Patent Publication Number: US-2023141615-A1

Title: Grating position dithering for improved distributed acoustic sensing engineered fiber performance

Description:
BACKGROUND 
     In the resource recovery industry, acoustic downhole measurements can be obtained from a wellbore using an optical system including an optical interrogator and an optical fiber extending from the optical interrogator into the wellbore. The optical fiber has a plurality of light scatterers along its length. The optical interrogator transmits a light into the optical fiber and records a reflection of the light from the scatterers. An acoustic signal impinging on the optical fiber changes a wavelength of the reflected light, which can be read at the optical interrogator to measure the acoustic signal. However, destructive interference can occur due to uniform spacing between gratings, thereby resulting in a weak optical signal being received at the optical interrogator. Therefore, it is desirable to be able to reduce the effects of destructive interference in an optical fiber used in acoustic downhole measurements. 
     SUMMARY 
     Disclosed herein is an optical system for measuring an acoustic signal. The optical system includes an optical interrogator, an optical fiber and a plurality of gratings formed in the optical fiber. The optical interrogator is configured to transmit a light pulse. The optical fiber receives the light pulse. The optical fiber has a plurality of nominal sites uniformly spaced apart along a longitudinal axis of the optical fiber. The plurality of gratings are formed in the optical fiber, each of the plurality of gratings associated with a nominal site and separated from its associated nominal site by an offset distance. The offset distance is selected to reduce a destructive interference between reflections from the plurality of gratings. 
     Also disclosed herein is a method of measuring an acoustic signal in a wellbore. An optical fiber is disposed in the wellbore, the optical fiber having a plurality of gratings formed therein, each of the plurality of gratings being located at a selected offset distance from a respective one of a plurality of nominal sites of the optical fiber, the plurality of nominal sites being uniformly spaced along a longitudinal axis of the optical fiber, wherein each offset distance is selected to reduce a destructive interference between reflections from the plurality of gratings. An optical interrogator transmits a light pulse into the optical fiber to measure the acoustic signal via a reflection of the light pulse from at least one of the plurality of gratings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
         FIG.  1    shows an optical system for obtaining a measurement of a downhole parameter from a wellbore in an embodiment; 
         FIG.  2    shows a section of an optical fiber of the optical system, in an illustrative embodiment; 
         FIG.  3    shows an apparatus for manufacturing the optical fiber of  FIG.  2   , in an illustrative embodiment; and 
         FIG.  4    shows a flowchart for manufacturing the optical fiber. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
     Referring to  FIG.  1   , an optical system  100  for obtaining a measurement of a downhole parameter from a wellbore  102  is shown in an embodiment. In various embodiments, the optical system  100  is a Distributed Acoustic Sensing (DAS) system that uses light to measure acoustic signals or acoustic waves within the wellbore  102 . 
     The optical system  100  includes an optical interrogation unit  104 , generally at a surface location, and an optical fiber  106  extending from the optical interrogation unit  104  into the wellbore  102 . A plurality of scatterers  108  are distributed along a length of the optical fiber  106 . The scatterers  108  can be scattering sites within the optical fiber that reflect light due to Rayleigh scattering, or they can be gratings, such as Fiber Bragg Gratings (FBGs), that are formed therein intentionally during a manufacturing process). In one embodiment, the optical fiber  106  is disposed on or in relation to a carrier or tool  120 , such as a drill string segment, downhole tool or bottomhole assembly. As described herein, a “carrier” refers to any structure suitable for being lowered into a wellbore or for connecting a drill or downhole tool to the surface and is not limited to the structure and configuration described herein. Examples of carriers include casing pipes, wirelines, wireline sondes, slickline sondes, drop shots, downhole subs, bottomhole assemblies, drill string inserts, modules, internal housings and substrate portions thereof. 
     The optical interrogation unit  104  includes a laser  110  or other light source, a detector  112  and a processor  114 . The detector  112  may be any suitable type of photodetector such as a diode assembly. The detector  112  is configured to receive return signals reflected from the scatterers  108  and generate measurement data. Exemplary parameters that can be measured using the optical fiber  106  include temperature, strain, pressure, position, and vibration. 
     In operation, the laser  110  transmits a light pulse into the optical fiber  106 . In various embodiments, a wavelength of the light pulse is 1550 nanometers (nm). However, those having skill in the art would recognize that the optical system  100  and optical fiber  106  can be constructed to operate with respect to any selected wavelength of the light pulse. The light pulse has a spatial length within the optical fiber  106  is referred to herein as a light pulse length. In various embodiments, the light pulse length is in a range from about 1 meter to about 15 meters. The light pulse travels along the optical fiber  106  and is reflected back to the optical interrogation unit  104  from various locations within the optical fiber by the scatterers  108  therein. The reflected signal is received by the detector  112  and is analyzed at the processor  114  to estimate desired parameters. 
       FIG.  2    shows a section  200  of the optical fiber  106  in an illustrative embodiment. The optical fiber  106  extends from a first end  202  (also referred to herein as a “launched end”) to a second end  204  along a longitudinal axis. The first end  202  is coupled to the optical interrogation unit  104 . The optical fiber  106  includes gratings  208   a - 208   c  at various grating locations in the optical fiber  106 . The light pulse generated at the optical interrogation unit  104  enters the optical fiber  106  at the first end  202  and is reflected from the gratings  208   a - 208   c  to exit the optical fiber  106  at the first end  202 . 
     The distance between gratings  208   a - 208   c  is selected so that there is at least one grating within a light pulse length  210 . In practice, at least 3 gratings within a light pulse length is generally preferred to reduce variation of signal due to natural variation in the strength of the grating reflections. In various embodiments, the gratings  208   a - 208   c  can be chirped Fiber Bragg gratings (FBGs). A chirped FBG is designed to have a reflection band that has a bandwidth greater than a bandwidth of the light from the laser  110 . The bandwidth of the FBG is selected, in part, based on the temperature range of the environment in which the optical fiber  106  is deployed. The central wavelength of an FBG shifts by ˜12 pm/C (picometers per degree Celsius). Multiplying this temperature shift by a desired temperature range of the optical fiber  106  sets a lower limit on the bandwidth for the FBG. The laser  110  used in DAS generally has a linewidth that is &lt;&lt;1 pm. In various embodiments, a linewidth of the laser  110  can be within a narrow band of about 10{circumflex over ( )}−7 nanometers, while the bandwidth of the chirped FBG is in a range from about 1 nm to about 10 nm. The wavelength of the laser  110  therefore falls within the reflection band of the chirped FBG. In other words, the bandwidth of the FBG is wide enough to fully reflect light from the laser  110 . The gratings  208   a - 208   c  having a variable spacing between them, as described herein. 
     The optical fiber  106  includes a plurality of nominal sites  206   a - 206   c  therein. Although only three nominal sites  206   a - 206   c  are shown in  FIG.  2   , the number of grating locations can be in the hundreds or thousands, in various embodiments. The nominal sites  206   a - 206   c  are spaced uniformly apart from each other by a nominal separation distance. In other words, a nominal separation distance d norm  between the first nominal site  206   a  and the second nominal site  206   b  is the same as the nominal separation distance d norm  between the second nominal site  206   b  and the third nominal site  206   c.    
     Each of the plurality of gratings  208   a - 208   c  is associated with a respective one of the plurality of nominal sites  206   a - 206   c  and is offset from its associated nominal sites  206   a - 206   c  by an offset distance. For example, first grating  208   a  is separated from the first nominal site  206   a  by first offset distance Δd 1 , second grating  208   b  is separated from the second nominal site  206   b  by second offset distance Δd 2 , third grating  208   c  is separated from the third nominal site  206   c  by third offset distance Δd 3 , etc. In various embodiments, the offset distance is greater than or equal to a quarter wavelength of the light from the laser  110 . When adjacent gratings are separated by the nominal separation distance, destructive interference occurs between the light reflected from these adjacent gratings. The offset distance is introduced and selected to reduce the occurrence of this destructive interference. In one embodiment, introducing an offset distance of a quarter wavelength results in a constructive interference between the light reflected form adjacent gratings. 
     Each offset distance (Δd 1 , Δd 2 , Δd 3 , . . . ) can be a value selected using a dithering function. In one embodiment, the dithering function can be a function that has an average value of zero and varies within a dithering range defined by a range limit. The offset distance is for a grating is a value selected from within the dithering range using the dithering function. The range limit can be between about 1 micron to about 1 centimeter in length. The offset distance can be a positive number or a negative number, so that the grating can be on either side of its associated nominal site. A positive offset distance is represented in  FIG.  2    as an offset to the right from its associated nominal site, and a negative offset distance is represented in  FIG.  2    as an offset to the left from its associated nominal site. In various embodiments, the dithering function can be a periodic function oscillating within the range limit or a combination of periodic functions. The dithering function generates the offset distance based on a distance along the optical fiber. Examples of dithering functions include, but are not limited to, a triangle wave function, a sawtooth wave function, a sinusoidal function. In another embodiment, the offset distances can be random distances selected from within the dithering range. 
     By introducing offset distances, the intra-grating spacing (Δg 1 , Δg 2 , . . . ) can be different for any two adjacent gratings. Thus, the first intra-grating spacing Δg 1  between the first grating  208   a  and the second grating  208   b  is different from the second intra-grating spacing Δg 2  between the second grating  208   b  and the third grating  208   c , etc. In particular Δg 1 =d norm −Δd 1 +Δd 2 ; and Δg 2 =d norm −Δd 2 +Δd 3 . Additionally, successive offset distances or adjacent offset distances (e.g., Δd 1  and Δd 2 ) cannot be the same or else the resulting intra-grating space (e.g., Δd 1 ) remains the same as the nominal distance d norm . 
     Destructive interference can occur when gratings are located at the nominal sites  206   a - 206   c  (i.e., evenly spaced from each other by d norm ) By introducing the offset distance (Δd 1 , Δd 2 , Δd 3 , . . . ) for each of the gratings, the occurrence of destructive interference is reduced or minimized Thus, the signal-to-noise ratio for a signal from the optical fiber  106  disclosed herein having gratings located at selected offset distances (Δd 1 , Δd 2 , Δd 3 , . . . ) from their associated nominal sites  206   a - 206   c  is greater than the signal-to-noise ratio for a signal from an optical fiber in which the gratings are located at the nominal sites. 
       FIG.  3    shows an apparatus  300  for manufacturing the optical fiber  106  of  FIG.  2   , in an illustrative embodiment. The apparatus  300  includes a drawing device  302  that draws the optical fiber  106  along a selected direction at a constant draw speed v (or a known draw speed). An optical engraver  304 , typically an ultraviolet (UV) pulsed laser, emits a light beam  308  to engrave a grating at selected locations in the optical fiber  106 . A processor  306  controls the operation of the optical engraver  304 . The processor  306  receives a signal from the drawing device  302  indicating the draw speed. The processor  306  determines or selects an offset distance using the dithering function and determines a grating location based on the defined distance between nominal sites and the selected offset distance. Using the draw speed of the optical fiber  106 , the processor  306  activates the optical engraver  304  when the determined grating location passes in front of the optical engraver  304 , thereby forming the grating in the optical fiber  106 . This process is repeated for subsequent gating locations until the optical fiber is complete. 
       FIG.  4    shows a flowchart  400  for manufacturing the optical fiber  106 . In box  402 , an optical fiber is drawn at a known draw speed. In box  404 , the processor determining a grating location based on a distance between nominal sites and an offset distance selected using a dithering function. In box  406 , the processor activates the optical engraver at a time based on the draw speed and the determined grating location to form a grating in the optical fiber  106  at the determined grating location. 
     Set forth below are some embodiments of the foregoing disclosure: 
     Embodiment 1: An optical system for measuring an acoustic signal. The optical system includes an optical interrogator, an optical fiber and a plurality of gratings formed in the optical fiber. The optical interrogator is configured to transmit a light pulse. The optical fiber receives the light pulse, the optical fiber having a plurality of nominal sites uniformly spaced apart along a longitudinal axis of the optical fiber. The plurality of gratings are formed in the optical fiber, with each of the plurality of gratings associated with a nominal site and separated from its associated nominal site by an offset distance. The offset distance is selected to reduce a destructive interference between reflections from the plurality of gratings. 
     Embodiment 2: The optical system of any prior embodiment, wherein the light pulse transmitted by the optical interrogator defines a light pulse length. 
     Embodiment 3: The optical system of any prior embodiment, wherein the plurality of nominal sites are spaced apart from each other to at a distance of about one or more nominal sites within a light pulse length. 
     Embodiment 4: The optical system of any prior embodiment, wherein the plurality of gratings further comprises a plurality of chirped Fiber Bragg gratings. 
     Embodiment 5: The optical system of any prior embodiment, wherein the offset distance is greater than a quarter wavelength of the light pulse. 
     Embodiment 6: The optical system of any prior embodiment, wherein the offset distance is selected using a dithering function defined within a dithering range, wherein a range limit of the dithering range is between about 1 micron and about 1 centimeter. 
     Embodiment 7: The optical system of any prior embodiment, wherein the plurality of gratings includes a first grating separated from a first nominal site by a first offset distance and a second grating separated from a second nominal site by a second offset distance, wherein the first offset distance is a first value selected using the dithering function and the second offset distance is a second value selected using the dithering function. 
     Embodiment 8: The optical system of any prior embodiment, wherein a signal-to-noise ratio obtained from the plurality of gratings located at the offset distances is greater than the signal-to-noise ratio obtained from the plurality of gratings located at the plurality of nominal sites. 
     Embodiment 9: A method of measuring an acoustic signal in a wellbore. An optical fiber is disposed in the wellbore, the optical fiber having a plurality of gratings formed therein, each of the plurality of gratings being located at a selected offset distance from a respective one of a plurality of nominal sites of the optical fiber, the plurality of nominal sites being uniformly spaced along a longitudinal axis of the optical fiber, wherein each offset distance is selected to reduce a destructive interference between reflections from the plurality of gratings. An optical interrogator transmits a light pulse into the optical fiber to measure the acoustic signal via a reflection of the light pulse from at least one of the plurality of gratings. 
     Embodiment 10: The method of any prior embodiment, wherein the light pulse defines a light pulse length. 
     Embodiment 11: The method of any prior embodiment, wherein the plurality of nominal sites are spaced apart from each other at about one or more nominal sites within a light pulse length. 
     Embodiment 12: The method of any prior embodiment, wherein the plurality of gratings further comprises a plurality of chirped Fiber Bragg gratings. 
     Embodiment 13: The method of any prior embodiment, wherein the offset distance is greater than a quarter wavelength of the light pulse. 
     Embodiment 14: The method of any prior embodiment, wherein the offset distance is selected using a dithering function defined within a dithering range, wherein a range limit of the dithering range is between about 1 micron and about 1 centimeter. 
     Embodiment 15: The method of any prior embodiment, wherein the plurality of gratings includes a first grating separated from a first nominal site by a first offset distance and a second grating separated from a second nominal site by a second offset distance, wherein the first offset distance is a first value selected using the dithering function and the second offset distance is a second value selected using the dithering function. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Further, it should be noted that the terms “first,” “second,” and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “about”, “substantially” and “generally” are intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. For example, “about” and/or “substantially” and/or “generally” can include a range of ±8% or 5%, or 2% of a given value. 
     The teachings of the present disclosure may be used in a variety of well operations. These operations may involve using one or more treatment agents to treat a formation, the fluids resident in a formation, a wellbore, and/or equipment in the wellbore, such as production tubing. The treatment agents may be in the form of liquids, gases, solids, semi-solids, and mixtures thereof. Illustrative treatment agents include, but are not limited to, fracturing fluids, acids, steam, water, brine, anti-corrosion agents, cement, permeability modifiers, drilling muds, emulsifiers, demulsifiers, tracers, flow improvers etc. Illustrative well operations include, but are not limited to, hydraulic fracturing, stimulation, tracer injection, cleaning, acidizing, steam injection, water flooding, cementing, etc. 
     While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. Also, in the drawings and the description, there have been disclosed exemplary embodiments of the invention and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention therefore not being so limited.