When engineers talk about porosity, they do not always mean the same thing. A rock can contain pores that are connected to each other and pores that are completely sealed off. This is why we use different terms: total (absolute) porosity, which counts every pore, and effective porosity, which counts only the connected pores. In this article you will learn what each term means, how they are related, how they are measured, and which one matters for producing oil and gas.
If you are new to the topic, read what porosity is first.
Quick Definitions
| Term | Definition | What it includes |
|---|---|---|
| Total (absolute) porosity | Total pore volume ÷ bulk volume | All pores: connected, isolated, and often the space held by clay-bound water |
| Effective porosity | Interconnected pore volume ÷ bulk volume | Only pores that are connected and can contribute to flow |
| Isolated (ineffective) porosity | Isolated pore volume ÷ bulk volume | Closed pores that are sealed off from the rest |
Total (Absolute) Porosity
Total porosity is the ratio of all the void space in a rock to the bulk volume of the rock. It counts every pore, whether the pore is connected to other pores or not.
Total porosity (%) = (Total pore volume ÷ Bulk volume) × 100
Total porosity tells you how much space the rock has to store fluid. It does not tell you how much of that fluid can be produced.
Effective Porosity
Effective porosity is the ratio of the interconnected void space to the bulk volume. Only these pores can contribute to flow into a well. In reservoir engineering, effective porosity is the figure that is normally used for reserves and flow calculations.
Effective porosity (%) = (Interconnected pore volume ÷ Bulk volume) × 100
In clean, well-sorted sandstones, effective porosity is close to total porosity. In rocks with a lot of cement, clay or isolated vugs, effective porosity can be much lower than total porosity.
How Total and Effective Porosity Are Related
Total porosity = Effective porosity + Isolated porosity
In shaly sands, log analysts also separate the water that is bound to clay minerals. This clay-bound water sits in very small pores that do not contribute to flow, so it is counted in total porosity but not in effective porosity.
Worked Example
A core sample has a bulk volume of 100 cm³. Tests show that the total pore volume is 22 cm³, of which 3 cm³ are isolated pores with no connection to the others.
- Total porosity = 22 ÷ 100 × 100 = 22%
- Isolated pore volume = 3 cm³
- Interconnected pore volume = 22 − 3 = 19 cm³
- Effective porosity = 19 ÷ 100 × 100 = 19%
The rock stores 22% of its volume as void space, but only 19% is available to flow.
Which One Is Used in the Field?
- Reserves and volumetrics: effective porosity, because it represents the pore space that holds producible hydrocarbons.
- Flow calculations: effective porosity, together with permeability.
- Rock characterization and log calibration: both. Total porosity is often computed first, then corrected to effective porosity.
How Each Type Is Measured
Different methods measure different things. See laboratory measurement of porosity for the full list.
| Method | Typically measures | Notes |
|---|---|---|
| Boyle’s law (gas expansion) porosimeter | Effective porosity | The gas only enters connected pores |
| Liquid resaturation | Effective porosity | Liquid fills connected pores only |
| Grain density method (crushed sample) | Total porosity | The sample is reduced to grains, so isolated pores are opened |
| Density, neutron and sonic logs | Total porosity, then corrected | Corrections are needed for clay and fluids |
Primary and Secondary Porosity
Total and effective porosity are different from primary and secondary porosity. Primary and secondary describe when and how the pores formed (at deposition or later). Total and effective describe whether the pores are connected. A rock can have both secondary porosity and good effective porosity, for example when dissolved vugs are connected by fractures. See the classification of porosity.
Why Porosity Alone Is Not Enough
Even high effective porosity does not guarantee good flow. A rock also needs pore throats big enough for fluids to pass. This is described in the relation between porosity and permeability.
Frequently Asked Questions
Is total porosity always bigger than effective porosity?
Total porosity is always greater than or equal to effective porosity. They are equal only when every pore is connected and no clay-bound water is counted.
Which porosity do reservoir engineers use?
Effective porosity, because only connected pores can store producible fluid and contribute to flow.
Can a rock have high total porosity and be a poor reservoir?
Yes. Shales and clay-rich rocks can have high total porosity, but the pores are tiny and poorly connected, so effective porosity and permeability are low.
What is the difference between absolute and total porosity?
The terms are used with the same meaning: the total void space divided by the bulk volume.
Key Takeaways
- Total (absolute) porosity includes all pores; effective porosity includes only connected pores.
- Total porosity = effective porosity + isolated porosity.
- Effective porosity is the one used for reserves and flow.
- Different lab and log methods measure different porosity types, so always check which one a number refers to.
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