Method for characterizing shear wave formation anisotropy

Abstract

A method of characterizing shear wave anisotropy in a formation includes obtaining crossed-dipole waveforms from a borehole penetrating the formation over a range of depths and frequencies, determining far-field slowness in a fast-shear and slow-shear direction using a low-frequency portion of the crossed-dipole waveforms, and determining near-wellbore slowness in the fast-shear and slow-shear directions using a high-frequency portion of the crossed-dipole waveforms. The method also includes marking a selected depth of the formation as having intrinsic anisotropy if at the selected depth the far-field slowness in the fast-shear direction is less than the far-field slowness in the slow-shear direction and the near-wellbore slowness in the fast-shear direction is less than the near-wellbore slowness in the slow-shear direction. The selected depth is marked as having stress-induced anisotropy if the far-field slowness in the fast-shear direction is less than the far-field slowness in the slow-shear direction and the near-wellbore slowness in the fast-shear direction is greater than the near-wellbore slowness in the slow-shear direction.

1 .- 17 . (canceled)  
   
   
       18 . A system configured to characterize shear wave anisotropy in a formation, comprising: 
 a logging tool configured to: 
 obtain crossed-dipole waveforms from a borehole penetrating the formation over a range of depths and frequencies; and  
   a surface unit operatively connected to the logging tool and configured to: 
 determine far-field slowness in a fast-shear direction and slow-shear direction using a low-frequency portion of the crossed-dipole waveforms;  
 determine near-wellbore slowness in the fast-shear direction and slow-shear direction using a high-frequency portion of the crossed-dipole waveforms;  
 select a depth in the formation;  
 characterize the depth of the formation as having intrinsic anisotropy when at the depth the far-field slowness in the fast-shear direction is less than the far-field slowness in the slow-shear direction and the near-wellbore slowness in the fast-shear direction is less than the near-wellbore slowness in the slow-shear direction; and  
 characterize the depth of the formation as having stress-induced anisotropy when at the depth the far-field slowness in the fast-shear direction is less than the far-field slowness in the slow-shear direction and the near-wellbore slowness in the fast-shear direction is greater than the near-wellbore slowness in the slow-shear direction.  
   
   
   
       19 . The system of  claim 18 , wherein the surface unit is further configured to: 
 determine the fast-shear direction prior to determining the far-field slowness and the near-wellbore slowness, wherein the slow-shear direction is orthogonal to the fast-shear direction.    
   
   
       20 . The system of  claim 19 , wherein determining the fast-shear direction comprises Alford Rotation processing of the crossed-dipole waveforms.  
   
   
       21 . The system of  claim 19 , wherein determining the fast-shear direction comprises parametric inversion of the crossed-dipole waveforms.  
   
   
       22 . The system of  claim 18 , wherein obtaining crossed-dipole waveforms comprises firing a plurality of dipole sources located on the logging tool to generate dipole acoustic signals which are transmitted into the formation.  
   
   
       23 . The system of  claim 22 , wherein obtaining crossed-dipole waveforms further comprises firing the plurality of dipole sources at different azimuthal positions in the borehole.  
   
   
       24 . The system of  claim 22 , wherein obtaining crossed-dipole waveforms further comprises detecting dipole acoustic signals from the formation using a plurality of dipole receivers located on the logging tool.  
   
   
       25 . The system of  claim 24 , wherein a first set of the dipole receivers selected from the plurality of dipole receivers are inline with a first one of the plurality of dipole sources and a second set of the dipole receivers selected from the plurality of dipole receivers are inline with a second one of the plurality of dipole sources.  
   
   
       26 . The system of  claim 22 , wherein a first one of the plurality of dipole sources fires at a low frequency and a second one of the plurality of dipole sources fires at a high frequency.  
   
   
       27 . The system of  claim 26 , wherein the low frequency is in a range from approximately 1 to 3 kHz.  
   
   
       28 . The system of  claim 26 , wherein the high frequency is in a range from approximately 4 to 7 kHz.  
   
   
       29 . The system of  claim 26 , wherein the low frequency and the high frequency are selected such that dispersion crossover would be detectible if dispersion curves were generated from the crossed-dipole waveforms.  
   
   
       30 . The system of  claim 26 , wherein the high frequency is selected to probe into the formation a radial distance of approximately one-half the borehole diameter.  
   
   
       31 . The system of  claim 26 , wherein the low frequency is selected to probe into the formation a radial distance of approximately two to three times the borehole diameter.  
   
   
       32 . The system of  claim 18 , wherein determining far-field slowness involves processing the crossed-dipole waveforms using slowness-time-coherence.  
   
   
       33 . The system of  claim 18 , wherein determining near-wellbore slowness involves processing the crossed-dipole waveforms using slowness-time coherence.  
   
   
       34 . A system configured to characterize shear wave anisotropy in a formation, comprising: 
 a logging tool configured to: 
 obtain crossed-dipole waveforms from a borehole penetrating the formation over a range of depths and frequencies; and  
   a surface unit operatively connected to the logging tool and configured to: 
 determine far-field slowness in a fast-shear direction and slow-shear direction using a low-frequency portion of the crossed-dipole waveforms;  
 determine near-wellbore slowness in the fast-shear direction and slow-shear direction using a high-frequency portion of the crossed-dipole waveforms;  
 select a depth in the formation; and  
 characterize the depth as having isotropic anisotropy when at the depth the far-field slowness in the fast-shear direction is substantially the same as the far-field slowness in the slow-shear direction.  
   
   
   
       35 . The system of  claim 34 , wherein the surface unit is further configured to: 
 characterize the depth as having isotropic anisotropy when at the depth the near-wellbore slowness in the fast-shear direction is substantially the same as the near-wellbore slowness in the slow-shear direction.    
   
   
       36 . A system configured to characterize shear wave anisotropy in a formation, comprising: 
 a logging tool configured to: 
 obtain crossed-dipole waveforms from a borehole penetrating the formation over a range of depths and frequencies; and  
   a surface unit operatively connected to the logging tool and configured to: 
 determine far-field slowness in a fast-shear direction and slow-shear direction using a low-frequency portion of the crossed-dipole waveforms;  
 determine near-wellbore slowness in the fast-shear direction and slow-shear direction using a high-frequency portion of the crossed-dipole waveforms;  
 select a depth in the formation; and  
 characterize the depth as having isotropic anisotropy when at the depth the far-field slowness in the fast-shear direction is substantially the same as the far-field slowness in the slow-shear direction and the near-wellbore slowness in the fast-shear direction is substantially the same as the near-wellbore slowness in the slow-shear direction.