Formula & Calculator
Porosity
Measures the percentage of a rock or soil's total volume that consists of open pore space, which can potentially hold water.
Interpretation
n = (Vv / Vt) × 100. Porosity is the percentage of void space in a rock or soil. Used to estimate groundwater storage and contaminant transport capacity.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| n | Porosity | % |
| Vv | Volume of void (pore) space | m3 |
| Vt | Total volume of the sample | m3 |
What it means
Porosity is the fraction of the total volume of a material that is occupied by void spaces (pores). It is expressed as a percentage. It is a fundamental property of soils and rocks that controls their ability to hold and transmit fluids. It is used in hydrogeology to estimate water storage capacity, in petroleum engineering for oil and gas reserves, and in soil science for plant‑available water. Porosity can be determined from core samples or from well logs. Understanding porosity is essential for evaluating groundwater resources and for designing water supply systems.
Worked example
Porosity – Two Detailed Examples
Real‑World| Parameter | Value |
|---|---|
| Void Volume (Vv) | 30 |
| Total Volume (Vt) | 100 |
| Parameter | Value |
|---|---|
| Vv | 15 |
| Vt | 100 |
Common mistakes
- Porosity n: n = (V_v / V_t) × 100 – the fraction of void space in a rock or sediment.
- V_v: Volume of voids (pores) – not the volume of water.
- V_t: Total volume of the sample.
- Effective porosity: The interconnected pores available for fluid flow – may be less than total porosity.
- Units: Expressed as a percentage – or decimal.
Applications
Porosity, n = (Vv/Vt)×100, is the percentage of void space in a rock or soil, determining its water‑holding capacity and permeability. This is a fundamental property in hydrogeology, petroleum engineering, and soil science. Geologists use porosity to evaluate reservoir quality, to assess groundwater storage, and to understand fluid movement. By measuring porosity, engineers can estimate the amount of oil, gas, or water that a formation can contain. Porosity also affects the strength and compressibility of geological materials. Understanding porosity is essential for resource evaluation, geotechnical engineering, and environmental studies.
- Reservoir characterisation for oil and gas exploration
- Groundwater storage assessment and aquifer evaluation
- Soil science – water retention and plant‑available water
- Geotechnical engineering – strength and deformation properties
- Environmental remediation – contaminant distribution and migration
Frequently Asked Questions
n = (V_v / V_t) × 100, where V_v is the volume of voids (pores) and V_t is the total volume of the rock or soil. Porosity is expressed as a percentage and represents the fraction of open space in the material.
- Clay: 40–60% (high, but mostly micropores).
- Silt: 30–50%.
- Sand: 25–40%.
- Gravel: 20–35%.
- Sandstone: 5–30%.
- Limestone: 5–20% (fracture and matrix).
Primary porosity is the original pore space formed during deposition (e.g., between sediment grains). Secondary porosity develops later due to fracturing, dissolution, or weathering.
No. Permeability depends on pore connectivity, not just total pore volume. Clay has high porosity but low permeability because pores are isolated and very small (capillary forces).
Common methods: gravimetric (saturate and weigh), mercury injection (for small pores), gas expansion (helium pycnometry), and image analysis (thin sections).
Bulk density = solid density × (1 − n). As porosity increases, bulk density decreases. This is used in geophysics (e.g., neutron porosity logs).
As sediments are buried, compaction reduces porosity by reorienting grains and expelling water. In sandstones, porosity can decrease from ~40% to 10–20% with depth.
Effective porosity is the portion of the total porosity that is interconnected and can transmit fluids. It is the relevant porosity for flow and storage.
Higher porosity generally lowers seismic velocity (especially V_p) because pores are weak zones. This is used in seismic reservoir characterization.
Porosity determines the maximum amount of water that can be stored. However, the usable storage is limited by specific yield (in unconfined aquifers) or storativity (in confined aquifers).