Apparent Porosity and Effective Porosity

The next key input to calculating effective permeability and net pay is apparent porosity. Apparent porosity is the raw porosity input with no shale corrections. In most cases this value is obtained from the average of neutron and density porosity values or the square root of the average of the values squared for gas wells. Density porosity alone can often be used, as it is in the exercise in this module. Sonic porosity can also be used, as can neutron or MRIL data.

In the case where no porosity logs are available, a correlation can often be obtained in fluvial-deltaic systems between an assumed value of maximum porosity as a function of depth and as a function of shale volume.

This correlation has been successfully applied in the Miocene in several reservoir characterization studies. Once the apparent porosity value is determined, it should be corrected for shale volume. There are two main techniques applied here. The first quick-look technique is to reduce the apparent porosity by the percentage of shale in the rock or multiply it by one minus the shale percentage. A more technically correct method is to estimate the porosity in the shales and reduce the apparent porosity by the product of the shale porosity and shale volume. This is the technique applied in the exercise. Asquith provides an excellent discussion of the techniques and equations involved.