ResvEngine

Black-oil correlations · ResvEngine standard pack

Vasquez–Beggs (1980)

The ResvEngine calculator uses Vasquez–Beggs for solution gas-oil ratio Rs, oil formation volume factor Bo, and bubble-point pressure Pb. Gas compressibility Z and Bg come from a separate DAK pack — see gas Z correlations. For Pb from a measured Rs, see the bubble-point calculator.

What VB covers

  • Rs — dissolved gas below bubble point
  • Bo — saturated oil FVF; undersaturated branch via oil compressibility
  • Pb — from measured value or inverse of Rs at bubble
  • Not in VB: gas Z, water Bw, viscosity

Solution GOR (field form)

Original correlation is fitted in field units: pressure in psia, temperature in °F, Rs in scf/STB. ResvEngine converts Metric inputs (bar, °C, Sm³/Sm³) internally before applying the same coefficients.

Rs=C1 γg pC2exp⁡ ⁣(C3 APIT+460)R_s = C_1 \, \gamma_g \, p^{C_2} \exp\!\left( \frac{C_3 \, \mathrm{API}}{T + 460} \right)

Coefficients C1, C2, C3 depend on oil API gravity (≤30 vs >30 API). Below bubble point, Rs follows this curve; above bubble, Rs = Rsi (constant).

Oil FVF (saturated branch)

Bo=1+A1Rs+A2(T−60)APIγg+A3Rs(T−60)APIγgB_o = 1 + A_1 R_s + A_2 (T - 60)\frac{\mathrm{API}}{\gamma_g} + A_3 R_s (T - 60)\frac{\mathrm{API}}{\gamma_g}

For undersaturated oil (p > Pb), Bo is extended with oil compressibility co from the same VB family.

Try it

Defaults follow a typical black-oil demo (API 35, γg = 0.65, T = 60 °C, Pb ≈ 105 atm). Charts show Rs(p) and Bo(p); open the full calculator for tables, reference overlay, and export.

Interactive Rs and Bo (Vasquez–Beggs)

Black-oil flash at fixed T and Pb. Below bubble, Rs follows the VB saturation branch; Bo uses saturated and undersaturated VB.

Auto-updates as you edit
Adjust inputs to plot.
Adjust inputs to plot.
Open full calculator with these inputs

Full calculator: results table and PVTO/PVTG (.inc) export on the same METRIC basis.

Applicability & limits

  • Black-oil tables for material balance or simulator drafts — not lab EOS tuning. Tank energy plot: Havlena–Odeh.
  • Standard conditions in ResvEngine: METRIC (1.01325 bar, 15.5 °C), not editable.
  • High sour gas or extreme P/T may fail on the gas side (DAK), not VB alone.
  • We do not auto-rank dozens of correlations — one documented pack per property class.

Original paper

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

Open calculator