ACOUSTICAL PROPERTIES OF CLAY-BEARING ROCKS.

摘要

Occurrences of clay-rich shaly formations in association with petroleum reservoirs have posed numerous problems over the years for geophysicists and reservoir engineers. The dominant method of identifying subsurface lithologies and detailing subsurface structural geometries has been through the correlation of seismic reflection data with data obtained from strictly controlled laboratory measurements of rock properties. Most rock types, including sandstones, limestones, and igneous and metamorphic rocks, have been comprehensively studied in the laboratory, and, hence, there exists a large body of information concerning the physical and acoustical properties of a wide range of materials. Shales, however, constitute a large class of rocks to which little study has been devoted. This dissertation seeks to substantially broaden existing knowledge of the acoustical and the physiochemical behavior of shales under test conditions that were designed to address important problems of current research and applications interest.;The first experiment deals with the acoustical identification of lithologies based on systematic dependencies of compressional- and shear-wave velocities on pore fraction and volumetric clay content. Velocities are seen to vary linearly with porosity and clay content within a few percent error in the detrital silicate rocks selected for the study over ranges of porosity and clay content that span 4 - 20 percent and 0 - 72 percent, respectively.;The second experiment focuses primarily on the delineation of abnormal pore pressure in subsurface shaly lithologies. Variation with depth of the transmitted first-arrival, zero-to-peak P-wave amplitude, inversely related to attenuation through the sample, is found to be the most sensitive acoustic parameter for delimiting pore pressures in clay-rich rocks.;The final experiment treats intrinsic acoustical anisotropy, a property that is strongly characteristic of shales due to high degrees of preferred orientation of morphologically platy clay minerals. The five independent elastic constants determined for a hexagonally symmetrical shale are seen to diverge with increasing differential pressure. This increase in anisotropy with depth should continue, tending toward the intrinsic crystallographic anisotropy of the statistically weighted volume fractions of the constituent minerals.;The work is divided into three experiments, all involving ultrasonic-frequency, transient-pulse measurements through cylindrical cores of rocks that are isolated at elevated confining and pore pressures.