Patent Publication Number: US-2004042703-A1

Title: Method and apparatus for sensing an environmental parameter in a wellbore

Description:
CROSS REFERENCE TO RELATED APPLICATION  
     [0001] This claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Serial No. 60/406,542, entitled “Method and Apparatus for Sensing an Environmental Parameter in a Wellbore,” filed Aug. 28, 2002. 
    
    
     
       TECHNICAL FIELD  
       [0002] This invention relates to methods and apparatus for sensing environmental parameters in wells for the production of petroleum products.  
       BACKGROUND  
       [0003] Wells for the production of petroleum products are drilled through the earth&#39;s subsurface. Wellbores may be vertical, deviated, or horizontal and may or may not be lined with a casing or other liner. Various operations are performed in the wellbore to complete the well as well as to produce hydrocarbons from target reservoirs. For example, a perforating gun can be lowered into the wellbore and fired to perforate openings through any surrounding casing or liner and to extend perforations into a surrounding reservoir. Also, valves and pumps are installed in the wellbore to control flow of hydrocarbons.  
       [0004] In performing operations in a wellbore, environmental parameters of the wellbore and/or the surrounding formation may be monitored. For example, temperature and pressure within the wellbore may be monitored. Wellbore temperature and pressure may be monitored in the vicinity of downhole equipment such as submersible pumps and logging modules to provide equipment operators with information regarding the equipment&#39;s performance. Moreover, pressure measurements in particular may be used to keep the equipment operating at peak performance levels, which in turn, improves the efficiency and longevity of the equipment. Downhole pressures may also be monitored during water, steam and carbon dioxide injection, drilling operations, or to optimize hydrocarbon extraction from the wellbore.  
       [0005] Conventional sensors are susceptible to electromagnetic noise or may be susceptible to damage due to the harsh environment in a wellbore. Moreover, conventional sensors may be unsafe under certain conditions and may be costly to maintain. Thus, there is a continuing need for improved methods and apparatus to sense environmental parameters within and surrounding a wellbore.  
       SUMMARY  
       [0006] In general, according to one embodiment, a system for using a wellbore comprises a device adapted to perform an operation in the wellbore and a sensor adapted to sense pressure in the wellbore. The sensor includes a strain sensitive member, and an optical fiber having a portion that is bonded to the strain sensitive member. The sensor further includes a housing defining a first chamber, the housing attached to the strain sensitive member. The first chamber is at a first pressure. The sensor further includes a second chamber proximate to the first chamber, with the second chamber at a second pressure.  
       [0007] Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008]FIG. 1 illustrates a fiber optic pressure sensor assembly according to one embodiment of the present invention;  
     [0009]FIG. 2 illustrates a fiber optic pressure sensor assembly according to an alternate embodiment of the present invention;  
     [0010]FIG. 3 is an enlarged cross-sectional view of the fiber optic sensor for use with the assembly of FIG. 1 or FIG. 2;  
     [0011]FIG. 4 is an enlarged cross-sectional view of the fiber optic sensor according to an alternate embodiment for use with the assembly of FIG. 1 or FIG. 2.  
    
    
     DETAILED DESCRIPTION  
     [0012] In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.  
     [0013] Referring to FIG. 1, a fiber optic sensor assembly  10  may be utilized to measure an environmental parameter such as temperature or pressure at various depths within a wellbore  12 . The assembly  10  may be utilized alone or in conjunction with downhole equipment. Moreover, the subsurface or distal portion of the assembly  10  may lie within a casing  16  or an annular space  17  between the casing  16  and the wellbore  12  wall. Further, in some embodiments, the assembly  10  may be fixed to either the interior or exterior wall of the casing  16 . For purposes of clarity, the assembly  10  of FIG. 1 is shown alone and suspended within the interior of the casing  16 .  
     [0014] The assembly  10  includes equipment  22  located at the earth&#39;s surface  14 , an optical fiber  18  and a pressure sensor  20 . The surface equipment  22  includes a light source  24  such as a light-emitting diode (LED) or laser diode and equipment  26  suitable for processing a returned light signal. The light source  24  produces a light signal such as a pulse of light that is transmitted into the fiber  18 . The processing equipment  26  evaluates the light signal that is returned from the sensor  20  via fiber  18  to determine the measured environmental parameter (e.g., pressure or temperature).  
     [0015] The proximal end of the optical fiber  18  is coupled or in close proximity to the light source  24 . Thereafter, the fiber  18  descends to a depth within the wellbore  12 . Thus, in this embodiment, the fiber  18  originates at the surface  14  and terminates within the wellbore  12 . In other embodiments, the equipment  22  may be positioned downhole in the wellbore. In this case, information pertaining to the measured environmental parameter.  
     [0016] The fiber  18  has a light or wave-guiding core that guides a light signal from the proximal end of the fiber  18  to the distal end of the fiber  18 . Typically, the optical fiber  18  includes two concentric layers of highly purified glass, such as silica glass, known as the core and the cladding. Generally, the cladding has a lower refractive index thereby confining light signals to the core of the fiber  18 . Although most optical fibers are glass, some suitable fibers may be made from plastic.  
     [0017] In some embodiments, a sheath for support and/or protection may surround the outside of the fiber  18 . The sheath may be multi-layered, including strengthening and insulating layers made from materials such as polyvinyl chloride (PVC), stainless steel and the like. In this way, the fiber  18  may be prepared to withstand the harsh environment of a wellbore.  
     [0018] The sensor  20  is positioned at a point on the subsurface portion of the optical fiber  18 . Although only one sensor  20  is shown in the embodiment depicted in FIG. 1, several sensors  20  may be distributed along the subsurface length of the fiber  18 . When several of the point sensors  20  are arranged on the fiber  18 , the sensors  20  may be multiplexed using time divisional multiplexing (TDM), wavelength divisional multiplexing (WDM), or frequency divisional multiplexing (FDM). In this way, multi-point sensing may be achieved.  
     [0019] Generally, a fiber optic sensor modulates a property of a light signal such as its wavelength, frequency or polarization. That is, fiber optic sensors may act as transducers that convert a measurement such as strain into an altered light signal. In embodiments of the present invention, the sensor  20  subjects the fiber  18  to strain in response to downhole environmental parameters, such as pressure, temperature, and so forth. Thus, a property of the light signal is altered.  
     [0020] In some embodiments of the present invention, a sensor element  28  is disposed within the pressure sensor  20 . For example, the sensor element  28  may be a diffraction grating, such as a Bragg grating. Generally, diffraction gratings are reflective surfaces having parallel grooves that are closely spaced. In the case of Bragg gratings, the optical fiber is exposed to intense ultraviolet light to produce grooves in the core of the fiber. The spacing between the grooves determines which wavelength of light is reflected back along the length of the fiber. Thus, Bragg gratings with different groove spacing will reflect back different wavelengths or frequencies of light. Wavelengths of light that are not reflected move though the grating.  
     [0021] As stated above, the sensor  20  subjects the fiber  18  to strain. Specifically, the sensor  20  is responsive to an environmental parameter such as pressure and/or temperature that causes the sensor  20  to compress or elongate a portion of the fiber  18  in the axial direction. In embodiments of the present invention that utilize Bragg gratings  28 , strain on the fiber  18  causes the spacing between the grooves of the grating  28  to be altered. Accordingly, the wavelength of light back-reflected by the grating  28  in the strained fiber  18  will be different from the wavelength of light back-reflected by the grating  28  in the fiber  18  that is not strained or placed under a reference strain. Because the strain on the fiber  18  is directly related to the wavelength of back-reflected light, the environmental parameter to be sensed can be quantified.  
     [0022] Thus, according to some embodiments of the present invention, a light signal generated by the light source  24  is guided to the subsurface portion of the fiber  18  where the sensor  20  is positioned. In response to the pressure within the wellbore  12 , the sensor  20  strains the fiber  18  in the axial direction. The strain causes the preset spacing between the grooves of the grating  28  to either compress or expand. In turn, the change in grating  28  spacing causes a change in the wavelength of light reflected by the grating  28  back to the earth&#39;s surface  14 . Wavelengths of light that are not back-reflected pass through the grating  28  and continue on through the fiber. To reduce back reflection of the non-reflected light, the most distal end of the fiber  18  may be cut at an angle or treated with an antireflective coating.  
     [0023] The wavelength or frequency of light reflected back to the earth&#39;s surface  14  may be monitored by any known technique, such as spectroscopy, optical filtering, tracking with a tunable filter, or detecting with an interferometer. In this way, the wellbore  12  pressure and/or temperature (or other downhole environmental parameter) at the point where the sensor  20  is positioned may be determined.  
     [0024] Referring to FIG. 2, an alternate embodiment of the fiber optic sensor assembly  10  includes surface equipment  30 , a fiber  18  having two arms  48  and  46 , a collecting fiber  32 , a sensor  20  and a beam splitter  34 . The surface equipment  30 , located at the earth&#39;s surface  14 , includes two light sources  36  and  38  and analysis equipment  40 . One light source  36  or  38  may be a dye laser pumped by a Q-switched, frequency-doubled Nd: YAG laser, as an example. The other light source  36  or  38  may also be a dye laser, such as a continuous wave Argon-pumped dye laser. Each light source  36  and  38  transmits a light signal into an end  42  and  44 , respectively, to travel through arms  48  and  46  of the fiber  18 . Counter propagating light signals propagate down the fiber  18  and meet at an area of interaction. The analysis equipment  40  may be any suitable equipment to analyze a light signal returned to the earth&#39;s surface  14  via fiber  32  (such as a Glan Thompson analyzer or a photodetector).  
     [0025] The fiber  18  of this embodiment is a polarization maintaining fiber such as a birefringent fiber. Generally, a highly birefringent fiber has different refractive indices for two orthogonal linear directions of optical polarization. In some embodiments, the core and cladding of the fiber  18  may have different strain-optic properties. Each end  42  and  44  of the fiber  18  is coupled or in close proximity to the light source  36  and  38  respectively whereas the bend  47  of the fiber  18  (at which the arms  48  and  46  meet) is at a depth within the wellbore  12 . Thus, each arm  46  and  48  of the fiber  18  extends from a light source  36  and  38  respectively to the bend  47 .  
     [0026] The fiber  32  is also a polarization maintaining fiber that may or may not have a core and cladding with different strain-optic properties. The distal end  50  of the fiber  32  is coupled to one arm  46  or  48  of the fiber  18  by the splitter  34  whereas the proximal end  52  of the fiber  32  is coupled to the analyzer  40 .  
     [0027] The beam splitter  34  is disposed on one arm  46  or  48  of the fiber  18  and is proximate to fiber  32 . The beam splitter  34  reflects the light signal that is generated after the two counter-propagating waves of light traveling down the arms  46  and  48  interact. That is, the beam splitter  34  reflects the light signal that results from the interaction of the two light signals launched from light sources  36  and  38  onto the collecting fiber  32 .  
     [0028] The sensor  20  is disposed on the arm  46  or  48  of the fiber  18  opposite that of the beam splitter  34 . In some embodiments, the sensor  20  and beam splitter  34  are separately housed. However, in other embodiments the sensor  20  and beam splitter  34  are enclosed within the same housing. As with other embodiments of the present invention, the sensor  20  strains the fiber  18 , which alters a property of a light signal.  
     [0029] According to some embodiments, the sensor  20  modulates the local birefringence of the fiber  18 . For example, a pulse of light that is linearly or circularly polarized is launched from light source  36  into arm  48  of the fiber  18 . This polarization state is maintained as the pulse is guided down fiber  18  to sensor  20 . The sensor  20  strains the fiber  18 , which alters the birefringence of the light signal. Thus, the light pulse that exits the sensor  20  is altered from its original form.  
     [0030] The modulated light may be probed with a second light signal that is either co-propagating or counter-propagating. In some embodiments of the present invention, a counter-propagating wave is utilized to probe the sensor  20  modulated light signal. However, the assembly  10  may be adapted to utilize a co-propagating wave to probe the modulated light signal. The counter-propagating light signal is launched from the light source  38  into the arm  46  of the fiber  18 . The counter-light signal is the same wavelength as the light signal originally launched from light source  36 . Moreover, the counter-signal may be linearly polarized at an angle to the fiber&#39;s  18  birefringent axes. At the region of interaction on the fiber  18 , the sensor  20  modulated light signal and the counter-light signal interact and couple. For example, if the sensor  20  does not modulate the original light signal, the coupled power is maintained. However, if the original light signal is sensor  20  modulated, then there will be an oscillatory variation in the signal power that emerges from the region of interaction. This variation may be measured as a shift in beat length frequency as a result of time. This shift in frequency is indicative of the strain placed on the fiber  18  by the sensor  20 . Thus, the emerging light signal is reflected into fiber  32 , by beam splitter  34 , which is returned to the earth&#39;s surface  14  for analysis.  
     [0031] In an alternate embodiment, the strain on the fiber  18  is measured by the ratio of the pulse energy before and after the interaction of the counter-propagating pulses. For example, if the original light signal and the counter-light signal begin with equal energies and remain unaltered, the interaction between the signals will be relatively great. As a result, the ratio will be small because only a portion of the original energy remains after the interaction. However, if the sensor  20  alters the original light signal, then the ratio will large due to the negligible interaction between the modified signal and the counter-signal. The beam splitter  34  may reflect the post-interaction signal onto the fiber  32 , which carries the signal to the earth&#39;s surface  14  for analysis.  
     [0032] Referring to FIG. 3, according to some embodiments, the sensor  20  includes an open chamber  60  (“open” or in communication with the wellbore environment), a sealed chamber  62  and a strain sensitive member  64  that envelops a portion of the fiber  18 . In embodiments where the fiber  18  is supported and/or protected by a sheath, the portion of the fiber  18  enclosed within the sensor  20  may have the sheath removed.  
     [0033] The open chamber  60  is in communication with the wellbore  12  via an inlet  66 . Thus, the environmental parameter to be sensed permeates the chamber  60 . In some embodiments, the inlet  66  may include a bellows. In another embodiment, the inlet  66  includes a port. An outer housing  68  that defines the open chamber  60  has two openings  70  and  72  for the fiber  18  to traverse the sensor  20 . The housing  68  may be made from any rigid material (such as plastic or metal) capable of withstanding the environmental conditions present in the wellbore  12 .  
     [0034] In this embodiment, the chamber  62  is enclosed by chamber  60  and is substantially sealed. That is, chamber  62  is not in communication with the open chamber  60 , hence the wellbore  12 . Although the chamber  62  has openings  74  and  76  for the fiber  18  to pass through, the openings  74  and  76  are sufficiently sealed to prevent exchange of fluids between chambers  60  and  62 . The openings  74  and  76  may be sealed in any conventional manner. An inner housing  67  that defines sealed chamber  62  may be made from a material that allows the chamber  62  to expand or contract in the axial direction, such as a flexible metal.  
     [0035] The strain sensitive member  64  supports the fiber  18  portion as it traverses through the sensor  20 . Generally, the member  64  is bonded to the interior walls  78  and  80  of housing  67  and extends between the two openings  74  and  76 . Moreover, in some embodiments, the member  64  may be bonded to the interior walls  82  and  84  of housing  68  and extend to the exterior walls  86  and  88  of the inner housing  67 . In this way, the member  64  may extend across the entire length of the sensor  20 . The member  64  may be bonded to the walls  78 ,  80 ,  82 ,  84 ,  86  and  88  of the housings  67  and  68  by any known means such as with an adhesive.  
     [0036] The length of the fiber  18  portion that traverses the sealed sensor  20  is continuously bonded to the member  64 . Continuous bonding of the fiber  18  to the member  64  increases the sensitivity of the sensor  20  to downhole environmental conditions. The optical fiber  18  portion is bonded continuously to the strain sensitive member  64  at least substantially along the entire length of the chamber  60 . The length of the chamber  60  is defined along its longitudinal axis between a first end and second end of the chamber  60 . The optical fiber  18  portion is generally parallel to the longitudinal axis of the chamber  60  as the optical fiber  18  portion extends through the chamber  60 . In the illustrated example of FIG. 3, the fiber  18  portion is also continuously bonded to the strain sensitive member  64  in the region outside the first chamber  60 .  
     [0037] The material that the member  64  is made from may also enhance the sensitivity of the sensor  20 . For example, the member  64  may be made from a material that has a desirable modulus of elasticity in the direction of the axis of the fiber  18 . In some embodiments, the member  64  may be made from an anisotropic material having a lower modulus of elasticity in the axial direction than in the circumferential direction. In other embodiments, the member  64  may be made from a carbon fiber material with a similar coefficient of expansion as the fiber  18 . Alternatively, the member  64  can also be made of epoxy or PEEK.  
     [0038] A strain point  90  is typically, approximately at the midpoint of member  64 . The strain point  90  is the point at which the relative strain on the fiber  18  portion is the largest. In some embodiments, the sensing element  28  (e.g., Bragg grating) may be located at the strain point  90 . Alternately, the sensing element  28  may be disposed proximate the strain point  90 .  
     [0039] The sealed chamber  62  may be at a reference pressure or contain a vacuum. A pressure differential created between chambers  60  and  62  causes the member  64  to strain the fiber  18  axially. According to some embodiments of the present invention, a strain on the fiber  18  portion induces a change in a property of a light signal. For example, in embodiments that include the Bragg grating  28 , the strain on the fiber  18  portion induced by the strain sensitive member  64  will determine the spacing between the grooves of the grating  28 . The wavelength of light that is back-reflected by the grating  28  is space dependent. Thus, the magnitude of the strain is indicated by the wavelength of light that is reflected back to the earth&#39;s surface  14 .  
     [0040] In other embodiments, for embodiments including a highly birefringent fiber  18 , the strain on the fiber  18  portion induced by the strain sensitive member  64  alters the local birefringence of the fiber  18 . Thus, variations in the coupling of counter propagating waves may be detected as a shift in frequency due to the strain on the fiber  18 . The detected shift in the frequency of the returned wave is indicative of the strain placed on the fiber  18 . Alternately, variations in the coupling of counter propagating waves may be detected as the energy remains that after cross-coupling.  
     [0041] The amount of strain on the fiber  18  portion is an interaction of the pressure differential between the open chamber  60  and sealed chamber  62 .  
     [0042] Referring to FIG. 4, an alternate embodiment of the sensor  20  also includes an open chamber  92 , a sealed chamber  94  and a strain sensitive member  96 . However, in this embodiment, the strain sensitive member  96  is one wall of the housing  98  that defines the chamber  94 .  
     [0043] As with other embodiments, the open chamber  92  is in communication with the wellbore  12  via an inlet  66 . Moreover, the sealed chamber  94  is at a reference pressure or a vacuum. In some embodiments, the chamber  92  encloses chamber  94 . Alternately, the chamber  92  may be proximate to chamber  94 . In either embodiment, when the parameter to be sensed permeates the chamber  94 , a pressure differential is created between the two chambers  92  and  94 .  
     [0044] The housing  100  that defines chamber  92  may be of a sufficiently rigid, non-corrosive material such as a plastic or metal. However, the housing  98  that defines chamber  94  is made from a material having a desired modulus of elasticity along the axis of the fiber  18 . Thus, the pressure differential between chambers  92  and  94  causes the housing  98  to be strained in the axial direction of the fiber  18 .  
     [0045] The optical fiber  18  portion in the sensor is continuously bonded to wall  96  of the housing  98 . Thus, in this embodiment, the strain sensitive member is a wall of the housing  100  defining the reference chamber  94 . The wall is continuously bonded to the fiber  18  portion along substantially the entire length of the chamber  94  between its first and second ends along the longitudinal direction of the chamber  94 .  
     [0046] The fiber  18  portion is also strained axially in response to the pressure differential between chambers  92  and  94 . In some embodiments the fiber  18  is also continuously bonded to the housing  98 . The fiber  18  portion can either be in direct contact with the housing  98 , or an adhesive layer may be provided between the fiber  18  portion and the housing  98 . As examples, the adhesive layer includes epoxy, PEEK and the like.  
     [0047] A strain point  102  lies approximately at the midpoint of wall  96 . As with other embodiments, the strain point  102  is the point wherein the strain on the fiber line  18  portion is maximized. Likewise, the strain point  102  may include the element  28 , or alternately, the element  28  may be proximate the strain point  102 . Strain on the fiber  18  due to the pressure differential between chambers  92  and  94  induces a change in a property of the light signal being monitored as previously described.  
     [0048] While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.