Introduction
Tutorials
Improving a discharge simulation
An output simulation calculates wellhead pressure at different steady-state mass flowrates for a discharging well. The result is called an output curve; usually used to predict the well's discharge pressures and mass flowrates. For more on output simulations see here.
This tutorial shows you how to run a typical output simulation for a well:
An output simulation can be TopDown, BottomUp or TestData read more. This describes a TopDown output simulation.
This tutorial uses the discharge simulation Better TD 100 t/hr that you should have entered earlier.
WellSim stores output simulations and reservoir drawdown simulations in the same part of the database. Therefore, an output simulation can not have the same ID number as a reservoir drawdown simulation.
Enter data into the white fields only, not the coloured fields. The data required, and hence the field colours, change depending on what discharge test type and deepest feedzone relationship you choose.
In the sample database, open a new output simulation for 
.
Enter this data:
| Data | Enter |
|---|---|
| Description | From better TD 100 t/hr |
| Data | Enter | Note |
|---|---|---|
| Discharge test type | TopDown | Select from the dropdown list read more |
| Calc Drawdown | Tick | Tick: WellSim will calculate drawdown parameters; No tick: You will enter drawdown parameters read more. For a TopDown discharge test, WellSim always calculates the drawdown parameters, and so you can not untick this. |
| Discharge simulation | Better TD 100 t/hr | Select the discharge simulation to use from the drop-down list. |
| Discharge simulation (Quadratic) | None | If you specify a QUADRATIC drawdown relationship below, select a second discharge simulation to use from the drop-down list. |
| Minimum mass flow | 30 | [t/hr] The lowest mass flow for this simulation. |
| Maximum mass flow | 300 | [t/hr] The highest mass flow for this simulation. |
| Number of mass flows | 40 | The number of points on the calculated mass flow curve; 20 to 40 is typical. |
| Data | Enter | Note |
|---|---|---|
| Drawdown Relationship | Linear | Select from the dropdown list read more. |
| Constant parameter | ENTHALPY | From the dropdown list, select DRYNESS, TEMPERATURE or ENTHALPY. Normally select ENTHALPY, because this is easiest to measure. WellSim uses the enthalpy from the discharge simulation you specified; the value is in the Discharge parameters below. |
| Reservoir pressure | 200 | [barg] Enter the undisturbed reservoir pressure at the depth of the deepest feed read more. |
| Data | Enter | Note |
|---|---|---|
| Depth type | Measured | For Start depth and Finish depth. Choose from the dropdown list read more |
| Depth increment | 25 | [m] A small depth increment is more accurate but the output simulation takes longer; 50 m is usually a good value |
The window should look like this:
Measure all depths relative to the same point at the wellhead read more.
Click 
to check and, if necessary, adjust the data. Correct any errors and repeat.
Click 
to run the simulation:
Click 
(top of window):
If your graph does not look like this, change it.
This graph is typical: wellhead pressure increases as mass flowrate decreases; sometimes there is a 'nose' in the graph and for low pressures, pressure decreases as mass flowrate decreases.
Now, change the graph so it looks like:
Note
Red line: pressure at the deepest feedzone. The slope is the linear drawdown factor, 0.118 bar/t/hr. At zero mass flowrate, the pressure is 200 barg, the reservoir pressure.
Black line: pressure at the wellhead. This doesn't look like a normal output curve because the pressure axis is expanded; you can see the pressure maximum at 40 to 50 t/hr.
The TopDown discharge simulation you specified for this output simulation is Better TD 100 t/hr. And for this, the difference in pressure between the wellhead and deepest feed was about 125 barg see here. On the graph above the horizontal distance between the red and black lines at 100 t/hr is this 125 barg.
The difference in pressure between the wellhead and deepest feed is remarkably similar over the wide range of mass flowrates. This is because as mass flowrate increases:
and these two changes tend to cancel each other out. Note that friction does not increase linearly with flow rate because friction different flow regimes have different friction losses.
Save this new output simulation:
Click 
to save the output simulation and return to the output simulations window.
In the output simulations window, right-click the row with this output simulation, From better TD 100 t/hr, and click Edit to open this simulation again.
For very high mass flowrates, the well chokes because there is not enough pressure to overcome friction along the bore. If WellSim detects this during an output simulation, it gives an error:
WellSim will calculate valid results up to the highest mass flow rate the well can produce. For example, if you set Maximum mass flow to 600 t/hr, then the simulation fails above about 500 t/hr, and the result is:
Get rid of the error by decreasing Maximum mass flow, or just don't worry about it.
If you reduce Number of Mass Flows then the simulation runs faster, but the graph is less accurate. For example, for a mass flowrate of 30 to 300 in 10 steps, the graph is:
Finally:
to ignore these last simulations and return to the output simulations window.In MY WELL's measured output curves, tag 
Output 1.
Click Graph, click Apply to 2nd worksheet.
In MY WELL's output simulations, tag From better TD 100 t/hr.
Click 
to compare the measured and calculated output curve:
If your graph does not look like this, change it.
If your measured output curve was made after the well had reached stable conditions, you would believe this rather than the output simulation. If they differ, WellSim can be used to explore the reasons for it and to find out more about the well; for example try modelling different drawdown relationships or correlations. However, differences can often be caused by things such as diffuse feedzones over a wide depth range rather than a limited number of point feedzones, scaling or obstruction in the well affecting flow. Output tests are a blunt instrument for this - it would be better to run flowing PT runs and compare these with discharge simulations. If this is properly done, then output simulations are more likely to match measured data.
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