Acoustic Properties of Shale With Variant Water Activity

Date

2004-08

Authors

Popp, Nicholas Gene

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Abstract

This project further developed ultrasonic laboratory-scale equipment and established reliable testing procedures to investigate the effect of water activity on the acoustic properties of shale. The test equipment used in this study consists of two systems, one system for the study of ultrasonic energy propagated through shales utilizing the through-transmission technique, and the other for controlling the water activity that exists in shales. The effects of water activity on the acoustic properties of five intact shales were examined. The effects of water activity on intact shales were compared to reconstituted shale samples, which were made from drill cuttings using a hydraulic press and compaction cell. Reconstituted shale samples were also made from the five intact shales used in this study for a direct comparison of intact and reconstituted shale samples. Compressional and shear wave velocities were measured at three water activities: 0.75, 0.85, and 0.92. Using the acoustic wave velocities and the densities of the shales samples, the dynamic elastic moduli of the shales were calculated. Time dependent tests on intact and reconstituted shales were conducted to examine the effects of water activity on the acoustic properties of the shale as the shale is equilibrated from an initial water activity to another known water activity. The wave amplitude, frequency spectrum and frequency amplitude were also analyzed and presented. The frequency dependent measurements were made on the Johnson intact shale and the CR-3 reconstituted shale on a time dependent basis. The effects of water activity on the mass, length, and diameter of the intact and reconstituted shales were investigated. These measurements are useful in describing the physical changes of shales due to changes in water activity. In general, the acoustic wave velocities and elastic moduli of intact and reconstituted shales decreased with an increase in water activity. Also, the frequency spectrum amplitude and peak frequency decreased as water activity is increased. The results indicate that water activity has the same general affect on the acoustic properties of intact and reconstituted shales. However, water activity has a greater effect on intact shales than on reconstituted shales. Water activity did not affect the acoustic properties of the intact and reconstituted shales to the same extent. Empirical formulations between the acoustic velocities of intact and reconstituted shales could not be established. No correlations between the acoustic properties of shales and shale mineralogy could be ascertained with respect to variations in water activity.

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