Calculator inputs
Enter reservoir and standard conditions using the form below. The layout stays single column overall, while the input grid becomes three columns on large screens, two on smaller screens, and one on mobile.
Example data table
These sample cases use absolute pressure, standard conditions of 14.7 psi and 60°F, and a standard compressibility factor of 1.00.
| Case | Reservoir Pressure (psi) | Reservoir Temperature (°F) | z | Standard Gas Volume (scf) | Bg | Reservoir Volume (ft³) |
|---|---|---|---|---|---|---|
| Moderate reservoir | 2500 | 180 | 0.92 | 50000 | 0.00665876 | 332.94 |
| Lower pressure case | 1800 | 150 | 0.88 | 50000 | 0.00843130 | 421.57 |
| Higher pressure case | 3200 | 220 | 0.95 | 50000 | 0.00570770 | 285.39 |
Formula used
Main equation
Bg = (Psc / P) × (z / zsc) × (T / Tsc)
Volume conversion
Vres = Bg × Vsc
- Bg is the gas formation volume factor.
- P and Psc must be absolute pressures.
- T and Tsc must be absolute temperatures.
- z and zsc are gas compressibility factors.
- The result expresses reservoir gas volume per standard gas volume on the same cubic basis.
How to use this calculator
- Enter the reservoir pressure and choose its unit and basis.
- Enter reservoir temperature and the reservoir compressibility factor.
- Enter standard pressure, standard temperature, and standard compressibility factor.
- Optionally enter a standard gas quantity if you want the reservoir gas volume.
- Click the calculate button to show results above the form, then export CSV or PDF if needed.
FAQs
1) What does gas formation volume factor mean?
It measures how much space a gas occupies at reservoir conditions compared with the same gas at standard conditions. The factor links standard gas volume to in-situ reservoir volume.
2) Why must pressure be absolute?
Gas volume relations depend on the true thermodynamic pressure, not gauge pressure. Gauge values exclude atmospheric pressure, so they must be converted before using the equation.
3) Why must temperature be absolute?
The gas law uses absolute temperature scales only. Celsius and Fahrenheit must be converted to Kelvin or Rankine first to prevent incorrect volume-factor results.
4) What is the role of the z-factor?
The z-factor adjusts for real-gas behavior. When gas deviates from ideal behavior, z corrects the volume relationship so the result better matches actual pressure and temperature conditions.
5) Is the standard compressibility factor always 1.0?
Often it is very close to 1.0 under common standard conditions, but not always. If you have measured or specified standard-condition gas data, enter the more accurate value.
6) What does the optional standard gas volume do?
It converts the dimensionless factor into an actual reservoir gas volume. The calculator multiplies the entered standard gas quantity by Bg to estimate in-reservoir occupied volume.
7) Why does Bg usually decrease when pressure increases?
At higher reservoir pressure, gas becomes more compressed, so each standard unit of gas occupies less reservoir space. That lowers the formation volume factor, all else remaining unchanged.
8) Is this suitable for laboratory-certified design work?
It is useful for education, screening, and quick checks. For critical design or reserves work, confirm inputs and final values with validated PVT laboratory data and project standards.