Estimation of borehole fluid slowness using sonic array waveforms

Mud slowness is an important parameter controlling wave propagation in a fluid-filled borehole. However, there is no direct measurement of this quantity in the hole at sonic frequencies. The most common approach to measure fluid slowness consists of performing slowness measurements at the surface using mud sample or using empirical laws linking mud slowness as a function of mud density and type. However, these measurements at the surface does not reproduce real well conditions introducing erroneous mud slowness estimate, while empirical equations tend to provide reasonable results but can yield wrong estimates if assumptions are not valid or uncertainties in some parameters are too large. An alternative approach consists in analyzing dispersive modes contained in recorded data and from this study deriving an estimate of the mud slowness. The drawback of this non automatic method is that it relies heavily on the skill of the person that analyzes the dispersion plots. To overcome these limitations, a probabilistic approach combining the high frequency monopole data and outputs from the monopole radial profiling has been developed to get an estimate of the mud slowness. In addition, this algorithm provides an automatic procedure based on the Scholte wave slowness to automatically set the parameters (center and standard deviation) of the a-priori probability distribution function of the mud slowness. This point is usually critical while using Bayesian approach. Robustness and efficiency of this algorithm will be illustrated on real field data.