Kevin B. Hartley
Shell Oil Company
New Orleans, Louisiana
ABSTRACT
Acoustic velocity logging has long been one of the standard methods for obtaining sand porosity. But it has been frequently noted that velocity is a function of many factors other than true porosity many of which vary between different formations.
Works by Gassmann, Biot, and others have expressed sand velocities as a function of the rock and contained fluid compressibilities, which are themselves functions of porosity and other parameters. These theoretical numbers frequently fit actual core measurements very well, but the computations are necessarily more involved than with more widely used relationships.
Extensive work has been done by Wyllie and many others, empirically fitting actual porosities to velocities via equations for various formations. This approach sometimes allows reasonable predictions of porosities from velocity logs for some specific cases. But these empirical correlations may be misleading if used outside the context of the data set from which they were developed. Also, the use of these correlations tends to obscure the effects of various rock and fluid properties upon the compressibilities. The compressibilities seem to have the most direct relationship with velocity.