Use of Wet Shale and Effective Porosity in a Petrophysical Velocity Model

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 17643, "Using Wet Shale and Effective Porosity in a Petrophysical Velocity Model," by A. May, Kerr-McGee Corp., prepared for the 2005 Offshore Technology Conference, Houston, 2-5 May. 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. Introduction Petrophysical velocity modeling, to improve sonic logs and create better well ties to seismic sections, is important today for both amplitude-variation-with-offset studies and seismic inversions. These petrophysical velocity models begin with a petrophysical interpretation that describes the rock components. The interpretation provides the volumes of sand, shale, carbonate, and other components in the solid matrix, as well as the porosity of the rock and a description of the fluids filling the rock's pores. This description is used to compute compressional sonic velocity and shear sonic velocity. Various methods can be used to make these calculations, but great care must be taken to make certain that the volume definitions used in the petrophysical interpretation are the same as those used in the velocity model. This admonition is particularly important for the definitions of shale and porosity. Wet Shale The techniques for producing and presenting petrophysical interpretations fall into two categories, the wet-shale/effective-porosity method and the dry-clay/total-porosity method. In both models, total porosity is the same. The wet-shale model, which also can be called a rock model, divides the rock into two components: sand and shale. These are not minerals, but rock components. In this model, the shale has bound water associated with it according to a shale porosity determined by the log response of a "type shale" chosen by the analyst. The effective porosity is the porosity in excess of the shale porosity. The effective-porosity portion of the rock contains irreducible water associated with the sand and silt that depends on the height above the hydrocarbon/water contact, or capillary pressure.