fluid_properties · black oil
Oil Density (Reservoir Conditions)
ρo = (350·γo + 0.0764·γg·Rs) / (5.615·Bo) [lbm/ft³]
click formula to derive ↑
Inputs
°API
—
scf/STB
RB/STB
Description
Computes live (reservoir-condition) oil density from a mass-balance identity: the mass of stock-tank oil plus the mass of dissolved solution gas, divided by the reservoir oil volume. Unlike the other PVT calculators in this discipline, this is an exact identity given Rs and Bo — not a fitted correlation — so its accuracy is limited only by how well Rs and Bo represent the fluid, not by any regression range.
Variables
| Symbol | Unit | Description |
|---|---|---|
| ρo | lbm/ft³ | Oil Density (Reservoir) |
| °API | °API | Stock-tank oil API gravity, used to compute the stock-tank specific gravity γo = 141.5/(131.5+API). |
| γg | — | Solution gas specific gravity (air = 1.0), from a separator gas analysis. |
| Rs | scf/STB | Solution gas-oil ratio at the pressure of interest, from black_oil_pvt or a PVT report. |
| Bo | RB/STB | Oil formation volume factor at the same pressure as Rs, from black_oil_pvt or a PVT report. |
Assumptions
- Rs and Bo are evaluated at the same reservoir pressure and represent the same fluid state
- Stock-tank oil specific gravity is adequately represented by API gravity (γo = 141.5/(131.5+API))
- No water or solid content in the oil sample being characterized
Limitations
- This is an identity, not a correlation — it has no independent fittedRange; all uncertainty passes through from the Rs and Bo values supplied
- Does not account for non-hydrocarbon gas content (CO2, H2S, N2) beyond what is already folded into the input γg
- Not valid above the bubble point unless Rs and Bo are the correct undersaturated values at that pressure (Rs = Rsb, Bo from undersaturated expansion)
Use Cases
- → Material balance density input: Supply reservoir oil density to gradient, PVT-consistency, and gravity-segregation checks that require a density rather than a volume factor.
- → PVT report cross-check: Cross-check a lab-reported live oil density against Rs and Bo from the same PVT report — a large mismatch flags an inconsistency in the reported data.
- → Wellbore fluid gradient: Estimate the flowing gradient contribution from oil phase density for multiphase pressure-drop calculations.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring oils (API 35–48°, Rs 500–1,200 scf/STB, Bo 1.20–1.45) typically compute to ρo ≈ 42–47 lbm/ft³ (670–755 kg/m³) at bubble point.
Global
Heavier, low-GOR oils sit near 50–55 lbm/ft³; light, high-GOR volatile oils can fall below 35 lbm/ft³ — values outside roughly 30–55 lbm/ft³ usually indicate an Rs/Bo mismatch rather than a real fluid.
References
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