Permeability

The permeability of a rock is a measure of the ease with which the rock will allow the passage of fluids. This defines how easy it will be to extract present hydrocarbons.

The unit of permeability is $\text{D}$, or the darcy. Darcy's formula for calculating the flow rate:

$$Q = \frac{k A(P_\text{i} - P_\text{o})}{\mu L}$$

Where $Q$ is the flow rate, $P_\text{o}$ is the outlet fluid pressure, $P_\text{i}$ is the inlet fluid pressure, $\mu$ is the dynamic viscosity of the fluid, $L$ is the length of the tube the fluid is flowing through, $k$ is the permeability of the sample, and $A$ is the area of the sample.

Permeability depends on porosity. The higher the porosity, the higher the permeability. It also depends on the connectivity of the pores. There must be a pathway where the flow of fluid is possible.

Some generalizations about permeability:

• The higher the porosity, the higher the permeability
• The smaller the grains, the lower the permeability (due to smaller pores and pore throats)
• The smaller the grain size, the lower the permeability (the larger exposed area to the flowing fluid leads to a larger friction between the fluid and the rock)

Permeability is not easily correlated using any single log value. Some tools that help estimate permeability:

• Core analysis
• Resisitivity logs