Patent Publication Number: US-7903908-B2

Title: Multi-core strain compensated optical fiber temperature sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/531,145 filed Sep. 12, 2006 now U.S. Pat. No. 7,512,292, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention generally relate to optical based temperature sensors. 
     2. Description of the Related Art 
     Various approaches exist for intelligent wells that monitor temperatures within oil and gas wellbores. Some reasons for monitoring these temperatures include reducing operating costs and increasing yield from individual reservoirs. Cost effectively providing more accurate and reliable measurements over a period of time can therefore improve benefits provided by these intelligent wells. 
     Sensors for measuring the temperatures in the wellbore can include optical sensors, which avoid problems associated with electrically based systems. A plurality of the optical sensors can form an array of optical sensors disposed along an optical cable that includes an optical transmission waveguide such as an optical fiber. The array of sensors can include a plurality of optical Bragg gratings that each return a signal whose wavelength varies with applied temperature. These arrays can be interrogated by, for example, time division multiplexing or wavelength division multiplexing. 
     As another example of optical based temperature measurement that can be utilized in the wellbore, the optical waveguide itself can be employed as a distributed temperature sensor (DTS) to provide more than one measurement along its length. The DTS can be based on analysis (e.g., Raman scattering analysis) of reflected light that is altered in accordance with the temperature of the waveguide. Processing such reflections as a function of time derives temperature as a function of well depth with earlier reflections indicating the temperature at relatively shallow depths. 
     The signals from these sensors disposed at discrete points or from the DTS can undesirably be influenced by other parameters than temperature, thereby altering a response indicated by the signal. For example, strain or pressure applied to the sensor may dynamically change making calibration of the signal to account for the strain impossible since there is no way to tell which parameter is contributing to the response of the sensor. Accordingly, multiple parameters contributing to the response undermine accuracy and confidence in temperature measurements utilizing either DTS or Bragg grating based sensors. 
     Therefore, there exists a need for apparatus and methods that perform improved discrete point temperature sensing using optical waveguides. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention relate to a discrete point temperature sensor that can be part of an array of such sensors for location in a wellbore. A single unitary structure forms the temperature sensor that has separate optical cores possessing different characteristics such that one core is unique from another core. Each core has a reflective grating (e.g., a Bragg grating) disposed therein such that the wavelength of light reflected by the gratings is in response to temperature and any strain applied to the sensor from a surrounding environment. According to some embodiments, dimensions and structural configuration of the structure forming the temperature sensor can aid in ensuring that the cores at the gratings experience the same or substantially similar stress. For some embodiments, the responses to strain from each of the gratings are similar while the responses from each of the gratings to temperature are dissimilar due to the different characteristics of the cores. These responses provided separately by each grating therefore enable compensation for strain in order to provide an accurate temperature measurement at the sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is an end view of a dual core waveguide according to embodiments of the invention. 
         FIG. 2  is a schematic top view of the dual core waveguide shown optically coupled with optical fibers. 
         FIG. 3  is a partial section view of a wellbore utilizing the dual core waveguide to enable multiple discrete temperature measurements according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention relate to a discrete point temperature sensor having separate optical cores with different characteristics disposed within a single unitary structure. Each core has a reflective grating (e.g., a Bragg grating) disposed therein such that the wavelength of light reflected by the gratings is in response to temperature and any strain applied to the sensor from a surrounding environment. Due to the different characteristics of the cores, the responses to strain from each of the gratings are similar while the responses from each of the gratings to temperature are dissimilar. These responses provided separately by each grating therefore enable compensation for strain in order to provide an accurate temperature measurement. 
       FIG. 1  shows an end view of a dual core waveguide  100  according to embodiments of the invention. The dual core waveguide  100  includes a first core  102  and a second core  104  provided together within a unitary structure that forms the dual core waveguide  100 . The cores  102 ,  104  are each surrounded by cladding  106  with an appropriate refractive index relative to a refractive index of the cores  102 ,  104  such that light propagates through the cores  102 ,  104 . Light introduced into each of the cores  102 ,  104  propagates separately through the cores  102 ,  104  along a length of the waveguide  100  since the cores  102 ,  104  run parallel to one another and are spaced from one another by a distance (d) to prevent interference between the cores  102 ,  104 . Preferably, the cores  102 ,  104  once surrounded by the cladding  106  yield suitable single mode optical pathways. 
       FIG. 2  illustrates a schematic top view of the dual core waveguide  100  shown optionally optically coupled with first and second input optical fibers  206 ,  208  and first and second output optical fibers  210 ,  212 . The first core  102  includes a first Bragg grating  202  that is disposed adjacent to a second Bragg grating  204  imprinted in the second core  104 . While the first and second Bragg gratings  202 ,  204  can be exactly the same, the gratings  202 ,  204  react differently to temperature and strain depending on which one of the cores  102 ,  104  that the grating is in due to differences in the cores. 
     The first core  102  differs from the second core  104  in at least one characteristic. As discussed herein in further detail, this difference in the cores  102 ,  104  provides similar responses to strain and dissimilar responses to temperature in each core as light is transmitted through each core and across the Bragg gratings  202 ,  204 , which reflect light to provide the responses. The characteristic can be determined by one or more of a material composition, geometry or a fabrication technique of the cores  102 ,  104  and/or cladding  106 . For example, the geometry of the first core  102  can provide a more elliptical cross sectional shape than the second core  104  that is relatively more rounded in shape. Modifying intrinsic strain fixed in the cores  102 ,  104  during a manufacturing process of the waveguide  100  can, for some embodiments, be utilized to provide different characteristics between the cores since the first core  102  can have a first strain frozen in that is different from a second strain frozen in the second core  104 . 
     For some embodiments, doping of the first core  102  with different elements and/or compounds than what the second core  104  is doped with provides the cores with different characteristics as desired. A change in wavelength of light reflected by a Bragg grating due to a change in temperature can range from, for example, 10.2 picometer per degree Celsius to 12.9 picometer per degree Celsius depending on which of various differently doped cores that the grating is imprinted. The first core  102  can be heavy germanium (Ge) doped while the second core can be heavy boron (B) doped with heavy doping meaning greater than 10.0 mol % and up to about 30.0 mol % percent of a doping element. The first core  102  can therefore have a germanium concentration of about 25.0 mol % and no boron. Concentration of boron and germanium in the second core  104  can be about 25.0 mol % and about 10.0 mol %, respectively. With these doping concentrations for the first and second cores  102 ,  104 , reflected light form both the Bragg gratings  202 ,  204  changes wavelength substantially the same amount relative to the common strain applied to both the gratings. However, the first Bragg grating  102  provides a change in wavelength of about 11.99 picometer per degree Celsius while the second Bragg grating provides a change in wavelength of about 10.79 picometer per degree Celsius. 
     Therefore, the wavelength change attributed to strain can be compensated for in order to provide an accurate temperature measurement by analyzing the signals received from each of the Bragg gratings  202 ,  204 . In other words, any difference in the wavelength change of the response from the first Bragging grating  202  relative to the wavelength change of the response from the second Bragging grating  204  is directly attributed to temperature since only the temperature responses between the Bragg gratings  202 ,  204  differ. Accordingly, the wavelength responses can be analyzed to determine the temperature at the Bragg gratings  202 ,  204  that form a sensor even when the sensor is strained and the strain is dynamic. Even if the strain response is different between the Bragg gratings  202 ,  204 , the temperature measurement can still be compensated for strain applied to the sensor by calibrating the sensor and calculating an accurate temperature measurement since there are two responses and only two unknowns (i.e., temperature and stress). 
     Referring to  FIG. 1 , a height (h) of the waveguide  100  is greater than a width (w) thereof. The cores  102 ,  104  are disposed in a spaced relationship next to one another along the width and within about the same planar area defined across the width so that the cores  102 ,  104  are at about the same position along the height of the waveguide  100 . Thus, the waveguide  100 , which is a glass structure, defines a ribbon-like element having a generally rectangular cross section with a preferred axis of bending parallel to its width and not its height. These dimensions and structural configuration aid in ensuring that the cores  102 ,  104  at the Bragg gratings  202 ,  204  experience the same or substantially similar stress from bending of the waveguide  100 . By inhibiting bending along an axis parallel to the height of the waveguide  100  and favoring bending in a manner that does not require any more bending of one of the cores  102 ,  104  relative to the other, the cores  102 ,  104  tend to bend the same amount creating similar strain states in both the cores  102 ,  104 . 
     For some embodiments, the height is approximately 80.0 microns while the width is approximately 200.0 microns. However, precise dimensions are less relevant than generally preserving a ratio of height to width (h/w) since this ratio should provide a preferred bending axis for the waveguide  100 . The distance (d) between the cores  102 ,  104  as previously discussed prevents cross interference and is therefore typically greater than about 10.0 microns or greater than about 15.0 microns. Additionally, the distance separating the cores  102 ,  104  can depend on acceptable error of the measurements since larger distances make it more likely that one of the cores experiences a different sensing environment such as a different temperature. Any dimension of the distance between the cores  102 ,  104  less than 1.0 millimeter creates small to no difference in temperature between the core in most applications. The distance between the cores  102 ,  104  also enables attachment of the fibers  206 - 212  (shown in  FIG. 2 ) due to there being working space between the cores. 
     The input optical fibers  206 ,  208  are fused to one end of the dual core waveguide  100  such that light traveling through the first input optical fiber  206  aligns with the first core  102  and light traveling through the second input optical fiber  208  aligns with the second core  104 . Similarly, the output optical fibers  210 ,  212  are fused to the other end of the dual core waveguide  100  such that light exiting through the first core  102  aligns with the first output optical fiber  210  and light exiting through the second core  104  aligns with the second output optical fiber  212 . For some embodiments, the response from the Bragg gratings  202 ,  204  are at different wavelengths such that it is possible to couple the input optical fibers  206 ,  208  along a single pathway or fiber. The optical fibers  206 - 212  can be part of a standard optical cable and can further be optional in situations where the waveguide  100  also provides interconnecting optical pathways. As further described next, the input optical fibers  206 ,  208  transmit light from a broadband light source to the gratings  202 ,  204  and return reflections forming the response from the gratings  202 ,  204  to detection and analysis equipment. Additionally, the output optical fibers  210 ,  212  can connect as inputs to subsequent sensors multiplexed with the waveguide  100 . 
       FIG. 3  shows a partial section view of a wellbore  300  having first and second temperature sensors  308 ,  310 . Each of the sensors  308 ,  310  utilizes a dual core waveguide according to embodiments of the invention to enable multiple discrete temperature measurements at various points in the wellbore  300 . As shown, clamps secure the sensors  308 ,  310  to a conveyance member such as production tubing  302  for lowering the sensors into the wellbore  300 . The sensors are optically coupled via an optical cable  306  to a broadband light source  304  and signal detection and processing equipment  305 . 
     In operation, light from the broadband light  304  is introduced into cable  306  and hence into the first temperature sensor  308  where at least two Bragg gratings disposed in different separate cores reflect portions of the light to respectively provide first and second signals. These signals are indicative of temperature at the first temperature sensor  308  and are received by the signal detection and processing equipment  305 . Next, analyzing the signals with the signal detection and processing equipment  305  determines a temperature measurement that is compensated for strain at the sensor based on similar strain effects on both the first and second signals. Temperature at another discrete point in the wellbore  300  can be measured in a corresponding manner with the second temperature sensor  310  utilizing remaining light passed through the first temperature sensor  308 . 
     Compensation for strain in temperature measurements according to embodiments of the invention utilizes different characteristics of two or more cores so that the responses to at least one of strain and temperature from one grating in one of the cores is dissimilar from corresponding responses to at least one of strain and temperature from another grating in another one of the cores. While only two cores  102 ,  104  have been shown within the waveguide  100 , additional cores can be incorporated in the waveguide  100 . One or more of these additional cores can have a Bragg grating therein and can have a different characteristic yet to further improve accuracy and resolution in the measurements described heretofore. Further, other reflective structures than a Bragg grating and/or other optical sensor arrangements can be implemented and benefit from the foregoing description of the embodiments described above. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.