Patent Publication Number: US-2007108378-A1

Title: High pressure optical cell for a downhole optical fluid analyzer

Description:
FIELD OF THE INVENTION  
      The present invention relates generally to subterranean formation evaluation and testing in the exploration and development of hydrocarbon-producing wells, such as oil or gas wells. More particularly, the invention relates to methods and apparatuses for producing high pressure optical cells for a downhole optical fluid analyzer used to analyze fluids produced in such wells.  
     BACKGROUND OF THE INVENTION  
      In order to evaluate the nature of underground formations surrounding a borehole, it is often desirable to obtain and analyze samples of formation fluids from various specific locations in the borehole. Over the years, various tools and procedures have been developed to facilitate this formation fluid evaluation process. Examples of such tools can be found in U.S. Pat. No. 6,476,384 (“the &#39;384 patent”), the entirety of which is hereby incorporated by reference.  
      As described in the &#39;384 patent, Schlumberger&#39;s repeat formation tester (RFT) and modular formation dynamics tester (MDT) tools are specific examples of sampling tools. In particular, the MDT tool includes a fluid analysis module for analyzing fluids sampled by the tool.  FIG. 1  illustrates a schematic diagram of such a downhole tool  10  for testing earth formations and analyzing the composition of fluids from the formation. Downhole tool  10  is suspended in a borehole  12  from a logging cable  15  that is connected in a conventional fashion to a surface system  18 . Surface system  18  incorporates appropriate electronics and processing systems for control of downhole tool  10  and analysis of signals received from downhole tool  10 .  
      Downhole tool  10  includes an elongated body  19 , which encloses a downhole portion of a tool control system  16 . Elongated body  19  also carries a selectively-extendible fluid admitting/withdrawal assembly  20  (shown and described, for example, in U.S. Pat. Nos. 3,780,575, 3,859,851, and 4,860,581, each of which is incorporated herein by reference) and a selectively-extendible anchoring member  21 . Fluid admitting/withdrawal assembly  20  and anchoring member  21  are respectively arranged on opposite sides of elongated body  19 . Fluid admitting/withdrawal assembly  20  is equipped for selectively sealing off or isolating portions of the wall of borehole  12 , such that pressure or fluid communication with the adjacent earth formation is established. A fluid analysis module  25  is also included within elongated body  19 , through which the obtained fluid flows. The obtained fluid may then be expelled through a port (not shown) back into borehole  12 , or sent to one or more sample chambers  22 ,  23  for recovery at the surface. Control of fluid admitting/withdrawal assembly  20 , fluid analysis module  25 , and the flow path to sample chambers  22 ,  23  is maintained by electrical control systems  16 ,  18 .  
      Over the years, various fluid analysis modules have been developed for use in connection with sampling tools, such as the MDT tool, in order to identify and characterize the samples of formation fluids drawn by the sampling tool. For example, U.S. Pat. No. 4,994,671 (incorporated herein by reference) describes an exemplary fluid analysis module that includes a testing chamber, a light source, a spectral detector, a database, and a processor. Fluids drawn from the formation into the testing chamber by a fluid admitting assembly are analyzed by directing light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information (based on information in the database relating to different spectra) in order to characterize the formation fluids. U.S. Pat. Nos. 5,167,149 and 5,201,220 (both of which are incorporated by reference herein) also describe reflecting light from a window/fluid flow interface at certain specific angles to determine the presence of gas in the fluid flow. In addition, as described in U.S. Pat. No. 5,331,156, by taking optical density (OD) measurements of the fluid stream at certain predetermined energies, oil and water fractions of a two-phase fluid stream may be quantified. As the techniques for measuring and characterizing formation fluids have become more advanced, the demand for more precise formation fluid analysis tools has increased.  
      As known in the art, the optical hardware employed in conventional fluid analysis modules may be adversely affected by the high pressures experienced in downhole environments. For example, optical windows interfacing with produced fluids are not capable of sealing against extremely high pressures. Consequently, fluids produced in some deep wells cannot be optically analyzed downhole. The electronics associated with optical fluid analysis must be fluidly isolated from the downhole conditions, and current windows are not capable of withstanding the high pressures found in certain wells.  
      Accordingly, there exists a need for an apparatus and method allowing optical fluid analysis in high pressure subterranean environments. More particularly, there is a need for high pressure optical cells capable of withstanding pressures up to 30 kpsi and more.  
     SUMMARY OF THE INVENTION  
      The present invention provides a number of embodiments directed towards improving, or at least reducing, the effects of one or more of the above-identified problems. According to at least one embodiment, an apparatus for analyzing subterranean formation fluids comprising a downhole tool, a fluid analysis module disposed in the downhole tool, a formation fluid flow path through the fluid analysis module, first and second cavities disposed in the fluid analysis module, and first and second windows disposed in the first and second cavities of the fluid analysis module, respectively. The first and second windows each comprises a polished external sealing surface. In some embodiments, the polished external sealing surface comprises a specular polish such as a 0.15 a specular polish.  
      In certain embodiments, there is an O-ring seal and a backup seal disposed in an annulus between the cavities and windows. The backup seal may be a PEEK backup ring disposed in the cavities adjacent to each of the first and second windows. The first and second O-rings may be disposed around the polished external sealing surface of the first and second windows, respectively. The first and second windows each cooperate with their respective O-ring seals to hold pressures of 30 kpsi or more.  
      According to some embodiments, the windows comprise sapphire cylinders. In addition, some embodiments include first and second flanges enclosing the first and second windows, respectively. The first flange may comprise an input channel receptive of a first optical communication fiber, and the second flange may comprise an output channel receptive of a second optical communication fiber.  
      Some embodiments of the apparatus comprise a first internal flowline insert disposed in the formation fluid flow path. The first internal flowline insert holds the first and second windows, and the first internal flowline insert comprises a fluid channel interfacing the first and second windows.  
      Certain embodiments of the apparatus include a third window disposed in a third cavity spaced axially from the first and second cavities. The third window comprises an angular prism for gas detection. The third window includes a polished external sealing surface. The polished external sealing surface of the third window may comprise a specular polish such as a 0.15a specular polish. The apparatus may further comprise an O-ring and a PEEK back up seal ring disposed around the third window. The third window cooperates with the O-ring and PEEK back up seal ring to hold at least 30 kpsi. The apparatus may further comprise a second internal flowline insert disposed in the formation fluid flow path adjacent to the third window. The second internal flowline insert may comprise a generally V-shaped flow groove open toward the third window.  
      One embodiment of the apparatus includes a gas detector, the gas detector comprising the third window and the angular prism, an LED and lens adjacent to the angular prism, a monitor photodiode, and a detector array for detecting light from the LED reflected at an interface between the third window and fluids flowing through the second internal flowline. A fiber array plate may interface between the detector array and the angular prism.  
      In certain embodiments, the third window comprises a generally elongated circle portion adjacent to the angular prism portion. A third flange may enclose the third window.  
      Another embodiment provides an apparatus for analyzing subterranean formation fluids as well. The apparatus comprises a downhole tool, a fluid analysis module disposed in the downhole tool, the fluid analysis module comprising an optical cell spectrometer and a gas detection cell. The optical cell spectrometer comprises a formation fluid flow path through the fluid analysis module, first and second cavities disposed in the fluid analysis module, and first and second windows disposed in the first and second cavities of the fluid analysis module, respectively. The first and second windows each comprise a polished external sealing surface. The gas detection cell comprises a third window disposed in a third cavity spaced axially from the first and second cavities. The third window comprises an angular prism for gas detection. The third window also comprises a polished external sealing surface.  
      According to some embodiments, the polished external sealing surfaces of the first, second, and third windows comprise approximately a 0.15a specular polish. Further, the apparatus may include an O-ring seal and a PEEK backup seal disposed in the cavities adjacent to each of the first, second, and third windows. The O-ring seals and the PEEK backup seals of each of the first, second, and third windows are capable of isolating 30 kpsi of pressure.  
      Another aspect of the invention provides a method of making an apparatus for analyzing subterranean formation fluids. The method comprises providing a downhole tool, providing a fluid analysis module with a plurality of window cavities, polishing a plurality of windows to a specular polish, inserting the plurality of windows into the window cavities, and sealing the plurality of windows in the window cavities. Polishing may comprise polishing to a 0.15a specular polish. Sealing may comprise providing an O-ring for each of the plurality of windows, inserting the O-ring between each of the plurality of windows and each of the plurality of window cavities, and inserting a backup PEEK ring between each of the plurality of windows and each of the plurality of window cavities.  
      Features from any of the above-mentioned embodiments may be used in combination with one another in accordance with the present invention. These and other embodiments, features and advantages will be more fully understood upon reading the following detailed description in conjunction with the accompanying drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings illustrate exemplary embodiments of the present invention and are a part of the specification. Together with the following description, the drawings demonstrate and explain the principles of the present invention.  
       FIG. 1  illustrates an exemplary downhole tool in which a fluid analysis cell according to principles of the present invention may be implemented.  
       FIG. 2  is an assembly diagram of an exemplary fluid analysis module for analyzing extracted samples of formation fluids according to one embodiment of the present invention.  
       FIG. 3  is a cross sectional view of a portion of the fluid analysis module of  FIG. 2  illustrating the optical cell spectrometer.  
       FIG. 4A  is a perspective view of an unpolished fluid analysis window.  
       FIG. 4B  is a perspective view of a polished fluid analysis window according to one embodiment of the present invention.  
       FIG. 5A  is a perspective view of an unpolished gas cell window.  
       FIG. 5B  is a perspective view of a polished gas cell window according to one embodiment of the present invention.  
       FIG. 6  is a side cross-sectional view of the gas cell of  FIG. 2  according to one embodiment of the present invention.  
       FIG. 7  is a top view of the fluid analysis module of  FIG. 2  without the flanges in place.  
       FIG. 8  is a top view of a gas detection cell of the fluid analysis module of  FIG. 2  without the flange in place.  
       FIG. 9  is a cross-sectional view, taken along line  9 - 9  of  FIG. 7 , of the fluid analysis module.  
       FIG. 10  is a cross-sectional view, taken along line  10 - 10  of  FIG. 7 , of the fluid analysis module.  
       FIG. 11  is a side view of the fluid analysis module of  FIG. 2  with flanges in place over optical windows.  
       FIG. 12  is a top view of the fluid analysis module of  FIG. 2  with flanges in place over optical windows. 
    
    
      Throughout the drawings, identical reference characters and descriptions indicate similar, but not necessarily identical elements. While the present invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, one of skill in the art will understand that the present invention is not intended to be limited to the particular forms disclosed. Rather, the invention covers all modifications, equivalents and alternatives falling within the scope of the invention as defined by the appended claims.  
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      Illustrative embodiments and aspects are described below. One of ordinary skill in the art having the benefit of this disclosure will appreciate that in the development of any such embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Although such a development effort might be complex and time-consuming, the same would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
       FIG. 2  is a partial assembly diagram of an exemplary fluid analysis module  100  for analyzing extracted samples of formation fluids. As will be appreciated by those of skill in the art, exemplary fluid analysis module  100  may be adapted for use in a variety of environments and/or included in a number of different tools. For example, fluid analysis module  100  may form a portion of a fluid analysis module  25  housed in downhole tool  10 , as illustrated in  FIG. 1 . According to at least one embodiment, exemplary fluid analysis module  100  comprises a formation fluid flow path  102  ( FIG. 3 ) housing an extracted formation fluid sample  104  ( FIG. 3 ). Formation fluid sample  104  ( FIG. 3 ) may be extracted, withdrawn, or admitted into flowline  102  ( FIG. 3 ) in any number of ways known to those of skill in the art. For example, sample  104  ( FIG. 3 ) may be admitted into flowline  102  ( FIG. 3 ) by a fluid admitting/withdrawal assembly, such as fluid admitting/withdrawal assembly  20  illustrated in  FIG. 1 . As detailed above, fluid admitting/withdrawal assembly  20  may admit fluid samples by selectively sealing off or isolating portions of the wall of a borehole  12  ( FIG. 1 ).  
      In certain embodiments, fluid analysis module  100  comprises an optical cell spectrometer section  106  and a gas detection section  108 . The optical cell spectrometer section  106  is generally used for liquids analysis, and the gas detection section  108  is generally used to detect gas. The optical cell spectrometer section  106  includes a first cavity  110  and a second cavity  112  arranged opposite of the first cavity  110 . The second cavity  112  may be coaxial and contiguous with the first cavity  110 , and therefore the first and second cavities  110 ,  112  may comprise a single cavity through the optical cell spectrometer section  106  as shown in  FIG. 2 .  
      Each of the first and second cavities  110 ,  112  may be receptive of a window. For example, a first window  114  may be disposed in the first cavity  110 , and a second window  116  may be disposed in the second cavity  112 . The first and second windows  114 ,  116  may be substantially identical, and each may comprise a cylinder of optical grade sapphire or other optical grade material.  
      As mentioned in the background, windows in typical optical fluid analyzers are not capable of withstanding high pressures associated with some wells. In fact a standard window in a downhole optical fluid analyzer can withstand no more than 22 Kpsi. However, according one embodiment of the present invention, the first and second windows  114 ,  116  are polished and sealed within the cavities  110 ,  112 , and are capable of isolating pressure differences of 30 to 33 kpsi or more.  
       FIG. 4A  illustrates the first window  114  with an unpolished external sealing surface  118 . The unpolished sealing surface  118  of  FIG. 4A  may be incapable of cooperating with a seal to isolate pressure differences of 30 to 33 kpsi. However, as shown in  FIG. 4B , the external sealing surface  118  of the first window  114  (and likewise the second window  116 ) is polished to a specular polish. For example, the external sealing surface  118  may comprise a 0.15a specular polish.  
      Returning to  FIG. 2 , the external sealing surface  118  ( FIG. 4B ) of the first and second windows  114 ,  116  may cooperate with one or more seals to facilitate pressure isolations of 30 to 33 Kpsi or more. For example, a first O-ring  120  may be disposed in an annulus  122  ( FIG. 3 ) between the first cavity  110  and the first window  114 . In addition to the first O-ring  120 , the apparatus may include a first back up seal  124  in the annulus  122  ( FIG. 3 ) between the first cavity  110  and the first window  114 . The first back up seal  124  may comprise PEEK (polyetheretherkeytone), which resists deformation, even at very high pressures (including pressures of at least 30 kpsi).  
      Similarly, a second O-ring  126  may be disposed in an annulus  128  ( FIG. 3 ) between the second cavity  112  and the second window  116 . Again, in addition to the second O-ring  126 , the apparatus may include a second back up seal  130  in the annulus  128  ( FIG. 3 ) between the second cavity  112  and the second window  116 . The second back up seal  130  also comprises PEEK (polyetheretherkeytone).  
      According to some embodiments, the first and second windows  114 ,  116  fit at least partially in a shell  132 . The shell  132  slides in between the first and second cavities  110 ,  112 , and may include a first internal flowline insert  134 . The first internal flowline insert  134  reduces the flowthrough diameter of the flowline  102  ( FIG. 3 ), and interfaces with each of the first and second windows  114 ,  116 , presenting the sample  104  ( FIG. 3 ) to the windows  114 ,  116  and allowing the passage of light through the windows  114 ,  116 . The first internal flowline insert  134  is shown more clearly in cross-section in  FIG. 10 , which is described in more detail below.  
      As shown in  FIG. 3 , first and second flanges  136 ,  138  enclose the shell  132  and the first and second windows  114 ,  116  within the first and second cavities  110 ,  112 . Mating first and second recesses  140 ,  142  ( FIG. 2 ) in the optical cell spectrometer section  106  receive the first and second flanges  136 ,  138 . A plurality of bolts, for example four bolts  144 , may thread into mating threaded recesses  146  ( FIG. 2 ) and attach the first and second flanges  136 ,  138  to the optical cell spectrometer section  106 . The first and second windows  114 ,  116  may be flush with or recessed in the first and second cavities  110 ,  112 , respectively, to maintain a gap between the first and second flanges  136 ,  138  and the respective windows  114 ,  116 . Therefore, no matter how tightly the first and second flanges  136 ,  138  are fit to the optical cell spectrometer section  106 , there is little or no mechanical pressure exerted on the windows  114 ,  116  by the flanges  136 ,  138 .  
      The first flange  136  comprises an input channel  148  extending therethrough. The input channel  148  is receptive of a first optical communication fiber or fiber bundle  150 . The input channel  148  may curve approximately ninety degrees and lead the first optical communication fiber  150  to a normal orientation with respect to the first window  114 . Accordingly, the first optical communication fiber  150  may present a light source to the first window  114 , and the first window may pass the light through the sample  104 .  
      The second flange  138  comprises an output channel  152  extending therethrough. The output channel  152  is receptive of a second optical communication fiber or fiber bundle  154 . The output channel  152  may curve approximately ninety degrees and lead the second optical communication fiber  154  to a normal orientation with respect to the second window  116 . Accordingly, the second optical communication fiber  154  may collect light passing through the sample  104  and through the second window  116 , and present the collected light to a spectrometer for analysis.  
      Light passed through the sample  104  via the first and second windows  114 ,  116  is primarily analyzed for liquid components. However, as shown in  FIG. 2 , the fluid analysis module  100  also includes the gas detection section  108 . The gas detection section  108  comprises a third cavity  156 . The third cavity  156  is receptive of another window. For example, a third window  158  may be disposed in the third cavity  156 . The third windows  158  may comprise a generally elongated cylinder or circle  160  adjacent to an angular prism  162 . The elongated cylinder  160  and the angular prism  162  may comprise a unitary piece of optical grade sapphire or other optical grade material. According to one embodiment of the present invention, the third window  158  is polished and sealed within the third cavity  156  and is capable of isolating pressure differences of 30 to 33 kpsi or more.  
       FIG. 5A  illustrates the third window  158  with an unpolished external sealing surface  164 . The unpolished sealing surface  164  of  FIG. 5A  may be incapable of cooperating with a seal to isolate pressure differences of 30 to 33 kpsi. However, as shown in  FIG. 5B , the external sealing surface  164  of the third window  158  is polished to a specular polish. For example, the external sealing surface  164  may comprise a 0.15a specular polish.  
      Returning to  FIG. 2 , the external sealing surface  164  ( FIG. 5B ) of the third window  158  may cooperate with one or more seals to facilitate pressure isolations of 30 to 33 Kpsi or more. For example, a third (elongated) O-ring  166  may be disposed in an annulus  168  ( FIG. 6 ) between the third cavity  156  and the third window  158 . Further, in addition to the third O-ring  166 , the apparatus may include a third back up seal  170  in the annulus  168  ( FIG. 6 ) between the third cavity  156  and the third window  158 . The third back up seal  170  may comprise PEEK.  
      Referring to  FIGS. 2 and 6 , the third window  158  is arranged adjacent to a second internal flowline insert  172 . The second internal flowline insert  172  reduces the flowthrough diameter of the flowline  102  and presents the sample  104  to the third window  158 . The second internal flowline insert  172  is shown more clearly in cross-section in  FIG. 9 , which is described in more detail below. In addition, a pair of third window supports  174  may fit inside the third cavity  156  in between the second internal flowline insert  172  and third window  158  (see  FIG. 9 ).  
      As shown in  FIG. 6 , a third flange  176  encloses the third window  158  within the third cavity  156  ( FIG. 2 ). A mating third recess  180  ( FIG. 2 ) in the gas detection section  108  receives the third flange  176 . One or more pins  182  ( FIG. 2 ) may ensure proper alignment of the third flange  176  with respect to the mating third recess  180  ( FIG. 2 ). A plurality of bolts  184  may thread into mating threaded recesses  186  ( FIG. 2 ) and attach the third flange  176  to the gas detection section  108 .  
      The third flange  176  interfaces the third window  158  and may house a number of gas detection components known to those of ordinary skill in the art having the benefit of this disclosure. For example, as shown in  FIG. 6 , the gas detector structure may include a light source such as an LED  188  and a lens  190  adjacent to one surface of the angular prism  162 . A polarizer  192  may be arranged between the LED  188  and the lens  190 . A reflector  194 , which is also arranged adjacent to the prism  162 , may reflect a portion of the light emitted by the LED  188  to a reference or monitor photodiode  196 . Light emitted by the LED  188  may also pass through the angular prism  162  and the elongated cylinder  160 , where it tends to be reflected at a gas  198 /third window  158  interface (if gas is present at the interface) and detected by a detector array  200 . If the interface is adjacent to liquids, the angle of the angular prism  162  is such that the light tends to refract through the sample. A fiber array plate  202  may direct light reflected at the gas  198 /third window  158  interface.  
      Referring next to  FIGS. 7-10 , the fluid analysis module  100  is shown without the flanges  136 ,  138 ,  176  ( FIGS. 2 and 6 ) in a side ( FIG. 7 ) view and a top view ( FIG. 8 , representing the gas detector section  108 ). Cross-sections along lines  9 - 9  and  10 - 10  of  FIG. 7  illustrate the second and first internal flowline inserts  172 ,  134 , respectively. As shown in  FIG. 9 , the second internal flowline insert  172  comprises a generally V-shaped channel or groove  204  open to the third window  158 .  
      Similarly, as shown in  FIG. 10 , the first internal flowline insert  134  defines a sample path  206  that is generally rectangular and open to both of the first and second windows  114 ,  116 . Therefore, light may be transmitted through the first window  114 , through the sample contained by the sample path  206 , and through the second window  116 . Information related to the light transmitted through the sample is then relayed along the second optical communication fiber or fiber bundle  154  for processing and/or analysis.  
       FIGS. 11-12  illustrate the fluid analysis module  100  from a side and top view, respectively, with the first, second, and third flanges  136 ,  138 ,  176  installed. The fluid analysis module  100  is fully assembled and ready for use. Moreover, the flanges cover the first, second, and third windows  114 ,  116 ,  158  ( FIG. 2 ), which are arranged with seals sufficient to isolate the sample fluid  104  ( FIG. 3 ) from any sensitive components at pressures of up to 30-33 kpsi or more.  
      The preceding description has been presented only to illustrate and describe the invention and some examples of its implementation. This exemplary description is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. For example, one of ordinary skill in the art will appreciate that the principles, methods and apparatuses disclosed herein are applicable to many oilfield operations, including MWD, LWD, and wireline operations.  
      As used throughout the specification and claims, the terms “borehole” or “downhole” refer to a subterranean environment, particularly in a borehole. The words “including” and “having,” as used in the specification and claims, have the same meaning as the word “comprising.” The preceding description is also intended to enable others skilled in the art to best utilize the invention in various embodiments and aspects and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.