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Tutorial: Creating TETRAD tables

TETRAD is software that models a geothermal reservoir. TETRAD uses measurements or estimates in the reservoir to calculate predicted conditions (like temperature and pressure) throughout the reservoir. It is then necessary to compare the predicted conditions to measured conditions to check the accuracy of TETRAD's model.

One way of doing this is to use measured conditions at the wellheads of reservoir wells. To do this:

• You must supply TETRAD with tables of data for the wells, relating conditions at each well's feedzone to conditions at the wellhead.
• TETRAD will then take its predicted conditions at each well's feedzone and interpolate in these tables to predict each wellhead's conditions.
• You can then compare the predicted and measured conditions at each wellhead.

Each well is assumed to be discharging at a steady flowrate and to have no secondary feedzones.

This tutorial shows how to use WellSim to create such TETRAD data tables for wells. Note that supplying TETRAD with such tables is optional; you can check the accuracy of the model in other ways.

Overview

To calculate a TETRAD data table, you specify a well geometry, a feedzone depth, formation temperatures and ranges of values of feedzone data. For each combination of the feedzone data, WellSim then calculates a BottomUp discharge simulation to find the corresponding wellhead conditions.

You can enter feedzone data separately for:

• Single-phase liquid: enter temperatures, pressures and mass flowrates.
• Two-phase liquid and steam: enter drynesses, pressures and mass flowrates. For dry steam, enter dryness = 1 [mass ratio] in the two-phase data.

However, don't try too hard to guess expected values of feedzone data, usually just enter a wide range of feedzone data and WellSim will do all the calculations and TETRAD will use what data it needs.

For each combination of the feedzone data, WellSim calculates a BottomUp discharge simulation to find the corresponding wellhead conditions. For example, for single-phase, if you enter 10 values of temperature, 8 values of pressure and 6 values of mass flowrate, then WellSim will calculate 10 x 8 x 6 = 480 BottomUp discharge simulations.

Many of these BottomUp discharge simulations will fail for various reasons, for example a particular combination of feedzone data might:

• be physically impossible
• cause the well flow to choke
• cause WellSim's simulation to fail unexpectedly.

These don't matter to TETRAD, since they usually result from feedzone conditions that will not be encountered.

If you enter many values of feedzone data then WellSim will perform many bottomUp discharge simulations, which can take a long time. This has some implications:

• When you have finished editing a TETRAD table, WellSim writes the TETRAD tables when you click Save, not after you have clicked Run. It writes the table using the results of the last run. This allows to to write a TETRAD table quickly, without having to take the time to do the run each time.

• If you want to use many values of feedzone data, then you can:

  1. First do a run with the full range of feedzone data that you want, but with a few values of each, say five of each.

  2. Look at the results to find where the BottomUp discharge simulations has failed.

  3. Try to reduce the range of the feedzone data to eliminate BottomUp discharge simulations that fail, then rerun it. For example, if all simulations for a minimum pressure of 20 barg fail, then increase the minimum pressure.

  4. Finally, do the run with this range of feedzone data and the number of feedzone values you want.

1 - Entering data for a TETRAD table

This uses five values of each feedzone data type so it will run quickly.

1. Go to TETRAD tables in WellSim, right-click and create a new TETRAD table.

2. Enter this Header data:

   

   Input data type

   Choose Single-phase data only, Two-phase data only or Single-phase and two-phase data according to what feedzone data you want to enter.

   Enthalpy option:

   The enthalpy calculation field relates to the values that will be stored in the enthalpy differences section of the TETRAD Wellbore Table. TETRAD can determine wellhead enthalpies as well as wellhead pressures from the data in the Wellbore Tables. Rather than explicitly using wellhead enthalpy values in the Wellbore Table for trilinear interpolation of the actual enthalpy value, TETRAD uses the enthalpy difference between wellbottom and wellhead.

   The options are:

   • Adiabatic where the enthalpy changes from wellbottom to wellhead, but the effects of heat transfer between the well and the surrounding rock formation are ignored.
   • Isenthalpic where the enthalpy difference is set to zero for every entry in the TETRAD Wellbore Table. (Note however, that in WellSim's internal calculations, the simulations are all performed adiabatically, as far as calculating each wellhead pressure value is concerned).
   • Heat Transfer the effects of heat transfer are incorporated into each simulation if a static temperature profile exists for the specified well.

   Depth increment and Number of retries:

   The depth increment is used in just the same way as for any normal. However, unlike a normal discharge test simulation, WellSim may try the simulation again should it fail, due to the non-convergence of the solution. The benefit of this is that the TETRAD table should be as complete and as continuous as possible, and you do not want zero entries in the table except for cases where the flow would physically choke. In some cases the simulation may not converge, simply due to the choice of depth increment. Depending on the number of retries specified (which can be set to zero), WellSim will reproduce the unsuccessful simulation, but will use a smaller depth increment. At each retry, WellSim will multiply the depth increment by 0.67.

3. Enter the feedzone data. You can enter the feedzone data:

   • either as a minimum, maximum and number of values: this is easy for initial trials
   • or as a table of numbers: this is slower to enter but lets you concentrate values near the expected values.

   Enter a minimum, maximum and number of values here, because it is faster.

   1. Enter this single-phase feedzone data:

      

   2. Enter this two-phase feedzone data:

      

4. Enter these well names into the Assigned well names:

   

   If there is more than one well with a similar geometry, feedzone depth and formation temperature then the same TETRAD table will apply to each well. WellSim lets you enter the applicable well names for each TETRAD table. WellSim does not check that the wells exist.

2 - Creating a TETRAD table

1. Click Check to check the data you have input. Here there is an error message:

   

   

   If error messages like this about the feedzone data indicate a combination of conditions that is physically impossible. You can either adjust a minimum or maximum until it does not give these errors when you click Check, or you can just ignore it and still Run the table. Here we'll ignore it.

2. Click Run to calculate the wellhead data for the tables. At the end of a run WellSim displays a message to remind you that it has calculated the TETRAD table data, but it has not yet written the ASCII text file for TETRAD to disk yet:

   

   Click the tab Calculated single-phase results or Calculated two-phase results:

   

   or

   

3. Look at the calculated results and check there are not too many errors. The error codes are:

   Code	Meaning
   A	Solution did not converge
   C	Fluid properties possibly out of range
   D	Data violates specified fluid type
   E	Data is underspecified
   F	NaCl concentration is greater than liquid fraction: specifying dry or almost dry steam with salt concentration is not possible
   G	Unspecified computational error
   H	Preprocessor did not successfully complete calculations
   I	Simulator did not successfully complete calculations
   J	Divide-by-zero error
   K	Flow choked: pressure and temperature probably too low to initiate discharge, or flowrate too high.
   M	Temperature above critical point for specified pressure and dryness: WellSim tries to find a temperature that will result in the specified dryness at the specified temperature.

   In this tutorial, there are not too many errors. If your live data has too many errors then you might try to change the feed data until it doesn't give so many errors when you click Run:

   • For errors C, D, E, F, H, K and M, change minimum or maximum values.
   • For errors A and G, change the corresponding feed data slightly or change the depth increment.
   • For errors I and J, try both things suggested above.

   Remember that the reason you are changing the feed data to reduce errors is mostly to decrease the number of BottomUp discharge calculations to be done; TETRAD will ignore any data points with such errors.

3 - Save - writing the TETRAD tables

When you save the data set, you have the option of writing the calculated TETRAD data to a text file, for sending to TETRAD.

1. Click Save. WellSim shows where the text file will be written:

   

   The message above also says that the text file already exists and it will be overwritten. To save an existing file, switch to the Windows Explorer, rename the existing file and switch back to WellSim to write the new text file.

2. If the calculated TETRAD results have errors, as they probably will, then WellSim asks what to do with them:

   

   • To write an text file to send to TETRAD, click Yes to convert the error codes in the text file to zeroes, which TETRAD will ignore.
   • To write an text file so you can check for errors, click No to leave the error codes in the file. Do not send such a file to TETRAD!

   In this tutorial click No so we can look at the errors.

3. Finally, WellSim confirms that it has written the text file, and gives its path:

   

   Note the path name (the folders and file name).

4. If you are writing more than one TETRAD table for a geothermal field to send to TETRAD, write each as described above, then use a text editor to join them together in one text file and send this file to TETRAD.

4 - The ASCII text file

Use a text editor to open the ASCII text file path you noted above.

Start of text file

This has:

• lines of comments
• then lines of the calculated results, after 'GWBORE'.

End of text file

This has:

• the end of the lines of the calculated results
• a list of the wells assigned to the results, after 'IGTABW'
• the legend of error codes

Note This tutorial file has the error codes in the lines of data, plus the legend of error codes. These must not be sent to TETRAD. To write an ASCII text file to send to TETRAD, click Yes to convert the error codes in the text file to zeroes and to not write the legend.

5 - Graphing the TETRAD data

Graphing the single-phase results.

1. At TETRAD tables, tag the line you just entered and click the Detail table tab (for single-phase results).

2. Click Multiple graph.

3. You may now see an error like this, ignore it:

   or

   Such errors happen when you change a TETRAD table's Multiple Graph from single and two-phase, or back, and the default independent axis now does not exist. Go to Settings and change the independent axis, as we do below.

4. Click Group.

   Now move Liquid feed pressure and Feed temperature from Sort fields to Group fields:

   1. Control-click Liquid feed pressure and Feed temperature.

   2. Click to move these two fields to Group fields.

      

   3. Finally click OK.

5. Click Settings. Change the axes and visible fields to:

   

   Finally click OK.

6. The graph is:

   

Note Group by Liquid feed pressure and Feed temperature as above to to see the variation of wellhead pressure with mass flow. Group by other fields to see different graphs, for example to see the variation of wellhead pressure with temperature, then group by pressure and mass flowrate.

Graphing the two-phase results.

Repeat the above steps, but this time:

• When you tag the data set to graph, click the Detail table 2 tab (for two-phase results).
• In Group, move Dryness and Two-phase feed pressure and from Sort fields to Group fields.
• In settings, change dependent axis to Two-phase mass flowrate.