The Effect of Collar Structure on Acoustic Logging Response While Drilling

To acquire formation acoustic data effectively during acoustic logging while drilling, the finite difference method and slowness-time coherence method were used jointly to determine impacts of ID, OD, ID variation and external grooves on acoustic wave propagation. Simulation calculation results showed that no formation S-wave could be detected when the OD of the drill collar increased, or when the clearance between external wall of the drill collar and the borewall decreased. When ID of the drill collar decreased, no significant changes could be observed in amplitudes of the collar wave, S-wave and the Stoneley wave, but the amplitudes of the Stoneley wave increased dramatically with the thinning of wall thicknesses of the drill collar. The impact of ID changes of the drill collar on amplitudes may predominantly be determined by whether or not the acoustic source frequency is in collar stopband range. When a groove appeared on the outer wall of drill collars, the wave amplitude from the drill collar became smaller, thus the P-waves could be extracted from the acquired wave shapes. Research results showed that changes in IDs and OD of drill collars could affect the acceptance of S-wave and Stoneley wave. On the other hand, the shape of the internal and external walls of the drill collar (such as notch groove) may affect the wave amplitude of drill collar. The relationship between the structure of the drill collar and the responses of the LWD may provide a reliable foundation for the design of silencers in the LWD tools and for the interpretation of LWD data.