Abstract:
This invention provides a method and an apparatus for fiber optic tomographic analysis and imaging of fluids. This invention includes a method for providing information on downhole fluid flowing in a hydrocarbon well, utilizing at least one downhole tomograph chamber ( 10 ). Light is introduced into the tomograph chamber ( 10 ) by an optical fiber bundle ( 24 ), and portions of the light are collected in other optical fiber bundles ( 32, 34 ). The collected portions of light are conveyed through the optical fiber bundles ( 32, 34 ) to a surface system ( 14 ), where the light is detected to produce signals proportional to the portions of light to provide information on optical properties of downhole fluid flowing in the well. This invention allows the generation of two or three dimensional images of multiple phase flow in the wellbore and allows determination of production parameters of multiple zones on an individual zone basis.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to analyzing hydrocarbon fluids and, more specifically, to an apparatus and method of obtaining information relating to the composition of downhole fluids flowing in a hydrocarbon well using fiber optics. 
     BACKGROUND OF THE INVENTION 
     Hydrocarbon fluids, such as oil and gas, generally are produced by wells drilled into hydrocarbon producing zones. Often there are multiple hydrocarbon producing zones that are traversed by each well. Perforations in the casing at each of the producing zones allow the fluids to enter the borehole, where the fluids can be recovered. Generally, tubing is used within the casing, which tubing extends to near the bottom of the hole. Liquids generally enter the annulus between the tubing and the casing and flow to the surface up the interior of the tubing. 
     It can be seen that in conventional wells, when multiple zones produce fluids, the fluids from all the zones are mixed together and returned up the tubing to the surface. Thus, while information can be obtained regarding the total production and combined composition of fluid of the well after the fluid exits the tubing at the surface, little information is available about downhole fluid flow or composition in each of several production zones. As such, only the total quantities of gas, water and oil can be measured at the top showing the composite from all the producing zones, and give little or no information on what is being produced from any one of the zones. Also, it can be seen that such information does not give flow details, for example, on sizes of oil drops or gas bubbles. 
     Therefore, a need has arisen for an apparatus and method for obtaining information regarding the composition of downhole fluids produced from a well. A need has also arisen for such an apparatus and method that allows for the determination of the composition of downhole fluids from individual zones in a well having multiple zones producing into a single tubing string. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises an apparatus and method for obtaining information about the composition of downhole fluids produced into a wellbore by imaging the downhole fluids within the wellbore with optical tomography. Through the imaging process, quantitative values of oil, water and gas are obtained. A determination of the composition of downhole fluids from individual zones producing into a single well may also be obtained using the optical tomography apparatus and method of the present invention. 
     The tomograph of the present invention, generally utilizes a bundle optical of fibers that pass downhole and are arrayed with ends pointing into a chamber through which the well fluid flows. The optical fibers run from terminations in a downhole optical tomograph chamber, up the wellbore to a surface system. Light is transmitted down one fiber or a bundle of fibers and is introduced into the downhole tomograph chamber. The other optical fibers or bundles of fibers each pick up portions of the light from the optical tomograph chamber and conduct these light portions up to the surface, where the light portions are detected by a number of light detectors. Preferably, the surface system includes a light source that is sequentially introduced into a first bundle of the optical fibers and sent downhole using a commutation process that, in effect, rotates the light introducing fiber around the chamber. 
     Portions of light sent through the first bundle of optical fibers carry the light source that is then picked up by detecting optical fiber bundles after the light has traveled through the downhole fluid. The proportions of light picked up by the detecting optical fiber bundles are a function of the fluid flowing through the chamber. The resultant light portions may then can be analyzed to produce an image of fluid flowing in the well. The commutation is preferably fast enough that very little movement of the multi phase fluids occurs during a single revolution or scan of the system. 
     The tomograph can be used, for example, in combination with seal assemblies between the tubing and the casing when multiple producing zones are encountered. Thus, for example, when there are two zones, typically two sets of casing perforations allow fluid to enter the well and be mixed together. However, in one embodiment of this invention, a seal assembly below the upper zone is used to prevent fluid from the upper zone from mixing with that of the lower zone, and an opening is provided through the tubing above the seal assembly, such that fluid from the upper zone enters the tubing above an optical tomograph, and thus does not pass through the tomograph. In this manner, the tomograph can analyze the fluid being produced by the lower zone unmixed with fluid from the upper zone. The combined production can then be measured at the surface, or in another tomograph higher in the tubing with the production of the lower zone subtracted to obtain production information on the upper zone. 
     The present invention allows the generation of two or three dimensional images of multiple phase flow in oil wells. This invention also allows determination of the production of multiple zones on an individual basis. No down hole electrical power is required, as all such power to the system can be provided by the surface system. Only optical energy need be transmitted downhole, and thus the system is intrinsically safe and does not pose an environmental hazard. No moving parts are required downhole, any commutation of transmitted signal can be handled at the surface. 
     This invention provides a method and an apparatus for fiber optic tomographic analysis and imaging of fluids. This invention includes a method for providing information on downhole fluids flowing in a hydrocarbon well, utilizing at least one downhole tomograph chamber. Light is introduced into the chamber by an optical fiber bundle, and portions of the light are collected in second and third optical fiber bundles. The collected portions of light are conveyed through the optical fiber bundles to a surface system, where the light is detected to produce signals proportional to the portions of light to provide information on optical properties of downhole fluid flowing in the well. 
     The invention is also an apparatus for providing information on downhole fluid flowing in a hydrocarbon well. The apparatus utilizes at least three optical fiber bundles having lower terminations in a downhole tomograph chamber in the well, and having upper terminations in a surface system. The surface system contains a light source and at least two light detectors. A commutator in the surface system links the light source, preferably sequentially, to the light introducing optical fiber bundles, and links light detectors to the receiving optical fiber bundles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a schematic system overview; 
     FIGS. 2A and 2B are cross sectional views of a downhole optical tomography wherein the fluid is clear and the optical fibers do not have lenses; 
     FIG. 3 is a cross sectional view of a downhole optical tomograph wherein divergent lenses are utilized; 
     FIG. 4 is a cross sectional view of a downhole tomograph with the light source being commutated to an adjacent fiber; 
     FIG. 5 is a cross sectional view of a downhole tomograph with the divergent lenses illustrating the absorption, reflection and refraction that may occur in multi-phase fluids; 
     FIG. 6 is a schematic representation of a system elevation in which an upper and a lower hydrocarbon producing a zone is encountered, with provisions for tomographic monitoring of the production of the lower zone; 
     FIG. 7 is a schematic representation of a system elevation of a well with three producing zones and two optical tomographs; 
     FIG. 8 is a cross sectional view of a commutator which provides for sequential introduction of light into one fiber and the receiving and detection of light from other fibers; and 
     FIG. 9 is a top view of a commutator which can provide commutation of the light around a downhole tomograph chamber. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
     In FIG. 1, a downhole optical tomograph chamber  10  within a wellbore is depicted. A bundle of optical fibers  12  connects the downhole optical tomograph chamber  10  to the surface system  14 . The optical tomograph  10  is shown on the well tubing  16  and within the casing  18 . Casing perforations  19  are shown and fluid can flow from the producing zone  20  into the annulus between the tubing  16  and casing  18  and then returned up the center of tubing  16  to the surface. Seal assembly  22  prevents fluid flow to the surface between tubing  16  and casing  18 . The optical tomographic chamber  10  provides information on the fluids flowing up the tubing  16  with the information being conveyed up the optical fibers  12  and analyzed in the surface system  14 . 
     The surface system  14  contains at least one optical transmitter or light source, such as a low power laser, and multiple optical receivers or detectors. At any one time, there is one fiber bundle used to transmit light and one less than the total number of fiber bundles used to detect light in the tomograph chamber  10 . For example, the surface system  14  may contain one light source and two light detectors. The surface system  14  also includes a computer which analyzes the light received by each fiber bundle. 
     Differences in the optical properties of the production fluids cause differences in the amount of light transmitted to the receiving fiber bundles, as the light is, when there is anything other than a single phase transparent fluid, either absorbed, reflected or refracted. The optical properties of the multiple fluid phases and the relative sizes and shapes of the phases contained in the multiple fluids determine the pattern of light, and these patterns are detected by the detectors in the surface system  14  during a scan. The computer analyzes the fluid and determines the composition of the phases of the fluids. 
     Various types of optical fiber terminations can be used in the tomograph chamber  10 . If no lense is used, in a clear one phase fluid, the light will be received substantially in the optical fiber bundle directly across from the transmitting fiber bundle. Other fiber bundles will receive substantial light only when there is reflection or refraction. Divergent lenses can be used to provide greater amounts of information. Two types of divergent lenses which may be used are cylindrical and conical. A cylindrical lens will provide a fanned shaped pattern of light, while s spherical lenses or conical lenses, spread light in a three dimensional conical pattern. 
     FIGS. 2A and 2B show an embodiment in which the optical fibers are terminated within the tomograph chamber  10  where no lenses are used in the fiber terminations. Thus, when light is introduced through fiber bundle  24  into tomograph chamber  10  having a clear fluid such as water flowing therethrough, the light will be passed directly across, as shown by the dotted line. Fiber bundles  26 ,  28 ,  30  receive little or no light as the light generally goes into optical fiber bundle  32 , where it is collected and returned to the surface. Similarly optical fiber bundles  34 ,  36  and  38  collect little or no light. 
     FIG. 2B shows the effect of a single step of commutation where the light is now introduced into tomograph chamber  10  by fiber bundles  26 , rather than fiber bundle  24 . In this case, the light is generally collected by fiber bundle  34 , and not by fiber bundles  32  as in FIG.  2 A. This sort of commutation of course can be continued all around with the light being introduced in turn by fiber bundle  28 , then  30 , then  32 , then  34 , then  36 , then  38 , and the process continued by again introducing light into fiber bundle  24 . In this manner the light is commutated around the tomograph chamber  10  and more information is available in the s signals received in the surface system  14  for analysis of the phases of the flowing fluid. 
     FIG. 3 depicts tomograph chamber  10  passing light through divergent lenses  40  and a clear single phase fluid. The lenses  40  on the fiber terminations not only spread the light emitted into the fluid, but also, provide better angular sensitivity to the receiving fibers. It should be noted that a tomograph chamber  10  can utilize multiple layers of optical fiber bundles and thus the layer shown in FIG. 3 could have one or more layers of optical fiber bundles either above and/or below the eight optical fiber bundles shown. For illustration purposes, one can use a layer of eight fiber bundles as shown, but have another layer of eight fiber bundles above and below the fiber bundles shown. In practice, many optical fibers may be used. 
     A single layer is preferred when cylindrical lenses are used at the fiber terminations to provide a fan like pattern of light from the transmitting fiber bundle. This fan like pattern of light is basically two dimensional but provides higher intensity light to the receiving fiber bundles. Conversely, multiple layers of fiber bundles provide greater amounts of information, and are preferably used with conical lenses to provide for three dimensional transmission and three dimensional receiving of light. 
     FIG. 4 shows light patterns similar to FIG. 3, with the transmitting fiber bundle being commutated to fiber bundle  26 . As explained above, this sort of commutation of course can be continued all around with the light being introduced in turn by fiber bundles  28 , then  30 , then  32 , then  34 , then  36 , then  38 , and the process continued by again introducing light into fiber bundle  24 . In this manner the light is commutated around the tomograph chamber  10  and more information is available in the signals received in the surface system  14  for analysis of the phases of the flowing fluid. 
     FIG. 5 depicts a tomographic chamber  10  in use with a multi phase fluid flowing through tomograph chamber  10 . Typical multi phase fluid flow may be composed of a continuous phase fluid and one or more discontinuous phase fluids. In hydrocarbon production, the continuous phase may be oil or water while the discontinuous phase fluids may be oil, water or gas. In the illustrated example, the continuous phase fluid is water  42  and the discontinuous phase fluids are oil bubbles  44  and gas bubbles  46 . During operation of tomographic chamber  10 , light is introduced from fiber bundle  26  through a cylindrical lense  40  which provides a fan shaped pattern of light. The light passes directly to fiber bundles  28 ,  36 ,  38 , and  24 , and both directly and by reflection to fiber bundle  30 . It can be seen that the oil bubble  44  absorbs light which would otherwise be transmitted directly across. In addition, light that would normally reach fiber bundle  34  is refracted to receiver  32  by gas bubble  46 . It can also be seen that the commutation of light rapidly around the tomographic chamber  10  provides a quantity of information which allows the analysis of gas, oil and water flow up the tubing  16 . Even though FIG. 5 has depicted a multi phase flow regime having a continuous phase fluid with two discontinuous phase fluid, it should be understood by those skilled in the art that the tomograph chamber  10  of the present invention is equally well-suited for analyzing multi phase fluid in other flow regimes including a stratified flow regime having multiple continuous phase fluids or a mist flow regime. 
     FIG. 6 illustrates a use of the optical tomograph  10  to analyze the production from two producing zones  20 ,  48 . Zone  48  introduces fluid through casing perforations  50  into the wellbore, here with the liquid going into the annulus between the tubing  16  and the casing  18 , to the bottom and then from the bottom of the hole up through the tubing  16  to the surface. In this embodiment, fluids from the zone  20  flows through the casing perforations  19 , into the annulus between the tubing  16  and the casing  18  and into the tubing through a tubing opening  52  which is located above the tomograph chamber  10 . The fluid from zone  20  cannot flow to the bottom of the hole because of the seal assembly  54  and is prevented from flowing to the surface between tubing  16  and casing  18  by seal assembly  22 . 
     In this manner, the optical tomograph  10  sees flow only from zone  48 , but does not see the production from the zone  20 . The fluid from the zone  20  flows through perforations  19  and tubing opening  52  and then goes directly up to the tubing  16  to the surface, bypassing the tomograph chamber  10 . Thus, an analysis at the surface can determine the total quantities of gas, oil, and water and the tomograph chamber  10  can determine the production from zone  48 . The production from zone  48  may then be subtracted to provide production information on zone  20 . Thus, the production from each of two zones  20 ,  48  can be determined. 
     FIG. 6 also illustrates the use of a protective tube  56  around the optical fiber bundle  12 , where the protective tube  56  contains the optical fiber bundle  12  which are fed through a pressure bulkhead  58  and where a pump  60  is used to provide hydraulic pressure to prevent inward leakage into the protective tube  56 . 
     FIG. 7 illustrates the use of multiple optical tomograph chambers  10 ,  69 . In addition to zone  20  and zone  48 , there is an intermediate producing zone  62 . Zone  20  again produces into the annulus through perforation  19 , and fluid is again introduced into the tubing  16  from zone  20  through tubing opening  52  from where the fluid flows up the tubing  16 . Similarly, fluid from the zone  48 , again goes into the annulus, goes to the bottom of the hole and then up the center of the tubing  16 . In this example, the fluid from zone  62  enters the annulus through casing perforation  64  and then into the tubing  16  through tubing opening  66 . Fluid from zone  62  then flows up the tubing  16 , but bypassing optical tomograph chamber  69  as seal assembly  68  prevents liquids from zone  62  from flowing to the bottom of the hole. Fluid from zone  62  is mixed within the tubing  16  with fluid from the zone  48  and the mixed fluid from these two zone  62 ,  64  is then analyzed by tomograph chamber  10 . Thus, it can be seen that the results from the analysis by tomograph chamber  10  can be subtracted from the total production measure on the surface to provide the production of zone  20 , and the information from optical tomographic chamber  69  can be utilized to directly indicate the production of zone  48 . Subtracting the production of zone  48  from the results obtained from tomograph chamber  10  gives the production from zone  62 . 
     FIG. 8 shows one embodiment of a commutator  71  for commutating the light introduction around the tomograph chamber  10 . Light source  76  generates light which is to be fed through optical fiber bundle  32  down into the hole and into the tomograph chamber  10 . A portion of the light is picked up by one of the other fiber bundles, here fiber bundle  24 , brought back up the hole and detected by light detector  78 . 
     The commutator  71  is preferably part of the surface system  14  and includes the cylindrical outer support  70  which holds the upper end of the optical fibers (illustrated here with optical fiber bundles  24  and  32 ), and showing the upper optical fiber termination  72 . A rotating mirror table  74  is used to provide motion for the commutation. Here the rotating table  74  holds transmitting mirror  80  and receiving mirror  82 . Light comes from light source  76  and is reflected off transmitting mirror  80  into optical fiber  32 , where light is introduced in the tomograph chamber  10  and collected as previously described. Light returning through optical fiber bundle  24  is reflected by receiving mirror  82  and is detected by detector  78 . 
     Generally, an electrical signal is produced by the light detector  78  which electrical signal is sent to the computer where information is gathered on the optical properties of the downhole fluid flowing in the well and images or quantitative values of phases flowing in the well can be generated. In this example, four fiber bundles and four detectors are used, as best seen in FIG. 9, and in such a configuration, there would be one transmitting mirror  80  and three receiving mirrors  82 . 
     FIG. 9 shows a top view of the rotating mirror table  74  of FIG. 8, showing light detectors  78 , three of which are beneath receiving mirrors  82 , and are shown as dotted circles. Light reflects off transmitting mirror  80 , and goes past detector  78  into optical fiber bundle  32 . Light returning from the tomograph chamber  10  through optical fiber bundles  24 ,  28 , and  36  is reflected by the three receiving mirrors  82  to the light detectors  78  shown as dotted circles beneath the mirrors  82 . 
     Generally the light source  76  can be a laser. A pulsed laser could be used, timed to the rotation of the rotating mirror table  74 . Alternate light sources, e.g. an incandescent lamp or super radiant LED could also be used. In addition, even though the optical tomograph chamber embodiments shown herein are used as a part of the tubing or a pipe string in a cased wellbore, it should be apparent to those of ordinary skill in the art that the system could also be used at the surface. 
     While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.