
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 17/19
I01 • Alternate Simulation Without the Injector
We are now going to create a new run by copy in which we will delete the injector to show the
impact of the gas cycling on the total oil production. To achieve so, rename the present run to
‘with cycling’, and create a new run by copy called ‘no cycling’:
Fig. I01.1 • New run dialog
In this new run, click on the Wells option and delete the injector by clicking on the icon
after its selection:
Fig. I01.2 • Deleting the injector
Once the injector has been deleted, initialize and run the simulation.
Inspection of the final oil saturation result field in the cross section plot allows us to check that
in that case condensate is being formed in the gas cap as illustrated on Fig. I01.4, where the
color scale has been changed to a maximum of 5 percent, as shown in Fig. I01.3:
Ecrin v4.30 - Doc v4.30.01 - © KAPPA 1988-2013 Rubis Guided Session #7 • RubGS07 - 18/19
Fig. I01.3 • Changing the Min-Max of the oil saturation field
Fig. I01.4 • Final oil saturation field: oil has formed in the higher part of the reservoir
A closer look at the producer logs hints that the fluid composition being produced is now
changing with time: due to this condensate bank being formed in the reservoir, the produced
well stream is becoming leaner and leaner (Fig. I01.5). This can be compared directly to
Fig.7.1 where the composition of the produced fluid was kept almost constant by applying a
large pressure support.
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Fig. I01.5 • Oil and pressure logs output at the producer (run without injector)
Finally, one can use the browser to compare the cumulative surface oil produced in each case
– with and without the presence of the injector. This comparison is displayed in the Fig. I01.6
below, hinting that the total oil production is increased by approximately 13% when the
injector is active:
Fig. I01.6 • Cumulative surface oil produced with and without lean gas cycling.