Acoustic Velocity in Porous Media

Abstract

Published in Petroleum Transactions, AIME, Volume 216, 1959, pages 262–270. Abstract An evaluation is made of the acoustic velocity log for measurement of formation porosity. Plots of field-observed velocities vs core-measured porosities of sandstones and limestones with intergranular porosity show that the velocity log gives a useful measure of porosity, in agreement with published data. A new method of plotting electrical and velocity log data makes it easier to recognize hydrocarbon-water contacts from log data and to make semi-quantitative estimates of hydrocarbon saturation. Two examples of this interpretation technique are given. This paper also discusses the various factors that affect acoustic velocity and shows how corrections can be made for some of these factors. Introduction Although the continuous velocity log was developed primarily as an aid to seismic interpretation, it is finding widespread use for measurement of porosity. The purpose of this paper is to evaluate this use of the log. Results are presented for the velocity-porosity relationship in many sandstones and one limestone. Practical applications of porosity values derived from the velocity log to improve interpretation of electrical surveys are presented. Various parameters which affect velocity in porous media are considered. Porosity, composition, cementation, pressure difference (overburden pressure minus fluid pressure), fluid saturation, and wettability are discussed. Some of the ideas are opinions based on general considerations of well logging problems while others are supported by published data and our own observations. Major conclusions from these studies are given in the body of the paper and detailed discussions of some of the various factors affecting porosity are given in the Appendix. Porosity Sandstones The primary factor affecting the velocity of sound in porous media is porosity. Wyllie, et al, have reported a time average equation for the relationship between velocity and porosity. This has found considerable acceptance in the industry. Other relationships between velocity and porosity have been proposed, none of which are entirely satisfactory. It is doubtful that any one relationship will completely describe all situations.