Abstract:
A system of transmitting optical signals during a subsea oilfield operations comprises a subsea fiber optical signal carrier. The fiber optical signal carrier includes a first optical fiber having a least one doped section that acts as an amplifier to optical signals passing there through when the doped section is supplied with optical energy. A second optical fiber is disposed alongside the first optical fiber for carrying optical energy. An optical coupler between the second optical fiber and the at least one doped section supplies optical energy from the second optical fiber to the first optical fiber. At least one sensor provides optical signals to the first optical fiber. An optical energy source supplies optical energy to the second optical fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/392,697, filed Jun. 27, 2002. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    This invention relates generally to oilfield operations and more particularly to an in-line fiber optic amplifier for use with sensors monitoring the condition of downhole equipment, monitoring certain geological conditions, reservoir monitoring and remedial operations.  
           [0005]    2. Description of the Related Art  
           [0006]    A variety of techniques have been utilized for monitoring wellbores during completion and production of wellbores, reservoir conditions, estimating quantities of hydrocarbons (oil and gas), operating downhole devices in the wellbores, and determining the physical condition of the wellbore and downhole devices.  
           [0007]    Reservoir monitoring typically involves determining certain downhole parameters in producing wellbores at various locations in one or more producing wellbores in a field, typically over extended time periods. Wireline tools are most commonly utilized to obtain such measurements, which involves transporting the wireline tools to the wellsite, conveying the tools into the wellbores, shutting down the production and making measurements over extended periods of time and processing the resultant data at the surface. Seismic methods wherein a plurality of sensors are placed on the earth&#39;s surface and a source placed at the surface or downhole are utilized to provide maps of subsurface structure. Such information is used to update prior seismic maps to monitor the reservoir or field conditions. Updating existing  3 -D seismic maps over time is referred to in industry as “4-D Seismic”. The above described methods are very expensive. The wireline methods are utilized at relatively large time intervals, thereby not providing continuous information about the wellbore condition or that of the surrounding formations.  
           [0008]    Placement of permanent sensors in the wellbore, such as temperature sensors, pressure sensors, accelerometers and hydrophones has been proposed to obtain continuous wellbore and formation information. A separate sensor is utilized for each type of parameter to be determined. To obtain such measurements from the entire useful segments of each wellbore, which may have multi-lateral wellbores, requires using a large number of sensors, which requires a large amount of power, data acquisition equipment and relatively large space in the wellbore: this may be impractical or prohibitively expensive.  
           [0009]    Once the information has been obtained, it is desirable to manipulate downhole devices such as completion and production strings. Prior art methods for performing such functions commonly rely on the use of electrically operated devices with signals for their operation communicated through electrical cables. Because of the harsh operating conditions downhole, electrical cables are subject to degradation. In addition, due to long electrical path lengths for downhole devices, cable resistance becomes significant unless large cables are used. This is difficult to do within the limited space available in production strings. In addition, due to the high resistance, power requirements also become large.  
           [0010]    In production wells, chemicals are often injected downhole to treat the producing fluids. However, it can be difficult to monitor and control such chemical injection in real time. Similarly, chemicals are typically used at the surface to treat the produced hydrocarbons (i.e., to break down emulsions) and to inhibit corrosion. However, it can be difficult to monitor and control such treatment in real time.  
           [0011]    Systems for using fiber optic sensors are known in the art. For example, see U.S. Pat. No. 6,281,489 to Tubel et al, assigned to the assignee of this application and incorporated herein by reference. Tubel et al provide apparatus and methods which utilize sensors (such as fiber optic sensors), to monitor downhole parameters and to perform a variety of functions. The sensors are used to measure parameters related to various downhole parameters of interest.  
           [0012]    Present day sub-sea production systems can comprise multiple wells connected by flow conduits to a single processing station that can be sub-sea or at the surface. The wells may be separated from the processing station by tens of kilometers. The sensor and communications cables are typically run adjacent, or inside, such flow conduits to a central controller. Optical signals traveling along optical fibers in these cables experience attenuation over the long distances causing poor detection at the central control.  
           [0013]    Optical amplifiers are commercially available for the telecommunications industry. These devices, known as erbium doped fiber amplifiers are powered by an electrically energized laser pumping diode to excite erbium ions doped in a small section of the optical fiber to an energized state. An incoming signal causes the excited ions to drop to a lower energy state and emit photons. The pumping diode and associated electronics are connected to optical fiber in the vicinity of the erbium doped section. These commercially available devices are not well suited for the oilfield environment due to their size and the need for electrical power, at the amplifier, to power the laser diode.  
           [0014]    The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by providing an optical amplifier that requires no electrical energy at the amplifier and is suitable for use in an oilfield environment.  
         SUMMARY OF THE INVENTION  
         [0015]    In one aspect of the present invention, a fiber optic signal system comprises a first optical fiber having a plurality of spaced apart doped sections that have a material that amplifies optical signals passing therethrough when optical energy is supplied to each of the plurality of spaced apart doped sections. A second optical fiber is adjacent the first optical fiber and is optically coupled to each of the plurality of doped sections. The second optical fiber has optical energy pumped therethrough for supplying optical energy to each of the plurality of doped sections to amplify optical signals carried by the first optical fiber.  
           [0016]    In another aspect of the present invention, a method of transmitting optical signals comprises providing a first optical fiber having a plurality of spaced apart sections doped with a material that acts as an optical amplifier upon supply of optical energy thereto. A second optical fiber is provided adjacent the first optical fiber. The second optical fiber is coupled to each of the plurality of spaced apart doped sections. Optical energy is pumped from a remote source through the second optical fiber to each of the plurality of doped sections in the first optical fiber to amplify optical signals passing through the first optical fiber.  
           [0017]    In yet another aspect of the present invention, a system of transmitting optical signals during a subsea oilfield operations comprises a fiber optical signal carrier placed at least a certain distance under water. The fiber optical-signal carrier includes a first optical fiber having a least one doped section that acts as an amplifier to optical signals passing therethrough when the doped section is supplied with optical energy. A second optical fiber is disposed alongside the first optical fiber for carrying optical energy. An optical coupler between the second optical fiber and the at least one doped section supplies optical energy from the second optical fiber to the first optical fiber. At least one sensor provides optical signals to the first optical fiber. An optical energy source supplies optical energy to the second optical fiber.  
           [0018]    Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    For detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein:  
         [0020]    [0020]FIG. 1 is a schematic drawing of a fiber optic signal carrier according to one embodiment of the present invention;  
         [0021]    [0021]FIG. 2 is a schematic drawing of a two-fiber fiber optic amplifier according to one embodiment of the present invention;  
         [0022]    [0022]FIG. 3A is a schematic drawing of a three-fiber fiber optic amplifier according to one embodiment of the present invention;  
         [0023]    [0023]FIG. 3B is a schematic drawing of an alternative three-fiber fiber optic amplifier according to another preferred embodiment of the present invention; and  
         [0024]    [0024]FIG. 4 is a schematic drawing of sub-sea production operation according to one embodiment of the present invention.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0025]    The present invention contemplates a fiber optic amplifier for boosting signal strength of optical signals on a long optical fiber.  
         [0026]    According to one preferred embodiment, see FIG. 1, a cable  100  extends from a surface platform  306  through a sub-sea wellhead  309  into a wellbore  305  in a subterranean formation  310 . The cable  100  comprises a signal optical fiber  101  and optical pumping fiber  102 . The optical fibers  101  and  102  are positioned adjacent each other. Optical pump  308  is a light source for injecting light into optical pumping fiber  102 . The cable  100  may contain other electrical or optical conductors. Signal fiber  101  is connected to sensor  303  in wellbore  305  through sensor interface  313 . Sensor  303  detects a parameter of interest related to the flow of production fluid from formation  310 . Sensor  303  may be one of (i) a temperature sensor; (ii) a pressure sensor; (iii) a flow measurement sensor; and (iv) a sensor providing a measure of a fluid characteristic. Such fluid characteristic sensors may be used to measure electrical and acoustic conductivity, density and to detect various light transmission and reflection phenomena. All of these sensor types are available commercially in various ranges and sensitivities which are selectable by one of ordinary skill in the art depending upon particular conditions known to exist in a particular well operation. Multiple sensors  303  may be located in the borehole  305  and optically coupled to signal fiber  101  through multiple sensor interfaces  313 . Signal fiber  101  may also contain multiple sensors  304  distributed within signal fiber  101 . See U.S. Pat. No. 6,281,489, incorporated herein by reference, for a more detailed description of such sensors. Sensors  304  may include (i) a temperature sensor; (ii) a pressure sensor; (iii) a flow measurement sensor; and (iv) a sensor providing a measure of a fluid characteristic. Sensors  304  are commonly formed in the signal fiber  101  during manufacture. A key issue regarding the use of fiber optic sensors in deep wells is the degradation of the optical signal strength over the required length of fiber. Fiber optic amplifiers  110  are suitably located in-line in cable  100  to boost the transmitted signal thereby providing suitable signal strength at surface receiver  307 .  
         [0027]    [0027]FIG. 2 shows one preferred embodiment of fiber optic amplifier  110 . As previously described, cable  1100  has a signal fiber  101  and an optical pumping fiber  102  encased in a protective cover  105 . Protective cover  105  may be a tubing of a material including but not limited to (i) polyurethane (ii) steel; or (iii) a composite material. The signal fiber  101  transmits optical signals from downhole sensors  303  and  304  to the surface receiver  307 , see FIG. 1. In long cables, optical attenuation in fiber  101  decreases the signal strength below acceptable levels. Optical amplifier  110  serves to boost the signals suitably above the background noise to enhance detection. Referring again to FIG. 2, a relatively short (on the order of a few meters) section of signal fiber  101  is doped with a rare-earth material such as erbium. Such doping techniques are known in the art. Optical pumping fiber  102  is optically coupled to signal fiber  101  using a commercially available optical coupler. The connected sections of fibers  101  and  102  are encapsulated using a suitable encapsulation material including, but not limited to, epoxy and silicone. The fibers  101 ,  102 , and the encapsulated section may be further encapsulated within protective cover  105  by a common filler material  107  used in the cable art.  
         [0028]    In operation, referring to FIGS. 1 and 2, optical power, which may be on the order of several watts, is injected into optical pumping fiber  102  by optical pump  308 . Optical sources are commercially available for pumping up to tens of watts into such a fiber. The optical power is injected to the erbium doped section  106  of signal fiber  101  causing the erbium ions to be raised to an energized state. When an optical sensor signal passes through the doped section, the energized ions are stimulated to fall back to a ground energy state while simultaneously emitting photons that boost the optical sensor signal. The number of such optical amplifiers required is application dependent and is related to several factors including, but not limited to, initial sensor signal levels, background optical system noise, transmission distance, and receiver sensitivity. These factors may be analyzed using techniques common in the art to determine the number of fiber optic amplifiers for a given application and the total power to be pumped down pumping fiber  102 .  
         [0029]    In another preferred embodiment, FIG. 3A shows a single optical pumping fiber  202  used to energize two separate signal fibers  201  and  204 . Pumping fiber  202  is coupled to signal fibers  201  and  204  by commercially available optical couplers  208  and  209 . The pumping fiber  202  conveys suitable laser energy to the doped sections  207  and  206  of signal fibers  201  and  204  for amplifying signals traveling in fibers  201  and  204 , as described previously. As shown in FIG. 3A, signal fibers  201  and  204  convey signals in opposite directions. Typical signals include, but are not limited to, sensor signals from a downhole location to a surface receiver/controller, and data command signals from a surface controller to a downhole sensor, such as sensor  303  of FIG. 1, for changing a sensor parameter. Typical sensor parameters include, but are not limited to, sampling frequency and sensor scaling. Alternatively, the data command signals may operate a production flow control device (not shown). While the pumping fiber  202  is shown being connected to two signal fibers  201  and  204 , the number of signal fibers and doped sections that may be connected to a single pumping fiber is application dependent. Analytical techniques are available in the optical arts to determine the number of required amplification sections for each signal cable and optical pumping capacity of the pumping fiber. Alternatively, as shown in FIG. 3B, the optical pumping fiber  252  may be optically fused in optically transparent material  260  to signal fibers  254  and  251  at doped sections  256  and  257  respectively. As previously described, the number of doped sections and connections is application dependent.  
         [0030]    It should be noted that all of the optical fiber embodiments disclosed may be contained in any of a number of cable types known in the art. These include, but are not limited to (i) electro-optical cables containing optical fibers and electrical conductors; (ii) electro-optical hydraulic cables carrying optical fibers, electrical conductors, and hydraulic hoses; and (iii) optical fiber cables. Any of these cables may be reinforced with braiding and encapsulation techniques common in the art. In addition, redundant signal fibers and pumping fibers may be included in such cables.  
         [0031]    In one preferred embodiment, schematically shown in FIG. 4, a production system comprises sub-sea wells  401 - 403 , drilled and completed in an offshore formation with production control equipment for each well located at the seafloor. The production flow from wells  401 - 403  are transferred through pipelines  411 - 413 , respectively, to a common sub-sea processing unit  410 . The processing unit  410  is controlled by sub-sea controller  415  that may contain circuitry (not shown) to operate flow control devices (not shown) for controlling flow from wells  401 - 403  based on instructions from a surface controller  420  located on production platform  419 . Alternatively, for production systems that are connected by pipelines (not shown) to land based processing facilities, surface controller  420  may be land based. Umbilical cable  416  connects surface controller  420  with sub-sea controller  415 . Umbilical cable  416  may contain electrical conductors and optical fibers suitable for signal transmission and for pumping optical energy as described herein. Combined electro-optical cables are known in the art and will not be described here further. It should be noted that wells  401 - 403  may be tens of kilometers from processing unit  410 . In order to better control the flows from each of the wells  401 - 403 , optical fiber cables  405 - 407  are disposed in pipelines  411 - 413  respectively. Each optical fiber cable  405 - 407  has fiber optic sensors  408  and fiber optic amplifiers  430  embedded in-line in each cable. The sensors  408  measure production parameters and may include, but are not limited to, (i) a temperature sensor; (ii) a pressure, sensor; (iii) a flow measurement sensor; and (iv) a sensor providing a measure of a fluid characteristic. Fiber optic amplifiers  430  as previously described are disposed in the optical fiber cables  405 - 407  to amplify signals from sensors  407 . In one preferred embodiment, signals from sensors  407  are transmitted along optical fiber cables  405 - 407  and are optically coupled at sub-sea controller  415  to optical signal fibers (not shown) in umbilical cable  416 . The optical signals are received by an optical system (not shown) in surface controller  420 . Surface controller  420  interprets the received signals according to programmed instructions and may initiate command signals to sub-sea controller  415  for controlling flow from wells  401 - 403 . Surface located light source  425  provides optical power through optical pumping fibers (not shown) in the umbilical cable  416  that is then optically coupled to optical pumping fibers (not shown) in each of cables  405 - 407 , thereby energizing each of optical amplifiers  430 . The optical amplifiers  430  may be any combination of the single direction amplifier described with respect to FIG. 2 or the bi-directional amplifier described with respect to FIGS. 3A,B. Multiple optical amplifiers  430  may be used in a single optical fiber, as required. Multiple optical fiber systems may be contained in each cable. It should be noted that the use of three wells in the previous description is exemplary only as any number of sub-sea producing wells may be connected to processing unit  410  as described above.  
         [0032]    In another preferred embodiment, optical amplifiers  430  may be used to amplify digital control and communication signals between surface and/or subsea controllers and production controllers (not shown) located in each of the production wells  401 - 403 .  
         [0033]    Systems and methods have been described in which at least one remotely powered, optical amplifier is used in oilfield applications to boost optical signals traveling along an optical fiber. Both sensor and communications signals may be boosted. The, optical fibers may be disposed in subsea wells and pipelines. Controllers at the surface and/or subsea may use the signals to control subsea production flows.  
         [0034]    The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes.