Viscous Fluid Effects on Guided Wave Propagation in a Borehole

Abstract

In order to investigate the effect of borehole fluid viscosity on the attenuation and dispersion of the guided waves present in full waveform acoustic logs, the problem of wave propagation in a borehole containing a viscoelastic fluid surrounded by an infinite elastic formation is solved using boundary layer theory. The results indicate that the losses due to viscous drag along the borehole wall are a small component of the overall guided wave attenuation for the frequencies of interest in full waveform acoustic logging (2-15kHz) and for reasonable viscosity values (1-1000cP). These losses, however, can be significant at low frequencies. In addition, the variations in viscosity have a negligible effect on the guided wave dispersion for this range of frequency and viscosity. These findings indicate that friction between grains in fluid suspension may be the dominant attenuation mechanism in the drilling fluids present in boreholes.