Determining shear slowness from dipole source-based measurements aquired by a logging while drilling acoustic measurement tool

Abstract

A technique includes receiving data representing time domain waveforms acquired by receivers of a drilling string-disposed acoustic measurement tool in response to energy emitted by at least one dipole source of the tool. The technique includes processing the data to determine slowness values associated with a plurality of acoustic modes, including a formation flexural acoustic mode and a tool flexural acoustic mode. The technique includes identifying slowness-frequency pairs from the slowness values and determining a shear slowness based at least in part on the identified slowness-frequency pairs.

What is claimed is: 
     
         1 . A method comprising:
 receiving data representing time domain waveforms acquired by receivers of a drilling string-disposed acoustic measurement tool in response to energy emitted by at least one dipole source of the tool;   processing the data to determine slowness values associated with a plurality of acoustic modes including a formation flexural acoustic mode and a tool flexural acoustic mode;   identifying slowness-frequency pairs from the slowness values; and   determining a shear slowness based at least in part on the identified slowness-frequency pairs.   
     
     
         2 . The method of  claim 1 , wherein processing the data to extract the at least one slowness dispersion characteristic comprises:
 dividing a relatively larger frequency range into a plurality of relatively smaller frequency ranges, wherein each frequency range is associated with a plurality of frequencies; and   for each frequency range of the plurality of frequency ranges:
 applying beamforming to determine a spectrum that varies with a phase slowness; 
 identifying at least one peak of the spectrum; and 
 using a result of the identifying to determine at least one of the slowness values. 
   
     
     
         3 . The method of  claim 2 , wherein applying the beamforming comprises applying Capon beamforming. 
     
     
         4 . The method of  claim 2 , wherein applying the beamforming comprises transforming the time domain waveforms into frequency domain waveforms;
 de-dispersing the frequency domain waveforms, wherein the de-dispersing comprises assigning a group slowness to the frequency domain waveforms to remove the mode dispersion in a wavenumber domain; and   selectively combining the results of the de-dispersing to spatially smooth the de-dispersed frequency domain waveforms with respect to a receiver offset direction.   
     
     
         5 . The method of  claim 1 , wherein identifying the slowness-frequency pairs comprises basing the identification at least in part on an uncertainty versus frequency for at least some of the slowness values. 
     
     
         6 . The method of  claim 1 , wherein identifying the slowness-frequency pairs comprises determining at least one frequency range in which some of the slowness values are sensitive to the shear slowness. 
     
     
         7 . The method of  claim 1 , further comprising performing the receiving and processing multiple times for multiple dipole source firings, wherein:
 identifying the slowness-frequency pairs comprises comparing a consistency of slowness values determined from the firings.   
     
     
         8 . The method of  claim 1 , wherein:
 the acoustic measurement tool acquires the data in response to measurements in a fast formation; and   determining the shear slowness comprises:
 constructing a formal flexural dispersion based on at least some of the slowness values; and 
 determining an asymptote of the constructed formation flexural dispersion. 
   
     
     
         9 . The method of  claim 1 , wherein determining the dipole shear slowness comprises inverting for the shear slowness based at least in part on the identified slowness-frequency pairs. 
     
     
         10 . The method of  claim 9 , wherein inverting for the shear slowness comprises performing a one parameter inversion. 
     
     
         11 . The method of  claim 9 , wherein inverting for the shear slowness comprises inverting for the shear slowness and a mud slowness. 
     
     
         12 . The method of  claim 9 , wherein:
 the emitted energy propagates in a fast formation;   identifying the slowness-frequency pairs from the slowness values comprises identifying slowness-frequency pairs that are associated with the formation flexural acoustic mode and are not associated with the tool flexural acoustic mode; and   inverting for the shear slowness comprises performing model-based inversion to determine the shear slowness based on the identified slowness-frequency pairs.   
     
     
         13 . The method of  claim 9 , wherein:
 the emitted energy propagates in a fast formation;   identifying the slowness-frequency pairs from the slowness values comprises identifying slowness-frequency pairs that are associated with the tool flexural acoustic mode and are not associated with the formation flexural acoustic mode; and   inverting for the shear slowness comprises performing model-based inversion to determine the shear slowness based on the identified slowness-frequency pairs.   
     
     
         14 . The method of  claim 9 , wherein:
 the emitted energy propagates in a fast formation;   identifying the slowness-frequency pairs from the slowness values comprises identifying slowness-frequency pairs that are associated with the tool flexural acoustic mode and are associated with the formation flexural acoustic mode; and   inverting for the shear slowness comprises performing model-based inversion to determine the shear slowness based on the identified slowness-frequency pairs.   
     
     
         15 . An apparatus comprising:
 an interface to receive data representing data acquired by a logging while drilling (LWD) tool in a well in response to energy being emitted by at least one dipole source; and   a processor to:
 process the data to extract slowness estimates associated with a plurality of acoustic modes including a formation flexural acoustic mode and a tool flexural acoustic mode; and 
 determine a shear slowness based at least in part on selected slowness-frequency pairs identified from the slowness estimates. 
   
     
     
         16 . The apparatus of  claim 15 , wherein:
 the energy propagates in a fast formation;   at least some of the slowness estimates are associated with a formation flexural dispersion; and   the processor integrates energy associated with the formation flexural dispersion based at least in part on the identified slowness-frequency pairs to determine the shear slowness.   
     
     
         17 . The apparatus of  claim 15 , wherein the processor performs a model-based inversion based at least in part on the selected slowness-frequency pairs to determine the shear slowness. 
     
     
         18 . The apparatus of  claim 15 , wherein the slowness-frequency pairs are associated with the tool flexural acoustic mode. 
     
     
         19 . A method comprising:
 receiving data representing time domain waveforms acquired by receivers of a drilling string-disposed acoustic measurement tool in response to energy emitted by at least one dipole source of the tool propagating through a slow formation;   processing the data to determine slowness values associated with a plurality of acoustic modes including a formation flexural acoustic mode and a tool flexural acoustic mode;   identifying slowness-frequency pairs from the slowness values; and   determining a shear slowness based at least in part on the identified slowness-frequency pairs.   
     
     
         20 . The method of  claim 19 , wherein:
 identifying the slowness-frequency pairs from the slowness values comprises identifying slowness-frequency pairs associated with the tool flexural acoustic mode; and   determining the shear slowness comprises using model-based inversion to determine the shear slowness based at least in part on the slowness-frequency pairs associated with the tool flexural acoustic mode.