INTERPRETATION OF SONIC WAVEFORMS ACQUIRED IN HIGH-ANGLE AND HORIZONTAL WELLS

摘要

Increasingly, more high-angle (HA) and horizontal(HZ) wells have been drilled in recent yearsbecause of the accelerated development of offshoreexploration and onshore organic-shaledevelopment. Conventional waveform processingmethods are developed only for vertical wells.When used in HA/HZ wells, these methods mayyield biased estimations of P- and S-waveslownesses. New methods are needed to processsonic waveforms acquired in HA/HZ wells,specifically in the vicinity of layer boundaries andin the presence of thin layers.This paper examines several HA/HZ boreholemodels when a sonic tool operates across ahorizontal bed boundary. High-resolution 3Dnumerical simulations of sonic waveforms arecarried out with a time-domain finite-differencemethod coupled with a complex, frequency-shifted,perfectly matched layer to study the transientacoustic fields measured in and out of the borehole.These simulations assume a 3D sonic logging toolwith multipole sources and 13×8 azimuthalreceivers. Both monopole and dipole syntheticwaveforms are obtained in HA/HZ boreholespenetrating hard and soft formations. Instead of theconventional processing of monopole or dipolewaveforms that takes the average, or difference, ofwaveforms acquired with azimuthal receivers, weseparately analyze the sonic waveforms at eachazimuthal receiver. Waveforms are first processedwith a weighted spectral semblance method toyield modal dispersions; next, we reconstructformation shear slowness from these dispersioncurves with a dispersion correction method. Thetop and bottom receiver slowness is chosen whenthe tool measurement point is in the formationabove and below the well, respectively. Resultsindicate that this new procedure provides unbiasedestimations of rock formation shear slownesscompared to conventional processing. Moreover,the difference between top and bottom receiverslowness reflects the elastic property contrastbetween the formation section above and below thewell, enabling the accurate evaluation of formationheterogeneity due to bed boundary effects inHA/HZ wells.The new sonic waveform interpretation approachintroduced in this paper is ideal for theelastic/mechanical assessment of organic shale, inwhich rock heterogeneity is commonplace.