Interpretation of sonic waveforms acquired withlogging-while-drilling (LWD) tools is often non-trivialin HZ/HA wells, especially in the proximity ofboundaries between layers with high slownesscontrasts. Low-frequency components of thequadrupole mode can be affected by deleterioussuperposition effects originating from the two layers,thereby biasing the assessment of formation shearslowness. Operating companies continue to report nonnegligibledifferences between shear slownessesmeasured with LWD and wireline (WL) tools inHA/HZ wells.This paper undertakes the numerical simulation andanalysis of sonic waveforms acquired with LWDmeasurements in the vicinity of layer boundaries.Measurements simulated in HA/HZ wells are studiedfor quadrupole sources, and compared to WL loggingresults using dipole sources to estimate shear slowness.Estimations involve hard or soft formations withdifferent contrasts in shear slowness. The HZ wellsconsidered in this work have axes located at theinterface between the two layers of the formation. ForHA wells, we consider two different inclination angles.In LWD simulations with quadrupole sources in HZwells, only the first-order quadrupole wave is detected.At high frequencies, the slowness of the detected waveis the average of the slownesses of the quadrupolewaves of each layer of the formation, while at lowfrequencies the observed wave slowness approaches theslowness of the fastest formation. This is true whenboth layers are hard or soft. In WL logging simulationswith dipole sources in HZ wells, we find that thebehavior of the flexural wave is similar to that of thequadrupole wave only for the vertical dipole source andwhen both formations are soft.In HA wells, our results suggest that in formations withonly soft layers, shear slowness estimation using dipolesources in WL logging is more reliable than estimationusing LWD tool with quadrupole source. Oursimulations show that in formations with two soft layersthe effects of additional modes arising from thepresence of heterogeneities may not have a significantcontribution at low frequencies.
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