fluid_properties · gas properties
Gas Formation Volume Factor (Bg)
Bg = 0.02829 × Z × T(°R) / P(psia) [ft³/scf]
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Inputs
psi
°F
—
Description
Computes gas formation volume factor (Bg) — the volume of gas at reservoir conditions relative to its volume at standard surface conditions — directly from the real gas law with a user-supplied z-factor. Bg is a required input to GIIP, P/Z material balance, and gas recovery factor calculations.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Bg | ft³/scf | Gas FVF (Bg) |
| P | psi | Reservoir pressure at which Bg is evaluated. |
| T | °F | Reservoir temperature in °F (converted to Rankine internally). |
| Z | — | Gas compressibility (deviation) factor at P and T, from a PVT correlation (e.g., Hall-Yarborough, Standing-Katz chart) or lab measurement. Z = 1.0 for ideal gas; real gas Z deviates below 1 at high pressure and above 1 at very high pressure. |
Assumptions
- Z-factor is accurately known at the reservoir P and T of interest — all uncertainty in Bg enters through Z
- Standard conditions are defined as 14.7 psia and 60°F (520°R); some jurisdictions use slightly different standard conditions (e.g., 14.65 psia in Texas) — adjust P_sc/T_sc accordingly if required
- The gas composition is constant (no phase condensation or compositional changes that would alter Z across the pressure range of interest)
Limitations
- This calculation is only as accurate as the supplied Z-factor — a Z error of ±5% propagates directly into a ±5% Bg error
- For gas condensate or retrograde condensate systems, liquid drop-out below the dew point alters the effective composition and Z-factor in ways this single-phase Bg equation does not capture
- Standard-condition definition varies by regulatory authority — confirm the basis matches the volumetric calculation (GIIP) that will consume this Bg
Use Cases
- → GIIP calculation input: Compute Bg at initial reservoir conditions to supply the required input to the Gas in Place (giip) volumetric calculator, avoiding manual Z-factor lookup and unit conversion.
- → P/Z material balance: Compute Bg at multiple historic pressure points to support a P/Z plot tracking reservoir depletion over time.
- → Gas recovery factor: Provide Bg at initial and abandonment conditions to the gas recovery factor calculator, which uses the Bg ratio to estimate ultimate recovery.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring gas-condensate: typical initial reservoir pressure 3,000–6,000 psi at 180–240°F. Bg at 4,000 psi and 200°F with Z≈0.88 is approximately 0.005–0.006 ft³/scf, consistent with the default in the giip calculator.
Haynesville
High-pressure (>10,000 psi) shale gas: Bg is very low (~0.003–0.004 ft³/scf) due to extreme compression. Z-factor at Haynesville conditions (>10,000 psi, >340°F) requires careful correlation selection — Hall-Yarborough or Dranchuk-Abou-Kassem give more reliable Z at these conditions than simpler correlations.
Appalachian Basin
Marcellus shale: lower reservoir pressures (2,000–4,500 psi) give Bg ≈ 0.006–0.012 ft³/scf. Z ≈ 0.85–0.95 for typical Marcellus gas compositions.
Gulf of Mexico
GOM deepwater gas reservoirs at 8,000–15,000 psi often exhibit Z > 1.0 (supercritical behavior) — ensure the Z-factor correlation being used is validated at ultra-high-pressure conditions.
References
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