System and method of distributed fiber optic sensing including integrated reference path

An apparatus for estimating a parameter includes: an optical fiber including at least one core configured to transmit an interrogation signal and including a plurality of sensing locations distributed along a measurement length of the optical fiber and configured to reflect light; a reference optical path configured to transmit a reference signal, the reference optical path disposed in a fixed relationship to the at least one core and extending at least substantially parallel to the at least one core, the reference optical path including a reference reflector that defines a cavity length corresponding to the measurement length; a detector configured to receive a reflected return signal; a reference interferometer configured to receive at least a reference signal and generate an interferometric reference signal; and a processor configured to apply the interferometric reference signal to the reflected return signal to compensate for one or more environmental parameters.

BACKGROUND

Fiber-optic sensors have been utilized in a number of applications, and have been shown to have particular utility in sensing parameters in various environments. Optical fiber sensors can be incorporated into environments such as downhole environments and be used to sense various parameters of an environment and/or the components disposed therein, such as temperature, pressure, strain and vibration.

Parameter monitoring systems can be incorporated with downhole components as fiber-optic distributed sensing systems (DSS). Examples of DSS techniques include Optical Frequency Domain Reflectometry (OFDR), which includes interrogating an optical fiber sensor with an optical signal to generate reflected signals scattered from sensing locations (e.g., fiber Bragg gratings) in the optical fiber sensor.

Swept-wavelength interferometric-based sensing systems, frequently used for distributed fiber-optic sensing, are so-called because they rely upon interferometry to encode the sensor information. In some applications, however, the sensing fiber (the fiber containing or consisting of the sensor(s)) is subject to vibrations. These vibrations can result in a smearing of data, and can ultimately reduce data fidelity or inhibit the ability to make a measurement altogether.

SUMMARY

An apparatus for estimating a parameter includes: an optical fiber including at least one core configured to be optically coupled to a light source and transmit an interrogation signal, the at least one core including a plurality of sensing locations distributed along a measurement length of the optical fiber and configured to reflect light; a reference optical path configured to transmit a reference signal, the reference optical path disposed in a fixed relationship to the at least one core and extending at least substantially parallel to the at least one core, the reference optical path including a reference reflector that defines a cavity length corresponding to the measurement length; a detector configured to receive a reflected return signal including light reflected from one or more of the plurality of sensing locations; a reference interferometer configured to receive at least a reference signal returned from the reference optical path and generate an interferometric reference signal; and a processor configured to apply the interferometric reference signal to the reflected return signal to compensate for one or more environmental parameters.

A method for estimating a parameter includes: disposing an optical fiber in a borehole in an earth formation, the optical fiber including at least one core having a plurality of sensing locations distributed along a measurement length of the optical fiber and configured to reflect light; disposing in the borehole a reference optical path configured to transmit a reference signal, the reference optical path disposed in a fixed relationship to the at least one core and extending at least substantially parallel to the at least one core, the reference optical path including a reference reflector that defines a cavity length corresponding to the measurement length; transmitting a first interrogation signal into the at least one core; transmitting a second interrogation signal into the reference optical path; receiving a reflected return signal including light reflected from one or more of the plurality of sensing locations; receiving, at a reference interferometer, a reference signal returned from the reference optical path, and generating an interferometric reference signal; applying the interferometric reference signal to the reflected return signal to compensate for one or more environmental parameters based on changes in the cavity length of the reference optical path; and estimating one or more environmental parameters based on the compensated reflected return signal.

DETAILED DESCRIPTION

Referring toFIG. 1, an exemplary embodiment of a downhole drilling, monitoring, evaluation, exploration and/or production system10disposed in a wellbore12is shown. A borehole string14is disposed in the wellbore12, which penetrates at least one earth formation16for performing functions such as extracting matter from the formation and/or making measurements of properties of the formation16and/or the wellbore12downhole. The borehole string14is made from, for example, a pipe, multiple pipe sections or flexible tubing. The system10and/or the borehole string14include any number of downhole tools18for various processes including drilling, hydrocarbon production, and measuring one or more physical quantities in or around a borehole. Various measurement tools18may be incorporated into the system10to affect measurement regimes such as wireline measurement applications or logging-while-drilling (LWD) applications.

In one embodiment, a parameter measurement system is included as part of the system10and is configured to measure or estimate various downhole parameters of the formation16, the borehole14, the tool18and/or other downhole components. The measurement system includes an optical interrogator or measurement unit20connected in operable communication with at least one optical fiber sensing assembly22. The measurement unit20may be located, for example, at a surface location, a subsea location and/or a surface location on a marine well platform or a marine craft. The measurement unit20may also be incorporated with the borehole string12or tool18, or otherwise disposed downhole as desired.

An optical fiber assembly22is operably connected to the measurement unit20and is configured to be disposed downhole. The optical fiber assembly22includes at least one optical fiber core24(referred to as a “sensor core”24) configured to take a distributed measurement of a downhole parameter (e.g., temperature, pressure, stress, strain and others) and at least one optical fiber core26(referred to as a “system reference core”26) configured to generate a reference signal. The sensor core24includes one or more sensing locations28disposed along a length of the sensor core, which are configured to reflect and/or scatter optical interrogation signals transmitted by the measurement unit20. Examples of sensing locations28include fibre Bragg gratings, Fabry-Perot cavities, partially reflecting mirrors, and locations of intrinsic scattering such as Rayleigh scattering, Brillouin scattering and Raman scattering locations. The system reference core26is disposed in a fixed relationship to the sensor core24and provides a reference optical path having an effective cavity length that is stable relative to the optical path cavity length of the sensor core24. The system reference core can be used to return reference signals used by a reference interferometer for compensating the distributed measurements based on changes in the cavity length caused by, e.g., vibration.

In one embodiment, a length of the optical fiber assembly22defines a measurement region30along which distributed parameter measurements may be taken. For example, the measurement region30extends along a length of the assembly that includes sensor core sensing locations28. The system reference core26is disposed relative to the sensor core24and provides a reference path having an effective cavity length that is stable relative to the optical path cavity length of the sensor core24in the measurement region30, which acts to moderate or reduce the effects of vibration and other movement in the system. For example, the sensor core24and the system reference core26are disposed in respective optical fibers that are disposed together in an optical fiber cable, adhered to one another or otherwise disposed so that at least the lengths of each core in the measurement region30deform together in response to downhole parameters. The reference optical path and the sensing path are thus configured so that they are in a fixed position relative to one another, so that the reference path experiences the same vibration or other movement as the sensing path. In one embodiment, the sensor core24and the system reference core26are disposed within a multi-core optical fiber32.

The measurement unit20includes, for example, one or more electromagnetic signal sources34such as a tunable light source, a LED and/or a laser, and one or more signal detectors36(e.g., photodiodes). Signal processing electronics may also included in the measurement unit20, for combining reflected signals and/or processing the signals. In one embodiment, a processing unit38is in operable communication with the signal source34and the detector36and is configured to control the source34, receive reflected signal data from the detector36and/or process reflected signal data.

In one embodiment, the measurement system is configured as a coherent optical frequency-domain reflectometry (OFDR) system. In this embodiment, the source34includes a continuously tunable laser that is used to spectrally interrogate the optical fiber sensing assembly22. In one embodiment, the interrogation signal has a wavelength or frequency that is modulated or swept (e.g., linearly) over a selected wavelength or frequency range. Scattered signals reflected from intrinsic scattering locations, sensing locations28and other reflecting surfaces in the optical fiber assembly22may be detected, demodulated, and analyzed. Each scattered signal can be correlated with a location by, for example, a mathematical transform or interferometrically analyzing the scattered signals in comparison with a selected common reflection location. Each scattered signal can be integrated to reconstruct the total length and/or shape of the cable. A modulator (e.g., function generator) in optical communication with the tunable optical source34may be provided that modulates the optical source34, such as by power, intensity or amplitude, using a modulation signal.

Referring toFIG. 2, an exemplary optical fiber assembly22includes a multi-core fiber32having the at least two cores24,26and a cladding40. The sensing core24is configured to guide light from the measurement unit20to the measurement locations28, and the at least one system reference core26is configured to guide a reference light signal from the measurement unit. The cores24,26may receive an interrogation signal from a single measurement unit20or a single source34, or receive individual signals from separate sources34. One or more sensor and/or reference reflectors42are positioned at selected axial locations to provide reference signals. In one embodiment, the reflector(s)42are disposed so that part of an interrogation signal in each core24,26is reflected from the reflector(s)42at substantially the same axial location for each core. In the example shown inFIG. 2, the reflectors42include a single reference reflector42such as a mirror, which is positioned at an axial location common to each core. The reference reflector may be disposed at an end of the optical fiber assembly22and/or at one or more locations along the length of the measurement region30. A cavity length is thus formed between a selected axial location and an axial location of each reflector42. For example, the reflector42may include multiple partially reflective mirrors disposed at different axial locations along the fiber optic assembly22and forming multiple respective cavity lengths.

In one embodiment, the sensing core24forms one or more components of a sensor interferometer. For example, the sensor interferometer may be formed from return signals reflected along a sensor path, i.e., a return signal path from a sensing location28and an axial location (e.g., the end of the sensing core24coupled to the detector36), and from a return signal reflected along a sensor reference path, i.e., a return signal path in the core24between the reflector42and the axial location. Each of these return signals may be returned to the measurement unit20where they can be combined to generate interferometric signals for parameter measurements. An additional interferometer (a reference interferometer) may be formed by a reference path return signal, i.e., a return signal in the system reference core26reflected along a system reference path between the reflector42and the axial location. It should be noted that, although the sensor path and the reference path are included in separate cores, these paths may be established in a single core. In addition, the sensor core24and the system reference core26may be included in separate optical fibers that are adhered together, disposed in a single cable and/or otherwise disposed so that the system reference path is disposed in a fixed relationship to the core24and extends at least substantially parallel to the core24.

The system reference core26and system reference return signal can be used to compensate for, e.g., the effects of non-linearities in the case that the system10utilizes swept-wavelength interferometry (SWI). Because the SWI-based interrogation unit (e.g., the optical fiber assembly22) may be subject to vibration, and because the sensing core24is often subject to different stimuli, the vibration can potentially produces reduced data fidelity. This happens because the effective cavity length of the interferometer formed by the sensor core24and the reflector42(and corresponding to the measurement length30) changes during the course of an acquisition. The configurations of the cores24and the26relative to one another allows for compensation of vibration effects.

Referring toFIG. 3, an embodiment of the system10is shown, in which the system interferometer is configured as a trigger interferometer. In this embodiment, a tunable laser or other light source34(e.g., swept-wavelength light source) is coupled to a beam splitter44configured to split light from the light source into at least one sensor beam and at least one reference beam. A coupling device46is configured to direct the sensor beam into the sensor core24and direct the reference beam into the reference core26.

In one embodiment, the measurement unit20includes a processing assembly50that is configured to receive input light beams as well as return signals from the optical fiber assembly22. For example, light reflected and/or scattered from each sensing location28(the “sensor return signal”) and light in the sensor core24reflected from the reflector42(the “sensor reference return signal”) are combined to generate a sensor interferometric signal in the form of an interference pattern indicative of phase differences between the sensor return signal and the sensor reference return signal. The interference of the sensor reference return signal with the sensor return signal occurs at a particular optical path length of the sensor, also known as the spatial frequency of the sensor.

Light in the system reference core26reflected from the reflector42(system reference return signal) is used in a reference interferometer. For example, the system reference return signal is directed to the measurement unit20and is combined with the initial sensor beam or the split sensor beam to generate an interference pattern indicative of changes in the cavity length formed between an axial location (e.g., the circulator44location) and the reference reflector42. This change in cavity length can be used as indicative of changes in the overall measurement path30, produced by parameters such as temperature, stress and vibration. This reference interferometer may be used to compensate the sensor interferometer data for parameter changes occurring for the entire length of the measurement region30, allowing for higher quality measurements of local parameters measured using the measurement locations28.

Referring again toFIG. 3, in one embodiment, the processing assembly50includes a detector52such as an optical-electrical converter (OEC) that receives the reflected light from core24(e.g., the sensor return signal, the sensor reference return signal, or a combined signal) via the circulator46. The detector52may be any suitable detector for converting an optical signal into an electrical signal, such as a photodetector, or a charge-coupled device. In one embodiment, the detector52produces an electrical signal54that corresponds to the waveform of the received light. The electrical signal54is sent via an optional filter56(e.g., a programmable anti-aliasing filter) that filters out the noise signals.

In one embodiment, the processing assembly50includes a sampler56such as an analog-to-digital converter (ADC). The sampler56receives the electrical signal54and samples the signal according to selected sampling parameters, such as sampling frequency and duration, which produces a sampled signal58that may be sent to a processor such as the processor38or a remote processor. The sampler56may receive sampling parameters from an external clock or a waveform corresponding to a particular sensor, a wavelength shift at the particular sensor, a strain at the sensor, a temperature at the sensor, or a deformation of a member coupled to the fiber optic assembly22. Alternatively, the parameter may be determined at any processor including processor38.

In one embodiment, the processing assembly includes a system reference interferometer58configured to generate a system reference interferometric signal using the system reference return signal received from the system reference core26. The system reference interferometric signal may be used with or applied to the signal52to compensate for parameters such as downhole temperatures and vibration along the measurement path30.

In one embodiment, the system interferometer58is configured as a trigger interferometer58for generating sampling parameters based on an interferometric signal derived from the system reference return signal received from the system reference core26. The trigger interferometer58receives an interference pattern signal or combines signals therein to generate the interference pattern signal that is used to establish sampling parameters. For example, the trigger interferometer58receives a portion of the reference beam from the beam splitter44and also receives the system reference return signal from the reference core26, and combines these beams to generate the interference pattern signal.

The trigger interferometer58provides a trigger signal60based on the interference pattern signal. For example, the trigger interferometer58produces a trigger signal using a negative-to-positive zero-crossing of an interference fringe pattern of the interference pattern signal, such as a transition from a dark region of the fringe pattern to an adjacent illuminated region of the fringe pattern. In an alternate embodiment, the trigger signal60may be produced from a positive-to-negative zero-crossing. Any suitable part of the fringe pattern may be used to produce the trigger signal. In one embodiment, an OEC62is included to convert the trigger signal60from an optical signal to an electrical trigger signal. The trigger signal is sent to the sample56to provide sampling parameters, such as a sampling rate corresponding to the frequency of negative-to-positive zero crossings and/or a sampling duration corresponding to time windows during which the interference pattern has an amplitude or magnitude above a selected value.

FIG. 4illustrates a method70of measuring downhole parameters. The method70includes one or more stages71-74. Although the method70is described in conjunction with the system10and the measurement system described above, the method70is not limited to use with these embodiments, and may be performed by the measurement unit20or other processing and/or signal detection device. In one embodiment, the method70includes the execution of all of stages71-74in the order described. However, certain stages may be omitted, stages may be added, or the order of the stages changed.

In the first stage71, the optical fiber assembly22along with the borehole string12, tool18and/or other components are lowered downhole. The components may be lowered via, for example, a wireline or a drillstring.

In the second stage72, light from the light source34is sent to the beam splitter44which may split the light into the sensor beam for obtaining signals from one or more sensing locations28and the reference beam for use in a system interferometer58such as the trigger signal interferometer58. In an exemplary embodiment, the beam splitter44splits the received light so that the sensor beam includes about 90% of the light and the reference beam includes about 10% of the light. However, any splitting ratio may be used. The reference beam may also be further split so that a portion of the reference beam is directed to the system reference interferometer58and another portion of the reference beam is directed to the reference core26. The circulator46directs the sensor beam into the sensor core24and directs the reference beam into the reference core26.

In the third stage73, the beams propagate through their respective cores and return signals are generated and received by the detector36and/or the measurement unit20. For example, light reflected and/or scattered from each sensing location28(sensor return signal) and light in the sensor core24reflected from the reflector42(sensor reference return signal) are combined to generate interferometric data. Light in the system reference core26reflected from the reflector42(system reference return signal) is used in the system reference interferometer58, for example to generate a trigger signal.

The reflected signals (reference and sensor) reflected from the sensing core24are combined and directed to the detector36(e.g., via the circulator46). In one embodiment, the signals are converted to an electronic signal via the OEC36. The reflected reference signal from the reference core26is combined with the input signal (e.g., via the trigger interferometer58) to produce an interferometric reference signal. The interferometric reference signal is combined with or otherwise applied to the sensor interferometric signal to produce a resultant signal that is compensated for vibration or other downhole parameters experienced by the measurement path.

In the fourth stage74, the reflected signal data is utilized to estimate various parameters along the optical fiber22, such as along the measurement path30. The reflected signal data is correlated to locations of sensing locations28, and parameters are estimated for one or more sensing locations28. Examples of such parameters include temperature, pressure, vibration, strain and deformation of downhole components, chemical composition of downhole fluids or the formation, acoustic events, and others.

The systems and methods described herein provide various advantages over prior art techniques. The systems and methods provide for integration of either or both the system reference and the sensor reference with the sensing fiber, such that the system interferometer and the sensing fiber experience substantially the same vibration environment, resulting in greater data fidelity. This configuration may also have advantages in providing more localized vibration correction by establishing multiple cavity lengths in the reference path (e.g., core26). The systems and methods are thus useful in subterranean hydrocarbon exploration, drilling and production operations, due to downhole vibrations that may be involved.

The optical fiber assembly22and/or the measurement system are not limited to the embodiments described herein, and may be disposed with any suitable carrier. The measurement system, optical fiber assembly22, the borehole string14and/or the tool18may be embodied with any suitable carrier. A “carrier” as described herein means any device, device component, combination of devices, media and/or member that may be used to convey, house, support or otherwise facilitate the use of another device, device component, combination of devices, media and/or member. Exemplary non-limiting carriers include drill strings of the coiled tube type, of the jointed pipe type and any combination or portion thereof. Other carrier examples include casing pipes, wirelines, wireline sondes, slickline sondes, drop shots, downhole subs, bottom-hole assemblies, and drill strings.

In support of the teachings herein, various analysis components may be used, including a digital and/or an analog system. Components of the system, such as the measurement unit20, the processor38, the processing assembly50and other components of the system10, may have components such as a processor, storage media, memory, input, output, communications link, user interfaces, software programs, signal processors (digital or analog) and other such components (such as resistors, capacitors, inductors and others) to provide for operation and analyses of the apparatus and methods disclosed herein in any of several manners well appreciated in the art. It is considered that these teachings may be, but need not be, implemented in conjunction with a set of computer executable instructions stored on a computer readable medium, including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, hard drives), or any other type that when executed causes a computer to implement the method of the present invention. These instructions may provide for equipment operation, control, data collection and analysis and other functions deemed relevant by a system designer, owner, user or other such personnel, in addition to the functions described in this disclosure.

Further, various other components may be included and called upon for providing for aspects of the teachings herein. For example, a power supply (e.g., at least one of a generator, a remote supply and a battery), cooling unit, heating unit, motive force (such as a translational force, propulsional force or a rotational force), magnet, electromagnet, sensor, electrode, transmitter, receiver, transceiver, antenna, controller, optical unit, electrical unit or electromechanical unit may be included in support of the various aspects discussed herein or in support of other functions beyond this disclosure.