fluid_properties · pvt
Gas-Oil Ratio (GOR)
GOR = Qg (scf/d) / Qo (STB/d)
click formula to derive ↑
Inputs
Mscf/d
STB/d
Description
Computes the producing gas-oil ratio — surface gas rate divided by surface oil rate — a fundamental fluid characterization and production surveillance metric. Its magnitude classifies the reservoir fluid type, and trends over time reveal changes in reservoir drive mechanism.
Variables
| Symbol | Unit | Description |
|---|---|---|
| GOR | scf/STB | Gas-Oil Ratio |
| Qg | Mscf/d | Surface gas production rate, measured at the separator or test facility. |
| Qo | STB/d | Surface oil production rate, measured at the separator or test facility, at the same time as the gas rate. |
Assumptions
- Gas and oil rates are measured concurrently at the same separator conditions
- No significant gas venting, flaring, or measurement error skewing one rate relative to the other
- Rates represent a representative, stabilized production period (not a transient test)
Limitations
- A single GOR measurement does not distinguish solution gas drive from gas cap breakthrough or coning — trend analysis over time is needed
- Separator conditions (pressure/temperature) affect the gas-liquid split and should be held consistent for trend comparisons
- Not a substitute for full PVT analysis (Rs, Bo, Bg) when fluid properties are needed for material balance
Use Cases
- → Fluid type classification: Use the producing GOR magnitude to classify the reservoir fluid as black oil, volatile oil, or gas condensate for development planning.
- → Production surveillance: Track GOR over time — a rising trend in a previously stable well signals gas cap breakthrough, coning, or reservoir pressure depletion below the bubble point.
- → Separator and facility design: Use expected GOR to size separators, compressors, and gas handling facilities for a new well or field.
Related Calculations
Region Notes
Permian Basin
Wolfcamp wells often start with GOR of 1,000-3,000 scf/STB and increase significantly over the well's life as reservoir pressure depletes below the bubble point.
Eagle Ford
Condensate-rich windows can show producing GOR of 3,000-10,000+ scf/STB; the volatile-oil to gas-condensate transition occurs across the play from south (oil) to north (gas).
Gulf of Mexico
Deepwater Miocene oil reservoirs commonly produce at GOR of 500-1,500 scf/STB near the bubble point, with sharper increases once free gas develops.
Global
A sudden step-change increase in GOR (rather than a gradual trend) often indicates a mechanical issue (e.g., gas lift valve failure or tubing leak) rather than a reservoir effect — check surface equipment first.
References
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