Topside Interrogation For Distributed Acoustic Sensing Of Subsea Wells

Abstract

A distributed acoustic system (DAS) method and system. The system may comprise an interrogator and an umbilical line comprising a first fiber optic cable and a second fiber optic cable attached at one end to the interrogator. The DAS may further include a downhole fiber attached to the umbilical line at the end opposite the interrogator and a light source disposed in the interrogator that is configured to emit a plurality of coherent light frequencies into the umbilical line and the downhole fiber. The method may include generating interferometric signals of the plurality of frequencies of backscattered light that have been received by the photo detector assembly and processing the interferometric signals with an information handling system.

What is claimed is: 
     
         1 . A distributed acoustic system (DAS) comprising:
 an interrogator   an umbilical line comprising:
 a first fiber optic cable; 
 a second fiber optic cable; 
 a proximal circulator connected to the first fiber optic cable and the second fiber optic cable; and 
 an optical amplifier assembly disposed after the proximal circulator; 
   a downhole fiber attached to the first fiber optic cable; and   a light source disposed in the interrogator that is configured to emit a plurality of coherent light frequencies into the umbilical line and the downhole fiber.   
     
     
         2 . The DAS of  claim 1 , wherein the optical amplifier assembly is attached to the first fiber optic cable or the second fiber optic cable. 
     
     
         3 . The DAS of  claim 2 , wherein the first fiber optic cable and the second fiber optic cable are connected to a distal circulator. 
     
     
         4 . The DAS of  claim 3 , wherein the umbilical line further comprises one or more distal optical amplifier assemblies disposed between the proximal circulator and the distal circulator. 
     
     
         5 . The DAS of  claim 4 , further comprising at least one Fiber Bragg Grating that is attached between the optical amplifier assembly and the distal circulator. 
     
     
         6 . The DAS of  claim 4 , further comprising at least one Fiber Bragg Grating that is attached between at least one of the one or more distal optical amplifier assemblies and the distal circulator. 
     
     
         7 . The DAS of  claim 1 , further comprising at least one Fiber Bragg Grating that is attached between the umbilical line and the end of the downhole fiber. 
     
     
         8 . The DAS of  claim 1 , wherein the optical amplifier assembly is an Erbium doped fiber amplifier. 
     
     
         9 . The DAS of  claim 8 , wherein the erbium doped fiber amplifier further comprises a Fabry Perot filter. 
     
     
         10 . The DAS of  claim 1 , wherein one or more remote circulators form the proximal circulator or the distal circulator. 
     
     
         11 . The DAS of  claim 10 , further comprising at least one Fiber Bragg Grating attached to the proximal circulator or the distal circulator. 
     
     
         12 . The DAS of  claim 10 , wherein the interrogator is configured to receive a backscattered light from a first sensing region and a second sensing region. 
     
     
         13 . The DAS of  claim 12 , wherein an interrogator receiver arm is configured to receive the backscattered light from the first sensing region or the second sensing region. 
     
     
         14 . The DAS of  claim 13 , wherein the optical amplifier assembly is attached to the first fiber optic cable or the second fiber optic cable at the proximal circulator. 
     
     
         15 . The DAS of  claim 13 , wherein the optical amplifier assembly is attached to the first fiber optic cable or the second fiber optic cable at the distal circulator. 
     
     
         16 . A method for interrogating a formation with a distributed acoustic system (DAS comprising:
 transmitting a coherent light that includes a plurality of frequencies from a light source in an interrogator to a downhole fiber through an umbilical cord connected to the interrogator, wherein the umbilical cord comprises:
 a first fiber optic cable; 
 a second fiber optic cable; 
 a proximal circulator connected to the first fiber optic cable; and 
 an optical amplifier assembly disposed after the proximal circulator; 
   receiving the plurality of frequencies as a backscattered light with the interrogator from the downhole fiber through umbilical line with a photo detector assembly;   generating interferometric signals of the plurality of frequencies of backscattered light that have been received by the photo detector assembly; and   processing the interferometric signals with an information handling system.   
     
     
         17 . The method of  claim 16  wherein the optical amplifier assembly is an Erbium doped fiber amplifier. 
     
     
         18 . The method of  claim 17 , wherein the erbium doped fiber amplifier further comprises a Fabry Perot filter. 
     
     
         19 . The method of  claim 16 , further comprising changing each of the plurality of frequencies to another frequency during the transmitting of the coherent light. 
     
     
         20 . The method of  claim 16 , wherein the light source and the photo detector assembly are disposed in the interrogator.