Casing collar location using elecromagnetic wave phase shift measurement

Abstract

A method for locating casing collars in a cased wellbore includes moving a well logging instrument coupled within a drill string through the cased wellbore. The instrument includes at least one electromagnetic transmitter and at least two spaced apart electromagnetic receivers. The at least one electromagnetic transmitter is energized with alternating current. A phase difference between electromagnetic signals detected by each of the at least two electromagnetic receivers is measured. A position of at least one casing collar is determined when a change in the measured phase shift is detected.

What is claimed is: 
     
         1 . A method for locating casing collars in a cased wellbore, comprising:
 moving a well logging instrument coupled within a drill string through the cased wellbore, the instrument including at least one electromagnetic transmitter and at least two spaced apart electromagnetic receivers;   energizing the at least one electromagnetic transmitter with alternating current;   measuring a phase difference between electromagnetic signals detected by each of the at least two electromagnetic receivers; and   identifying a position of at least one casing collar when a change in the measured phase difference is detected.   
     
     
         2 . The method of  claim 1  further comprising energizing the at least one transmitter with alternating current at at least two different frequencies and confirming that the measured phase difference corresponds to at least one casing collar by comparing the phase shift measurements made at each of the at least two different frequencies. 
     
     
         3 . The method of  claim 2  wherein the at least two different frequencies comprise 2 MHz and 400 KHz. 
     
     
         4 . The method of  claim 1  wherein the identifying position comprises recording the measured phase difference in the well logging instrument with respect to time, making a record at the surface of position in the wellbore of the well logging instrument with respect to time and correlating the recorded phase shift measurements with respect to the position record with respect to time. 
     
     
         5 . The method of  claim 1  wherein the identifying position comprises detecting measurements of phase shift transmitted to the surface from the well logging instrument. 
     
     
         6 . The method of  claim 1  further comprising moving the well logging instrument into an uncased portion of the wellbore and detecting phase shift measurements corresponding to formations surrounding the wellbore. 
     
     
         7 . The method of  claim 1  further comprising energizing with alternating current each of a plurality of spaced apart electromagnetic transmitters disposed on the well logging instrument, measuring phase shift between the at least two receivers corresponding to the energizing of each of the plurality of spaced apart electromagnetic transmitters. 
     
     
         8 . The method of  claim 7  further comprising energizing each of the plurality of spaced apart transmitters with alternating current at a plurality of frequencies and, measuring phase shift between the at least two receivers corresponding to the energizing of each of the plurality of spaced apart electromagnetic transmitters at each of the plurality of frequencies. 
     
     
         9 . The method of  claim 1  further comprising using the measured phase shift to determine a position of a casing shoe in the wellbore. 
     
     
         10 . A method for well logging, comprising:
 moving a well logging instrument coupled within a drill string through the wellbore, the instrument including at least one electromagnetic transmitter and at least two spaced apart electromagnetic receivers, the wellbore including a cased portion having jointed steel pipe therein and an uncased portion therein;   energizing the at least one electromagnetic transmitter with alternating current;   measuring a phase difference between electromagnetic signals detected by each of the at least two electromagnetic receivers;   using the measured phase difference to determine a resistivity of formations in the uncased portion and;   identifying a position of at least one casing collar in the cased portion when a change in the measured phase difference is detected.   
     
     
         11 . The method of  claim 10  further comprising energizing the at least one transmitter with alternating current at at least two different frequencies and confirming that the measured phase difference in the cased portion corresponds to at least one casing collar by comparing the phase shift measurements made at each of the at least two different frequencies. 
     
     
         12 . The method of  claim 11  wherein the at least two different frequencies comprise 2 MHz and 400 KHz. 
     
     
         13 . The method of  claim 10  wherein the identifying position comprises recording the measured phase difference in the well logging instrument with respect to time, making a record at the surface of position in the wellbore of the well logging instrument with respect to time and correlating the recorded phase shift measurements with respect to the position record with respect to time. 
     
     
         14 . The method of  claim 10  wherein the identifying position comprises detecting measurements of phase shift transmitted to the surface from the well logging instrument. 
     
     
         15 . The method of  claim 10  further comprising energizing with alternating current each of a plurality of spaced apart electromagnetic transmitters disposed on the well logging instrument, measuring phase shift between the at least two receivers corresponding to the energizing of each of the plurality of spaced apart electromagnetic transmitters. 
     
     
         16 . The method of  claim 10  further comprising using the measured phase shift to identify a position of a casing shoe in the wellbore.