Engineering lesson
Explore gas in place sensitivity to porosity and saturation
For fixed geometry and Bg, volumetric gas in place is proportional to porosity and to 1 − Sw. Saturation sensitivity depends on the gas-filled fraction, not on Sw alone.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Read multiplicative sensitivities correctly
A 10% relative increase in porosity gives a 10% relative increase in G when everything else is fixed. A 0.05 absolute increase in Sw reduces the gas fraction by 0.05, which is a different relative change.
At Sw = 0.25, changing to 0.30 reduces gas-filled pore fraction from 0.75 to 0.70, a 6.6667% decrease.
Avoid an unsupported probability interpretation
The scenario table changes saturation deterministically. It is not a P10/P50/P90 distribution and does not assume independent random errors.
Porosity, saturation, facies and net pay can be correlated. Combining isolated low and high values does not automatically create credible probabilistic bounds.
Preserve the fluid and classification basis
Bg is held at 0.005 reservoir ft³/scf in the example. A change in pressure or gas composition would require reconsidering that property.
The output remains initial gas in place under a volumetric model. Applying a recovery factor or assigning reserves requires separate evidence.
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.
Water saturation (fraction) = 0.25. Gas in place (scf): 7840800000; Gas-filled pore fraction: 0.75.
- Gas in place (scf)
Worked example
Gas occupancy sensitivity
160 acres, 50 ft net pay, φ = 0.15 and Bg = 0.005 reservoir ft³/scf; only Sw varies.
Relation and unit basis
G = 43,560 × 160 × 50 × 0.15(1 − Sw)/0.005
Sw is the water fraction of the same pore volume represented by φ.
1. Define the baseline
Water saturation (fraction) = 0.25
160 acres, 50 ft net pay, φ = 0.15 and Bg = 0.005 reservoir ft³/scf; only Sw varies.
2. Select the relation
G = 43,560 × 160 × 50 × 0.15(1 − Sw)/0.005
Sw is the water fraction of the same pore volume represented by φ.
3. Substitute the values
G = 348,480,000 × 0.15 × (1 − 0.25)/0.005 = 7,840,800,000 scf
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Gas in place (scf) = 7840800000; Gas-filled pore fraction = 0.75
An absolute saturation change must be compared with the remaining gas fraction to obtain its relative volume effect.
Result and interpretation
Gas in place (scf) = 7840800000; Gas-filled pore fraction = 0.75
An absolute saturation change must be compared with the remaining gas fraction to obtain its relative volume effect.
Saturation varies at fixed porosity. Use the gas-fraction ratio to check the output ratio.
Exact scenario data
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
| Water saturation (fraction) | Gas in place (scf) | Gas-filled pore fraction |
|---|---|---|
| 0.1 | 9.4090e+9 | 0.9 |
| 0.2 | 8.3635e+9 | 0.8 |
| 0.25 | 7.8408e+9 | 0.75 |
| 0.3 | 7.3181e+9 | 0.7 |
| 0.5 | 5.2272e+9 | 0.5 |
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. At Sw 0.5, what does a rise to 0.6 do to gas-filled fraction?
Show the worked answer
It falls from 0.5 to 0.4, a 20% relative decrease, despite only a 0.1 absolute saturation change.
2. Reproduce the baseline Gas in place (scf) without rounding intermediate values.
Show the worked answer
G = 348,480,000 × 0.15 × (1 − 0.25)/0.005 = 7,840,800,000 scf. Gas in place (scf) = 7840800000. An absolute saturation change must be compared with the remaining gas fraction to obtain its relative volume effect.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Water saturation (fraction) changes from 0.1 to 0.5. Other controls remain as stated in the premise. Gas in place (scf) changes from 9408960000 to 5227200000.
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. Sw is the water fraction of the same pore volume represented by φ. An absolute saturation change must be compared with the remaining gas fraction to obtain its relative volume effect.
Before using the result
- Distinguish absolute and relative changes.
- Keep Bg and geometry fixed in this sensitivity.
- Do not label deterministic scenarios as probability bounds.
An absolute saturation change must be compared with the remaining gas fraction to obtain its relative volume effect.
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.