Compositional PVT

#Overview

The compositional workspace on the PVT page builds an equation-of-state (EOS) fluid model from a composition. It splits and lumps the plus fraction, computes the phase envelope, flashes and lab experiments, tunes the model to your lab data, and writes black-oil and compositional tables for the rest of the project and for your simulator. Use it for volatile oils, gas condensates and wet gases, or any time you have a full lab composition.

Switch to it with the workspace toggle Compositional (equation of state) next to the PVT title. A project can hold several EOS models; one of them can be the project fluid.

#Inputs

The Fluid panel on the left holds the model definition. The EOS engine works in absolute pressure (psia); in Metric, the page shows bara.

The panel subtitle reads, for example, "PR · 11 components entered · 15 after the heavy-end split", so you can see what the model holds after characterisation.

#Methods

Hydrate screening is a correlation, valid for 100 to 4,000 psia and gas gravity 0.55 to 0.90. A sour or CO₂-rich gas needs a thermodynamic hydrate model, which Converge does not include.

#Workflow

1. On PVT, select Compositional (equation of state). Pick a model with the Model switcher, or choose New model from a sample….
2. In Fluid, set Start from, the Equation of state, Reservoir temp. and Reservoir pressure, then enter the composition and plus fraction.
3. Read the tiles: Bubble point or Dew point, Fluid type, Separator GOR (or Condensate-gas ratio for a gas), and Stock-tank liquid in °API.
4. Open Heavy-end split to set Highest carbon number, the Critical-property method and the Lumping scheme.
5. Use Phase envelope for the envelope, critical point, cricondenbar and cricondentherm, and the hydrate line with an optional Inhibitor. Use Flash for a single pressure and temperature.
6. Use Lab experiments to simulate Constant composition expansion, Differential liberation, Constant volume depletion and Separator test. Set the Pressure schedule (psia) and the separator stages; the last stage is the stock tank, kept at 14.7 psia and 60 °F.
7. Run Data QC on the raw lab data before you tune. Choose the sample type (Bottomhole, Formation tester, Separator (recombined) or Wellhead) and review Checks passed, Warnings and Failures.
8. Tune the model to your lab data on Lab data & regression or Live tuning. See Tuning to lab data.
9. On Black-oil & export, tick Use this compositional fluid for the whole project if this model should be the project fluid, and download the tables you need.

#Tuning to lab data

On Lab data & regression, enter measurements under Measured lab data in the tabs Psat & separator, CCE, DL, CVD and Swelling. Tick the Parameters to adjust:

You can also tune individual pseudo-components under Individual pseudo-components. Viscosity is matched separately through the LBC critical volume, so it never disturbs the phase behaviour.

Press Run regression, or Quick advice for a deterministic recommendation based on each parameter's sensitivity (this works without the AI). The result table shows Measured, Before (error) and After (error) per measurement. Then choose Save as new model (keeps the current model and adds the tuned one to the library), Apply to this model or Discard.

Live tuning gives a full-width view with sliders and charts that update as you drag. It adds Regression weights presets (EOS defaults, Coats & Smart (SPE 11197), Saturation pressure only, Separator and densities first (practitioner default)), the options Keep heavy-end properties monotonic and No negative interaction coefficients, and three actions: Diagnose (sensitivity and correlation of the candidate parameters), Staged fit (saturation pressure by plus-fraction MW, then all phase behaviour, then viscosity) and Run regression. Apply writes the parameters into the active model; Save as new model keeps it and adds a tuned copy.

The AI Engineer can run the same tuning for you. See Fluid tuning with the AI Engineer.

#Interpreting results

• Saturation tile: the foot says whether the reservoir is saturated or undersaturated at the reservoir pressure you entered.
• Fluid type: McCain classification. Check it agrees with what you expect before you trust the separator numbers.
• Regression result: "Objective reduced N% in M model runs" heads the result. Parameters that hit a bound appear as warnings. A bound hit means the data may be inconsistent; do not force it.
• Lab experiments: the CCE liquid percent is 100 above the saturation pressure (single-phase liquid).
• Library and Compare: the Model view toggle switches between Workspace, Library and Compare. Compare overlays the phase envelopes of two or more models and lists saturation pressure, fluid type, GOR or CGR, critical point and whether each model is tuned to lab data.

#Common pitfalls

#Exports

On Black-oil & export:

Under Simulator exports, choose Deck units (FIELD or METRIC):

Phase envelope and Lab experiments each have a CSV button (eos-phase-envelope.csv, eos-cce.csv, eos-differential-liberation.csv, eos-cvd.csv).

#Related

• Black-oil PVT
• Project fluid
• Fluid tuning with the AI Engineer
• Simulation
• Glossary