Time and depth correction of MWD and wireline measurements using correlation of surface and downhole measurements

Abstract

Differences in times of a time clock at the surface and a downhole time clock are determined using a correlation technique. These differences can be used to provide better correspondence between downhole formation evaluation (FE) sensor measurements and drilling measurements, as well as between different FE sensors.

1 . A method of determining a difference between a first clock at a surface location and a second clock at a downhole location of a downhole assembly, the method comprising: 
 (a) making first measurements of a first parameter of the downhole assembly at the surface location and associating a first time stamp with the first measurements;    (b) making second measurements, similar to the first measurements, of the downhole assembly at the downhole location and associating a second time stamp with the second measurements; and    (c) determining the time difference by matching a feature of the first measurements with a corresponding feature of the second measurements.    
   
   
       2 . The method of  claim 1  wherein the downhole assembly comprises a bottomhole assembly (BHA) conveyed on a drillstring and the first parameter is at least one of (i) a rotational speed of the drillstring, (ii) a torque, and, (iii) a pressure.  
   
   
       3 . The method of  claim 2  wherein the second measurements comprise a measurements of at least one of (i) a rotational speed of the drillstring, (ii) a torque, and, (iii) a pressure.  
   
   
       4 . The method of  claim 2  wherein the second measurements comprise a rate of penetration (ROP) of the drillstring.  
   
   
       5 . The method of  claim 4  further comprising determining the ROP using accelerometer measurements.  
   
   
       6 . The method of  claim 1  wherein the matching further comprises performing a cross-correlation of the first and second measurements  
   
   
       7 . The method of  claim 1  further comprising determining the time difference at a plurality of drilling times.  
   
   
       8 . The method of  claim 7  further comprising: 
 (i) making measurements with a formation evaluation (FE) sensor of a property of the earth formation, the FE sensor being associated with the second clock; and    (ii) converting the FE measurements to drilling depth using the determined time difference at the plurality of drilling times.    
   
   
       9 . The method of  claim 8  further comprising: 
 (i) making measurements with an additional FE sensor of an additional property of the earth formation, the additional FE sensor being associated with a third clock having an additional difference of time relative to the first clock; and    (ii) converting the additional FE measurements to drilling depth using the additional time difference at the plurality of drilling times.    
   
   
       10 . The method of  claim 8  further comprising evaluating the earth formation by jointly processing the converted FE measurements in conjunction with measurements made using sensors on at least one of (i) a wireline, and, (ii) a slickline.  
   
   
       11 . The method of  claim 7  further comprising: 
 (i) making measurements with a formation evaluation (FE) sensor of a property of the earth formation, the FE sensor being associated with the second clock; and    (ii) converting the FE measurements to drilling time using the determined time difference at the plurality of drilling times.    
   
   
       12 . The method of  claim 8  further comprising determining a lithology of an earth formation from the converted FE measurements.  
   
   
       13 . A measurement while drilling (MWD) system comprising: 
 (a) a first clock at a surface location;    (b) a second clock on a bottomhole assembly (BHA) at a downhole location in a borehole in an earth formation, the second clock having a time difference relative to the first clock; and    (c) a processor which: 
 (A) uses first measurements of a first parameter of the MWD system at the surface location and second measurements, similar to the first measurements at the downhole location; and  
 (B) determines the time difference by matching a feature of the first measurements with a corresponding feature of the second measurements.  
   
   
   
       14 . The system of  claim 13  further comprising a drilling tubular which conveys the BHA into the borehole.  
   
   
       15 . The system of  claim 13  wherein the first parameter is at least one of (i) a rotational speed of a drillstring, (ii) a torque, and, (iii) a pressure.  
   
   
       16 . The system of  claim 15  wherein the second measurements comprise at least one of (i) a rotational speed of the drillstring, (ii) a torque, and, (iii) a pressure.  
   
   
       17 . The system of  claim 15  wherein the second measurements comprise a rate of penetration (ROP) of the drillstring.  
   
   
       18 . The system of  claim 17  further comprising an accelerometer which gives an output indicative of the ROP, 
 wherein the processor further determines the ROP using the output of the accelerometer.    
   
   
       19 . The system of  claim 13  wherein the processor performs the matching by further performing a cross-correlation of the first and second measurements  
   
   
       20 . The system of  claim 13  wherein the processor further comprising determines the time difference at a plurality of drilling times.  
   
   
       21 . The system of  claim 20  further comprising a formation evaluation (FE) sensor which makes measurements of a property of the earth formation, the FE sensor being associated with the second clock, 
 and wherein the processor further converts the FE measurements to drilling depth using the determined time difference at the plurality of drilling times.    
   
   
       22 . The system of  claim 20  further comprising a formation evaluation (FE) which makes measurements of a property of the earth formation, the FE sensor being associated with the second clock 
 and wherein the processor further converts the FE measurements to drilling time using the determined time difference at the plurality of drilling times.    
   
   
       23 . The system of  claim 21  wherein the processor further determines a lithology of an earth formation from the converted FE measurements.  
   
   
       24 . A computer readable medium for use with a measurement while drilling (MWD) system, the MWD system comprising: 
 (a) a first clock at a surface location; and    (b) a second clock on a bottomhole assembly (BHA) at a downhole location in a borehole in an earth formation, the second clock having a time difference relative to the first clock;    the medium comprising instructions which enable:    determining from first measurements of a first parameter of the MWD system at the surface location and second measurements similar to the first measurements at the downhole location, the time difference by matching a feature of the first measurements with a corresponding feature of the second measurements.    
   
   
       25 . The medium of  claim 24  wherein the medium is selected from the group consisting of (i) a ROM, (ii) an EPROM, (iii) an EAROM, (iv) a Flash Memory, and, (v) an Optical disk.