fluid_properties · phase behavior
Bubble Point Pressure (K-Value / Compositional)
Solve Σ(zi·Ki(P)) = 1 for P, using Wilson K-values for a two-pseudo-component mixture
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Inputs
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°F
°F
psi
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°F
psi
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Description
Computes bubble point pressure for a simplified two-pseudo-component mixture (a light and a heavy fraction) by solving Σ(zi·Ki) = 1 for P using Wilson K-values — a compositional, EOS-adjacent method. This is a materially different approach from bubble_point_pressure (Standing / Vasquez-Beggs), which is an empirical correlation fit directly to measured Pb, Rs, °API, and gas gravity with no composition input at all. Use this calculator when you have (or can estimate) a two-fraction mixture composition and component critical properties; use bubble_point_pressure when you have field PVT data (solution GOR, API gravity, gas gravity) instead.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Pb | psi | Bubble Point Pressure |
| z1 | — | Overall mole fraction of the light pseudo-component (e.g., methane / gas-cap fraction). The heavy pseudo-component's mole fraction is z2 = 1 − z1. |
| T | °F | Reservoir (or system) temperature at which the bubble point is evaluated. |
| Tc1 | °F | Critical temperature of the light pseudo-component. Default shown is methane (Tc = 190.6 K = −82.55°C). |
| Pc1 | psi | Critical pressure of the light pseudo-component. Default shown is methane (Pc = 4600 kPa = 667.0 psia). |
| ω1 | — | Acentric factor of the light pseudo-component. Default shown is methane (ω = 0.008). |
| Tc2 | °F | Critical temperature of the heavy pseudo-component. Default shown is n-heptane (Tc = 540.2 K = 267.05°C), a common single-carbon-number stand-in for a stock-tank-oil pseudo-component. |
| Pc2 | psi | Critical pressure of the heavy pseudo-component. Default shown is n-heptane (Pc = 2740 kPa = 397.3 psia). |
| ω2 | — | Acentric factor of the heavy pseudo-component. Default shown is n-heptane (ω = 0.349). |
Assumptions
- The mixture can be adequately represented by two lumped pseudo-components (light and heavy) with single representative critical properties each — a simplification of a real multi-component reservoir fluid
- Wilson K-values apply (ideal solution, ideal gas, low-to-moderate pressure, away from the true mixture critical point)
- z1 given is the true overall (bubble-point liquid) composition, not a surface/producing-stream composition
Limitations
- This is a two-pseudo-component simplification, not a full N-component compositional bubble point — a real reservoir fluid characterization typically uses 6–20+ components/pseudo-components; results should be treated as an order-of-magnitude / screening estimate, not a substitute for a proper EOS flash with full composition
- Inherits all of Wilson K-value's limitations: no real-mixture fugacity corrections, and unreliable near the mixture's true critical point or at high pressure
- Distinct from bubble_point_pressure (Standing / Vasquez-Beggs): that calculator is an empirical correlation fit to measured field PVT data (Rs, °API, gas gravity) with no composition required, and is generally more reliable for typical black-oil reservoir work when field PVT data are available. Use this calculator only when composition and component critical properties are known or reasonably estimated, e.g. from a lab compositional analysis lumped into two fractions
Use Cases
- → Compositional screening: Given a lab compositional analysis lumped into a light and heavy fraction, get a quick bubble point estimate without running a full EOS flash package.
- → Sensitivity to composition: Explore how bubble point shifts as the light-fraction mole fraction z1 changes — useful for understanding gas-injection or blending scenarios conceptually.
- → Cross-check against empirical correlations: Compare against bubble_point_pressure (Standing/Vasquez-Beggs) computed from the same fluid's field PVT data as a rough consistency check between compositional and empirical methods.
Related Calculations
Region Notes
Global
Two-pseudo-component bubble point behavior is a teaching/screening simplification with no regional calibration — for basin-specific work, prefer bubble_point_pressure (calibrated empirical correlations) or a full compositional EOS model with basin-representative fluid composition.
References
Primary source
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