True Porosity Calculator

What is True Porosity?

True porosity is a fundamental property in geology and materials science that measures the fraction of void space in a material. It represents the ratio of the volume of pores (voids) to the total volume of a material. True porosity is crucial in understanding fluid storage capacity, permeability, and other physical properties of rocks, soils, and porous materials.

Formula

The formula for true porosity is:

\[ \phi = \frac{V_p}{V_b} = \frac{V_b - V_g}{V_b} \]

Where:

• \( \phi \) is the true porosity (dimensionless or as a percentage)
• \( V_p \) is the pore volume (in cubic units, e.g., cm³)
• \( V_b \) is the bulk volume of the material (in cubic units, e.g., cm³)
• \( V_g \) is the grain volume (solid material volume) (in cubic units, e.g., cm³)

Calculation Steps

Let's calculate the true porosity for a rock sample:

1. Given:
   • Bulk volume (\( V_b \)) = 100 cm³
   • Grain volume (\( V_g \)) = 75 cm³
2. Apply the true porosity formula: \[ \phi = \frac{V_b - V_g}{V_b} \]
3. Substitute the known values: \[ \phi = \frac{100 \text{ cm³} - 75 \text{ cm³}}{100 \text{ cm³}} \]
4. Perform the calculation: \[ \phi = \frac{25 \text{ cm³}}{100 \text{ cm³}} = 0.25 \]
5. Convert to percentage (optional): \[ \phi = 0.25 \times 100\% = 25\% \]

Example and Visual Representation

Let's visualize the concept of true porosity:

This diagram illustrates:

• The bulk volume (\( V_b \)) represented by the entire rectangle
• The grain volume (\( V_g \)) shown as gray circles (solid material)
• The pore space (\( V_p \)) as the white areas between grains
• True porosity (\( \phi \)) is the ratio of pore space to bulk volume