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EOS Tuning

After having successfully run a C7+ characterization, the fluid model is usually tuned to match experimental results. This means that certain parameters are altered so that the fluid model can more accurately predict the fluid behavior. Note that if you do not want to complete this step, you can save the fluid model without performing any kind of tuning, but you will get a warning in whitsonPVT*. Further theory on EOS tuning is described in the modules/EOS Tuning*** section.

In whitsonPVT, the default parameters set to be tuned are limited to the Binary Interaction Parameters (Commonly referred to as BIPs or kij). Generally, you want to perform as little tuning as possible, while still capturing the fluid behavior, which is why we recommend starting with a simple BIPs tuning and then adding further parameters progressively.

Tuning parameter order

In the EOS Tuning Module, the following order for tuning parameter addition is recommended. You can find more information on the various parameters in that section.

  1. Critical temperature (Tc) ramping.

  2. Critical pressure (pc) ramping.

  3. Critical temperature heaviest fraction (Tc) CN+ multiplier.

  4. Critical pressure heaviest fraction (pc) CN+ multiplier.

  5. Normal boiling point temperature (Tb) ramping.

  6. Normal boiling point temperature heaviest fraction (Tb) CN+ multiplier.

Hence it may be advisable to use a case with minimal tuning if it yields satisfactory results, as this means the risk of overtuning is reduced. Overtuning simply means that the model has been forced to fit experimental results, by no longer representing the original fluid properties.

Case Selection

The first step in the EOS tuning, is to choose the C7+ characterization case you would like to build on. The software should auto-select your most recent case, so you may only need to verify the case selected is the characterization you would like to develop. As seen below, you can select between your saved C7+ characterization cases by clicking on the dropdown menu icon.


Additionally, it is on this tab that we could select our weight factors by hitting the + symbol, where we decide what experimental values we would like to be prioritized during the tuning process. We could also remove certain experiments from the weighting through the 'IGNORE' button.

Tuning Parameters

The 'Parameter Tuning' tab allows for selection of what properties should be tuned to by selecting the 'lock' sign. As for the BIPs tuning seen below, an unlocked sign means the property will be altered.


In contrast, a locked sign means the property will not be changed from its inital value. Simply toggle the lock symbol to add or remove tuning properties. In the figure below, we see critical pressure (Pc) with the default of settings.


If you wish to revert your changes to the default parameters, you can simple select the 'Default' button located at the bottom of the various tabs.

Weight Factors

As previously described, there is an option to value certain experimental values more than others in the 'EOS Tuning Basis Information'. Once the '+' has been clicked, the following pop-up will appear.


In the pop-up, we see the various experiments, along with their associated properties. Currently, the CCE is selected with its default values. One may observe that the bubblepoint & saturation pressures are set to values of 15. This means that the regression will prioritize matching these values more than lower numbers, and changing this number would affect this priority, referred to as a weighting. For inexperienced users, it is generally adviced to utilize the standard weighting here - though it could be wise to investigate the effects.

Ignore Experiments

Clicking the 'IGNORE' button will generate the following pop-up.


In this selection, we can choose to not factor for certain experiments in the tuning by ticking off the 'Ignore' checkboxes to the right. This can be adventageous if we have a faulty experiment or data of uncertain quality.

Compositional Adjustment

The last tuning parameters we can play with in whitsonPVT, is the compositional adjustments. The software allows us to slightly alter individual samples' GOR and molecular weights (MW). If we suspect some inaccuracy here, this feature will do a regression on these properties, so that the sample data better matches that of fluid model predictions.


In the picture above, we see that the gas sample has been selected for MW tuning, while the oil sample has been selected for GOR tuning. To the right, you see an option named 'Bounds', which allows the user to set the max and minimum limits of these variables. The default settings ensure whatever tuning performed will not change C1 more than 1 mole% and MW 5%, but these can be increased to 3 mole% for C1 (GOR) and 20% for the MW.

Next Steps

The next step in the fluid model development is to develop a viscosity model!