Use of wet shale and effective porosity in a petrophysical velocity model

Sonic logs can be improved by use of a petrophysical model to create theoretical compressional and shear waves. The measured sonic logs are used only as a guide. Measured sonic logs are subject to serious error resulting from borehole conditions and invasion. When the results of a petrophysical velocity model are used to compare wells to seismic or to build a rock-strength model for pore-pressure prediction or a well completion, the results usually are better than with the measured data alone. One debate is about how to add porosity and what porosity to use. Many petrophysicists prefer to use total porosity and volume of dry clay to build models. This author prefers to use effective porosity and wet-shale volume. Copyright 2005 Offshore Technology Conference.