Investigation of Acoustic Boundary Waves And Interference Patterns as Techniques for Detecting Fractures
Authors: Koerperich E.A. 
Published: December 2007

Acoustic waveform amplitudes (compressional, shear, and later arrivals) and patterns of reflected and mode converted waves recorded on an axially oriented acoustic logging tool are unreliable indicators of in-situ fractures in Altamont Field, UT. These negative results are believed to result fromvertical travel path of the acoustic waves, whereby vertical fractures provide negligible interruption to the path, andtool frequency.

Introduction

Following the discovery of Altamont Field in the Uinta Basin of Utah in 1970, Shell Research initiated a program to investigate ways to locate fractured zones effectively. Location of fractures was considered important since the reservoir at Altamont Field is very tight sandstone and carbonate that requires fractures for commercial transmissibility of fluids. While many techniques for detecting fractures have been proposed and tested, no commercial tool is available that can accurately and consistently respond to fractured zones. Fractures at Altamont Field are usually vertical or almost vertical. The vertical traveling acoustic energy detected by conventional sonic logs is probably less sensitive to vertical fractures than to horizontal ones.

Three proposed means for deterring these vertical fractures were selected for further study. One proposal (described by Vogel) was to use circumferentially traveling shear waves that, in vertically fractured zones, would require recorded signals to cross the fracture plane. At our request, a prototype tool was built by Schlumberger (their Circumferential Microsonic Log - CMS log). Results of field tests of this tool have indicated that it is a viable way to locate vertically oriented fractures. The second proposed method was to measure amplitudes of the low-frequency boundary wave traveling along the borehole wall between transmitter and receiver of an axially oriented array. The third proposal was to investigate patterns of reflected or mode converted energy that may originate from acoustic discontinuities, such as fractures or bed boundaries. These last two proposed methods were investigated and the results are reported in this study. Amplitudes of compressional (P) and shear (S) waves also were measured since they were readily available from the same wave forms used for studying boundary wave amplitudes and interference patterns.

Procedures Procedures A single-transmitter/single-receiver acoustic tool with about 70-in. transmitter-receiver (T-R) spacing was run in two cored wells at Altamont Field. The data were recorded on the field film in a conventional variable density (X-Z) presentation and all wave forms were recorded on magnetic tape for laboratory processing. The tapes were played back in the laboratory and the data were displayed on an oscilloscope. A 35-mm, continuous time drive camera was attached to the face of the scope. By varying tape and film speed, suitable presentations were obtained for measuring amplitudes and observing interference patterns. Peak-to-peak measurements of compressional, shear, and boundary wave amplitudes were made every 6 in. through cored intervals of the two wells. These data were keypunched, normalized, and played back as amplitude curves on a flatbed plotter. played back as amplitude curves on a flatbed plotter. Amplitude curves were correlated with borehole logs and core depth, and the data were examined for relative amplitude attenuation in the fractured zones.

A considerably different display was required for studying interference patterns.

JPT

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