Wireline Optical Fiber Sensing

Abstract

The high sensitivity provided by an enhanced DAS system comprising a DAS interrogator and a high reflectivity fiber allows for the deployment of such a high reflectivity fiber as part of a wireline intervention cable which can be temporarily lowered into a well, thus avoiding the need to permanently cement such a high reflectivity optical fiber cable into the well. Instead, such a wireline cable incorporating the high reflectivity optical fiber has been found to be sensitive enough to detect micro-seismic activity and low frequency strain with many more measurement points and channels than conventional wireline deployed geophones and tiltmeters. Additionally, the cable requires no clamping and can be easily and quickly removed from one well and placed in another well.

What is claimed is: 
     
         1 . An optical fiber distributed sensor system, comprising:
 an optical source arranged in use to produce optical signal pulses;   an optical fiber deployable in use in an environment to be sensed and arranged in use to receive the optical signal pulses; and   sensing apparatus arranged in use to detect light from the optical signal pulses reflected or backscattered back along the optical fiber and to determine any one or more of an acoustic, vibration, temperature or other parameter that perturbs the path length of the optical fiber in dependence on the reflected light;   
       the system being characterized in that the optical fiber is encased in a wireline cable or a slickline cable for deployment downhole. 
     
     
         2 . A system according to  claim 1 , wherein the optical fiber is adapted so as to have higher reflectivity along its length to the optical signal pulses than conventional optical fiber 
     
     
         3 . A system according to  claim 2 , wherein the optical fiber comprises a plurality of reflector portions distributed along its length in at least a first sensing region thereof to thereby provide the higher reflectivity. 
     
     
         4 . A system according to  claim 3 , wherein the reflectivity of the reflector portions is:
 i) inversely dependent on the number of reflector portions in the at least first sensing region; and   ii) non-inversely dependent on a selected amount of crosstalk between the reflector portions in the at least first sensing region.   
     
     
         5 . A system according to  claim 2 , wherein the optical fiber has a higher backscatter coefficient than conventional optical fiber. 
     
     
         6 . A system according to  claim 1 , wherein the optical fiber is conventional single mode or multimode fiber with conventional reflection or backscatter characteristics. 
     
     
         7 . A system according to  claim 1 , wherein the sensing apparatus further comprises a means for processing the reflected or backscattered light to measure the relative phase, frequency and amplitude of the received light from along the length of the optical fiber to detect the acoustic perturbations, wherein in use the relative phase, frequency and amplitude measurements taken from along the length of the optical fiber are synchronized to enhance signal sensitivity. 
     
     
         8 . A system according to  claim 7 , wherein the sensing apparatus further comprises an interferometer arranged in use to receive backscattered and/or reflected light from along the sensing optical fiber, the interferometer comprising at least two optical paths with a path length difference therebetween, the backscattered and/or reflected light interfering in the interferometer to produce interference components, and wherein the means for processing comprises plural photodetectors to measure the interference components, and a processor arranged to determine optical phase angle data therefrom. 
     
     
         9 . A system according to  claim 1 , wherein the wireline cable or slickline cable is heavier than any liquid encountered downhole such that in use the cable sinks through any such liquid until it reaches a solid support surface. 
     
     
         10 . A system according to  claim 9 , wherein the solid support surface is the bottom of steel casing that is cemented in a lateral oil or gas well. 
     
     
         11 . A wireline cable having encased therein an optical fiber adapted so as to reflect or backscatter any optical pulses travelling therealong to a greater extent than conventional optical fiber. 
     
     
         12 . A cable according to  claim 11 , wherein the optical fiber comprises a plurality of reflector portions distributed along its length in at least a first sensing region thereof. 
     
     
         13 . A cable according to  claim 12 , wherein the reflectivity of the reflector portions is:
 i) inversely dependent on the number of reflector portions in the at least first sensing region; and   ii) non-inversely dependent on a selected amount of crosstalk between the reflector portions in the at least first sensing region.   
     
     
         14 . A cable according to  claim 11 , wherein the optical fiber has a higher backscatter coefficient than conventional optical fiber. 
     
     
         15 . A cable according to  claim 11 , wherein the cable is heavier than any liquid encountered downhole such that in use the cable sinks through any such liquid until it reaches a solid support surface. 
     
     
         16 . A cable according to  claim 15 , wherein the solid support surface is the bottom of steel casing that is cemented in a lateral oil or gas well. 
     
     
         17 . A cable according to  claim 12 , wherein a product of the number of reflector portions and the average reflectivity of the reflector portions is 0.1 or less. 
     
     
         18 . A cable according to  claim 11 , the cable being so arranged that in use it is capable of being either pumped or tractored into a lateral section of an oil or gas well. 
     
     
         19 . An optical fiber distributed sensor system according to  claim 1 , wherein the system is an optical fiber distributed acoustic sensor system arranged to sense acoustic signals incident upon the cable. 
     
     
         20 . An optical fiber distributed sensor system according to  claim 3 , wherein a product of the number of reflector portions and the average reflectivity of the reflector portions is 0.1 or less. 
     
     
         21 . A method of downhole acoustic surveying, comprising:
 deploying a wireline or slickline cable containing an optical fiber downhole into a well;   connecting the surface end of the optical fiber to a distributed acoustic sensor interrogator;   operating the interrogator to send optical pulses along the optical fiber and measuring the optical reflections and/or backscatter received from along the length of the optical fiber;   after the interrogator operation, disconnecting the surface end of the optical fiber from the interrogator and retrieving the cable from within the well.   
     
     
         22 . A method according to  claim 21 , and further comprising processing the optical reflection and backscatter to determine properties of any acoustic signals incident on the cable along its length. 
     
     
         23 . A method according to  claim 22 , and further comprising processing the determined properties of any acoustic signals to determine properties of any microseismic, low frequency strain and/or drilling induced vibrations present in the vicinity of the well. 
     
     
         24 . A method according to  claim 21 , and further comprising moving location to another well, and repeating the steps of the method at that other well. 
     
     
         25 . A method according to  claim 21 , wherein the optical fiber is adapted so as to have higher reflectivity along its length to optical signal pulses travelling therealong than conventional optical fiber. 
     
     
         26 . A method according to  claim 25 , wherein the high reflectivity optical fiber comprises a plurality of reflector portions distributed along its length in at least a first sensing region thereof. 
     
     
         27 . A method according to  claim 21 , wherein operating the interrogator further comprises processing the measured optical reflections and/or backscatter received from along the length of the optical fiber to measure the relative phase, frequency and amplitude of the received light from along the length of the optical fiber to detect acoustic perturbations, wherein in use the relative phase, frequency and amplitude measurements taken from along the length of the optical fiber are synchronized to enhance signal sensitivity. 
     
     
         28 . A method according to  claim 21 , and further comprising deploying respective cables containing optical fiber into multiple wells in the same field, connecting the respective cables to respective DAS interrogators, and operating the DAS interrogators simultaneously to obtain DAS data from multiple wells simultaneously. 
     
     
         29 . A method according to  claim 28 , and further comprising processing the DAS data from the multiple wells to obtain data indicative of cross-well strain. 
     
     
         30 . A method according to  claim 29 , wherein at least one of the optical fibers in at least one of the wells is permanently deployed in the at least one well, and at least one of the other optical fibers deployed in another of the wells is retrievable from the another well. 
     
     
         31 . A method according to  claim 22 , and further comprising processing the determined properties of any acoustic signals to determine any one or more of:
 i) a poroelastic effect within the rock surrounding the well;   ii) a hydraulic fracturing pump start and/or stop time;   iii) rock fractures opening around or in the well during hydraulic fracturing operations;   iv) rock fractures closing around or in the well after hydraulic fracturing operations have ceased.