Leaky Flexural Wave Semblance Based Annular Stacking Velocity Determination In Cased Wells

Abstract

Disclosed herein are systems and methods of determining cement integrity behind a casing string using acoustic signals in the field of well drilling and completions. Specifically, the systems and methods evaluate the interface between cement and formation and/or cement-second casing and determine the thickness of the cement. Data are processed to determine annular compression wave velocities in a cased well using a semblance method or by stacking the amplitude of the leaked A0 mode energy that has reflected from the cement-formation or cement-second casing interface. The annular compression wave velocity is necessary to convert time domain images of the annulus into radial distance domain images for better interpretation of annular conditions. A semblance method is used to determine a velocity estimate for the annulus and annular thickness. The semblance method is used for all depths and azimuths to create detailed radial distance domain images of the annulus in cased wells.

1 . A method of determining a cement integrity behind a conduit string using acoustic signals comprising:
 transmitting an acoustic signal into at least part of a conduit string;   measuring a return signal from at least part of the conduit string;   analyzing the return signal;   computing travel times for leaked and reflected waves for a range of annular thicknesses and compressional wave velocities;   comparing the computing travel times with the analyzed return signal; and   plotting a semblance map of annular thicknesses as a function of compressional wave velocities.   
     
     
         2 . The method of  claim 1 , wherein the acoustic signals are obtained from an acoustic logging tool in a pitch-catch (P-C) configuration. 
     
     
         3 . The method of  claim 1 , wherein the acoustic signals are obtained from an acoustic logging tool in a pitch-catch (P-C) receiver configuration, wherein a distance between a P-C transmitter and a P-C receiver is from about 6 inches to about 20 inches. 
     
     
         4 . The method of  claim 1 , wherein the acoustic signals are obtained from an acoustic logging tool in a pitch-catch (P-C) receiver configuration, wherein a distance between a P-C transmitter and a P-C receiver is from about 8.5 inches to about 15 inches. 
     
     
         5 . The method of  claim 1 , further determining if a compressional wave velocity of the cement is close to a shear wave velocity of the conduit string to select a travel time equation to create a semblance map. 
     
     
         6 . The method of  claim 1 , further computing travel times for leaked and reflected waves for a range of annular thicknesses and compressional wave velocities and selecting the annular thicknesses and compressional wave velocities for which a semblance is maximized. 
     
     
         7 . The method of  claim 1 , further plotting a semblance map for a range of annular thicknesses and cement velocities, picking a top solution based on semblance values from the semblance map, and taking the corresponding annular thickness and cement velocity. 
     
     
         8 . The method of  claim 1 , further creating a velocity profile based on a combination of conduit string material properties, logging fluid layer thickness, logging fluid sound speed, annular thickness, and compressional wave velocity, and converting a time domain structural image into a radial distance domain image of an annulus. 
     
     
         9 . The method of  claim 8 , further repeating the creation for every depth and azimuth to create a velocity profile as a function of depth and azimuth in a logging operation. 
     
     
         10 . A method of determining a cement thickness and velocity of a wave propagation comprising:
 a). transmitting an acoustic signal into at least part of a conduit string;   b). measuring a return signal from at least part of the conduit string;   c). computing one or more amplitudes of a resonate signal from the return signal;   d). calculating a semblance for a range of annular thicknesses and cement velocities;   e). plotting a semblance map as a function of cement thickness and cement velocity;   f). calculating a mean cement thickness and velocity based on semblance values; and   g). converting a time domain image to a radial distance domain image using the mean cement thickness and velocity.   
     
     
         11 . The method of  claim 10 , further repeating steps a) through g) at several depths and azimuths to obtain a 3D radial distance domain image. 
     
     
         12 . The method of  claim 10 , further determining if a compressional wave velocity of the cement is close to a shear wave velocity of the conduit string to select a travel time equation to create the semblance map. 
     
     
         13 . The method of  claim 10 , wherein the acoustic signal is obtained from an acoustic logging tool in a pitch-catch (P-C) configuration. 
     
     
         14 . The method of  claim 10 , wherein the acoustic signals are obtained from an acoustic logging tool in a pitch-catch (P-C) configuration, wherein a distance between a P-C transmitter and a P-C receiver is from about 4 inches to about 30 inches. 
     
     
         15 . The method of  claim 10 , wherein the acoustic signals are obtained from an acoustic logging tool in a pitch-catch (P-C) configuration, wherein a distance between a P-C transmitter and a P-C receiver is from about 6 inches to about 20 inches. 
     
     
         16 . The method of  claim 10 , wherein the acoustic signals are obtained from an acoustic logging tool in a pitch-catch (P-C) configuration, wherein a distance between a P-C transmitter and a P-C receiver is from about 8.5 inches to about 15 inches. 
     
     
         17 . A system to determine a cement integrity behind a conduit comprising:
 an acoustic logging tool in a pitch-catch (P-C) configuration to transmit an acoustic signal into at least part of a conduit string and measure a return signal from at least part of the conduit string;   a digital telemetry system; and   an information handling system to analyze the return signal, compute travel times for leaked and reflected waves for a range of annular thicknesses and compressional wave velocities, compare the computed travel times with the analyzed return signal, and plot a semblance map of annular thicknesses as a function of compressional wave velocities.   
     
     
         18 . The system of  claim 17 , wherein the acoustic signals are obtained from an acoustic logging tool in a pitch-catch (P-C) configuration, wherein a distance between a P-C transmitter and a P-C receiver is from about 4 inches to about 30 inches. 
     
     
         19 . The system of  claim 17 , wherein the acoustic signals are obtained from an acoustic logging tool in a pitch-catch (P-C) configuration, wherein a distance between a P-C transmitter and a P-C receiver is from about 6 inches to about 20 inches. 
     
     
         20 . The system of  claim 17 , wherein the acoustic signals are obtained from an acoustic logging tool in a pitch-catch (P-C) configuration, wherein a distance between a P-C transmitter and a P-C receiver is from about 8.5 inches to about 15 inches.