Engineering lesson
Porosity versus permeability why storage is not flow capacity
Porosity describes void volume; permeability describes how readily a connected pore system transmits fluid. One cannot be inferred uniquely from the other without rock-specific evidence.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Separate a storage calculation from a flow calculation
A rock volume multiplied by porosity gives pore volume. Darcy flow additionally depends on permeability, viscosity, geometry and pressure gradient. Two samples with equal porosity can therefore store similar fluid volumes and transmit very different rates.
Pore throats, connectivity, cementation and fractures help explain the difference. The teaching comparison varies permeability while holding porosity fixed; it does not propose a universal porosity–permeability law.
Use calibration only within its evidence
The site's permeability-calibration workflow fits a log-permeability trend to supplied core data. Its synthetic default data are not a regional geological model.
Core orientation, stress, fluid type, sampling and facies affect a fitted relationship. Mixing populations can produce an attractive regression line that represents neither population well.
Audit what a prediction can support
A porosity-derived permeability is a model estimate, not a direct measurement. Retain residuals and the sampled range with the fitted coefficients.
Before applying a core relationship to reservoir flow, consider scale and directional permeability. Storage at one scale and connected flow at another are not interchangeable observations.
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.
Permeability (mD) = 100. Darcy local rate (bbl/day): 2.8175; Pore volume (m³): 200.
- Darcy local rate (bbl/day)
Worked example
Equal storage, different flow capacity
Two synthetic rocks have porosity 0.2 and bulk volume 1,000 m³. A fixed linear Darcy geometry and pressure gradient are used to compare k.
Relation and unit basis
Vpore = φ Vbulk; qDarcy ∝ k at fixed A, ΔP, μ and L
Pore volume is 200 m³ in every row; flow uses the published field-unit linear Darcy defaults except for k.
1. Define the baseline
Permeability (mD) = 100
Two synthetic rocks have porosity 0.2 and bulk volume 1,000 m³. A fixed linear Darcy geometry and pressure gradient are used to compare k.
2. Select the relation
Vpore = φ Vbulk; qDarcy ∝ k at fixed A, ΔP, μ and L
Pore volume is 200 m³ in every row; flow uses the published field-unit linear Darcy defaults except for k.
3. Substitute the values
Vpore = 0.2 × 1,000 = 200 m³; at k = 100 mD, q = 0.001127 × 100 × 500 × 100/(2 × 1,000) = 2.8175 bbl/day
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Darcy local rate (bbl/day) = 2.8175; Pore volume (m³) = 200
Permeability changes flow capacity without changing the stipulated void volume.
Result and interpretation
Darcy local rate (bbl/day) = 2.8175; Pore volume (m³) = 200
Permeability changes flow capacity without changing the stipulated void volume.
Equal pore volume is the control; changing permeability alters Darcy rate without changing storage.
Exact scenario data
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
| Permeability (mD) | Darcy local rate (bbl/day) | Pore volume (m³) |
|---|---|---|
| 10 | 0.28175 | 200 |
| 25 | 0.704375 | 200 |
| 100 | 2.8175 | 200 |
| 250 | 7.04375 | 200 |
| 1,000 | 28.175 | 200 |
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. If permeability doubles at fixed porosity, does pore volume double?
Show the worked answer
No. Pore volume is unchanged. Darcy rate doubles only if the other flow inputs and linear assumptions are fixed.
2. Reproduce the baseline Darcy local rate (bbl/day) without rounding intermediate values.
Show the worked answer
Vpore = 0.2 × 1,000 = 200 m³; at k = 100 mD, q = 0.001127 × 100 × 500 × 100/(2 × 1,000) = 2.8175 bbl/day. Darcy local rate (bbl/day) = 2.8175. Permeability changes flow capacity without changing the stipulated void volume.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Permeability (mD) changes from 10 to 1000. Other controls remain as stated in the premise. Darcy local rate (bbl/day) changes from 0.28175 to 28.175.
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. Pore volume is 200 m³ in every row; flow uses the published field-unit linear Darcy defaults except for k. Permeability changes flow capacity without changing the stipulated void volume.
Before using the result
- Keep storage and flow units separate.
- Record facies and measurement conditions.
- Avoid extrapolating a fitted trend beyond its data.
Permeability changes flow capacity without changing the stipulated void volume.
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.