Logging while drilling acoustic measurement in unconsolidated slow formations

With the world-wide application of LWD acoustic technology in recent years, various formation types and drilling environments are encountered in the practice. A challenge for the LWD acoustic measurement is the shallow, unconsolidated, and acoustically very slow formations that are frequently encountered in drilling a deep-water reservoir. In this environment, the borehole is usually very large, varying in the range of 12 ? 18 inches. Consequently, LWD acoustic devices that are designed for high-frequency (~ 10 kHz or higher measurement often fail to excite the desirable acoustic propagation and to correctly measure formation velocity. By analyzing the acoustic wave phenomena in this environment, we have designated a procedure that incorporates the low-frequency measurement in the 2–4 kHz frequency range. The acoustic compressional waves in this frequency range have a good signal-to-noise ratio and the data quality is good even in the presence of drilling noise. The waves, however, are usually attenuative (or leaky, from radiating energy into formation and dispersive. The degree of dispersion depends on the softness (or more specifically, Poisson?s ratio of the formation and the possible radial alteration of the formation velocity. A dispersion analysis is then applied to the measured data to estimate and to correct for the dispersion effect. The measured dispersion can also be compared with theoretical modeling to assess the possible formation alteration. Field data examples demonstrate that the above procedure yields reliable compressional wave slowness for the unconsolidated slow formations.