Abstract:
An illustrative distributed acoustic sensing system includes a multi-mode optical fiber cable for distributed sensing and a distributed acoustic sensing interrogator coupled to the multi-mode optical fiber cable via a single mode optical fiber. The interrogator derives distributed acoustic measurements from Rayleigh backscattering light that is initiated with a substantially under-filled launch configuration that is designed to excite only the lowest-order modes of the multi-mode optical fiber. Mode conversion within the multi-mode optical fiber is anticipated to be negligible. For elastic scattering (i.e., Rayleigh scattering), it is further anticipated that the scattered light will be primarily returned in the incident propagation mode, thereby escaping the extraordinarily large coupling loss that would otherwise be expected from coupling a single-mode optical fiber to a multi-mode optical fiber for distributed sensing. Experiments with graded index multi-mode optical fiber have yielded positive results.

Description:
BACKGROUND 
     Distributed optical sensing technology is turning out to be suitable for a number of downhole applications ranging from temperature sensing to passive seismic monitoring. As engineers develop new and improved systems to increase performance and sensitivity, they have encountered certain obstacles. For example, recent distributed acoustic sensing system designs specify the use of single-mode optical fiber to achieve adequate sensing performance, yet many existing well installations employ multi-mode optical fiber, which would be largely infeasible to replace. The inventors are unaware of any existing system that exploits multi-mode optical fiber for distributed acoustic sensing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Accordingly, there are disclosed in the drawings and the following description various systems and methods that employ an under-filled launch configuration to exploit a multi-mode fiber for distributed acoustic sensing. In the drawings: 
         FIG. 1  shows an illustrative distributed acoustic sensing system in a production well. 
         FIG. 2  shows an alternative distributed acoustic sensing system embodiment. 
         FIG. 3  shows an illustrative heterodyne system with an under-filled launch configuration. 
         FIG. 4  shows an illustrative homodyne system with an under-filled launch configuration. 
         FIG. 5  is a flowchart of an illustrative distributed acoustic sensing method employing a multi-mode optical fiber for sensing. 
     
    
    
     It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims. 
     DETAILED DESCRIPTION 
     Certain disclosed system and method embodiments employ an under-filled multi-mode optical fiber for distributed interferometric phase sensing applications in the downhole environment. Many existing cable installations for distributed temperature sensing (“DTS”) in borehole environments and other asset monitoring applications employ graded index multi-mode fiber. The inventors have discovered that, contrary to accepted wisdom, many of the advantages provided by the use of a single-mode optical fiber for sensing (including coherency preservation and minimal dispersion) can also be achieved with multi-mode optical fiber for sensing, so long as the system employs an under-filled launch configuration to excite only the lowest-order modes in the multi-mode optical fiber. 
     This approach further enables a single distributed sensing interrogator design for sensing with both single-mode and multi-mode optical fibers, without the extraordinarily high coupling losses that would generally be expected to occur in connections between multi-mode optical fibers and single-mode optical fibers. As an illustrative example, a splice between a typical filled multi-mode optical fiber (having a core diameter of 50 microns) and a typical single-mode optical fiber (having a core diameter of 8 microns) would exhibit a theoretical transmission loss of approximately 16 dB, making it infeasible to analyze the weak optical signals typical of distributed sensing. The disclosed systems are suitable for detection of distributed acoustical and vibrational energies (DAS/DVS). 
     Turning now to the figures,  FIG. 1  shows a well  10  equipped with an illustrative embodiment of a distributed downhole sensing system  12 . The well  10  shown in  FIG. 1  has been constructed and completed in a typical manner, and it includes a casing string  14  positioned in a borehole  16  that has been formed in the earth  18  by a drill bit. The casing string  14  includes multiple tubular casing sections (usually about 30 foot long) connected end-to-end by couplings. One such coupling is shown in  FIG. 1  and labeled ‘ 20 .’ Within the well  10 , cement  22  has been injected between an outer surface of the casing string  14  and an inner surface of the borehole  16  and allowed to set. A production tubing string  24  has been positioned in an inner bore of the casing string  14 . 
     The well  10  is adapted to guide a desired fluid (e.g., oil or gas) from a bottom of the borehole  16  to a surface of the earth  18 . Perforations  26  have been formed at a bottom of the borehole  16  to facilitate the flow of a fluid  28  from a surrounding formation into the borehole and thence to the surface via an opening  30  at the bottom of the production tubing string  24 . Note that this well configuration is illustrative and not limiting on the scope of the disclosure. 
     The downhole optical sensor system  12  includes an interface  42  coupled to a multi-mode optical fiber cable  44  for distributed downhole sensing. The interface  42  is located on the surface of the earth  18  near the wellhead, i.e., a “surface interface”. In the embodiment of  FIG. 1 , the multi-mode optical fiber cable  44  extends along an outer surface of the casing string  14  and is held against the outer surface of the of the casing string  14  at spaced apart locations by multiple bands  46  that extend around the casing string  14 . A protective covering may be installed over the multi-mode optical fiber cable  44  at each of the couplings of the casing string  14  to prevent the cable from being pinched or sheared by the coupling&#39;s contact with the borehole wall. In  FIG. 1 , a protective covering  48  is installed over the multi-mode optical fiber cable  44  at the coupling  20  of the casing string  14  and is held in place by two of the bands  46  installed on either side of coupling  20 . 
     In at least some embodiments, the multi-mode optical fiber cable  44  terminates at surface interface  42  with an optical port adapted for coupling the multi-mode optical fiber cable to a distributed sensing interrogator having a light source and a detector. (In the illustrated embodiment, the interrogator is assumed to be part of the interface  42  and is not shown separately in the figure. In practice, the interrogator may be a separate portable unit removably coupled to the interface  42 .) The light source transmits light pulses along the multi-mode optical fiber cable  44 , which contains scattering impurities. As the pulse of light propagates along the fiber, some of the pulse energy is scattered back along the fiber from every point on the fiber. The optical port communicates the backscattered light to the detector, which responsively produces electrical measurements of differences in backscattered light phase at each point in the fiber. As will be explained in greater detail below, the interrogator employs an under-filled launch configuration to excite only the lowest-order modes in the multi-mode optical fiber. 
     The illustrative downhole optical sensor system  12  of  FIG. 1  further includes a computer  60  coupled to the surface interface  42  to control the interrogator and obtain distributed sensing measurements. The illustrated computer  60  includes a chassis  62 , an output device  64  (e.g., a monitor as shown in  FIG. 1 , or a printer), an input device  66  (e.g., a keyboard), and information storage media  68  (e.g., magnetic or optical data storage disks). However, the computer may be implemented in different forms including, e.g., an embedded computer permanently installed as part of the interrogator, a portable computer that is plugged into the interrogator as desired to collect data, and a remote desktop computer coupled to the interrogator via a wireless link and/or a wired computer network. The computer  60  is adapted to receive the digitized measurement signals produced by the interrogator and to responsively determine a distributed parameter such as, e.g., distributed acoustic sensing along the length of the casing string. 
     The computer may be configured for application specific operation by software stored, for example, on the information storage media  68  for execution by computer  60 . The instructions of the software program may cause the computer  60  to collect phase differences of backscattered light derived from the electrical signal from surface interface  42  and, based at least in part thereon, to determine downhole parameters such as acoustic signals at each point on the fiber  44 . The instructions of the software program may also cause the computer  60  to display the acoustic waveforms or envelopes associated with each point on the fiber via the output device  64 . The software may further provide a user interface that enables the user to configure operation of the interrogator including, for example, pulse width, pulse spacing, and measurement sampling rates. 
       FIG. 2  shows an alternative embodiment of downhole optical sensor system  12  having the multi-mode optical fiber cable  44  strapped to the outside of the production tubing  24  rather than the outside of casing  14 . Rather than exiting the well  10  from the annular space outside the casing, the multi-mode optical fiber cable  44  exits through an appropriate port in the “Christmas tree”  100 , i.e., the assembly of pipes, valves, spools, and fittings connected to the top of the well to direct and control the flow of fluids to and from the well. The multi-mode optical fiber cable  44  extends along the outer surface of the production tubing string  24  and is held against the outer surface of the of the production tubing string  24  at spaced apart locations by multiple bands  46  that extend around the production tubing string  24 . The downhole optical sensor system  12  of  FIG. 2  optionally includes a hanging tail  40  at the bottom of a borehole. In other system embodiments, the multi-mode optical fiber cable  44  may be suspended inside the production tubing  24  and held in place by a suspended weight on the end of the fiber. 
       FIG. 3  shows an illustrative distributed sensing system that employs an under-filled launch configuration for a multi-mode sensing fiber. In interrogator  301 , a high coherence (ultra monochromatic) single transverse mode (TEM00) laser  302  emits a beam of coherent light. A gas laser (e.g., HeNe) may be preferred, though a erbium doped fiber laser or a vertical cavity surface emitting laser (VCSEL) may be acceptable alternatives. An optional erbium-doped fiber amplifier (EDFA)  304  amplifies the signal. A pulse generator  306  turns the beam into pulses with an adjustable width and adjustable spacing. An illustrative pulse width of 1 nanosecond would offer a spatial resolution of about 1 foot, and an illustrative pulse spacing of 0.1 milliseconds would offer a sampling rate of 10 kHz on a 10 kilometer fiber. These values can be tailored to the particular details of each installation. 
     Compensator  308  converts each pulse into a double pulse, using a dual path system with a delay coil  312  in one path and an acousto-optic modulator (AOM)  310  in the other path. The modulator  310  provides a frequency shift, so that the two pulses exiting the compensator  308  are at slightly different frequencies. A circulator  314  directs the interrogating beam to a coupling  316  with a multi-mode optical fiber, and returns backscattered light received via the coupling  316  to an EDFA  323  which amplies the signal prior to its conversion to an electrical signal by a photodetector or other form of optical receiver  324 . 
     The backscattered light is a combination of light from the two pulses scattered from different points on the fiber  318 . The frequency difference of the dual pulses creates a beat frequency in the combined backscatter. An oscillator  328  is tuned to demodulate this beat frequency to baseband in-phase and quadrature-phase signals. Multipliers  326 A,  326 B each take the product of the electrical beat frequency signal and the oscillator signal, with multiplier  326 B employing a 90°-shift on the oscillator signal. Lowpass filters  330 A and  330 B forward the baseband component of the product signals to respective analog-to-digital converters  332 A,  332 B. Based on the time lag from each pulse signal launch, the digitized in-phase and quadrature-phase measurements are associated with a spatial position (“channel”) on the fiber, and tracked as a function of pulse number to obtain a time-dependent measurement of channel phase, from which a corresponding acoustic signal can be readily determined. The optical phase of the returned light changes as the relevant portion of the fiber is stretched or compressed. 
     A computer collects the channel measurements from the analog-to-digital converters  332  and processes the in-phase and quadrature-phase components to determine and track phase. Abbreviating the in-phase signal as “I” and the quadrature-pase signal as “Q”, we have the following relations
 
 I =cos(phase)
 
 Q =sin(phase)
 
phase=arctan( Q/I )
 
 QF=I   2   +Q   2  
 
where the quality factor (“QF”) is a convenient representation of the channel noise level.
 
     Though there are various acceptable optical path configurations within interrogator  301 , ranging from single-mode optical fiber to free-space propagation, it is contemplated that the optical path will primarily comprise single-mode fiber, and in any event it is anticipated that the communication between the interrogator  301  and coupling  316  will be performed via single-mode fiber. The illustrated coupling  316  takes the form of a mode field adapter, but as explained further below it is expected that a standard splice will provide sufficient performance. 
     Coupling  316  provides an under-filled launch configuration for multi-mode optical fiber  318 , meaning that the interrogating beam does not enter the high-order propagation modes supported by the multi-mode optical fiber, but rather enters only the lowest-order propagation modes. For maximum performance, it is desired to excite only the single lowest order mode of the multi-mode fiber. However, it is expected that adequate performance can nevertheless be achieved with excitation of multiple low order modes, though the reverse coupling loss is expected to increase with the number of excited modes. Coupling  316  can be a splice (e.g., a fusion splice), or something more complex such as a fiber taper or even a free-space optical device utilizing collimators, lenses, etc., that can function as a mode field adapter. 
     While some limited degree of mode conversion may be expected as the interrogating beam propagates along the multi-mode optical fiber  318 , it is anticipated that this conversion will be negligible. Moreover, the scattering of light from the interrogating beam only occurs from those scattering centers that couple to that propagation mode. For elastic scattering (i.e., Rayleigh scattering), it is anticipated that the scattered light will be primarily returned in the propagation mode that initially coupled to the scattering center to cause the scattering. In other words, light elastically scattered from a given propagation mode may be expected to return in that propagation mode, particularly when dealing with low-order propagation modes. Again, as the backscattered beam propagates along the multi-mode fiber, mode conversion is anticipated to be negligible. Finally, as the coupler  316  communicates light into the single-mode fiber, a high coupling coefficient is anticipated for the lowest order propagation mode and perhaps for the few next-to-lowest order modes. (A mode field adapter may be employed to convert additional low-order modes in the multi-mode fiber to the fundamental propagation mode for the single-mode fiber.) Initial experiments, albeit with graded index multi-mode optical fiber, seem to support these expectations. Though graded index multi-mode optical fiber may be preferred as this type of fiber generally causes less mode dispersion, it is not expected to be necessary. Rather, it is believed that the proposed operating principles will similarly apply to step index multi-mode optical fibers and other types of multi-mode optical fibers including “holey” and “photonic crystal” multi-mode optical fibers. 
     Many existing distributed temperature sensing installations employ graded index multi-mode optical fiber for sensing. With the disclosed techniques, these existing installations can be readily adapted for distributed acoustic sensing. That is, the interrogator  301  may be configured to share an existing multi-mode fiber with a distributed temperature sensing interrogator  322  as shown in  FIG. 3 . As distributed temperature sensing is often preformed at a shorter wavelength than distributed acoustic sensing (e.g., 1064 nm versus 1550 nm), the two interrogators can operate in parallel without causing interference. A wavelength division multiplexer  320  may be provided to couple both interrogators to the sensing fiber. If some loss is acceptable, the multiplexer  320  can be replaced with a beam splitter. It is common for DTS systems to rely on inelastic (Raman) scattering rather than the elastic (Rayleigh) scattering being employed by interrogator  301 . 
     The internal configuration of interrogator  301  is termed a heterodyne configuration.  FIG. 4  shows an alternative, “homodyne” configuration. Components with functions similar to those of  FIG. 3  are labeled similarly. The illustrated configuration lacks  FIG. 3 &#39;s compensator  308 , so circulator  314  sends isolated light pulses (rather than the double-pulses generated by compensator  308 ) to the sensing fiber  318 . Circulator  314  returns the backscattered light to optional EDFAs  434 ,  438 , each of which is provided with a filter  436 ,  440  to block out-of-band noise. The amplified signal enters a combined 3×3 coupler/compensator unit  444  via an input  442 . A second circulator  446  directs the input light into a 3×3 coupler  448 . The light exits the coupler on three ports. Port  450  is coupled to a Faraday rotator mirror (FRM) that returns the light to port  450 . Port  452  is coupled via a delay line to a second FRM, which returns the light to port  452  with an added delay. (The time delay causes light returning to ports  450  and  452  to be from different positions on the sensing fiber.) Port  454  is terminated with an absorber. 
     The light returning to ports  450  and  452  is combined by the 3×3 coupler to obtain an interference signal that is directed to output ports  456 ,  458 ,  460 , with a 120° phase separation between the outputs. This interference signal enables an interference measurement between backscattered light from spaced-apart locations on the fiber. The three phase-separated output measurements can then be combined to determine the in-phase and quadrature components. Referring to the coupler outputs for a given wavelength as A, B, and C, we have the following relations:
 
 A =Cos [Phase]
 
 B =Cos [Phase−120°]
 
 C =Cos [Phase+120°]
 
 I =√{square root over (3)}( A−B )
 
 Q=A+B− 2 C  
 
The calculation of phase and quality factor can then proceed as described previously.
 
     Receiver electronics  462  convert the optical signals to electrical signals, which are then digitized and buffered for retrieval by a personal computer  464  or other form of data acquisition device. The signal phase can be determined for each spatial mode measurement and combined as outlined previously. 
     In both interrogator embodiments, the digitized signals are timed relative to the launched pulses to determine an associated position on the sensing fiber for each measurement. Moreover, the measurements are repeated to obtain a time-dependent measurement of interference phase, from which the system derives a distributed acoustic sensing signals. 
       FIG. 5  shows an illustrative method for employing a multi-mode optical fiber for distributed acoustic sensing. In block  502 , a distributed acoustic sensing interrogator is coupled via a single-mode optical fiber to a multi-mode optical fiber that has been deployed in a borehole. In some embodiments, the coupling is performed with a mode field adapter. 
     In block  504 , a pulse generator converts a beam from a high coherence, single transverse mode (TEM 00 ) into a pulse sequence with an adjustable pulse width and adjustable pulse spacing. In block  506 , the system couples the pulses to low-order modes of the multi-mode optical fiber and receives Rayleigh backscattered light via the single-mode optical fiber from the multi-mode optical fiber. In block  508  optionally diverts inelastically scattered light to another interrogator for distributed temperature sensing. 
     In block  510 , the system takes an interferometric signal and coherently measures its phase, both as a function of position along the sensing fiber, and as a function of pulse launch number. In block  512 , the measurements are digitized and tracked. The method repeats blocks  504 - 512 , communicating the measurements to a computer that, in block  514 , derives acoustic signals for each of multiple positions along the fiber and displays the signals to a user. the it Th the beroptic cable is deployed in the borehole, either by being strapped to a tubing string as the tubing string is lowered into the borehole, or by being transported into place with a weighted end and/or frictional fluid flow. The deployment is completed by connecting the fiberoptic cable to an interface that enables the cable to be interrogated by distributed sensing electronics. 
     Taken as a whole, the distributed acoustic sensing display reveals a surprising amount of information about ongoing downhole processes, including fluid flows patterns, fluid flow compositions, chemical processes (including curing of cement), formation treatments, operations of mechanical components, and seismic survey signals. The disclosed systems and methods are expected to be widely valued for enabling such monitoring in existing optical fiber installations. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the figures show system configurations suitable for production monitoring, but they are also readily usable for monitoring treatment operations, cementing operations, active and passive seismic surveys, and field activity monitoring. As used herein, the term “acoustic sensing” encompasses “vibration sensing” and “seismic sensing”. It is intended that the following claims be interpreted to embrace all such variations and modifications.