Weekly Petroleum Geoscience Digest
Relationship between rock spaces known as Porosity and Permeability – 22 August 2017
- By Dr Livinus Nosike
Fluid flow through rocks because rocks have small spaces in-between their grains; these spaces are known as pores or interstitial spaces. How much of such spaces exist in a rock is known as Porosity. Porosity is measured by adding water into a rock in a container and seeing how much of the water is absorbed. Whatever amount of water is absorbed will be equivalent to the available pores or interstitial spaces which is a measurement of the porosity.
Sometimes, we can estimate the permeability by just measuring the porosity; we provide a curve that relates one to the other. But not all spaces between rock grains are connected. So having high porosity does not mean you will have high permeability. Remember, Permeability is how easily a fluid like water, oil or gas passes through something like a rock.
A rock that has little or no spaces between the grains may as well have a crack, we say it is fractured. Cracks create secondary spaces, that is, spaces that occur after the sediments or grains have come together and solidify to form the rock. The more the fractures the more fluid will flow. Over years, certain rocks such as limestone begin to dissolves due to aging or because acidic fluid flow into them. Such dissolution also creates connected spaces and enhances permeability.
Because cracks happen in a given direction, permeability may also have direction. This relates to what we have discussed in the next episode, known as permeability anisotropy.
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For more info, contact Dr Livinus Nosike: contact@iesog.com