Method for borehole measurement of formation properties

Abstract

The present invention is a method of estimating formation properties by analyzing acoustic waves that are emitted from and received by a bottom hole assembly.

We claim:  
     
         1 . A method of using a bottom hole assembly deployed in a borehole to estimate a formation property comprising the steps of: 
 (a) generating a source signal from said bottom hole assembly;    (b) detecting at least one receiver signal using said bottom hole assembly;    (c) computing a frequency dependent characteristic of said at least one receiver signal; and    (d) using said frequency dependent characteristic to estimate said formation property.    
     
     
         2 . The method of  claim 2  wherein said tool is a bottom hole assembly of a drilling apparatus.  
     
     
         3 . The method of  claim 2  wherein said source signal is a noise spectrum generated by a drill bit of said drilling apparatus.  
     
     
         4 . The method of  claim 3  wherein said step of determining frequency dependence is carried out by cross-correlation analysis.  
     
     
         5 . The method of  claim 4  wherein said at least one receiver signal comprises a direct formation signal, and wherein said formation surrounds said borehole.  
     
     
         6 . The method of  claim 4  wherein said at least one receiver signal comprises a reflected signal, and wherein said formation is ahead of said borehole.  
     
     
         7 . The method of  claim 1  wherein said frequency dependent characteristic is amplitude attenuation.  
     
     
         8 . The method of  claim 7  wherein the formation property is pore pressure.  
     
     
         9 . The method of  claim 8  wherein said pore pressure is estimated from a frequency dependent attenuation relationship.  
     
     
         10 . The method of  claim 1  wherein said frequency dependent characteristic is wave propagation velocity.  
     
     
         11 . The method of  claim 10  wherein said formation property is pore pressure.  
     
     
         12 . The method of  claim 1  wherein said formation property is lithology.  
     
     
         13 . The method of  claim 1  wherein said formation property is fluid content.  
     
     
         14 . The method of  claim 1  wherein said formation property is rock strength.  
     
     
         15 . The method of  claim 1  wherein said tool is a bottom hole assembly of a measurement while well logging system.  
     
     
         16 . The method of  claim 1  wherein said source signal is generated by an active source located on said bottom hole assembly.  
     
     
         17 . The method of  claim 16  wherein said step of determining frequency dependence is carried out by a frequency component analysis.  
     
     
         18 . The method of  claim 1 , wherein said at least one receiver signal comprises a direct borehole signal.  
     
     
         19 . The method of  claim 18  wherein said formation property is permeability.  
     
     
         20 . A method of continuously estimating the pore pressures of formations ahead of a bottom hole assembly, comprising the steps of 
 a) generating a source signal from said bottom hole assembly;    b) detecting at least one receiver signal using said bottom hole assembly;    c) using said source signal and said receiver signal to estimate a pore pressure of at least one said formation; and    d) repeating steps a), b), and c) as said bottom hole assembly moves sequentially downward through said formations.    
     
     
         21 . A method of continuously monitoring the wellbore pressure safety margin corresponding to formations ahead of a bottom hole assembly, comprising the steps of 
 a) generating a source signal from said bottom hole assembly;    b) detecting at least one receiver signal using said bottom hole assembly;    c) using said source signal and said receiver signal to determine a pore pressure of said formation;    d) using said pore pressure to monitor said wellbore pressure safety margin; and    e) repeating steps a), b), c) and d) as said bottom hole assembly moves sequentially downward through said formations.    
     
     
         22 . A method of continuously optimizing the weight of drilling mud used in a drilling operation, comprising the steps of 
 a) generating a source signal from a bottom hole assembly;    b) detecting at least one receiver signal using said bottom hole assembly;    c) using said source signal and said receiver signal to determine a pore pressure of a formation ahead of said bottom hole assembly; and    d) using said pore pressure to specify a weight of said drilling mud which corresponds to a target wellbore pressure safety margin.