Porosity tells us how much fluid a rock can store, and permeability tells us how easily that fluid can flow. The two are related, but one does not simply predict the other. This article explains how porosity and permeability are connected, what controls the link, and why a rock can have high porosity and still be a poor reservoir.
Porosity vs. Permeability
| Property | What it describes | Question it answers |
|---|---|---|
| Porosity | Void space as a fraction of the rock volume | How much can the rock hold? |
| Permeability | Ability of the rock to transmit fluids | How easily can fluids flow through it? |
Do Porosity and Permeability Go Together?
Often, but not always. In general, rocks with more connected pore space tend to be more permeable. But permeability depends on how the pores are connected and how large the pore throats are, not only on how many pores there are. Two examples show the difference:
- A clean sandstone with well-connected pores can have high porosity and high permeability.
- A shale or clay-rich rock can have fairly high porosity, yet its pores and throats are so small that permeability is extremely low.
This is why engineers always look at both properties together when judging reservoir rocks.
The Kozeny Equation
Many investigators have attempted to correlate permeability with porosity, grain size and shape, and packing. The most frequently used relation was developed by Kozeny:
k = Φ3 / (5 × Sv × (1 − Φ)2)
- k = permeability, cm² (1 cm² = 1.013 × 10⁸ Darcies)
- Φ = effective porosity (fraction)
- Sv = total grain surface per unit volume of reservoir, cm²/cm³
The equation shows two things. Permeability rises quickly as effective porosity rises. And it falls as the grain surface area (Sv) increases, which happens when grains are smaller. It is a theoretical relation for idealized packing, so real rocks, with cement, clay and irregular grains, usually need laboratory measurements.
Effect of Grain Size
The following images show how grain size and sorting relate to porosity and permeability. In an idealized packing, porosity is not affected by grain size, but permeability increases as grain size increases. Larger grains leave larger pore throats, so fluid meets less resistance.
Effect of Sorting
Porosity and permeability are both affected by sorting, and both increase with better sorting. In a poorly sorted rock, small grains fill the spaces between large grains, which lowers both properties.
Other Factors That Break the Link
- Cementation: cement reduces both porosity and permeability, and can seal pore throats.
- Clay minerals: clays can hold a lot of water and keep porosity high, but they block the throats.
- Fractures: open fractures add little to porosity but can raise permeability greatly (see secondary porosity).
- Dissolution: vugs and moulds may add porosity without connecting, so permeability stays low.
Why This Matters in the Field
A reservoir can only be produced economically if fluids reach the well at a useful rate. Porosity sets the volume of hydrocarbons in place, while permeability controls the flow rate. Both are measured on core samples in the laboratory; see laboratory measurement of porosity.
Frequently Asked Questions
Can a rock have high porosity and low permeability?
Yes. Clay-rich rocks and shales often do, because their pores are tiny and poorly connected.
Can a rock have low porosity and high permeability?
Yes. A fractured rock with low matrix porosity can still flow well through its fractures.
Does better sorting improve both properties?
Yes. Well-sorted grains leave more open and better-connected space.
Key Takeaways
- Porosity measures storage; permeability measures flow.
- They often rise together, but pore connection and throat size decide permeability.
- Larger grains raise permeability without changing ideal porosity, and better sorting raises both.
- Cement, clay and fractures can break the simple link between the two.
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