Distributed acoustic sensing for coiled tubing characteristics

Abstract

Distributed Acoustic Sensing systems may be employed for monitoring wellbore events occurring within the interior of a tubular conveyance such as a coiled tubing strand. A signal cable carried by the coiled tubing strand may provide a measure of acoustic energy as a function of position in the wellbore such that a location of a ball dropped through the coiled tubing strand may be monitored in real time. The signal cable can be used to determine whether the ball encounters an obstruction within the coiled tubing strand, and/or can confirm that the ball has properly seated on a downhole tool actuated by engagement with the ball. In other embodiments, the location and density of a liquid sand flow or other fluid mixture can be monitored, and/or the location of a leak through a lateral wall of the coiled tubing strand can be detected.

What is claimed is: 
     
         1 . A method of monitoring a downhole operation, the method comprising:
 acquiring downhole distributed acoustic sensing measurements from a signal cable deployed into a wellbore on a tubular conveyance;   detecting an acoustic signature of a target flowing within the tubular conveyance by processing the distributed acoustic sensing measurements; and   determining a location of the target within the tubular conveyance based on the acoustic signature.   
     
     
         2 . The method according to  claim 1 , further comprising deploying a downhole tool into the wellbore on the tubular conveyance, and flowing an actuator through the tubular conveyance to actuate the downhole tool, wherein the actuator is the target. 
     
     
         3 . The method according to  claim 2 , wherein determining the location of the target includes tracking the actuator by determining a plurality of locations of the actuator corresponding to a plurality of times as the actuator flows through the tubular conveyance. 
     
     
         4 . The method according to  claim 3 , further comprising verifying a seated position of actuator at the downhole tool. 
     
     
         5 . The method according to  claim 2 , wherein detecting the acoustic signature of the target includes detecting a dual peak signature of the actuator representing a differential flow at leading and trailing ends of the actuator. 
     
     
         6 . The method according to  claim 2 , further comprising transmitting an acoustic signal from a transmitter carried by the actuator, and wherein detecting the acoustic signature of the target includes detecting the acoustic signal with the signal cable or detecting an echo of the acoustic signal with the signal cable. 
     
     
         7 . The method according to  claim 1 , wherein the tubular conveyance is a coiled tubing strand and wherein detecting an acoustic signature of the target includes detecting a leak in the coiled tubing strand by detecting an acoustic signature of fluid flowing through an opening in a lateral wall of the coiled tubing strand. 
     
     
         8 . The method according to  claim 7 , wherein processing the distributed acoustic sensing measurements includes determining a downhole bulk flowrate of a fluid inside the coiled strand and comparing the downhole bulk flow rate to a bulk flow rate of the fluid into the coiled tubing strand at a surface location. 
     
     
         9 . The method according to  claim 1 , wherein detecting an acoustic signature of the target includes detecting a volume of fluid within the coiled tubing strand by detecting a change in density of fluid flowing through the coiled tubing strands at the leading and trailing ends of the volume of fluid. 
     
     
         10 . The method according to  claim 9 , further comprising detecting changes in density within the volume of fluid. 
     
     
         11 . A wellbore monitoring system, comprising:
 a tubular conveyance extending into a wellbore;   a signal cable extending along a length of the tubular conveyance within the wellbore;   a distributed acoustic sensing unit operably coupled to the signal cable for acquiring downhole distributed acoustic sensing measurements from the signal cable;   a processor operably coupled to the distributed acoustic sensing unit for processing the distributed acoustic sensing measurements; and   a memory operably coupled to the acoustic sensing unit, the memory having a set of instructions stored thereon that, when executed by the processor cause the processor to compare the processed distributed acoustic sensing measurements to a predetermined acoustic signature of a downhole event within the tubular conveyance.   
     
     
         12 . The system according to  claim 11 , wherein the predetermined acoustic signature includes at least one of the set consisting of an actuator dropped through the tubular conveyance, a volume of fluid moving through the tubular conveyance inducing a change in fluid properties at a fixed location within the tubular conveyance, and a leak of fluid flowing through a sidewall of the tubular conveyance. 
     
     
         13 . The system according to  claim 12 , wherein the predetermined acoustic signature is an acoustic signature of an acoustic transmitter carried by the actuator. 
     
     
         14 . The system according to  claim 11 , wherein the tubular conveyance comprises a coiled tubing strand. 
     
     
         15 . The system according to  claim 14 , wherein signal cable extends through an interior of the coiled tubing strand in a helical pattern. 
     
     
         16 . A method for monitoring a downhole operation, the method comprising:
 deploying a signal cable into a wellbore on a tubular conveyance, wherein the signal cable extends along a length of the tubular conveyance;   dropping an actuator through an interior passageway of the tubular conveyance into the wellbore;   acquiring downhole distributed acoustic sensing measurements from the signal cable;   detecting, in the distributed acoustic sensing measurements, an acoustic signature of the actuator moving through the tubular conveyance, wherein the acoustic signature includes a dual peak pattern indicative of relatively high levels of acoustic energy at leading and trailing ends of the actuator.   
     
     
         17 . The method according to  claim 16 , further comprising suspending further downhole operations until a location of the acoustic signature corresponds with a location of a downhole tool actuated by the actuator. 
     
     
         18 . The method according to  claim 16 , wherein deploying the signal cable into the wellbore includes deploying the signal cable into the wellbore on a coiled tubing strand by unwinding the coiled tubing strand from a reel. 
     
     
         19 . The method according to  claim 18 , further comprising detecting an acoustic signature of a pinhole leak including fluid flowing a lateral wall of the coiled tubing strand at a downhole location. 
     
     
         20 . The method according to  claim 19 , further comprising comparing a downhole bulk flowrate of a fluid inside the coiled strand to a downhole bulk flow rate of the fluid into the coiled tubing strand at a surface location.