Initial Results from an Acoustic Logging-While-Drilling Tool

Abstract This paper presents the initial results from field tests of an acoustic logging-while-drilling tool. Over 60,000 ft of well have been logged with 6.75-in.- and 8-in.-diameter versions of the tool. Various types of formations were encountered with compressional slowness values ranging from 55 µsec/ft to 170 µsec/ft. The tool is configured as a borehole-compensated device; transmitters are located on opposite sides of an array of four receivers. An ultrasonic standoff transducer provides a tool-to-borehole wall distance measurement for data quality evaluation and processing. Slowness values are computed downhole using real-time waveform processing. Full-waveform data are stored for retrieval and re-processing at the surface. Interactive processing software allows an engineer to reprocess log intervals of poorer log quality and improve the slowness log computed downhole. Downhole-processed and post-drilling-processed slowness logs are compared, and generally show close agreement. These logs also agree well with wireline slowness logs. Rugose boreholes and attenuating formations can reduce waveform quality. A new rapid-fire technique acquires multiple estimates of slowness as the drillstring rotates through only a fraction of a revolution, and improves slowness quality in rugose boreholes. Shear-wave energy is clearly apparent in most waveform data. Waveforms can be processed to yield both shear and compressional slowness values. These can then be used to estimate porosity, compute elastic moduli of the rock (for example, Poisson’s ratio, Young’s modulus, bulk compressibility, and shear modulus), and to compute synthetic seismograms for correlation with seismic profiles. Some of these applications of compressional and shear slowness data are discussed with examples.