Patent Publication Number: US-6336408-B1

Title: Cooling system for downhole tools

Description:
BACKGROUND 
     The invention relates to cooling systems for downhole tools. 
     A wellbore is typically a hostile environment, with downhole temperatures capable of reaching well over 500° F. Such elevated temperatures can damage heat-sensitive components of tools lowered into the wellbore to perform various activities, such as logging, perforating, and so forth. Examples of such heat-sensitive components include explosives and detonating cords used in a perforating apparatus or batteries and electronic circuitry in other devices. 
     Conventionally, to avoid damage to heat-sensitive components in tools lowered into wellbores having elevated temperatures, the tools must be quickly inserted and retrieved from the well to perform the desired activities. Generally, this is practical only in vertical wells. In highly deviated or horizontal wells, in which insertion and retrieval of tools are relatively slow processes, the length of time in which the tools are kept in the wellbores at elevated temperatures could cause damage to heat-sensitive equipment. 
     In some logging tools, dewar flasks have been used to protect heat-sensitive equipment. A dewar flask is generally tubular and contains a vacuum layer that reduces heat transfer. Heat-sensitive components are placed in the inner bore of the dewar flask. By using the dewar flask, the rate of temperature rise is reduced to allow the logging tools to stay downhole longer. However, a need continues to exist for more effective techniques of reducing the rate of temperature rise of components lowered into a wellbore. 
     SUMMARY 
     In general, in one embodiment, an apparatus for cooling a component inside a tool includes a heat sink positioned next to the component. An insulation layer surrounds the component to reduce heat transfer to the component. 
     Other features and embodiments will become apparent from the following description and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a perforating apparatus that includes a passive cooling system. 
     FIGS. 2 and 3 are enlarged views of the perforating apparatus of FIG.  1 . 
     FIGS. 4 a ,  4   b , and  4   c  are cross-sectional views of different sections of the perforating apparatus of FIG.  1 . 
     FIG. 5 is a graph showing the temperature rise with respect to time inside the perforating apparatus of FIG. 1 as compared to the ambient temperature of the wellbore. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it is to be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
     Referring to FIG. 1, a perforating apparatus  12  according to one embodiment includes a “passive” cooling system for protecting heat-sensitive components by maintaining the temperature of the components below the ambient temperature of the wellbore for some period of time. The cooling system keeps the heat-sensitive equipment at a reduced temperature long enough to allow the equipment to operate properly. In further embodiments, other types of downhole tools may be protected using the same or variations of the cooling system. 
     In one embodiment, the passive cooling system includes layers located inside a loading tube  48  that surround heat-sensitive components (also inside the loading tube  48 ) to reduce heat conduction, convection and radiation. Heat insulation sheets (e.g., mica layers) may be used to reduce conduction; a vacuum layer (e.g., a dewar flask such as the Pyroflask product made by Vacuum Barrier Corporation of Woburn, Mass.) may be used to reduce conduction and convection; reflective layers (e.g., shiny foils, thin sheet metals, or metal coatings or platings) may be used to reduce radiation; and heat sinks (e.g., chambers containing a eutectic material or liquid) may be used to further slow down the rate of temperature increase of the protected components. 
     In the illustrated embodiment of FIG. 1, the perforating apparatus  12  is lowered through a tubing  22  and positioned in a cased wellbore. The perforating apparatus  12  contains heat-sensitive components (including shaped charges  14 , a detonating cord  16  and a detonator  39 ) located inside the loading tube  48  that need to be protected from high temperatures. In other types of downhole tools, other types of heat-sensitive components may be present, such as electronic circuitry, batteries, sensors, and so forth. 
     The perforating apparatus  12  includes a perforating gun  26  coupled to a firing module  28 . As further shown in FIGS. 2 and 3, to protect the heat-sensitive components in the perforating apparatus  12 , the passive cooling system includes a dewar flask  30  (a tube having a hollow wall filled with vacuum), insulating and reflective layers  32  and  34  made of shiny foils (or sheet metals) and heat insulation material (such as mica), and heat sink bars  36  and a heat sink tube  41  each filled with an eutectic material. The shiny foil or sheet metal used in layers  32  and  34  reflect radiated heat coming from the wellbore through the housing  38  of the perforating gun  26 , and the insulation material reduces heat conduction. 
     The dewar flask  30  is a metal container having a hollow wall  30   a . A vacuum region  30   b  is drawn inside the wall  30   a  of the dewar flask  30 , with the wall extending around the bottom of the flask  30 . A space  114  (also filled with vacuum) in the bottom portion of the dewar flask  30  contains a radial spacer  70  that supports the weight of the components in the dewar flask  30 . 
     An evacuation tube  73  is located at the bottom of the dewar flask  30  to allow air to be evacuated from the vacuum chamber inside the wall  30   a  of the dewar flask  30 . To further isolate the components in the loading tube  48 , a thermal storage material  71  (e.g., nickel, copper, or other suitable materials) is placed at the bottom of the inner bore of the dewar flask  30 . The loading tube  48  sits on top of the thermal storage material  71 . 
     The shaped charges  14  and heat sink bars  36  are located inside the loading tube  48  (FIG.  3 ). Shelves  31 , which can be made of a metallic material, are used to create multiple chambers in the bottom portion of the loading tube  48  for alternately storing the charges  14  and the heat sink bars  36 . The inner wall of the loading tube  48  is coated or plated with a thin layer of reflective material, such as chrome, to reflect radiated heat transferred from outside the loading tube  48  and also to improve heat conduction between the heat sink bars  36  and the shaped charges  14 . The shelves  31  also aid in transferring heat from the shaped charges  14  to the heat sink bars  36 . The heat sink bars  36  draw heat from the detonating cord  16  and shaped charges  14  inside the loading tube  48  to maintain a temperature below that of the wellbore for an extended period of time. 
     The insulating and reflective layers  32  and  34 , the dewar flask  30 , and the loading tube  48  each extends upwards along the inner bore of the perforating gun  26  into the bore of the firing module  28 . The loading tube  48  is sealed at its top end  13  (FIG. 1) (seal not shown) to prevent well fluid from entering the tube  48 . As shown in FIGS. 2 and 3, the detonating cord  16  extends from the shaped charges  14  in the perforating gun  26  into the firing module  28  and is ballistically connected to a percussion detonator  39  in the firing module  28 . The percussion detonator  39  is activated when a firing pin  46  is driven into the detonator  39  by hydrostatic pressure generated by fluid pressure above the firing pin  46 . 
     The firing pin  46  is held in position by a release sleeve  33 , which holds ball bearings  100  in a circumferential groove in the firing pin  46 . When the release sleeve  33  is lifted (by a sufficient force to break a shear pin  102 ) by a release mechanism (not shown) in the firing module  28  to free the ball bearings  100 , well fluid hydrostatic pressure drives the firing pin  46  into the percussion detonator  39  to initiate a detonation wave in the detonating cord  16  to fire the shaped charges  14 . 
     The detonating cord  16 , the percussion detonator  39 , and the firing pin  46  are protected against excessive heat by enclosing them in the layers  32  and  34  and the dewar flask  30  inside the loading tube  48 . In addition, a heat sink tube  41  is attached (e.g., welded) to the inner wall of the loading tube  48  to draw heat from the protected components. The heat sink tube  41  includes a hollow wall that encloses a space into which a eutectic material is injected. The tube  41  is sealed after the eutectic material has been poured into the space. 
     The detonating cord  16  is enclosed inside the heat sink tube  41 . Further, the percussion detonator  39  is fixed inside the tube  41  by a sleeve  104  threadably connected at its top to the heat sink tube  41 . The detonator  39  is retained against a shoulder  108  in the sleeve  104  by a retainer ring  106 . 
     The heat sink tube  41  also reduces the temperature of the firing pin  46  to a certain extent as a portion of the firing pin  46  extends into the heat sink tube  41 . The heat sink tube  41 , like the heat sink bars  36  in the perforating gun  26 , draw heat away from the firing pin  46 , the detonator  39 , and the detonating cord  16  to maintain a reduced temperature inside the heat sink tube  41 . 
     Referring to FIGS. 4 a - 4   c , cross sections are taken at reference lines A—A, B—B, and C—C (FIG.  3 ), respectively, along the perforating apparatus  12 . In FIG. 4 a , the outermost layer is the perforating gun housing  38 . The insulating and reflective layer  32  is immediately inside the housing  38 , followed by the dewar flask  30 , the second insulating and reflective layer  34 , and the loading tube  48 , which encloses the shaped charge  14  and the detonating cord  16 . 
     The dewar flask  30  is a metal tube enclosing a vacuum layer  30   b  inside its wall  30   a . The vacuum layer  30   b  significantly reduces heat transfer due to convection and conduction. 
     Each of the layers  32  and  34  can include a number, e.g., four, sub-layers of alternating insulating materials and reflective materials. The insulating sub-layers reduce heat conduction and the reflective sub-layers reduce heat radiation from the wellbore. The insulating materials can be mica sheets, and the reflective materials can be sheets of metal, such as chrome, copper, aluminum, or silver. 
     In addition, the inner wall  54  of the housing  38  is coated or plated with a reflective material to further reduce radiated heat transfer. For example, the reflective material can be chrome, nickel, or any other suitable material that reduces heat radiation. Other surfaces that are similarly coated or plated with reflective materials are the inner surface  52   a  and external surface  52   b  of the dewar flask  30 , and the inner surface  50   a  and external surface  50   b  of the loading tube  48 . 
     In FIG. 4 b , the inner layers of the cross section of the perforating gun  26  along reference line B—B (FIG. 3) are shown. The heat sink bar  36  positioned inside the loading tube  48  includes an eutectic material  56  (initially in solid form). The external surface of the eutectic material  56  is plated with chrome or some other suitable material. The plating  60  is of sufficient thickness to form a container when the eutectic material  56  melts at higher temperatures once the perforating apparatus  12  is lowered downhole. Alternatively, the plating  60  can represent a fabricated metal container  60  into which eutectic material  56  is initially poured or placed. 
     The latent heat of fusion of the eutectic material  56  will maintain the temperature at its fusion temperature (or melting temperature) until the eutectic material is totally melted. A longitudinal groove  62  is provided on the outside surface  58  of the heat sink bar  36  to allow the detonating cord  16  to pass through. A second longitudinal groove  63  is provided to compensate for the increase in volume due to heat expansion of the eutectic material  56  and plating  60 . The eutectic material can be a cerro metal alloy, such as a tin/zinc composition that is about 91% tin and about 9% zinc by weight manufactured by Cerro Metal Products Corporation. The melting temperature of this tin/zinc composition is approximately 390° F. Alternatively, depending on the desired melting temperature, the ratio of tin to zinc in the composition can be varied. 
     Alternative heat sinks can also be used. For example, the eutectic material (initially heated to liquid form) can be poured into cavities inside a loading tube having a hollow wall and sealed. Additionally, instead of using eutectic materials, canisters can be provided that store liquids. If liquids are used, then the latent heat of vaporization controls the heat sink effect, that is, the vaporization temperature of the liquid maintains the temperature inside the loading tube  48 . 
     FIG. 4 c  shows the cross-section of the firing module  28  along reference line C—C (FIG.  3 ). The outermost layer is the housing  35  of the firing module  28 . The housing  35  encloses the following layers in order from the outside in: the insulating and reflective layer  32 , the dewar flask  30 , the insulating and reflective layer  34 , and the loading tube  48 . The loading tube  48  in turn encloses the heat sink tube  41  that encloses the detonating cord  16  and the percussion detonator  39 . The heat sink tube  41  includes a metal wall  57  that encloses an eutectic material  59 . A longitudinal bore runs in the center of the heat sink tube  41  through which the detonating cord  16  extends. 
     The inner wall of the housing  35  is coated or plated with a reflective material to further reduce radiated heat transfer. In addition, as described above, the walls of the dewar flask  30  and the loading tube  48  are coated or plated. The inner wall  61  of the heat sink tube  41  is also coated or plated. 
     As with the heat sink bars  36 , the heat sink tube  41  can be filled with other types of materials, e.g., liquid. In addition, the bore of the dewar flask  30  can be filled with a liquid (so that a portion of the loading tube  48  is immersed in liquid) to further reduce the rate of temperature increase. The liquid in the dewar flask  30  would be sealed inside. 
     Referring to FIG. 5, a graph illustrates the approximate temperature behavior inside the loading tube  48  versus the ambient temperature of the wellbore. As shown in the graph, the wellbore temperature quickly rises (within a few hours) to about 500EF as the tool is being lowered downhole. In contrast, the rise in temperature inside the loading tube  48  is more gradual, requiring more than about 30 hours before the internal temperature reaches about the melting temperature of the eutectic material, which is 390EF for a 91%/9% tin/zinc eutectic composition. Thereafter, the internal temperature remains at the eutectic material melting temperature until all the material melts. When that occurs, the internal temperature rises to the environment temperature (not shown on graph). Thus, a period of over 100 hours can be achieved during which the passive cooling system maintains the internal temperature at or below the tin/zinc melting temperature. 
     Other embodiments are within the scope of the following claims. For example, other components in other types of downhole tools can be protected using the cooling system described. Examples of such components include batteries and electronic circuitry.