Acoustic wave propagation in oil shale: 1. Experiments

Abstract

This study examines the propagation characteristics of compressional (P) and shear (S) acoustic waves through cored and broken Green River oil shale specimens. Both compressional and shear wave velocities ( V p and V s ) are sensitive to shale organic content, showing a monotonic decrease with increasing organic content in the range 30–200 L/t. Trends in the dependence of V p and V s on temperature are explained by thermal alterations in the shale matrix which include release of free and bound water molecules, pyrolysis of organic matter and re-cementation of the shale matrix by the products of pyrolysis. The temperature dependence of V p and V s is less pronounced when the acoustic energy propagates in a plane parallel to the shale bedding planes. Channelling of acoustic energy via mineral-rich layers has been invoked to explain the anisotropy. The degree of anisotropy is also dependent on shale organic content; the differences becoming less pronounced at high levels of organic content. Data on V p and V s for burnt and retorted shales show minimal temperature dependence, although these values are sensitive to the initial organic content of the material. Finally, measurements on broken pieces of shale reveal the expected increase in V p and V s with increasing compaction pressure. The decrease in V p and V s with decreasing particle size down to 0.6−0.3 mm is rather anomalous and can be explained only by the presence of a large number of narrow voids in the material.