ResvEngine

Black-oil PVT · ResvEngine standard pack

Bubble-point pressure Pb

Pressure at which oil is saturated with dissolved gas. Below Pb free gas comes out of solution; above it, Rs stays at Rsi. Same Vasquez–Beggs pack as the full calculator and VB notes.

Try it

Demo: API 35, γg = 0.65, T = 60 °C, Rs = 80 Sm³/Sm³. Drag a slider; Pb sits beside the inputs.

Interactive bubble-point pressure

Drag Rs, API, gravity, or T. Pb updates in the browser (Vasquez–Beggs inverse).

Moves as you drag

Calculating Pb…

Equation with these numbers · Vasquez–Beggs (1980) · API > 30

API 35 · γg 0.65 · T 60 °C · Rs 80 Sm³/Sm³ (449.17 scf/STB). C₁=0.0178 · C₂=1.187 · C₃=23.93.

pb=[449.20.0178 0.65 exp⁡ ⁣(23.93⋅35/(140+460))]1/1.187p_b=\left[\frac{449.2}{0.0178\,0.65\,\exp\!\left(23.93\cdot35/(140+460)\right)}\right]^{1/1.187}

What Pb means

  • Saturated — p < Pb: Rs follows the VB saturation curve.
  • Undersaturated — p ≥ Pb: Rs = Rsi (constant).
  • Not EOS — black-oil correlation, not a lab CCE or cubic flash.

Inverse Vasquez–Beggs

The published fit is in field units: pressure in psia, temperature in °F, Rs in scf/STB. Metric inputs (bar, °C, Sm³/Sm³) are converted inside the engine. Coefficients C1, C2, C3 split at 30 API.

pb=[RsC1 γg exp⁡ ⁣(C3 API/(T+460))]1/C2p_b = \left[\frac{R_s}{C_1\,\gamma_g\,\exp\!\left(C_3\,\mathrm{API}/(T+460)\right)}\right]^{1/C_2}

Applicability & limits

  • Black-oil tables for material balance or simulator drafts — not lab EOS tuning.
  • Standard conditions in ResvEngine: METRIC (1.01325 bar, 15.5 °C), not editable.
  • Viscosity is not computed. Gas Z uses a separate DAK pack.

Original paper

M. Vasquez, H.D. Beggs, Correlations for Fluid Physical Property Prediction, J. Pet. Technol., SPE-6719-PA, 1980. Read SPE-6719-PA