What is Porosity?

Porosity is one of rock important rock properties and it is a measure of pore space in a rock.

Figure 1 – Porosity

(Ref: http://www.tulane.edu/~sanelson/images/pores.gif)

Figure 1 shows the illustration of porosity and it can be seen that grain size and distribution can affect porosity of rock.

Porosity (ɸ) is mathematically expressed as a ratio of total void space with a rock to a total volume of rock.

Porosity (ɸ) = (Vp ÷ Vb) × 100

Where;

Vp = pore volume

Vb = bulk volume of rock

Porosity (ɸ) is shown in percentage form.

How To Measure Porosity

There are several methods to determine porosity as listed below

• Direct methods: measurement from core flush by fluid or air
• Indirect methods: well logging tools (density, neutron, and sonic)

Porosity (ɸ) can be classified into several types based on criteria;

Primary porosity: this is porosity in rock that was formed during sedimentation. Primary porosity depends on several factors, such as depositional environment, grain size & shape, distribution of sand grain, cementation between sands, etc.

Secondary porosity: This is formed during rock diagenesis. Chemical reaction dissolves rock grains and it results in void spaces in the rock.

Absolute porosity: It is a ratio of total pore space to a rock bulk volume. This will not account for voids which don’t connect to others.

Effective porosity: It is a ration of interconnected pore spaces to a rock bulk volume. This is a proper figure to use to calculate fluid volume in a reservoir

Example: A core sample is 5 cm long and 3 cm diameter. In the lab, vacuum, 3.25 cm3, of air is removed from the pore spaces.

1. What is the bulk volume of the core sample?

Bulk volume = (π÷4) × Diameter2 × Core Length

Bulk volume = (π÷4) × 32 × 5

Bulk volume = 35.34 cm3

2. What is the pore volume?

This is the volume of air vacuumed so pore space is 3.52 cm2

3. What is the porosity of the rock?

Porosity (ɸ) = (Vp ÷ Vb) × 100

Porosity (ɸ) = (3.52 ÷ 35.34) × 100

Porosity (ɸ) = 10 %

4. Is it effective or total porosity?

This is effective porosity because it measures volume of air that can be removed from the core.

References

Abhijit Y. Dandekar, 2013. Petroleum Reservoir Rock and Fluid Properties, Second Edition. 2 Edition. CRC Press.

L.P. Dake, 1983. Fundamentals of Reservoir Engineering, Volume 8 (Developments in Petroleum Science). New impression Edition. Elsevier Science.

Tarek Ahmed PhD PE, 2011. Advanced Reservoir Management and Engineering, Second Edition. 2 Edition. Gulf Professional Publishing.

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