Some Phenomena Pertinent to Velocity Logging
- M.R.J. Wyllie (Gulf Research & Development Co.) | G.H.F. Gardner (Gulf Research & Development Co.) | A.R. Gregory (Gulf Research & Development Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- July 1961
- Document Type
- Journal Paper
- 629 - 636
- 1961. Original copyright American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Copyright has expired.
- 2.4.3 Sand/Solids Control, 1.14 Casing and Cementing, 5.6.1 Open hole/cased hole log analysis, 5.2 Reservoir Fluid Dynamics
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The main experimental facts relating to an understanding and interpretation of the Continuous Velocity Log are reviewed. Experience shows that the velocity through liquid-saturated rocks primarily depends on porosity, and that the "time-average" formula can be relied on to give the dependence within about 5 per cent. An exception occurs in the case of shallow unconsolidated sands. Data are given for the dilatational and shear velocities through dry rocks under a high confining stress. It is argued that a correction made for pressure increase or porosity reduction with depth in the interpretation of a velocity log is unwarranted because of the many uncertain parameters involved.
The application of Biot's theory is discussed, and illustrated with a numerical example. If tile dilatational and shear velocity in the dry material are known, then the change in velocity on saturating the material with any inert fluid is explicitly expressed in terms of the density and compressibility of the fluid. However, since the dry velocities of rocks in the ground are not known, and their correlation with porosity is small, the practical use of Biot's theory is very limited. As a didactic tool it achieves its greatest usefulness.
The introduction of the Continuous Velocity Log (CVL) as a routine logging device is a significant technical feat, but even more remarkable has been its wide acceptance as a tool to solve problems in correlation and to determine the porosities of subsurface beds. Indeed in the latter regard it has been recently described as the best wireline tool now available for the evaluation of porosity. While this statement seems to ignore definite drawbacks in the use of CVL data to calculate porosities, world-wide experience in the interpretation of CVL data by the so-called "time-average" technique has shown it to be extraordinarily successful.
The primary intention in this paper is to review the major experimental facts which must be considered when interpreting CVL data. A secondary purpose is to show how some of these data can be explained by modification of the classic theory of wave motion in continuous matter. The third aim, but by no means the least important one, is to remark on some of the misconceptions which appear to have crept into current interpretation procedures and to emphasize where experimental and theoretical lacunae exist.
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