A circumferential acoustic logging instrument that operates at a radial mode resonance of 120 k.Hz utilizes opposing pairs of acoustic transmitters and receivers mounted on pad assemblies which are pressed against the borehole wall. An important feature of the instrument is a baffle system which effectively minimizes the propagation and reception of the d:l.rect fluid wave. The incorporation of the baffle allows both types of boundary waves I the Rayleigh and the guided fluid wave, to be more easily recognized.
At present, the primary application of the Circumferential Acoustilog ® is in the locating of natural fracture systems. When run with an orientation section, the instrument is also capable of determining fracture direction. Canadian field examples are presented which demonstrate the capabilities and effectiveness of circumferential acoustic logs.
The Circumferential Acoustilog (CALog), which incorporates the basic concepts of generating and detecting circumferentially propagated acoustic boundary waves, has already been proven successful in locating vertical or near-vertical fractures in various lithologies in North America, on-and offshore Europe, and Canada. The special instrument design using a multilayered baffle system enhances the recorded Rayleigh and guided fluid waves by eliminating any direct fluid component. Figure 1 demonstrates the usefulness and effectiveness of the baffle system in (a) a water tank, (b) an aluminum fracture, and (c) a carbonate test well. Significant enhancement of the Rayleigh and guided fluid waves is clearly shown.
Incorporation of an orientation section into the logging tool string has further expanded the. usefulness of the Circumferential Acoustilog by providing the capability for determining major directional trends of subsurface fracture systems. Basically, this is accomplished by measuring the azimuth, relative bearing. and deviation, with both azimuth and relative bearing referenced to receiver R2. Generally speaking, the fracture direction can be determined to within 90 °. However, as the Circumferential Acoustilog instrument rotates while Logging up the borehole, additional azimuth readings in the fractured zones allow for 3 more precise determination of fracture direction. Also, several repeat runs over the subject interval will further enhance this fracture direction capability.
Furthermore, two independent caliper pairs of pads 1 and 3 (transmitters) and pads 2 and 4 (receivers) provide two independent borehole size measurements. Fractured zones are characterized by (1) an indication of thick mudcake buildup, especially in the presence of heavy drilling mud or lost circulation material, or (2) by borehole cross-sectional elongation observed preferentially in the main direction of in-situ fractures and microfracture systems. Such fracture-related "washout/breakout" indications result from "chipping" action during drilling operations in those fractured intervals.
The sampling rate of the Circumferential Acoustilog is six samples per foot (or two sample per decimeter) which generates a voluminous amount of data. Acoustic data representing 360 degree coverage of the borehole wall is recorded in four quadrants and plotted in a Variable Density (VDL) format. As an alternative, a signature waveform presentation is also available. While a gamma ray curve is provided for correlation 3rd lithology identification, the dual calipers record the two independent borehole diameter measurements between opposing pads.