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, sensor types such as those disclosed in U.S. Pat. No. 7,564,562 (“Method for demodulating signals from a dispersive white light interferometric sensor”) rely on spectral analysis of broadband light for proper operation, which makes it difficult for a single optical fiber to provide multiplexed access to multiple such sensors. (In the context of optical sensing systems relying on signals in the 1460-1675 nm range, broadband may be taken to mean a spectrum having a full-width at half maximum greater than 80 nm.) The conventional multiplexing approach, wavelength division multiplexing (WDM), apportions only relatively small portions of the spectrum to each sensor to avoid expanding the system bandwidth beyond what the typical communications fiber can handle.
Time division multiplexing (TDM) is another multiplexing approach that has been employed in optical sensing systems for signals that do not require spectral analysis. See, e.g., U.S. Pat. No. 7,221,815 (“Optical sensor multiplexing system”). However, commercially available spectrometers generally require a measurement time on the order of 1 ms, and in any event no less than 200 μs. Assuming a typical fiber delay of 5 ns/m, a fiber delay coil employed to provide such a long delay time would be on the order of 100 km long. Such inter-sensor spacings are simply infeasible in a downhole sensing system. The only other proposed solution known to the authors has been the use of one or more dedicated fibers for each sensor requiring spectral analysis. This approach becomes infeasible as the number of downhole sensors increases.
Accordingly, there are disclosed in the drawings and the following description certain methods and systems suitable for enabling spectral analysis of time-division multiplexed (TDM) signals from downhole sensors. In the drawings:
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.
The obstacles outlined above are at least in part addressed by the disclosed downhole sensing systems and methods. Some disclosed system embodiments include an array of downhole sensors, each of which provides a sequence of light pulses having spectra indicative of a measurand for that downhole sensor. An optical fiber conveys the sequences in a time-multiplexed fashion to a receiver having at least one gating element that passes only a selected one of the sequences and at least one spectrometer that receives the selected one of said sequences and responsively measures a light spectrum. Notably, the integration interval for the spectrometer measurement is substantially greater than the pulse period of each sequence, including multiple pulses within the measurement by the spectrometer.
Turning now to the figures,
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 the fiber optic cable 44 for communication with an array of downhole sensors as discussed further below. The interface 42 is located on the surface of the earth 18 near the wellhead, i.e., a “surface interface”. In the embodiment of
In at least some embodiments, the fiber optic cable 44 terminates at surface interface 42 with an optical port adapted for coupling the fiber(s) in cable 44 to a light source and a detector. The light source transmits a sequence of broadband light pulses along the fiber optic cable 44 to the spaced-apart downhole sensors, each of which returns a modified sequence of light pulses have a spectrum indicative of a measurand. In at least some embodiments, the sensors are extrinsic Fabry-Perot interferometers (EFPI), such as those described in U.S. Pat. No. 5,301,001 (“Extrinsic fiber optic displacement sensors and displacement sensing systems”), U.S. Pat. No. 7,564,562 (“Method for demodulating signals from a dispersive white light interferometer sensor”), and Choi, Cantrelle, Bergeron, and Tubel, “Minimization of temperature cross-sensitivity of EFPI pressure sensor for oil and gas exploration and production applications in well bores”, SPIE Vol. 5589 p. 337-344, Optics East 2004, Philadelphia.
The EFPI sensors provide a gap (such as an air gap) that can be configured to respond to temperature, pressure, stress, acceleration, and other measurands. The relevant literature provides a number of low-cost EFPI sensor designs having linear parameter dependencies with negligible cross-sensitivities to other parameters and no polarization sensitivity, which can be used to provide a low-cost, high performance remote sensing system, once the multiplexing obstacles have been overcome as set out below. The optical port communicates the modified pulse sequences to the detector. As will be explained in greater detail below, the detector measures the spectra of the modified pulse sequences.
The illustrative downhole optical sensor system 12 of
In at least some implementations, the non-transient information storage media 68 stores a software program for execution by computer 60. The instructions of the software program cause the computer 60 to collect spectra measurements received as a digital signal from surface interface 42 and, based at least in part thereon, to determine downhole parameters such as temperatures at each sensor position on the fiber 44. The instructions of the software program may also cause the computer 60 to display the parameter values associated with each sensor position via the output device 64.
As mentioned previously, each sensor may be an EFPI sensor having a gap in the fiber that introduces two index of refraction mismatches; one on each side of the gap. The light passing into the gap is partially transmitted and partially reflected, as is the light passing out of the gap on the other side. Any light in the cavity can be reflected back and forth multiple times inside the cavity before escaping. The constructive and destructive interference in the light leaving both sides of the gap produces a distinctive spectral pattern that reveals the width of the gap in the fiber, which gap may be designed to be sensitive to a selected measurand, e.g., temperature, pressure, stress, acceleration, etc. The reflected light from each sensor is a sequence of pulses having a modified spectrum, and the couplers DC1-DC(N−1) return the modified pulse sequences to the fiber as upgoing sequences 310. Alternatively, an upgoing signal fiber separate from the downgoing signal fiber can be employed to collect the modified pulse sequences (in the form of light that has transmitted through the EFPI sensor) with a second set of couplers and transport it to the surface. In either case, the signal propagation delay associated with the inter-sensor spacing causes each downgoing pulse to produce a series of upgoing pulses, each upgoing pulse having a unique delay as determined by the position of the couplers along the cable 44.
Circulator 306 directs the upgoing sequences to a detector 312 having a spectrometer 316. At the input to the spectrometer 316, a high-speed optical switch 314 gates the upgoing pulse sequences, enabling the spectrometer to receive only a selected one of the pulse sequences corresponding to a currently selected sensor. A processing unit 318 coordinates the operation of the source 302 and the receiver 312, adjusting the timing of switch 314 relative to the firing of source 302 to control which sensor-modified pulse sequence the spectrometer 316 is measuring. Processing unit 318 further initiates spectrometer measurements and receives the resulting spectra.
In at least some embodiments, the processing unit 318 performs the selection operation by generating clock signals for the source 302 and the switch 314, using an adjustable delay based on the spacing of the sensors to select any given one of the sensors. The pulse period, pulse width, and adjustable delay settings are expected to be set by the operator during system initialization, based on the given configuration of downhole sensors. Alternatively, these settings may be determined iteratively or adaptively, enabling the processing unit 318 to discover the optimal timing for array interrogation and sensor selections.
The high-speed optical switch may take the form of a High Speed Variable Attenuator available from Boston Applied Technologies. The spectrometer 316 may take the form of a miniature spectrometer from Ocean Optics of Dunedin, Fla.
To further explain the time-division multiplexing aspects of the system,
To separate out only the pulses from a given sensor, the processing unit 318 supplies a clock signal 502 to switch 314, causing the switch to pass only the pulses in the time window corresponding to the selected sensor. (In the figure, sensor #1 is selected.) Such gating is desirable because, depending on the model, the typical minimum integration time for a commercially available miniature spectrometer ranges from 200 μs to 1 ms. This integration time is far larger than the maximum pulse width that can be used without overlapping responses from the downhole sensor array. The switch 314 blocks all but the pulses from the selected sensor, enabling the spectrometer to analyze the pulses from the selected sensor without interference, using an integration time that can be extended over as many pulses as needed to achieve the desired signal-to-noise ratio.
In block 606, the receiver gates the received signal to pass only the upgoing pulses from one selected sensor. In block 608, the spectrometer measures the spectrum of the pulses from the selected sensor, e.g., by using a diffractive or refractive element to disperse the spectral components across a charge-coupled device (CCD) or other array of photodetectors. Once enough pulses have been collected to complete the spectrum measurement, the receiver adjusts the timing of the gate pulses relative to the source pulses in block 610, so as to select the pulses from another sensor. Blocks 604-610 are repeated to collect measurements from each sensor in turn, and then further repeated to collect subsequent measurements from each sensor.
If M pulse periods (M>1) are needed for an adequate spectrum measurement, the full measurement cycle using one spectrometer is approximately MN pulse periods, where N is the number of sensors. Moreover, since each pulse period is a minimum of N pulse widths, the full measurement cycle is at least MN2 pulse widths. To keep the measurement cycle from becoming prohibitively long, the number of sensors N for a given fiber may be limited. Additional fibers (and spectrometers) may be added to the system to support additional sensors. For example, one fiber having 10 sensors for pressure measurements may be paired with a second fiber having 10 sensors for temperature measurements. Illustrative values include M=1000, N=10, and a pulse width of 1 μs, yielding a single-spectrometer measurement cycle of 0.1 s, for a sensor logging rate of 10 Hz, which is more than enough for pressure and temperature profile monitoring.
In block 612, the receiver converts the spectrum measurement into a sensor measurand, e.g., pressure, temperature, strain, etc. In block 614, the measurand values are tracked as a function of time to obtain a log of the desired parameters for display to a user. One illustrative log display is given in
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, and field activity monitoring. It is intended that the following claims be interpreted to embrace all such variations and modifications.
Filing Document | Filing Date | Country | Kind |
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PCT/US2014/010399 | 1/7/2014 | WO | 00 |