Radial profiling of slowness: methods and apparatus for near-wellbore alteration estimation

Abstract

A method facilitating radial profiling of acoustic slowness of a formation traversed by a borehole is disclosed. According to some aspects of the invention, a 3D image of near, far, and intermediate formation slowness is provided.

1 . A method of logging an earth formation surrounding a borehole, comprising:
 providing a logging device that is moveable through the borehole;   transmitting sonic energy into the formation and receiving, at a plurality of transmitter-to-receiver spacings on the logging device, sonic energy that has traveled through the formation; and   providing a 3D image of near, far, and intermediate formation slowness.   
   
   
       2 . A method of logging an earth formation surrounding a borehole according to  claim 1 , wherein providing the 3D image of formation slowness comprises providing near, far, and intermediate formation slowness as a function of radius. 
   
   
       3 . A method of logging an earth formation surrounding a borehole according to  claim 1 , wherein providing the 3D image of formation slowness comprises providing the image in real time. 
   
   
       4 . A method of logging an earth formation surrounding a borehole according to  claim 1 , wherein providing the 3D image of formation slowness comprises providing formation slowness discreet length by discreet length as a function of radius. 
   
   
       5 . A method of logging an earth formation surrounding a borehole according to  claim 1 , wherein the providing a 3D image of formation slowness comprises:
 collecting multiple waveforms at each of multiple formation azimuthal locations;   generating one radial profile for each formation azimuthal location;   compensating formation azimuth for any tool rotation, and   combining the profiles at each formation azimuthal location.   
   
   
       6 . A method of logging an earth formation surrounding a borehole according to  claim 5 , wherein the generating one radial profile at each azimuthal location comprises:
 producing signals representative of the received sonic energy for the plurality of transmitter-to-receiver spacings;   determining, from the signals, differential transit time data for the respective transmitter-to-receiver spacings;   fitting the differential transit times data to a spline function profile; and   transforming spline fitted differential transit times versus transmitter-to-receiver spacings to an inverted profile of slowness as a function of borehole radius.   
   
   
       7 . A method of logging an earth formation surrounding a borehole according to  claim 6 , wherein the transforming comprises:
 (a) starting with an initial radially-layered formation model with an initial profile given by an inverted profile resulting from the transforming;   (b) computing a time of flight for a wave traveling from a transmitter to a receiver based on the initial formation model;   (c) computing a formation slowness for the initial model based on the time computed of flight;   (d) measuring formation slowness;   (e) comparing the computed formation slowness to the measured formation slowness;
 if a difference between the computed formation slowness and the measured formation slowness is within a predetermined error tolerance, assigning the initial model as a final model and stopping further iterations; 
 if the difference between the computed formation slowness and the measured formation slowness is outside the predetermined error tolerance:
 updating the initial formation model with a new radial formation layer thickness; and 
 repeating steps (b)-(e) for the updated formation model; 
 
   (f) repeating steps (a)-(e) for a plurality of radial layers.