The Micro CBL - A Second Generation Radial Cement Evaluation Instrument

A new "radial" cement bond instrument, the Micro CBL, has been developed to measure the quality of cement to casing bond. The instrument is capable of measuring the quality of the cement bond both vertically and circumferentially. Four arrays of sectored transmitters and receivers arranged longitudinally along the instrument body provide a compensated radial bond measurement with high azimuthal resolution. The sectored transmitter excites both a compressional wave and a shear wave in the casing. Unlike the standard CBL measurement, the beam pattern of the transmitter is focused in a radial direction. The sectored receiver provides additional directivity for enhanced detection of charnels in cement. Log examples recorded with the prototype instrument at a test well with manmade channels verify its ability to detect channeling in cement. Theoretical basis for the design indicates that the Micro CBL should allow radial cement evaluation in heavy mud and gasified cement environments. In these conditions, ultrasonic pulse-echo types of cement inspection tools have shown inconsistent results. Numerous laboratory experiments and field tests were conducted to determine and verify the tool configuration, including transmitter-receiver spacing, frequency, and transducer type. The Micro CBL provides measurement of the quality of cement bond in eight radial segments. The resolution of the instrument provides 5 to 10 percent overlap to insure "full" coverage of the borehole. Over 32 waveforms are acquired at each depth level. Subsurface digital conversion of the acoustic data insures data integrity. In its normal operational mode, amplitude information is recorded from the sectored receivers, an acoustic signature from a five-ft spaced transmitter-receiver is recorded, and a two axis orientation measurement (RB) is acquired. The system also has the flexibility to acquire and record acoustic signatures from the segmented receivers. This configuration, combined with more sophisticated full waveform processing, has the potential to provide "radial" cement-to-formation bond evaluation.