Abstract:
Evanescent waveguide sensors for measuring and determining the composition of wellbore fluids in situ are provided. The waveguides are provided on substrates have a thickness and strength sufficient to withstand wellbore pressures and a sufficient surface area to allow for broad range measuring of the wellbore characteristics. The optical sensors facilitate determination of the wellbore fluid composition without requiring in-tool sampling of the wellbore fluid.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to downhole wellbore logging devices and methods, and more particularly, to fiber optic sensors for measuring wellbore fluid characteristics in situ. 
       BACKGROUND 
       [0002]    In wellbore operations it is often desirable to identify characteristics, such as the composition, of the wellbore fluid at identifiable locations in the wellbore. For example, in some circumstances it may be desirable to identify at what location in the wellbore that water, gas, or oil is entering from the surrounding formation. It may further be a desire to identify the presence of hydrogen sulfide or carbon dioxide. 
         [0003]    It is a desire of the present invention to provide methods and apparatus for determining the composition and/or characteristics of wellbore fluids in the wellbore. It is a still further desire to provide optical fiber sensors for determining wellbore fluid characteristics. It is a still further desire to provide optical sensors and methods for determining the chemical composition of a wellbore fluid, in the wellbore, without requiring in-tool fluid sampling. 
       SUMMARY OF THE INVENTION 
       [0004]    In view of the foregoing and other considerations, the present invention relates to determining the composition of wellbore fluids using optical sensors. 
         [0005]    In an example of an optical apparatus for investigating wellbore fluids of the present invention, the apparatus includes an optical sensor having a waveguide formed on the surface of a substrate; a portion of the waveguide is open to the surrounding environment to emit an evanescent sensing field therein. The substrate has a thickness sufficient to withstand the pressures encountered in wellbores. More than one waveguide, or discreet sensor may be formed on the substrate. 
         [0006]    An optical system includes one or more optical sensors carried by a wellbore tool. The tool may be a logging tool or drilling tool. The tool may carry one or more optical sensors. Multiple sensors may be formed on a single substrate. Desirably the optical sensor system of the present invention provides an economical, rugged optical sensor system that can determine the composition of the wellbore fluid, as well as other characteristics, without in-tool sampling of the fluid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing and other features and aspects of the present invention will be best understood with reference to the following detailed description of a specific embodiment of the invention, when read in conjunction with the accompanying drawings, wherein: 
           [0008]      FIG. 1  is a wellbore schematic wherein an optical sensor system of the present invention is deployed; 
           [0009]      FIG. 2  is a side view of an example of a reflectance optical sensor of the present invention; and 
           [0010]      FIG. 3  is a plan view of a refractive index optical sensor of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
         [0012]    As used herein, the terms “up” and “down”; “upper” and “lower”; and other like terms indicating relative positions to a given point or element are utilized to more clearly describe some elements of the embodiments of the invention. Commonly, these terms relate to a reference point as the surface from which drilling operations are initiated as being the top point and the total depth of the well being the lowest point. 
         [0013]    Referring now to  FIG. 1 , an optical borehole composition sensor system of the present invention, generally denoted by the numeral  10 , is illustrated. A tool  12  is suspended in a well  14  or borehole by a conveyance  16 . In the illustrated example, tool  12  is a production logging tool and conveyance  16  is a cable. Cable  16  may include conductors (not shown), which may be electrical and/or optical, for communicating with data processing equipment  18 . Tool  12  may be utilized with or incorporated into a logging or measurement while drilling tool and conveyed by tubing or drill pipe. 
         [0014]    Tool  12  includes one or more optical sensors  20  for detecting and measuring various characteristics of the contents of the wellbore such as, but not limited to, water cut of the wellbore fluid  26 , gas holdup, oil composition, carbon dioxide content, hydrogen sulfide detection, temperature and pressure. An optical (light) source  23  and detection equipment  25  are in operational connection with the one or more optical sensors  20 . The light source  23  and detection equipment  25  may be located in various locations such as the surface  22  or in the electronics housing  24  of tool  12 . 
         [0015]    Tool  12  includes one or more evanescent waveguide sensors  20  that are operational in the harsh environment of wellbores. Evanescent sensors  20  include reflectance, transmission sensors  20   a  ( FIG. 2 ) and refractive index sensors  20   b  ( FIG. 3 ). Note that sensors  20  are optical fiber sensors and therefore are very small in relation to tool  12 . As noted with reference to  FIG. 1 , one or more sensors  20  may be carried by tool  12 . If tool  12  includes a plurality of sensors  20 , the sensors may be positioned proximate to one another or scattered along tool  12 . It should further be recognized that sensors  20  may be positioned in wellbore  14  in various manners including the logging tool example described herein. 
         [0016]    Sensors  20  include waveguides that are fabricated on the surface of a substrate  28  ( FIGS. 2 ,  3 ). The waveguides may be fabricated using in-diffusion or ion implantation and both processes may be based on reproducible and standard photo-lithographical/planar processing. These methods facilitate the accurate definition of both the waveguide path and the size of the windows  32  ( FIGS. 2 ,  3 ) to specify the properties of the sensor. 
         [0017]    Various materials such as sapphire, silicon, silica and diamond may be utilized for the substrate. Sapphire provides benefits that are desirable for utilization in wellbores. Some of the benefits of sapphire crystals include low cost and availability in relatively large sizes. Size of the substrate is important for various reasons. For example, the thickness of substrate  28  must be sufficient to withstand the pressures, in particular the pressure differential across substrate  28 , encountered in wellbores. Sapphire substrates can readily be obtained in thicknesses of 6 mm, providing suitable strength for many wellbore applications. Although substrate  28  is illustrated and described in terms of planar configurations, cylindrical or hemispherical sapphire substrates  28  may be utilize in particular for high pressure and/or high temperature wells. Additionally, sapphire substrates  28  are available with large surface areas (for example, diameters of 25 mm) providing for the placement of a number of discreet sensors on each substrate. 
         [0018]    Refer now to  FIG. 2  wherein a side view of a reflectance sensor  20   a  is illustrated. A sapphire substrate  28  is provided in connection with an optic fiber  30 . A window  32  is formed through the covering or overclad  34  exposing a portion of the surface  36  of substrate  28  on which the waveguide ( 40 ,  FIG. 3 ) is formed. Window  32  exposes fluid  26  directly to the evanescent field and forming a sensing field  38 . 
         [0019]    Sensor  20   a  measures the spectral content of the waveguide transmission. Fluid  26  in contact with the evanescent field of the waveguide will have a characteristic absorption fingerprint corresponding to molecular absorption bands that can be used to identify the constituent components of the fluid  26 . The molecular absorption bands and/or the constituent components of fluid  26  are communicated via display or the like to an operator. 
         [0020]    It is noted that the transmission should be interrogated over a relevant spectral band, for example by sweeping the wavelength of the illuminating light source and measuring the intensity on detector  25  ( FIG. 1 ). Tool  12  may include sensors  20   a  having different lengths, from microns to millimeters, of windows  32  to cover a desired range of transmission loss and sensitivity. 
         [0021]    Refer now to  FIG. 3 , wherein an example of a refractive index sensor  20   b  is illustrated. Sensor  20   b  is illustrated as a Mach-Zehnder interferometer. Waveguide  40  includes a first branch  42  and a second branch  44  etched on the surface of substrate  28 . Light travels along waveguide  40  in the direction indicated by the arrow. As indicated in  FIG. 2 , the majority of waveguide  40  is covered with overclad  36  so that the evanescent field does not penetrate into the wellbore fluid. Second branch  44  of waveguide  40  includes a window  32  in the overclad or covering to expose waveguide  44  as illustrated in  FIG. 2 . 
         [0022]    The interferometric configuration of sensor  20   b  measures the difference in optical path length between two or more waveguides or waveguide branches. Waveguide  42  is immune to the effect of fluid on its surface, as it is covered with an optical protective layer. Second waveguide  44  has a window  32  of a specific chosen length so that the refractive index of the fluid  26  in contact with waveguide  44  alters the optical path length which is detected with the interferometric arrangement. The magnitude of the change in the optical path length can be used to assess the nature and composition of wellbore fluid  26  in contact with waveguide  44 . 
         [0023]    An example of operation is now described with reference to  FIGS. 1 through 3 . Light is emitted from LED source  23  and travels along optical fiber  30  protected from the downhole environment by cladding  34 . When the light arrives at window  32 , some of the light interacts with wellbore fluid  26 , and the remaining light is reflected and travels back through the optical fiber. The reflected light travels through a Y coupler to a receiving photodiode  25  and is converted into an electrical signal. The amount of reflection depends on the refractive index of fluid  26 . 
         [0024]    From the foregoing detailed description of specific embodiments of the invention, it should be apparent that a novel and unobvious system and method for downhole testing and determination of the composition and characteristics of a wellbore fluid has been disclosed. Although specific embodiments of the invention have been disclosed herein in some detail, this has been done solely for the purposes of describing various features and aspects of the invention, and is not intended to be limiting with respect to the scope of the invention. It is contemplated that various substitutions, alterations, and/or modifications, including but not limited to those implementation variations which may have been suggested herein, may be made to the disclosed embodiments without departing from the spirit and scope of the invention as defined by the appended claims which follow.