A simple definition of porosity or the amount of pore space is the volume of space within a rock which can be occupied by gas (air) or a fluid (water, oil).

Measuring the porosity of a rock will give an indication of its storage potential for groundwater, oil or gas. An important quantity for investment purposes.

There are many types of porosity:

- Intergranular porosity, fracture porosity, secondary porosity.
- Total porosity, effective porosity, interconnected porosity.

Core samples from a borehole can be sent to a laboratory to measure porosity. This is commonly a slow and expensive process.

A number of wireline logging methods are able to measure porosity as the measurement technique is influenced by the porosity of a rock. The three most common are density, sonic and neutron porosity.

#### Density Porosity

The formation density measurement is a function of the density of the rock grain (matrix), the amount of pore space (porosity) and the fluid/gas filling the pore spaces. Using a fractional equation and assumptions for the density of the matrix and fluid, porosity values can be calculated.

#### Sonic Porosity

The velocity of an acoustic or sonic signal travelling through the rock is a function of its time spent in the rock grains (matrix) and time spent in the fluid within the pores. This relationship is expressed by a mathematical expression known as the “Time Average Equation”. In much the same way as the density porosity, by assuming a velocity (or transit time) for the matrix and the fluid, porosity values can be calculated.

#### Neutron Porosity

Neutron porosity is a more direct porosity measurement in that neutrons generated from the source at the bottom of the probe lose most of their energy when colliding with atoms of a similar size – this means hydrogen. In the rock, most hydrogen is present in water – within the pore space of a sandstone or limestone. Commonly known as the Hydrogen Index, the ratio of the long and short spaced neutron detectors provide a good measurement of the formation porosity.

#### Resistivity

The resistivity measurements are used in conjunction with porosity to calculate the presence and amount of hydrocarbons in a pore space through a mathematical expression known as Archie’s Equation.

Rocks with matrix grains only and no pore space are typically highly resistive (shales/clays and their associates are exceptions). Introducing pore space filled with fluid will result in a significant decrease in the measured resistivity which can be related to porosity. However, there are a number of other influencing factors which can affect the measurement, namely, the conductivity of the fluid in the pore space (fresh – saline water), the rock texture which can affect the resistivity measurements, clay minerals which can have a significant effect on the resistivity measurement and the formation temperature.