Using Wet Shale And Effective Porosity In A Petrophysical Velocity Model

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIt is well known that sonic logs can be improved by using a petrophysical model to create a theoretical compressional and shear sonic. The measured sonic logs are only used as a guide. There are numerous techniques for doing this, although the most popular are the Xu, White and Keys model (Keys and Xu, Geophysics, 2002) and the Greenberg and Castangna method (Greenberg and Castangna, Geophysical Prospecting, 1992).These methods and most others require that the subject well be analyzed accurately for porosity, water saturation, and major lithologic and/or mineral components.Using the petrophysical analysis the compressional and shear moduli are built up step by step. First the moduli of the rock mixture are computed, then porosity is added to the mixture and finally the fluids are added using the Gassmann equation (Gassmann, F., Vierteljahrschrift der Naturforschenden Gesellschaft, 1951). The last step is to use the final Gassmann bulk and shear moduli to compute the compressional sonic log and the shear modulus to compute the shear sonic log. Sonic logs containing any fluid mixture can then be computed once this rock framework is built (Batzle and Wang, Geophysics, 1992).Past experience has shown us that measured sonic logs are subject to serious error due to 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 are usually better than with the measured data alone.The best procedures for building these valuable models have yet to be agreed upon by the industry because the technology is still new. One of the debates is about how to add porosity and what porosity to use. Many petrophysicists prefer to use total porosity and dry clay volume to build their model. Others, like this author, prefer to use effective porosity and wet shale volume. The reasons behind this choice are presented.

Porosity Concepts in Gassmann Fluid Substitution: A Simulator to Seismic Modelling Perspective

“…Gassmann's model is by far the most common theory used for fluid substitution. It is appreciated in the literature that different porosity choices (total or effective) exist for Gassmann fluid substitution (May 2005). However, a thorough assessment of these models is not covered, and approaches attempt to validate Gassmann's model with little reference to the underlying porosity models.…”