Originally written by Aditya Ariewijaya (AA)
There’s a good chance that any of you who read this, never heard about porosity or might be unfamiliar with the term. However, there’s also a good chance that you already familiar with the introduction picture above (Figure 1). In a simplest explanation, this picture will be your guide to understand porosity. By the end of this series, We will (hopefully) understand: “What is porosity”; “Why is it important”; “How we measure it”; and “Who benefits from it”.
What is Porosity?
To put it simply as shown in the Figure 1, porosity is the “empty spaces” that lies between those pebbles. If we consider the whole dimension of the picture as the total volume, then the porosity is the comparison of the empty spaces between pebbles (illustrated by the yellow shading) vs the total volume (illustrated by multiplying length to width).
As illustrated in Figure 2 and 3, if we assume that the total volume is equal to total area of the picture (3 becomes 2 dimension), thus the porosity would be defined as total yellow area divided by total area (X*Y).
If we are assuming that the total area is equal to 6.5 unit, or 6.5x of most yellow area on the right side of the picture, then the porosity would be Φ= 1/(6.5) = 0.154
So, the porosity (Φ) of this rock in the picture is 15.4 percent*, 15.4 pu*, or just 0.154*.
*In the world of petrophysics, the porosity is symbolized by Φ. The porosity would be calculated as value in v/v unit, decp, p.u. (porosity unit), or percentage. It is basically unitless.
Why is is important?
Now that you get the idea of “what” is porosity, you were probably start questioning why should we understand it in oil and gas world? Why is it important to us? The short answer is, because it’s where your oil and gas kept beneath the earth. It gives the idea how much is your oil and gas.
The long answer is, it will become the one of input to calculate reserve calculation (through water saturation and volumetric calculation), the basis of your reservoir characterization through rock typing, and if the rock quality is well-known enough, you can even determine the saturation from only porosity (Buckle Constant*).
Take volumetric calculation as an example. Porosity is affecting two factors here, water saturation using Sw from Archie’s Eq. and STOIIP. Messing up porosity, will make double errors in our final calculation of reserve.
How we measure it?
Depend on how we measure it, porosity measurement can be classified into two groups. Direct measurement and Indirect measurement (which we will only discuss a bit).
1. Direct Measurement
Direct measurement of porosity is usually related to core data, where we have the real data of what we want to quantify (rock). Direct measurement relies on actual measurement, dimenstion, volume, pressure etc. It can be done using several methods, from a simplest one; caliper, to complicated one; using boyle’s law.
2. Indirect Measurement
Indirect measurement relies on physical properties of the rock that has impliecation to porosity. such as Hydrogen Index, Bulk Density and Slownees. This measurement usually related to logging data either Wireline or LWD. The benefir of this measurement compare to direct measurement is its continous measurement of data. Since core porosity is discontinuous and a luxury that not every company/ petrophysicist has. What usually happen is the logging data is being used as main data to calculate porosity of the rock. While direct measurement of core porosity being used as calibration point for porosity from log (see Track Porosity at Figure above). The most common indirect measurements are density, neutron, and sonic/acoustic.
Who benefits from it?
Now, straight to the answer. People who benefit the most from a reliable data of porosity is…… The Reservoir Engineer. They are people who calculate reserve calculation, doing simulation of our dynamic reservoir, doing historical matching from simulated model to production data. All of the model that they developed is based on static model which basically affected by porosity (Sw, Permeability, Porosity). However, make no mistake, although they take the most benefit from it, the person who determine the porosity is you, Petrophysicist.