Abstract:
An optical fiber extends down hole from an OFDR. A first set of sensors with a centrally-located reference reflector is disposed over a first fiber length, and a second set of sensors with a centrally-located reference reflector is disposed over a second fiber length. The sensors of the first and second sensing lengths are positioned at slightly offset positions from the reference reflectors so as to interleave the reflected signals. Additional sensing lengths may be similarly interleaved. The system is used by sending an optical signal along the optical fiber, detecting a reflected optical signal, separating the optical signal into component signals, and extrapolating a well condition therefrom. Another method includes creating a low frequency signal component in a reflected optical signal by placing at least one sensor beyond a Nyquist sampling distance limit, detecting the low frequency signal component, and extrapolating a well condition therefrom.

Description:
BACKGROUND  
       [0001]     The invention relates generally to fiber optic sensing technologies. In particular, fiber optic sensors are used to detect conditions within a well.  
         [0002]     Available electronic sensors measure a variety of values, such as, pH, color, temperature, or pressure, to name a few. For systems that require a string of electronic sensors over a long distance, e.g., twenty to thirty kilometers or longer, powering the electronic sensors becomes difficult. Conventionally, the powering of electronic sensors requires running electrical wire from a power source to each of the electronic sensors. Powering electronic sensors electrically has been a problem in the petroleum and gas industry. However, electric wires spanning such long distances create too much interference and noise, thereby reducing the accuracy of the electronic sensors.  
         [0003]     Optical fibers have become the communication medium of choice for long distance communication due to their excellent light transmission characteristics over long distances and the ability to fabricate such fibers in lengths of many kilometers. Further, the light being transmitted can also power the sensors, thus obviating the need for lengthy electrical wires. This is particularly important in the petroleum and gas industry, where strings of electronic sensors are used in wells to monitor down hole conditions.  
         [0004]     As a result, in the petroleum and gas industry, passive fiber optic sensors are used to obtain various down hole measurements, such as pressure or temperature. A string of optical fibers within a fiber optic system is used to communicate information from wells being drilled, as well as from completed wells. For example, a series of weakly reflecting fiber Bragg gratings (FBGs) may be written into a length of optical fiber, such as by photoetching. As is known in the art, the distribution of light wavelengths reflected from an FBG is influenced by the temperature and strain state of the device to which the FBG is attached. An optical signal is sent down the fiber, which is reflected back to a receiver and analyzed to characterize the length of optical fiber. Using this information, down hole measurements may be obtained.  
         [0005]     Many methods are utilized to characterize these sensor-containing lengths of optical fiber, including but not limited to optical reflectometry in time, coherence, and frequency domains. Due to spatial resolution considerations, optical frequency-domain reflectometry (OFDR), is a technique under investigation for use in oil well applications. OFDR is capable of spatial resolution on the order of 100 microns.  
         [0006]     In OFDR, the probe signal is a continuous frequency modulated optical wave, such as from a tunable laser. The probe signal, which is optimally highly coherent, is swept around a central frequency. The probe signal is split and sent down two separate optical paths. The first path is relatively short and terminates in a reference reflector at a known location. The second path is the length of optical fiber containing the sensors. The reference reflector and the sensors in the length of optical fiber reflect optical signals back toward the source of the signal. These optical signals are converted to electrical signals by a photodetector. The signal from the reference reflector travels a shorter path, and a probe signal generated at a particular frequency at a single point in time is detected at different times from the reference reflector and the sensors. As such, at any point in time, the signal at the receiver is a signal from the reference reflector and a signal from the sensors at slightly different frequencies due to the sweeping nature of the tunable laser source. A difference frequency component stemming from the time delay in receiving the signal from the reference reflector and the sensors in the optical fiber can be observed in the detector signal. The frequency of the difference frequency component determines the position of the sensor on the fiber and the amplitude is proportional to the local back scattering coefficient and optical power. Performing a Fourier transform of the detector signal, one can simultaneously observe the back scattered waves from all points along the fiber under test.  
         [0007]     The operational properties of an OFDR are governed by the wavenumber spacing, ν, the wavelength sweep range, R, the data acquisition frequency, ƒ, and the sensing fiber length, L S . As discussed in greater detail herein, the sensing length L S  is a simple function of L, but L also affects ν, R, and ƒ. Increasing the sensing length L S  by making L arbitrarily large consequently reduces the wavelength sweep range R and increases the data acquisition frequency ƒ to impractical values. Similarly, the wavelength sweep range R can be restored by increasing N, but this increase comes at the expense of the size of the data set required and the amount of time required for FFT computation. Maximizing the sensing length while maintaining speed and efficiency is a difficult challenge in the successful construction of an OFDR. Given these challenges, a typical OFDR system is currently limited to a sensing length of about 100 meters. Therefore, a need exists in the art for efficiently extending the useful sensing lengths for OFDR systems.  
       SUMMARY OF THE INVENTION  
       [0008]     Briefly, in accordance with one embodiment of the present invention, a system for monitoring a well includes an OFDR surface instrumentation unit having an optical fiber extending therefrom. At least one optical sensor is disposed along a length of the optical fiber, and means for increasing a sensing length of the optical fiber are included in the system.  
         [0009]     According to another aspect of the present invention, a system for monitoring a well includes an OFDR surface instrumentation unit with an optical fiber extending from the OFDR unit. A first set of sensors is disposed at a first set of locations over a first length of the optical fiber. A second set of sensors is disposed at a second set of locations over a second length of the optical fiber. The second set of locations corresponds to but is slightly offset from the first set of locations. Centrally located between each of the first and second lengths are first and second reference reflectors, respectively.  
         [0010]     According to yet another aspect of the present invention, a system for monitoring a well includes an OFDR surface instrumentation unit with an optical fiber extending therefrom. Along the length of the optical fiber, a reference reflector is disposed between a first plurality of sensors disposed at a first set of distances as measured from the reference reflector and a second plurality of sensors disposed at a second set of distances as measured from the reference reflector. The second set of distances is similar to but slightly offset from the first set of distances.  
         [0011]     According to yet another aspect of the present invention, a system for monitoring a well includes an OFDR unit having an optical fiber extending therefrom. A reference reflector is disposed along the length of the optical fiber, as are a plurality of sensors. At least one sensor is placed beyond the distance limit imposed by Nyquist sampling.  
         [0012]     According to another aspect of the present invention, a method for monitoring a condition within a well includes the steps of (1) transmitting an optical signal down an optical fiber, wherein the optical fiber contains at least two sensing lengths comprising a plurality of sensors with centrally located reference reflectors; (2) detecting an optical signal reflected from the at least two sensing lengths; (3) separating the optical signal into component signals; and (4) extrapolating the condition from the component signals.  
         [0013]     According to another aspect of the invention, a method for monitoring a condition within a well includes the steps of (1) sending an optical signal along an optical fiber to a plurality of sensors; (2) creating a low frequency signal component in a reflected optical signal by placing at least one sensor beyond a distance limit imposed by Nyquist sampling; (3) detecting the reflected optical signal; (4) converting the low frequency signal component into usable sensor data; and (5) extrapolating the condition from the usable sensor data. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:  
         [0015]      FIG. 1  is a schematic drawing of a sensor system according to the present invention;  
         [0016]      FIG. 2A  is a schematic drawing of a first and a second sensing region of  FIG. 1 ;  
         [0017]      FIG. 2B  is a schematic drawing of a virtual sensing region of  FIG. 1 ;  
         [0018]      FIG. 2C  is shows an idealized return signal from the sensor system of  FIG. 1 ;  
         [0019]      FIG. 3  is a diagram of a sensor system according to the present invention adapted to more than two sensing regions on separate sensing fibers;  
         [0020]      FIG. 4  is a graphical representation of raw data acquired from a sensor system according to the present invention;  
         [0021]      FIG. 5 . is a graphical representation of the raw data of  FIG. 4  after Fourier transformation;  
         [0022]      FIG. 6  is a graphical representation of one region of the data as shown in  FIG. 5 ;  
         [0023]      FIG. 7  is a graphical representation of the data from one sensor; and  
         [0024]      FIG. 8  is a graphical representation of the data of  FIG. 7  after inverse Fourier transformation. 
     
    
     DETAILED DESCRIPTION  
       [0025]     The operational properties of an OFDR are governed by the wavenumber spacing, ν, the wavelength sweep range, R, the data acquisition frequency, ƒ, and the sensing fiber length, L S . Each of these parameters is described by the following equations:  
             v   =     1     2   ⁢   nL               Eq   .           ⁢   1             
 
 where n is the refractive index of the fiber, and L is the length of a reference interferometer;  
             R   =         λ   O     ⁡     (     1   -       λ   O     ⁢   N   ⁢           ⁢   ν       )         (     1   +       λ   O     ⁢   N   ⁢           ⁢   ν       )               Eq   .           ⁢   2             
 
 where λ 0  is the initial wavelength of the wavelength sweep, and N is the number of data points acquired during a measurement;  
             f   =       N   ⁢           ⁢     v   .       R             Eq   .           ⁢   3             
 
 where {dot over (ν)} is the laser wavelength sweep rate; and  
               L   S     =       L   4     .             Eq   .           ⁢   4             
 
 N can be further described by the following equation: 
 
N= ρ   Eq. 5
 
 where ρ is a power of two for facilitating the use of fast Fourier transforms in the processing of the signal data. 
 
         [0026]     The sensing length L S  is a simple function of L, but L also effects ν, R, and ƒ. Achieving a long sensing length L S  by making L arbitrarily large reduces the wavelength sweep range R and increases the data acquisition frequency ƒ to impractical values. Additionally, the wavelength sweep range R can be restored by increasing N, but this increase also increases the size of the data set required and, therefore, the amount of time required for FFT computation. The data acquisition frequency ƒ can be restored by decreasing {dot over (ν)}; however, achieving slow and linear wavelength sweeping is difficult to achieve in practice and currently has a limit of about 4 nm/s. Finding an appropriate balance among these parameters for an application typically results in a relatively low sensing length L S  and a long reference interferometer. For example, if the desired sensing length is 10 km and the wavelength sweep range R is 45 nm, common parameters for a strain application, the other parameters work out to ƒ=190 MHz, N=2147483648, so that the required reference interferometer is 40 km. Given these challenges, a typical OFDR system is limited to about 100 meters of sensing length.  
         [0027]     As illustrated in the accompanying drawings and discussed in detail below, the present invention is directed to a system  10  for sensing conditions in a well  12  such as an oil or natural gas well. As shown in  FIG. 1 , generally, system  10  includes an OFDR surface instrumentation unit (SIU)  14  optically connected to a length of fiber  24  that extends into well  12 . Fiber  24  is connected to OFDR SIU  14  via a coupler  16 .  
         [0028]     OFDR SIU  14  includes a tunable laser  40 , an internal interferometer  42 , a detector  44  and a processing unit  46 . Preferably, laser  40  has a tuning range of over 120 nm, although any tuning range may be applied to the present invention. Laser  40  preferably has a coherence length of more than 1 km, although the present invention may be adapted to lasers with shorter coherence lengths. Laser  40  generates coherent light, preferably in the 750-1550 nm range, although other wavelengths of light may also be used.  
         [0029]     Interferometer  42  is preferably a length of coiled fiber and a photodetector internally located within SIU  14 . Interferometer  42  is used to trigger sampling of the return signals at equal wavenumber intervals. In another embodiment, more than one interferometer  42  is internal to SIU  14 . Detector  44  may be any type of photodetector known in the art capable of detecting the light signals in the wavelength range generated by laser  40 , such as InGAs, Si, and Ge photodiodes. Optionally, filters may be used in SIU  14  to further enhance the signal prior to detection.  
         [0030]     Fiber  24  is any type of optical fiber capable of transmitting light inside well  12 . Fiber  24  has sufficient length to provide information regarding engineering parameters within well  12  to the maximum depth, such as 7,000 to 9,000 meters. Typical engineering parameters include but are not limited to temperature, strain, pressure, position, shape and vibration.  
         [0031]     At a first location along the length of fiber  24  is a first sensing length  28 . Sensing length  28  includes a reference reflector  30  and pluralities of sensors  32 ,  34 . As discussed above, the operational distance of an OFDR system is practically limited to about 200 meters; specifically, the distance from reference reflector  30  to the farthest sensor in fiber  24  must be less than 200 meters, however, this distance can be measured on both sides of reference reflector  30 , so the total sensing length  28  may increased to 400 meters as long as reference reflector  30  is placed in the center of sensing length  28 .  
         [0032]     Reference reflector  30  is any type of reflector known in the art, preferably a Fresnel reflector. While reference reflector  30  may be chosen to reflect only a limited band of wavelengths, preferably reference reflector  30  is a broadband reflector that is not band limited.  
         [0033]     Sensors  32 ,  34  may be any type of optical sensor known in the art, preferably FBGs. FBGs, particularly weak FBGs, may be written directly onto fiber  24  via, for example, photoetching. Interpretation of Rayleigh scattering may also be used as sensors  32 ,  34 . For example, a reading may be taken of the Rayleigh scattering at various locations along the length of fiber  24 , such as at locations of “sensors”  32 ,  34 . This known Rayleigh scattering profile of fiber  24  is then stored in memory in processor  46 . First sensor length  28  may include any number of sensors  32 ,  34  up to the maximum allowable for OFDR sensing, or first sensor length  28  may include only one sensor along its length. The number of sensors is typically limited by the requirement that some light is reflected from all the sensors and sufficient light must be transmitted to the final sensor for reflection back to SIU  14 . For example, a minimum of 10% illumination of the last sensor may be required. If all of the sensors reflect at the same wavelength and all of the sensors at 0.0001% reflective, then the maximum number of sensors is 23,000 sensors. The maximum spacing then is simply the desired sensing length divided by 23,000. The minimum spacing is generally the width of a sensor, typically around 5 mm.  
         [0034]     Using a physical interleaving technique, pluralities of sensors  32 ,  34  may be placed on both sides of reference reflector  30 . As can be seen in  FIG. 2A , first plurality of sensors  32  is located before reference reflector  30  and second plurality of sensors  34  is located after reference reflector  30  along the length of fiber  24 . In order for the reflected signals not to overlap after FFT, the individual sensors within pluralities of sensors  32 ,  34  must be placed at slightly offset distances from reference reflector  30 . For example, as shown in  FIG. 2A , both first plurality of sensors  32  and second plurality of sensors  34  contain five individual sensors placed. For each sensor  32 A in first plurality of sensors  32 , a corresponding sensor  34 A is placed in second plurality of sensors  34 . Sensor  32 A is located a first distance D 1  from reference reflector  30 . Sensor  34 A is located a second distance D 2  from reference reflector  30 , where D 2  is similar to but either slightly greater or slightly less than distance D 1 . In general, the offset or difference between D 1  and D 2  may be as short as the width of a sensor. Offsets can be as long as the desired sensing length. The optimal spacing depends on several factors, including the particular range of temperatures and strains expected to be placed on fiber  24 .  
         [0035]     Expanding this physical interleaving technique, a second sensing length  128  may be added to fiber  24 . Second sensing length  128  is similar in proportion to first sensing length  28 , but is disposed further along fiber  24 , i.e., second sensing length  128  will sense a deeper location of well  12 . Second sensing length  128  includes a reference reflector  130  located in the center of second sensing length  128  and a plurality of sensors  132 ,  134  on either side of reference reflector  130 . Electronic filters are included in SIU  14  to reduce coherent interference of the light signals reflected from the various sensing lengths  28 ,  128 . If the coherence length of the source is limited, interference between the first and second sensing length may be insignificant.  
         [0036]     As a result, SIU  14  detects a signal as if from a virtual interleaved sensing length  228  as shown in  FIG. 2B  with a virtual reference reflector  230 , where the signals from first sensing length  28  and second sensing length  128  combine.  FIG. 2C  shows an idealized portion of the signal received from sensors  32 A,  34 A,  132 A, and  134 A. Due to the similar but slightly different locations of sensors  32 A,  132 A,  34 A,  134 A along fiber  24 , the signal from sensors  32 A,  34 A will not overlap the signals from sensors  132 A,  134 A. The signals received from first and second sensing lengths  28 ,  128  will be readily identifiable based upon their unique position. It is believed that no fundamental limit exists for the number of sensing lengths that may be interleaved in this manner. However, for the purposes of practicality and for monitoring conditions in a well, it is believed that up to five sensing lengths may be physically interleaved to create a maximized virtual interleaved sensing length  228 . As shown in  FIG. 3 , the physical interleaving technique of the present invention may be extended to n sensing lengths on separate fibers. This results in extending the overall sensing length for system  10  to 2 km (400-meter sensing lengths*5 interleaved sections). This is not, however, a fundamental limit and increases in data acquisition frequency, available memory, and reference interferometer length make sensing over 10 km possible without wavelength division multiplexing.  
         [0037]     After the maximum number of sensors have been placed using the physical interleaving technique described above, the length of fiber  24  for use in sensing system  10  may be further extended using wavelength division multiplexing (WDM) principles. An FBG responding to the maximum combination of temperature and strain will shift approximately 10 nm. Laser  40  may be tuned over a range that is greater than 10 nm, preferably 120 nm. For a tunable range of 120 nm, 12 wavelength divisions are available. In other words, as laser  40  cycles through its tunable range, each set of physically interleaved sensing lengths  228  may be designed to respond only to a wavelength that is 10 nm different from the next set of physically interleaved sensing lengths  228 . For example, given a 400 meter sensing length, a virtual interleaved sensing length  228  maximized to include five actual sensing lengths, and 12 wavelength divisions, a total sensing length of 24 km (400 meters*5 interleaved sensing lengths*12 wavelength intervals) may be achieved. This is not a fundamental limit and increases in data acquisition frequency, available memory, and reference interferometer length make sensing over 100 km possible  
         [0038]     Additional length can also be obtained by using an aliasing technique. If sensors  32 ,  34 ,  132 ,  134  are narrowband FBGs, they may be placed outside of the Nyquist sampling distance. This results in a low frequency signal component that, when sampling data, is typically regarded as undesirable noise. However, if intentionally placed, the low frequency signal component instead allows the signal from sensors  32 ,  34 ,  132 ,  134  placed “too far” away from the sampler to alias back into the passband. SIU  14  may then extrapolate the desired information from the low frequency component.  
         [0039]     In operation, laser  40  emits a light signal that is sent down fiber  24  and reflected back to detector  44 . The trigger interferometer provides a clocking signal from which the sampling of the data is triggered at equal steps in optical frequency. Due to the plurality of reflectors in fiber  24 , an interference pattern is created in the reflection as received on detector  44 . Processing unit  46  can calculate the interferometer optical path length difference between any given sensor and the related reference reflector is given by 
 
2nL  (Eq. 6)
 
 Where n is the effective index of fiber  24  and L is the distance between the reference reflector and the sensor. As the laser is tuned, the signal D at detector  44  from any one sensor is given by 
 
D=cos(k2nL)  (Eq. 7)
 
 where the constant k is the wavenumber of the light, given by 
 
k=2π/λ  (Eq. 8)
 
 where λ is the wavelength of the light. The response of each sensor with respect to the reference reflector is limited to the narrow wavelength range over which the sensor reflects. The total signal at detector  44  at any given point in time, then, is given by the sum of sensor responses: 
 
D tot =Σ i R i cos(k2nL i )  (Eq. 9)
 
 where R is the spectrum of the ith sensor. As such, each sensor  32 ,  34 ,  132 ,  134  is modulated by a signal with a unique frequency which is governed by the position of the sensor along fiber  24 . 
 
         [0040]     Processing unit  46  of SIU  14  takes the raw data from detector  44 , an example of which is shown in  FIG. 4 , and performs a Fast Fourier Transform on the data to obtain the bandpass-filtered signal as shown in  FIG. 5 . This signal shows the individual peaks from each sensor when the scale of the graph is altered to “zoom in” on a specific segment of the signal, as shown in  FIG. 6 , or on a single sensor, as shown in  FIG. 7 . The spectrum of a single sensor is then obtained by performing an inverse Fourier transformation on only that portion of the data that contains the information from a single sensor. The resultant spectrum from the current example is shown in  FIG. 8 . The reflected wavenumber is then determined using the spectral peak, which may be determined using a number of different methods known in the art, such as by using the apparent central peak. Information regarding the conditions within well  12  is then extrapolated from the reflected wavenumber using known relationships.  
         [0041]     A similar process is followed if Rayleigh scattering sensors are used in system  10 . In this case, the actual Rayleigh scattering at a location  32  is compared to the expected Rayleigh scattering as stored in the memory of processor  46 . The information regarding the conditions within well  12  is then extrapolated from the detected difference between actual and anticipated Rayleigh scattering using known relationships. One example of how to extrapolate the desired information is more fully described in U.S. Pat. No. 6,545,760, which is incorporated herein in its entirety by reference thereto.  
         [0042]     While it is apparent that the illustrative embodiments of the invention disclosed herein fulfill the objectives of the present invention, it is appreciated that numerous modifications and other embodiments may be devised by those skilled in the art. Additionally, feature(s) and/or element(s) from any embodiment may be used singly or in combination with feature(s) and/or element(s) from other embodiment(s). Therefore, it will be understood that the appended claims are intended to cover all such modifications and embodiments which would come within the spirit and scope of the present invention.