Abstract:
In one aspect, a method of extracting heat from a downhole device is disclosed, which method, in one non-limiting embodiment, may include: providing a heat exchange fluid that includes a base fluid and core-shell nanoparticles therein; circulating the heat exchange fluid in the downhole device proximate to a heat-generating element of the downhole to cause the core of the core-shell nanoparticles to melt to extract heat from the downhole device and then enabling the heat exchange fluid to cool down to cause the core of the core shell nanoparticles to solidify for recirculation of the heat exchange fluid proximate to the heat-generating element.

Description:
BACKGROUND 
     1. Field of the Disclosure 
     This disclosure relates generally to an apparatus and method for extracting heat from downhole devices and more particularly to extracting heat using core-shell nano particles. 
     2. Background of the Art 
     Wellbores are drilled in subsurface formations for the production of hydrocarbons (oil and gas). Wells often extend to depths of more than 1500 meters (about 15,000 ft.). Many such wellbores are deviated or horizontal. After a wellbore is formed, a casing is typically installed in the wellbore, which is perforated at hydrocarbon-bearing formation zones to allow the hydrocarbons to flow from the formation into the casing. A production string is typically installed inside the casing. The production string includes a variety of flow control devices and a production tubular that extends from the surface to each of the perforated zones. Some wellbores are not cased and in such cases the production string is installed in the open hole. Often, the pressure in the hydrocarbon-bearing subsurface formations is not sufficient to cause the hydrocarbons to flow from the formation to the surface via the production tubing. In such cases, one or more electrical submersible pumps (ESP) are deployed in the wellbore to lift the hydrocarbons from the production tubing to the surface. Power to the ESPs is supplied from the surface. Such pumps are often deployed at great depths, where the wellbore temperature can exceed 200° F. An ESP includes an electrical motor and a pump. The electrical motor includes magnets and windings, which generate heat. The temperature inside the motor of an ESP can often reach or exceed 300° C. ESP&#39;s are relatively expensive and can therefore also be prohibitively expensive to replace. It is therefore desirable to extract as much heat as practicable to reduce the temperature of the motor for efficient operation and the longevity of the motor. Other downhole devices and sensors also operate more efficiently and have longer operating lives at lower temperatures. 
     The disclosure herein provides apparatus and methods for removing or extracting heat from downhole devices including, but not limited to, electrical submersible pumps. 
     SUMMARY 
     In one aspect, a method of extracting heat from a downhole device that generates heat is disclosed, which method, in one non-limiting embodiment, may include: providing a heat exchange fluid that includes a base fluid and core-shell nano particles therein; circulating the heat exchange fluid in the downhole device proximate to a heat generating element to cause the cores of the core-shell nanoparticles to melt to extract heat from the downhole device and then enabling the heat exchange fluid to cool down to cause the cores of the core shell nanoparticles to solidify for recirculation of the heat exchange fluid proximate to the heat-generating member. 
     In another aspect, an apparatus for use in a wellbore is disclosed that in one non-limiting embodiment may include a downhole device that generates heat; a reservoir containing a heat exchange fluid having a base fluid and core-shell nanoparticles; a fluid circulation mechanism that circulates the heat exchange fluid in the downhole device to cause the cores of the core-shell nanoparticles to melt and then solidify before recirculating the fluid. 
     Examples of the more important features of the apparatus and methods of the disclosure have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features that will be described hereinafter and which will form the subject of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed understanding of the apparatus and methods disclosed herein, reference should be made to the accompanying drawings and the detailed description thereof, wherein like elements have generally been given like numerals and wherein: 
         FIG. 1  is a schematic line diagram of an exemplary production wellbore with an ESP deployed therein, made according to one non-limiting embodiment of the disclosure, for lifting formation fluid to the surface; 
         FIG. 2  shows a motor of an ESP that includes a heat exchange fluid according to one non-limiting embodiment of the disclosure; 
         FIG. 3  shows a cut-view of the motor section “A” shown in  FIG. 2 ; 
         FIG. 4  shows a cut-view of the motor section “B” shown in  FIG. 2 ; and 
         FIG. 5  shows a non-limiting embodiment of a heat-exchange fluid reservoir that includes or has associated therewith a device that mixes nanoparticles with a base fluid in the heat-exchange reservoir. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows an exemplary wellbore system  100  that includes a wellbore  110  that has been drilled from the surface  104  through the earth formation  102 . The wellbore  110  is shown formed through a production zone  120  that contains hydrocarbons (oil and/or gas) therein. The fluid in the production zone  120  may contain hydrocarbons (oil and/or gas) and water and is referred to herein as the formation fluid. The formation fluid  150  enters the wellbore  110  from the production zone  120  via perforations  116  and control equipment  130 , such as sand screens, valves, etc. known in the art. The formation fluid  150  then enters a pump  184  of an electrical submersible pump (ESP)  160  as shown by arrows  162 . The production zone  120  is shown isolated from the wellbore  110  above and below perforations  116  by packers  122   a  and  122   b . The wellbore section between the packers  122   a  and  122   b  is therefore filed with the formation fluid  150 . The ESP  160  is shown deployed on a production tubing  140  for lifting the formation fluid  150  from the production zone  120  to the surface  104  via the production tubing  140 . The fluid level in the wellbore is maintained a certain level above the ESP to provide a fluid head to the ESP. Power to the ESP  160  is supplied from a power source  162  at the surface and a controller  164  controls the operations of the ESP  160 . A fluid processor  170  at the surface  104  processes the formation fluid  150  received at the surface  104 . In general, the ESP  160  includes an electric motor  180  that drives a pump  184  that moves the formation fluid  150  to the surface. Seals  186  separate the motor  180  and the pump  184 . Various sensors  188  may be utilized for determining information about one or more parameters relating to the ESP  160 , including, but not limited to, temperature, pressure and vibration. As noted earlier, the disclosure herein provides apparatus and methods for removing heat from devices using core-shell nanoparticles as heat transfer particles. As an example, and not as a limitation, the concepts and the methods for removing heat using core-shell nanoparticles are described herein in reference to ESPs, which are known to generate significant amounts of heat during operation in wellbores. 
     In one aspect, the heat transfer particles may be nanoparticles or micro-particles or a combination thereof. The term “nanoparticle” is used herein to denote particles having nano and micro sizes or a combination thereof. In a non-limiting embodiment, the nanoparticles include a core and a shell surrounding the core. In one aspect, the core may include a metallic material and the shell may be made from a metallic or a non-metallic material. In another aspect, the core may be bismuth and the shell made from a metallic or non-metallic material. In another embodiment, the core may be bismuth and the shell may be made from aluminum, alumina or a combination thereof. Bismuth has a melting point of 271.5° C. and density of 9.78 gm/cc at the room temperature. When solid bismuth is heated, it starts to store heat or thermal energy and its temperature rises up to its melting point. At the melting point, further introduction of heat increases the enthalpy of bismuth but its temperature remains constant until all the material has become liquid. This change in enthalpy is commonly referred to as the “enthalpy of fusion” or “heat of fusion”. Once all of the bismuth has melted, further heating the liquid bismuth increases its temperature. Therefore, bismuth can be heated to a temperature above its melting point, for example 350° C., to store thermal energy, with the heat of fusion being a significant part of the total stored thermal energy. The melting point of aluminum or alumina is substantially higher than the melting point of bismuth and the steam temperature, thereby allowing the nanoparticles have bismuth as core to be heated to an elevated temperature to store thermal energy. In one aspect, the present disclosure utilizes the stored thermal energy to discharge heat to a selected section of the reservoir to decrease the viscosity of the fluids therein, such as heavy oils, typically present as bitumen. 
     In one aspect, the nanoparticles having a core and a shell may be made by heating nanoparticles of a core material, such as bismuth, with triethylaluminum. Triethylaluminum decomposes above 162° C., whereat the aluminum separates from the triethylaluminum compound. When the mixture of bismuth nanoparticles and triethylaluminum is heated between the decomposition temperature of triethylaluminum and melting point of bismuth, the aluminum separates from the triethylaluminum compound. The separated aluminum attaches to the bismuth nanoparticles forming a shell around the bismuth nanoparticles, thereby providing nanoparticles having a bismuth core and an aluminum shell. Oxygen present in the environment oxidizes at least some of the aluminum to alumina (Al 2 O 3 ), thereby providing a shell that is a combination of aluminum and alumina. If the mixture is heated to just below the melting point of bismuth, it attains its maximum volume. And when the aluminum and/or alumina attaches to bismuth nanoparticles, the cores of such nanoparticles have the maximum volume. When such core-shell particles are cooled down, bismuth core shrinks while the aluminum/alumina shell shrinks, but less than the core. When such shell-core nanoparticles are heated to or above the melting point of bismuth, the core expands to its maximum volume within the shell until it melts and then shrinks a bit because the density of the molten bismuth (10.05 gms/cc at the melting point) is greater than the density of the solid bismuth (9.78 gms/cc at room temperature). After bismuth shrinks at the melting point, further heating of core starts the liquid bismuth core to expand. To prevent cracking of the shell due to the expansion of the molten core, the temperature is not exceeded beyond when the volume of the molten core becomes equal to the maximum volume of the solid core when the core was contained within the alumina/aluminum shell. Another embodiment of a phase change heat exchange particle may comprise a core made of a commercially known material referred to as “Polywax,” which may include a polyethylene. The shell may comprise Nickel. In one aspect, a nanoparticle may include a Polywax core, formed as a sphere of polyethylene, and coated with a uniform layer of electroless Nickel shell. The coating or shell is continuous and porosity-free in order to confine the Polywax when it melts. Due to the difference in the thermal expansion coefficient of the Polywax core and the Nickel shell, the shell thickness is chosen to withstand the temperature oscillations during formation of the device containing such a material. This minimum thickness is a function of the thermal expansion coefficients and the mechanical properties of the core and the shell. Stress distribution calculation of the core (for example Polywax) and the shell (for example Nickel) may be used to determine the thickness of the shell. The dimensions of the Polywax-Nickel particles may exceed 2 microns. In addition to electroless deposition, the shell may be produced by Physical Vapor Deposition or Chemical Vapor Deposition processes and variations thereof. In such cases the particles can be suspended in a fluid bed or in a fluidized bed, or in a vibrating or rotating table, where they are free to rotate while the outer layer is deposited. Any suitable size of the heat exchange particles may be utilized for the purposes of this disclosure. As an example, core sizes between 1 nm and 40 nm and shell thickness of at least 0.3 nm may be utilized as heat exchange particles. 
       FIG. 2  shows a motor  180  of an ESP that includes a heat exchange fluid according to one non-limiting embodiment of the disclosure. Referring to  FIGS. 1 and 2 , the motor  180  includes a housing  210 , a base  212  and an upper threaded end  214  for connection to the seals  186 . The motor  180  includes stator laminations  220  and rotors  230  that rotate a shaft  240 . Bearings  250  support the rotors  230  and the shaft  240 . The motor  180  further includes a heat exchange reservoir or chamber  260  that includes a heat exchange fluid  270 . In one non-limiting embodiment, the heat exchange fluid  270  may include any fluid  272  used in ESPs and a selected amount of core-shell nanoparticles  280 . During operations, the rotor  230  rotates the shaft  240  at a relatively high rotational speed, which speed may exceed 3000 rpm. The heat exchange fluid  270  moves up the shaft  240  and circulates around the bearings  250 , thereby removing heat from the heat-generating elements, such as the stator laminations  220  and the rotor  230 . Details of the heat removal process are described in more detail below in reference to  FIGS. 3 and 4 . 
       FIG. 3  shows a cut-view  300  of motor section “A” shown in  FIG. 2 . View  300  shows the housing  210  containing stator laminations  220 , rotor  230  with end rings  332 , and shaft  240  supported by bearings  250   a . A bore  345  runs along the shaft  240 . The bore  345  is sufficient to allow the heat exchange fluid  270  to move from the heat exchange reservoir  260  up along the shaft  240 , as shown by arrow  370 , circulate around or proximate to bearings  250   a  and other heat-generating elements of the motor  180  and return back to the heat exchange reservoir  260  as described below in reference to  FIG. 4 . 
       FIG. 4  shows a cut-view  400  of motor section “B” shown in  FIG. 2 . View  400  shows housing  210  containing stator laminations  220 , rotor  230  with end rings  332 , and shaft  240  supported by bearings  250   a . Heat exchange fluid  270  moving along the gap  345  is shown by arrow  370 . The heat exchange fluid  270  moves from channel  345  and circulates around the bearing  250   a  via fluid passages  420  and returns to the reservoir  260  ( FIG. 1 ) via fluid passages  420  and  480  as shown by arrows  475  and  485  respectively. Typically, there are more than one set of bearings. The heat exchange fluid  270  that is circulated around bearings that are above bearing  250   a  return to the reservoir  260  via a passage, such as passage  488 . 
     In one aspect, the temperature around bearings  250  is greater than the melting point of the core of the core-shell nanoparticles  280  in the fluid  270 . The cores of such nanoparticles  280  melt, i.e. undergo a first phase transition from a solid state to a liquid state, when they are in such high temperature environment. The nanoparticles  280  return to the reservoir  260 , where they solidify, i.e. undergo a second phase transition, and recirculate as described above. The heat exchange system described herein is a closed loop system, in which the heat exchange fluid  270  containing the core-shell nanoparticles removes heat in excess of the heat that would have been removed by the base fluid  272  alone. In other aspects, the core of a nanoparticle may undergo other phase transitions to store and release energy, such as: transition from a crystal structure to amorphous structure; a transition from one allotrope of element to another allotrope; a peritectic transformation, in which a two-component single phase solid is heated and transforms into a solid phase and a liquid phase; eutectic transformation; a direct transition from a solid phase to a gas phase to a solid phase (sublimation/deposition); a transition to a mesophase between a solid and a liquid, such as one of the “liquid crystal” phase; etc. 
       FIG. 5  shows a non-limiting embodiment of a reservoir that includes or has associated therewith a device that mixes the nanoparticles  280  with the base fluid  272  in the reservoir. In one aspect, the shaft  240  may be extended, as shown by extension  510  and a mixer  520  attached to the shaft extension  510 . In one non-limiting embodiment, the mixer  520  may include any type of mixing mechanism, including, but not limited to, propellers and fins that continuously churn the fluid  270  in the reservoir  260 . 
     The foregoing disclosure is directed to certain exemplary embodiments and methods. Various modifications will be apparent to those skilled in the art. It is intended that all such modifications within the scope of the appended claims be embraced by the foregoing disclosure. The words “comprising” and “comprises” as used in the claims are to be interpreted to mean “including but not limited to”. Also, the abstract is not to be used to limit the scope of the claims.