fluid_properties · black oil
Black Oil PVT — Pressure/Volume/Temperature (Rs and Bo)
Rs = γg[(P/18.2+1.4)×10^(0.0125API−0.00091T)]^1.205 | Bo = 0.972+0.000147F^1.175
click formula to derive ↑
Inputs
psi
°F
—
°API
—
Description
Computes solution GOR (Rs) and oil formation volume factor (Bo) at reservoir pressure and temperature using selectable black-oil PVT correlations. Rs quantifies how much gas is dissolved in oil at reservoir conditions; Bo converts between reservoir and stock-tank oil volumes. Both are required inputs to OOIP, material balance, and IPR calculations.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Rs | scf/STB | Solution GOR (Rs) |
| Bo | RB/STB | Oil FVF (Bo) |
| P | psi | Reservoir pressure at which Rs and Bo are evaluated. Must be at or below the bubble point for these correlations to apply (above Pb, all gas remains in solution and Rs = Rsi). |
| T | °F | Reservoir temperature. Typical subsurface range 100–300°F (38–149°C) for most producing intervals. |
| γg | — | Gas specific gravity (relative to air = 1.0), from a separator gas analysis or surface gas composition. Values below 0.65 indicate dry gas; 0.65–0.75 is typical for associated gas. |
| °API | °API | Oil API gravity — the standard measure of oil density. Light oil: >31°API, medium: 22–31°API, heavy: <22°API. |
| Method | — | 1 = Standing (1947) — California oils, most widely cited; 2 = Vasquez-Beggs (1980) — worldwide dataset, split at 30°API. Select the correlation calibrated to your basin or fluid type. |
Assumptions
- Reservoir pressure is at or below the bubble point — above Pb, Rs equals the initial solution GOR (Rsi) and Bo should be calculated from the undersaturated expansion, not these equations
- Standing correlation is calibrated to California crude oils (API 16–58°, GOR 20–1425 scf/STB); accuracy degrades outside this range
- Vasquez-Beggs uses uncorrected gas gravity γg — separator-gas-gravity correction (requires separator T and P) is omitted; this introduces a small systematic error that can be manually corrected if separator conditions are known
Limitations
- Neither correlation accounts for CO2, H2S, or N2 content in the gas phase — these non-hydrocarbon components alter PVT behavior and require corrected Z-factor or dedicated correlations
- Results should be validated against measured PVT laboratory data wherever available — correlations can err by 10–20% for fluids far from the original calibration dataset
- For volatile-oil or condensate systems (GOR > 3,000 scf/STB), neither Standing nor Vasquez-Beggs is reliable — use a full EOS-based PVT model instead
Use Cases
- → OOIP and material balance input: Compute Bo to convert reservoir-condition oil volumes to stock-tank conditions, required as inputs to ooip and material_balance_undersaturated calculations.
- → IPR model selection: Determine Rs at current reservoir pressure; if Rs has declined from Rsi, the reservoir is below the bubble point and Vogel's IPR (ipr_vogel) is appropriate rather than the linear PI.
- → Production forecast PVT: Evaluate how Rs and Bo change as reservoir pressure depletes, to accurately convert reservoir-condition flow rates to surface-condition volumes in production forecasts.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring crude oils: API 35–48°, γg 0.65–0.75, initial reservoir pressures 3,000–6,000 psi at 180–240°F. Vasquez-Beggs performs well for this range; Standing also gives reliable results. Initial Bo typically 1.20–1.45 RB/STB; Rs 500–1,200 scf/STB at bubble point.
Eagle Ford
Condensate-rich windows (38–50°API) often have GOR > 2,000 scf/STB — approaching the upper reliability limit for these correlations. Consider EOS modeling for volatile-oil characterization in the Eagle Ford.
Gulf of Mexico
GOM Miocene oils: API 25–40°, Bo typically 1.15–1.60 RB/STB. Standing generally adequate; Vasquez-Beggs preferred for heavier (< 30°API) GOM crude types.
North Sea
Brent Group and other North Sea light oils (35–42°API): consider the Glaso (1980) correlation, developed specifically from 45 North Sea crude systems, as an alternative to the two built-in methods. Not implemented here — apply directly if regional accuracy is critical.
Middle East Carbonates
Carbonate reservoir crude oils can span a wide API range (20–45°). Al-Marhoun (1988) was specifically calibrated to Middle East crudes; Vasquez-Beggs is a reasonable alternative if Al-Marhoun is unavailable.
References
Primary source
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