Report Summary

Abstract

This report contains the results of work completed during the seventh year of the Full Waveform Acoustic Logging Consortium in the Earth Resources Laboratory at M.LT. During the past year, we have been concentrating on the problem of logging in fractured and anisotropic media. We have developed theories for wave propagation in boreholes in these media. One of the theories is for the propagation and attenuation of Stoneley waves across a horizontal fracture and along a vertical fracture. The theory has been checked against laboratory data obtained from scale models. The theoretical predictions and experimental results agree very well. We have also developed a theory for calculating the phase and group velocities of guided wave modes, including non-axisymmetric modes such as the flexural and screw modes, in a borehole embedded in a general, weakly anisotropic medium. This addresses the effects of logging in holes drilled into tilted blocks, non-horizontally drilled into sedimentary sequences, and drilled into formations with vertical fractures. We used a perturbation method based on Hamilton's Principle to calculate these velocities. We have generated techniques of inverting for the relevent parameters in these media such as the different elastic moduli and the degree of anisotropy from the full waveform logs. This year we have begun applying some of the techniques we have developed over the past years to logs in the field. In one example, we compare the shear wave velocity obtained in soft sediments by our inversion technique to those based on the difference between measured P-wave velocity and that predicted using Wood's equation. The results give us some handle on the limitations and accuracies of our technique. In another example, we restacked array data in various combinations to obtain a better depth resolution for the velocities picked from array data without sacrificing the use of a larger number of receivers to enhance the signal to noise ratio. In a third paper, we apply a general inversion to a number of different logs to obtain the lithology in a marine environment. Another paper deals with the implications of the differences in hydraulic conductivity estimated from Stoneley wave attenuation and packer tests in the field. Another focus of the past year's research is rock physics. Specifically, the relationship between permeability, velocity, including anisotropy, and pore geometry. The aim is to interpret measured velocities in terms of pore sizes and shapes, and then to use these pore geometries to model the flow properties of the rocks. The following is a summary of the papers in this report.