Intermediate Steps to Calculating Water Saturation and Effective Permeability

The basic inputs to calculating water saturation and effective permeability are shale volume, apparent porosity, and resistivity. Shale volume will be used to convert apparent porosity to an effective porosity value. It will also be used to correct the resistivity value for the effect of shale. A discussion of these three parameters is in order.

Shale Volume
The volume of shale or clay in the rock can be determined from gamma-ray (GR), spontaneous-potential (Sp), resistivity, neutron, neutron and density, sonic, or NMR data. In some cases a combination of indicators is required where either the minimum or a specialized average is used. In the exercise provided it is assumed that gamma ray will be used. A discussion of other techniques can be found in Asquith.

The basic process is to determine a value of the tool measurement that corresponds to sand and a value that corresponds to shale. The percentage of shale is determined by finding the difference between the actual value and the clean value divided by the total difference between the sand and shale values. In the case of the more commonly used gamma ray, there is not a linear relationship as the equation provided in the exercise indicates. The key calibration value for the volume of shale is the X-Ray Diffraction technique (XRD). This technique measures the bulk volume of shale as a percentage of total rock volume. The Scanning Electron Microscope technique also provides a clay volume by a direct examination of core slices. It is not recommended for the calibration of log-based shale volume if XRD data are available.