Have you ever wondered what the water saturation really meant when evaluating tight sands? Over the past 80 years, since the logging of Pechelbron #7, the formation evaluation industry has been very innovative in making a meaningful water saturation calculation that is indicative of the formation productivity. Conventional Archie reservoirs are defined as clean sandstone. Using the Archie equation to calculate water saturations, petroleum engineers and geologists can make effective well completion design and formation productivity decisions. If the water saturation is 30%, the well will produce water free hydrocarbons. As the water saturation increases, so increases the chances to make water. In these rocks, the conductivity of the rock matrix is very small compared to the conductivity of the pore fluid. This is a common understanding throughout most of the industry.

Now we enter an era where we are trying to understand non-Archie type rocks, tight sands. In tight shaly sands, it is a very common to have the conductivity path through the rock matrix larger than the conductivity path through the pore space. There have been numerous attempts to overcome this issue as one notices when doing literature searches on shaly sand water saturation equations. Water saturation calculations in shaly tight sands result in very high numbers, in the 40-80% range in sands that produce water free. That’s a difficult hurdle for many in the industry to grasp and understand.

The Crossplot in Figure 1 is a typical response between the Gamma Ray measurement that is often used as a shale indicator and the deep Resistivity measurement for shaly tight sand reservoirs. Notice the cloud of points that follow a relationship between the Gamma Ray and Resistivity measurements. By fitting equations to these data, the relationship between a shale indicator and resistivity can be defined for both the “wet” trend and the “irriducable” water saturation tend. Using this Psudo-RT as an overlay technique with the actual deep resistivity measurement gives a graphical analysis of gas productive sands and wet sands. Figure 2 is a cross section that clearly shows the differenced between water sands and gas productive sands. The blue shading highlights the sands with water in the pore space and the red shading represents the sands where the pore fluid conductivity is much lower than the matrix conductivity, thus indicating a zone at irreducible water saturation.

So what does water saturation calculations really mean in shaly tight sands? They are a great shale indicator. When using these SW calculations as an input when calculating permeability using wireline log inputs, the technique works out to identify the sands with higher shale volume (also higher water saturation) as having lower permeability. The cleaner sands would conversely have the lower water saturation indicating higher permeability. So, in age of complex calculations find meaning and purpose for water saturation in shaly tight sands, don’t forget the simple things in life are often the most useful.

Figure 1. Gamma-ray / resistivity cross plot.

Figure 2. Cross section showing water sands and gas-productive sands.

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