Engineering lesson
Gas expansion factor versus gas formation volume factor
Gas expansion factor is the reciprocal of gas formation volume factor only when the volume units and reference conditions are complementary. Always write the numerator and denominator units.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Reverse the mapping, not just the number
If Bg = 0.005 reservoir ft³/scf, its reciprocal is 200 scf/reservoir ft³. If Bg is expressed in reservoir bbl/scf, the reciprocal is scf/reservoir bbl instead.
The number changes when the reservoir volume unit changes. Omitting the units makes two correct reciprocals look inconsistent.
Keep the state definitions attached
Bg depends on reservoir pressure, temperature and gas behavior relative to specified standard conditions. The standard state is part of the definition of scf.
A reciprocal conversion does not simulate expansion through a separator or calculate sales-gas recovery. It relates two stated volume bases for the same gas amount.
Use the factor in the right direction
Reservoir gas volume divided by Bg gives standard volume. Equivalently, reservoir gas volume multiplied by the complementary expansion factor gives the same answer.
The worked table compares these two paths. It is a unit audit, while the PVT worked lesson derives Bg from a specified thermodynamic state.
Explore the example
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Bg (reservoir ft³/scf) = 0.005. Expansion factor (scf/reservoir ft³): 200; Standard volume (scf): 200000.
- Expansion factor (scf/reservoir ft³)
Worked example
Complementary volume factors
Fixed reservoir gas volume 1,000 ft³; Bg uses reservoir ft³/scf throughout.
Relation and unit basis
Eg = 1/Bg; standard volume = Vres/Bg = Vres Eg
The reciprocal units are scf/reservoir ft³. The site's separate field correlations use rounded coefficients and can differ slightly from an exact reciprocal.
1. Define the baseline
Bg (reservoir ft³/scf) = 0.005
Fixed reservoir gas volume 1,000 ft³; Bg uses reservoir ft³/scf throughout.
2. Select the relation
Eg = 1/Bg; standard volume = Vres/Bg = Vres Eg
The reciprocal units are scf/reservoir ft³. The site's separate field correlations use rounded coefficients and can differ slightly from an exact reciprocal.
3. Substitute the values
1/0.005 = 200 scf/ft³; 1,000/0.005 = 200,000 scf
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Expansion factor (scf/reservoir ft³) = 200; Standard volume (scf) = 200000
Exact reciprocity requires complementary units and the same state definitions.
Result and interpretation
Expansion factor (scf/reservoir ft³) = 200; Standard volume (scf) = 200000
Exact reciprocity requires complementary units and the same state definitions.
The reciprocal sweep holds definitions fixed. Compare both conversion paths for the same reservoir gas volume.
Exact scenario data
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
| Bg (reservoir ft³/scf) | Expansion factor (scf/reservoir ft³) | Standard volume (scf) |
|---|---|---|
| 0.002 | 500 | 500,000 |
| 0.004 | 250 | 250,000 |
| 0.005 | 200 | 200,000 |
| 0.008 | 125 | 125,000 |
| 0.01 | 100 | 100,000 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. What is the reciprocal of 0.004 reservoir ft³/scf?
Show the worked answer
250 scf/reservoir ft³. The reciprocal unit is essential to the answer.
2. Reproduce the baseline Expansion factor (scf/reservoir ft³) without rounding intermediate values.
Show the worked answer
1/0.005 = 200 scf/ft³; 1,000/0.005 = 200,000 scf. Expansion factor (scf/reservoir ft³) = 200. Exact reciprocity requires complementary units and the same state definitions.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Bg (reservoir ft³/scf) changes from 0.002 to 0.01. Other controls remain as stated in the premise. Expansion factor (scf/reservoir ft³) changes from 500 to 100.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. The reciprocal units are scf/reservoir ft³. The site's separate field correlations use rounded coefficients and can differ slightly from an exact reciprocal. Exact reciprocity requires complementary units and the same state definitions.
Before using the result
- Write both volume units.
- Record standard conditions.
- Use the factor in the intended direction.
Exact reciprocity requires complementary units and the same state definitions.
Continue learning
Start with the broader workflow guide · Related method guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.