Introduction

Tutorials

Reference

Tutorial: Output simulations

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:

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.

Entering a TopDown output 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.

  1. In the sample database, open a new output simulation for .

  2. Enter this data:

Common headers

Data Enter
Description From better TD 100 t/hr

Output simulation parameters

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.

Drawdown parameters

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.

Discharge parameters

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.

Running the output simulation

  1. Click to check and, if necessary, adjust the data. Correct any errors and repeat.

  2. Click to run the simulation:

    • WellSim has calculated the linear drawdown factor at the deepest feedzone: 0.118 bar/t/hr.

    • To stop the run, click .

    • If you get the error Solution did not converge see here.

    • If you get this error, see below.

Interpreting an output 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

Now

Save this new output simulation:

  1. Click to save the output simulation and return to the output simulations window.

  2. 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.

Choked error: Output at high mass flow rates

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.

Changing the number of points on the output curve

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:

Comparing with a measured output curve

  1. In MY WELL's measured output curves, tag Output 1.

    Click Graph, click Apply to 2nd worksheet.

  2. In MY WELL's output simulations, tag From better TD 100 t/hr.

  3. Click to compare the measured and calculated output curve:

    If your graph does not look like this, change it.

If your output simulation does not match your measured output curve

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.