Water Saturation Sw Concept

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List of equations assignment

Water Saturation (Sw)

Saturation of a fluid is defined as the ratio between the volume that occupies a fluid in a pore space and that the total volume of that porous media.

Saturation = fluid volume / pore volume

So, saturation of water is defined as the ratio between the volume of water accumulated in a rock porous media, and the total pore volume of that rock.

The value of water saturation decreases to minimum values at irreducible water saturation zones, where in oil-wet rocks (read about wettability) lower values are around 0.15 (15%), and in water-wet rocks (water wettable), water saturation values are higher than 0.2 (20%). These values help us as a guide when we calculate water saturation, because if we get a water saturation value lower than 20% in a water-wet rock, we can infer that something is wrong with our calculations. Usually, sedimentary rocks are water-wet rocks, but in the case of carbonates, there is a considerable proportion of oil-wet rocks. However, mixed wettability also exists.

It is important to know how to get a reliable water saturation value, due to that it will impact considerably the hydrocarbon reserves estimation.

Saturation of a porous media is expressed the following way:

Where:
1. St = total saturation, equals 1 or 100%
2. So = oil saturation
3. Sg = gas saturation
4. Sw = water saturation

The equation to calculate oil saturation could be expressed the following way if the reservoir only accumulates gas and oil:

To calculate water saturation from well logs, we can use resistivity and spontaneous potential (SP) well logs.

In this section, you can find the different equations used to calculate water saturation.

You can calculate the following Saturation Equations:

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Made by Henry Herrera