Using Wet Shale and Effective Porosity in a Petrophysical Velocity Model

摘要

It is well known that sonic logs can be improved by using arnpetrophysical model to create a theoretical compressional andrnshear sonic. The measured sonic logs are only used as arnguide. There are numerous techniques for doing this, althoughrnthe most popular are the Xu, White and Keys model (Keys andrnXu, Geophysics, 2002) and the Greenberg and Castangnarnmethod (Greenberg and Castangna, Geophysical Prospecting,rn1992).rnThese methods and most others require that the subject well bernanalyzed accurately for porosity, water saturation, and majorrnlithologic and/or mineral components. Using thernpetrophysical analysis the compressional and shear moduli arernbuilt up step by step. First the moduli of the rock mixture arerncomputed, then porosity is added to the mixture and finally thernfluids are added using the Gassmann equation (Gassmann, F.,rnVierteljahrschrift der Naturforschenden Gesellschaft, 1951).rnThe last step is to use the final Gassmann bulk and shearrnmoduli to compute the compressional sonic log and the shearrnmodulus to compute the shear sonic log. Sonic logsrncontaining any fluid mixture can then be computed once thisrnrock framework is built (Batzle and Wang, Geophysics, 1992).rnPast experience has shown us that measured sonic logs arernsubject to serious error due to borehole conditions andrninvasion. When the results of a petrophysical velocity modelrnare used to compare wells to seismic or to build a rockrnstrength model for pore pressure prediction or a wellrncompletion, the results are usually better than with thernmeasured data alone.rnThe best procedures for building these valuable models havernyet to be agreed upon by the industry because the technologyrnis still new. One of the debates is about how to add porosityrnand what porosity to use. Many petrophysicists prefer to userntotal porosity and dry clay volume to build their model.rnOthers, like this author, prefer to use effective porosity andrnwet shale volume. The reasons behind this choice arernpresented.