Cooling Systems for Downhole Tools

Abstract

A cooling system for a downhole tool includes an insulating chamber disposed in the downhole tool, wherein the insulating chamber is adapted to house an object to be cooled; a thermoacoustic cooler disposed in the downhole tool, wherein the thermoacoustic cooler has a cold end configured to remove heat from the insulating chamber and a hot end configured to dissipate heat; and an energy source for generating an acoustic wave in the thermoacoustic cooler. A method for constructing a downhole tool includes disposing a to-be-cooled object in an insulating chamber in the downhole tool; and disposing a thermoacoustic cooler in the downhole tool proximate the insulating chamber such that the thermoacoustic cooler is configured to remove heat from the insulating chamber.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to techniques for maintaining downhole tools and their components within a desired temperature range in high-temp environments, and, more specifically, to a thermoacoustic cooling system for use with downhole tools.

[0003] 2. Background Art

[0004] Various well logging and monitoring techniques are known in the field of hydrocarbon and water exploration and production. These techniques employ downhole tools or instruments equipped with sources adapted to emit energy through a borehole traversing the subsurface formation. The emitted energy passes through the borehole fluid ("mud") and into the surrounding formations to produce signals that are detected and measured by one or more sensors, which typically are also disposed on the downhole tools. By processing the detected signal data, a profile of the formation properties is obtained.

[0005] A downhole tool, comprising a number of emitting sources and sensors for measuring various parameters, may be lowered into a borehole on the end of a cable, a wireline, or a drill string. Data collected by the sensors are sent to a processing center at the surface through the cable/wireline. With this type of wireline logging, it becomes possible to measure borehole and formation parameters as a function of depth, e.g., while the tool is being pulled uphole.

[0006] An alternative to wireline logging techniques is collecting data in downhole conditions during the drilling process. By collecting and processing information during the drilling process, an operator can modify or correct key steps of the operation to optimize performance in real time. Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are known as Measurement While Drilling (MWD) techniques. Similar techniques focusing more on measurement of formation parameters than on movement of the drilling assembly are know as Logging While Drilling (LWD).

[0007] Logging While Tripping (LWT) is an alternative to LWD and MWD techniques. In LWT, a small-diameter "run-in" tool is sent downhole through the drill pipe, at the end of a bit run, just before the drill pipe is pulled. The run-in tool is used to measure downhole physical quantities as the drill string is extracted or tripped out of the hole. Measured data is recorded in tool memory, as a function of time, during the trip out. At the surface, a second set of equipment records bit depth versus time for the trip out, and this allows the measurements to be placed on depth.

[0008] FIG. 1 shows a conventional logging tool 12 disposed in a borehole 11 that penetrates a subsurface formation 10. The logging tool 12 may be deployed on a wireline 13 via a wireline control mechanism 14. In addition, the logging tool 12 may be connected to surface equipment 15, which may include a computer (not shown).

[0009] Downhole tools are exposed to extreme temperatures (up to 260.degree. C.) and pressures (up to 30,000 psi and possibly up to 40,000 psi in the future). An operation temperature of 473 K (245.degree. C.) or higher, which is likely to be encountered in deep wells, already exceeds the operating temperatures of most logging tools. In order to drill deeper, new tools will need to be developed.

[0010] The downhole tools are typically equipped with sensitive components (e.g., electronics packages and mechanical seals) that often are not designed for such harsh environments. The trend among manufacturers of electronic components is to address the high volume commercial market, making it difficult to find components for downhole tools. At the same time, the oilfield industry is moving toward the exploration of deeper and hotter reservoirs as more-easily accessible resources are being depleted. As a result, there is an urgent need for methods or devices that permit the sensitive electronic components to be operated at high temperatures.

[0011] Redesigning silicon chips to operate at high temperatures is costly, and has a significant impact on the development time and thus the time to market. The alternative is to have systems to protect the electronic components from the high temperature environments. Conventional techniques include those that insulate the sensitive components from the hot environments, such as putting them in Dewar flasks. This technique protects the tool only for a limited duration of time, and the nature of the flasks makes them intrinsically fragile.

[0012] Alternatively, existing equipment can be modified to include active cooling systems. Cooling is not only required by electronics, but also is needed in some mechanical parts within the system, or for reservoir hydrocarbon samples. A cooling system capable of providing multi-watt refrigeration for thermally-protected electronic components in downhole tools would enable the use of electronic and sensor technologies that are otherwise not suitable for high-temperature applications. This would reduce the ever-increasing costs associated with the development and implementation of high-temperature electronics, and make it possible to introduce new technologies to subsurface exploration and production. A cooling system for use in a downhole tool needs to fit in the limited space within the tool. Examples for the use of active cooling in downhole tools may be found in U.S. Patent Application Publication No. 20050097911 by Revellat et al., which discloses downhole tools with Stirling cooling systems.

[0013] Although some cooling systems for use in downhole tools have been proposed, a need remains for improved cooling/refrigeration techniques for downhole tools.

SUMMARY OF INVENTION

[0014] One aspect of the invention relates to cooling systems for downhole tools. A cooling system in accordance with one embodiment of the invention includes an insulating chamber disposed in the downhole tool, wherein the insulating chamber is adpated to house an object to be cooled; a thermoacoustic cooler disposed in the downhole tool, wherein the thermoacoustic cooler has a cold end configured to remove heat from the insulating chamber and a hot end configured to dissipate heat; and an energy source for generating an acoustic wave in the thermoacoustic cooler.

[0015] One aspect of the invention relates to methods for constructing a downhole tool. A method in accordance with one embodiment of the invention includes disposing a to-be-cooled object in an insulating chamber in the downhole tool; and disposing a thermoacoustic cooler in the downhole tool proximate the insulating chamber such that the thermoacoustic cooler is configured to remove heat from the insulating chamber.

[0016] One aspect of the invention relates to methods for cooling a portion of a downhole tool. A method in accordance with one embodiment of the invention includes providing a thermoacoustic cooler in the downhole tool proximate to the portion to be cooled; and energizing the thermoacoustic cooler to generate an acoustic wave such that heat is removed from the portion to be cooled.

[0017] Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0018] FIG. 1 shows a conventional downhole tool disposed in a borehole.

[0019] FIG. 2 shows a downhole tool including a thermoacoustic cooler in accordance with one embodiment of the invention.

[0020] FIG. 3 shows a downhole tool including a thermoacoustic cooler in accordance with another embodiment of the invention.

[0021] FIG. 4A shows a basic structure of a thermoacoustic cooler to be disposed in a downhole tool in accordance with one embodiment of the invention.

[0022] FIG. 4B shows a diagram illustrating a thermoacoustic cycle.

[0023] FIG. 5 shows a schematic illustrating a thermoacoustic cooler having a cylindrical geometry.

[0024] FIG. 6 shows a schematic illustrating a thermoacoustic cooler including a sonic compressor.

[0025] FIG. 7 illustrates a traveling-wave refrigerator.

[0026] FIG. 8 illustrates a pulse-tube refrigerator.

[0027] FIG. 9A shows a heat-driven thermoacoustic refrigerator; FIG. 9B shows a loudspeaker driven thermoacoustic refrigerator similar to that in FIG. 9A.

[0028] FIG. 10 shows a generalized schematic illustrating heat transfer using a thermoacoustic cooler in accordance with some embodiments of the invention.

[0029] FIG. 11 illustrates a method for manufacturing a downhole tool in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

[0030] Embodiments of the invention relate to cooling systems for use in downhole tools. In particular, cooling systems in accordance with some embodiments of the invention are based on thermoacoustic cooling. A thermoacoustic cooling system functions by generating an acoustic wave to oscillate or resonate within a space (a resonator). The fluid molecules that oscillate or resonate within a resonator contact a plurality of thermal conductive plates and transfer heat to these plates. The heat can then be removed from these plates.

[0031] Before describing thermoacoustic cooling in detail, various techniques that can potentially be used in downhole tools are summarized as follows.

(1) Thermoelectric Cooling (TEC)

[0032] TEC systems are advantageous over conventional refrigerating systems in many aspects, such as being more compact and involving only solid-state components. TEC is based on the Peltier effect, which occurs when electric current flows through two dissimilar materials. Depending on the current flow directions, cooling or heating could be achieved at the junction between the two dissimilar materials. However, TEC coolers cannot handle much power and are typically used to cool electronic circuitry that requires a cooling power below 100 W.

(2) Isentropic Expansion

[0033] In a thermally insulated container, a fast, but not explosively-fast, expansion of gas is considered an isentropic expansion. In isentropic expansion, the temperature T of the gas varies with the pressure p as:

( .differential. T .differential. p ) s = T ( .differential. V .differential. T ) p / C p = TaV m / C p , m , ( 1 ) ##EQU00001##

wherein S is the entropy, V is the volume, C.sub.p is the heat capacity at a constant pressure, a is the isothermal expansivity, and the subscript m indicates a molar quantity. For an ideal gas,

pV.sub.m=RT. (2)

and Equations (1) and (2) result in

T.sub.2=T.sub.1(p.sub.2/p.sub.1).sup.R/C.sup.p,m, (3)

wherein R is the gas constant. For an ideal monoatomic gas, C.sub.p,m=5R/2.

[0034] At T.sub.1=473 K, a temperature likely to be encountered in a deep well, an isentropic expansion from p.sub.1=140 MPa to p.sub.2=35 MPa yields a cooled temperature T.sub.2=271 K. Thus, using this cooling mechanism, it is possible to achieve a net cooling of 200 K without increasing temperatures elsewhere in the system.

[0035] However, this process needs a large volume of gas, which has a relatively low heat capacity (compared to, e.g., that of water), to cool a massive metal tool. Although it is possible to feed the large volume of gas through a tube from the surface, it is inconvenient.

(3) Enthalpy of a Phase Transition

[0036] Alternative to feeding a large volume of gas through a tube from the surface to the downhole tool, liquids such as liquid nitrogen may be fed to the downhole tool through a tube. Liquids have much smaller volumes than the corresponding gases. For example, liquid nitrogen may be used to provide gaseous nitrogen as a refrigerant. The liquid-to-gas phase transition provides the cooling. Other phase transitions may also be used in this type of cooling to provide the enthalpy of evaporation.

(4) Vapor-to-Liquid Compression

[0037] Vapor-to-liquid compression is typically used in home refrigerators and air-conditioning systems. This process often uses halogenated hydrocarbons that can be readily condensed to liquid at the hot side of the system. For downhole tools at an ambient temperature of about 200.degree. C., conventional refrigerants and lubricants cannot be used.

(5) Liquid Compression

[0038] Refrigerators working under this principle are referred to as Malone refrigerators. See, e.g., G. W. Swift, Los Alamos Sci. 1993, 21, 112-123 (hereinafter "Swift (1993)," which is incorporated by reference in its entirety. Liquids without undergoing a vapor-to-liquid phase transition as cooling agents have certain advantages. For example, the isothermal compressibility, -(.differential.V/.differential.p).sub.1/V, is much smaller for liquid than for gases. Because the amount of energy transferred is proportional to the pressure change, a low-compressibility cooling agent requires only a small volume change to achieve a desired pressure change. In addition, the heat capacity C.sub.p,m, of a typical liquid is orders of magnitudes greater than that of a gas at pressures typically encountered in refrigerators. Consequently, the volume of the cooling agent required is much smaller for a liquid than for a gas. With a liquid cooling agent, the dimensions of a piston compressor can be smaller, the system is more compact, and the cooling efficiency can be improved. Moreover, the mechanical power required to pump a liquid through a heat exchanger is much smaller. A liquid cooling agent to be used in the downhole environment needs to endure an ambient temperature of 473 K or higher. One possibility is to use pressurized water (see, e.g., F. J. Malone, J. Soc. Arts 1931, 79, 679).

(6) Stirling Refrigerators

[0039] Stirling refrigerators are based on the Stirling cycle, which is a well known thermodynamic cycle. A Stirling refrigerator typically requires no lubrication and can function at relatively low pressures, as compared to a vapor compression system. A Stirling engine uses heat (temperature difference) as the energy source to provide mechanical work. A Stirling cooler operates in reverse; it uses mechanical energy to produce temperature difference--e.g., as a cooler or refrigerator. A Stirling refrigerator may use one or two moving pistons to cause gas to compress and expand, and thus is mechanically similar to an internal combustion engine.

[0040] Due to engineering challenges, Stirling cycle engines are rarely used in practical applications. Stirling cycle coolers are often limited to the specialty field of cryogenics and military use. The development of Stirling engines/coolers involves practical considerations such as efficiency, vibration, lifetime, and cost. Using Stirling engines/coolers on downhole tools presents additional difficulties because of the limited space available in a downhole tool (typically 3-6 inches in diameter) and the harsh downhole environments (e.g., temperatures up to 260.degree. C. and pressures up to 30,000 psi or more). Stirling engines have been proposed for use as electricity generators for downhole tools (see, e.g., U.S. Pat. No. 4,805,407 issued to Buchanan). More recently, a version of a Stirling refrigerator has been incorporated into a Schlumberger high-temperature wire-line conveyed sampling tool, see U.S. Patent Application Publication No. 20050097911.

(7) Giant Magnetocaloric Effect

[0041] In this cooling mechanism, a magnetic material is spun through a permanent magnet that causes the temperature of the material to increase. The temperature decreases when the material leaves the magnetic field. The hot portion of the magnet is usually cooled with a circulating fluid. Materials having a large magnetocaloric effect and a high magnetic ordering temperature are suitable for this type of operations. Gd.sub.5(Si.sub.2Ge.sub.2) is known to order ferromagnetically around T=299 K. Upon cooling, this material undergoes a first order phase transition from the high temperature ferromagnetic (I) to a second ferromagnetic structure (II) at about 276 K (see, e.g., V. K. Pecharsky and K. A. Gschneidner, Jr., Phys. Rev. Lett. 1997, 78, 4494-4497). Another suitable material, Ni.sub.2MnGa, has been studied by Zhou et al. (J. Phys-Condens Mat. 2004, 16, L39-L44).

[0042] Refrigeration process using these materials may reach an efficiency of about 60%, significantly higher than vapor-compression cycles that have a typical efficiency of 40%. However, finding a material suitable for use in the downhole environment requires further research. The Curie temperature, i.e., the temperature above which a ferromagnetic material loses its permanent magnetism, needs to be sufficiently high for the material to function properly in the downhole environment.

[0043] The above description illustrates that these cooling technologies are not all suitable for downhole use. On the other hand, devices based on the thermoacoustic cooling are more suitable for the downhole environments. Accordingly, embodiments of the invention are based on the thermoacoustic cooling.

[0044] In accordance with an embodiment of the invention, a thermoacoustic cooling system uses a sonic compressor to generate high acoustic pressure (about 0.8 MPa) waves at a resonance frequency of a cavity to compress (and decompress) a refrigerant. The sonic compressor compresses gaseous refrigerant without sliding parts, and thus does not require lubricating oil. In addition, a sonic compressor can reach a higher frequency than a conventional compressor. A basic structure of a sonic compressor has been disclosed by Swift (1993), which is hereby incorporated by reference in its entirety.

[0045] A cooling system in accordance with embodiments of the invention may be used to cool a downhole tool, such as tool 12 shown in FIG. 1. Alternatively, a cooling system of the invention may be used to cool a critical part within a downhole tool. FIG. 2 shows a downhole tool (such as 12 in FIG. 1) in accordance with one embodiment of the invention. As shown, the downhole tool 20 is deployed in a borehole 27 traversing a formation 28. The tool 20 may be deployed via a wireline, a drill pipe or tubing, or other means known in the art. The tool 20 includes an elongated housing 21 that protects various components of the instrument. The components may be located in different modules within the tool 20.

[0046] The components may include electronics 23 that need to be protected from high temperatures. The electronics 23 are disposed in an insulating enclosure or chamber 24 and are connected to a thermoacoustic cooler 22. The controller 25 may include other electronics for controlling the thermoacoustic cooler 22 or mechanisms to remove heat from the hot end of the thermoacoustic cooler 22, and may also include a power source to power the thermoacoustic cooler 22. The thermoacoustic cooler 22 may be connected directly to the working module or the temperature-sensitive components; alternatively, it may be connected to these components via ducts or channels 26. A cooling fluid can be circulated through the channels 26 using a pump (not shown).

[0047] One of ordinary skill in the art would appreciate that the channels 26 may use any heat transport mechanism known in the art, including circulating fluid or gas. Therefore, the term "channels" as used herein is intended to include any suitable heat transport mechanism, which may or may not include a "channel" structure. In this manner, the heat removed from the object to be cooled (the electronics 23) is effectively "pumped" to the other end (the hot end) of the thermoacoustic cooler 22 and dissipated by the controller 25 into, for example, a mud flow.

[0048] FIG. 3 shows a schematic of a system for heat removal using a thermoacoustic cooler in accordance with another embodiment of the invention. As shown, instead of using a channel 26, a thermoacoustic cooler 32 is in direct contact with a module or component 33 to be cooled.

[0049] A thermoacoustic cooler may use high-intensity (e.g., 170 dB) standing acoustic waves to provide cooling and requires no moving parts, except for a single flexing moving diaphragm that forms the sound source (for example, that described in U.S. Pat. Nos. 5,745,438 and 5,600,610). This property gives the thermoacoustic cooler advantages of simplicity, reliability, lower cost, and requiring no sealant.

[0050] The principles of thermoacoustic cooling have been reviewed by Swift (J. Acoust. Soc. Am. 1988, 84, 1145-1180, hereinafter "Swift (1988)," which is incorporated by reference in its entirety. FIG. 4A illustrates a schematics of a basic thermoacoustic cooler, which is similar to a structure disclosed in Swift (1993). The thermoacoustic cooler 40 has a loudspeaker 41, which maintains a standing sound wave within a resonator 42. The sound wave interacts with a stack of solid plates 43, causing gases to oscillate between the plates 43.

[0051] The resonator 42 has a length of about one-fourth of the sound wavelength so that all gas molecules are in resonance, i.e., compressed and decompressed at essentially the same time. When the gases are compressed by the loudspeaker 41, they heat up and move away from the loudspeaker 41. When they are decompressed by the loudspeaker 41, they cool down and move toward the loudspeaker 41. When the gas molecules oscillate back and forth between the stack of solid plates 43, they contact these plates and heat transfer results. As a result, heat is transferred from the cold-side heat exchanger 44 to the hot-side heat exchanger 45.

[0052] The thermodynamic cycle of the cooling process in the thermoacoustic cooler 40 of FIG. 4A is illustrated in FIG. 4B. At state 46 in the cycle, gas near the cold-side heat exchanger 44 absorbs heat from the cooler part of the plates 43. When compressed by the loudspeaker 41, the gas is warms up and move to the hot-side heat exchanger 45, shown as state 47 in the cycle. At state 48 in the cycle, the gas transfer heat to the warmer part of the plates 43 near the hot-side heat exchanger 45. Upon decompression by the loudspeaker 41, the gas expands and cools while moving back toward the cold side, shown as having lower temperature and pressure at state 49 in the cycle.

[0053] As described by Swift (1988), a few important parameters relating to the cooling efficiency of a thermoacoustic cooler includes: the thermal penetration depth

.delta. s = ( .kappa. M .rho. C p , m .pi. f ) 1 / 2 , ( 4 ) ##EQU00002##

the viscous penetration depth

.delta. s = ( .eta. .rho. .pi. f ) 1 / 2 , ( 5 ) ##EQU00003##

and the Prandtl number

Pr = .eta. C p , m .kappa. M . ( 6 ) ##EQU00004##

wherein .kappa. is the thermal conductivity, .eta. the viscosity, .rho. the mass density, M the molar mass, and f the frequency.

[0054] As discussed in Swift (1988), a practical refrigerator requires a working fluid with a large thermal expansion coefficient. In addition, all available cross-sectional area needs to be filled with plates to maximum the cooling efficiency. The plates are spaced apart with a distance from by 2 .delta. .kappa. to 4 .delta. .kappa..

[0055] The thermoacoustic cooler shown in FIG. 4 is one example. One of ordinary skill in the art would appreciate that other types of thermoacoustic coolers may also be used in accordance with embodiments of the invention. For example, FIG. 5 shows another thermoacoustic cooler for use in a downhole tool in accordance with another embodiment of the invention. As shown in FIG. 5, the resonator 52 of a thermoacoustic cooling system 50 to be used in a downhole tool has a cylindrical shape. A plurality of plates 53, in a shape similar to circular washers, are used to confine gas oscillations in a fundamental radial-breathing mode. A basic cylindrical radial-wave refrigerator has been described in Swift (1988).

[0056] The cylindrical geometry of the resonator shown in FIG. 5 is particularly suitable for a downhole tool. For example, heat can be easily removed using, e.g., a cooling fluid flowing along the axial direction of the cylinder 52, such as in a pipe 56.

[0057] FIG. 6 shows another embodiment of a thermoacoustic cooler 60, which includes a wall 61 for enclosing the resonator cavity 62. The compressor 61 is powered by a magnet 63 and a coil 64 for compressing the gas in the resonator cavity 62. The resonator 62 is anchored to the compressor wall 61 using an elastic support 65. Low-pressure vapor enters through an inlet 66 having a one-way intake valve 67, which opens at the resonant frequency, into the resonator cavity 62, while the compressed gas exits through a one-way outflow valve 68 at an outlet 69 into the ambient atmosphere, carrying away heat.

[0058] In accordance with yet another embodiment of the invention, a thermoacoustic cooling system in a downhole tool may use a traveling wave refrigerator. In a Stirling engine, the time phasing between pressure and velocity is the same as for a traveling acoustic wave (see, e.g., S. Backhaus and G. W. Swift, J. Acoust. Soc. Am. 2000, 107, 3148-3166). Based on this fact, Ceperley has proposed (H. Ceperley, J. Acoust. Soc. Am. 1979, 66, 1508; H. Ceperley, J. Acoust. Soc. Am. 1985, 77, 1239-1244) to eliminate pistons from Stirling engines and use acoustical techniques to drive the waves. This proposal led to the development of traveling-wave heat engines. In a traveling-wave heat engine, regenerators are used to add acoustic power to a traveling wave in a loop structure, in which heat is pumped from one side to another.

[0059] A traveling wave refrigerator 70 similar to that described in Swift (1988) is illustrated in FIG. 7. Refrigerant is confined to a loop structure 71. The path length around the loop structure 71 approximately equals an integer multiple of the wavelength of the sound wave. Thus, a traveling wave can run around the loop structure 71.

[0060] A regenerator 72 and heat exchangers 73 and 74 function as a prime mover, adding acoustic power to the traveling wave as heat flows from the hot-side heat exchanger 73 to the room-temperature-side heat exchanger 74. Another regenerator 75 and heat exchangers 76 and 77 function as a heat pump, using acoustic power from the traveling wave to pump heat from the cold-side heat exchanger 76 to the room-temperature-side heat exchanger 77.

[0061] In accordance with an embodiment of the invention, a thermoacoustic cooling system in a downhole tool uses a pulse-tube refrigerator as described in Swift (1988). The pulse-tube refrigerator is a combination of a Stirling refrigerator and a thermoacoustic refrigerator. The basic structures of a pulse-tube refrigerator have been suggested by Gifford and Longsworth (W. E. Gifford and R. C. Longsworth, Adv. Cryog. Eng. 1966, 11, 171). Improvements to a basic pulse-tube refrigerator, including an orifice pulse-tube refrigerator with an added flow impedance, and using an adjustable needle valve to provide the impedance, have been proposed by various authors (see, e.g., E. I. Milulin, A. A. Tarasov, and M. P. Shkrebyonock, Adv. Cryog. Eng. 1984, 29, 629; R. Radebaugh, J. Zimmerman, D. R. Smith, and B. Louie, Adv. Cryog. Eng., 1986, 31, 779; R. Radebaugh, Jpn. J. Appl. Phys. Suppl. 1987, 26, 2076, G. W. Swift, D. L. Gardner, and S. Backhaus J. Acoust. Soc. Am. 1999, 105, 711-724). A prototype using this technology was reported to achieve cooling down to 60 K.

[0062] A pulse-tube refrigerator 80 as shown in FIG. 8 includes a pulse tube 81, a regenerator 82, and a rotary valve 83 that has a high-pressure gas intake 84 and a vent 85. Heat is pumped from the cold end 86 to the room temperature heat sink 87.

[0063] A thermoacoustic cooler may use a power transducer to convert electrical power into acoustic power that provides the energy for cooling. Thermoacoustic engines are capable of high power densities. For example, a refrigerator operating at a frequency of 1 kHz, with an acoustic pressure of 1 MPa and a Mach number of 0.1, has a power density of 8 Wcm.sup.-3, i.e., almost 3 times that of a typical automobile engine, which typically produces a power density of about 3 Wcm.sup.3.

[0064] The thermoacoustic engine described above can also be used as prime movers. In this case, the transducer extracts acoustic power from the resonator, converting it into, e.g., electrical power. The energy source for this down-hole electrical generator can come from, for example, burning hydrocarbon in the resonator.

[0065] Although the transducer can be located anywhere in the system, it is preferably disposed at the stack end of the resonator for the high-Q operation of the refrigerator. The transducer needs to have a high impedance, i.e., a large force and small displacement, because it is at a location of high acoustic impedance, i.e., large pressure and small velocity, in the standing wave. A number of different transducers, including electro-acoustic, electro-dynamic, electrostatic, magnetic, magnetostrictive, and piezoelectric transducers, have been described by Hunt (Electroacoustics: The Analysis of Transduction, and Its Historical Background, Acoustical Society of America, New York, 1982, Chapter 1), and by Goodwin et al. (Sound Speed, Ch. 6 in Experimental Thermodynamics, Vol. VI. Measurement of the Thermodynamic Properties of Single Phases, Ch. 5, Goodwin et al. Eds; for International Union of Pure and Applied Chemistry, Elsevier; Amsterdam, 2003). Examples of transducer for operation at elevated temperatures are also described in U.S. Pat. Nos. 5,745,438 and 5,600,610.

[0066] Thermoacoustic cooling systems typically use helium as a medium. However, fluids other than helium, such as a mixture of Ar and He (see, e.g., Jin et al., Rev. Sci. Instrum. 2003, 74, 677-679), or liquid sodium (see, e.g., Migliori et al., Appl. Phys. Lett. 1988, 53, 355-357) can also be used. Among these, liquid sodium has a low Prandtl number and a moderate density and expansion coefficient, and particularly has a high electrical conductivity that allows magneto-hydrodynamic transduction.

[0067] A thermoacoustic refrigerator is not necessarily powered by a loudspeaker. In accordance with some embodiments of the invention, a thermoacoustic cooling system may use a heat-driven thermoacoustic refrigerator. Swift (1988) describes the working principles of a heat-driven thermoacoustic refrigerator. The sound source is replaced with a heat source to cause gas to oscillate, resulting in a cooling effect. As illustrated in Swift (1988), the refrigerator includes a tube with an approximate length of 37 cm. The tube is closed at the top. The bottom is connected to a bulb of an approximate spherical shape. The working fluid in the example is helium at a pressure of about 0.3 MPa. Near the top, a stack of plates are disposed with a spacing about 0.08 cm between the plates.

[0068] A heat-driven thermoacoustic refrigerator 90 similar to that described in Swift (1988) is shown in FIG. 9A. The refrigerator 90 includes a resonator sphere 91, a resonator tube section 92 about 37 cm long, a cold-side heat exchanger 94, a heat pump stack 95, a room-temperature heat exchanger 96, a prime mover stack 97, a hot-side heat exchanger 98, and a thermal energy source 99. The thermal energy source 99 may derive its thermal energy from burning hydrocarbons.

[0069] When the temperature of the hot-side heat exchanger 98 is sufficiently high, the helium gas oscillates spontaneously at about 580 Hz, with a pressure antinode at the closed top of the case and a velocity antinode a the tube-bulb junction. As the oscillating helium interact with the heat pump stack 95, heat is pumped from the cold-side heat exchanger 94 to the room-temperature heat exchanger 96. Such a device has been shown to be able to achieve cooling down to 273 K or lower. In fact, at an applied power of 380 W, one such device was able to cool down to a temperature of 262 K.

[0070] In an alternative embodiment, the thermal energy source 99 may be replaced by an acoustic source 99B, as shown in FIG. 9B. That is, the heat-driven thermoacoustic refrigerator 90 shown in FIG. 9A can be converted to a loudspeaker-driven thermoacoustic refrigerator, shown in FIG. 9B, while maintaining a similar geometrical structure. Such a refrigerator has been described by Hofler ("Thermoacoustic refrigerator design and performance." Ph.D. dissertation, Physics Department, University of California at San Diego, 1986), hereby incorporated by reference in its entirety.

[0071] The lowest temperature achieved by a refrigerator, as shown in FIG. 9B, was 200 K, and the highest thermal efficiency was 12%. In a particular example, the plates in the heat pump stack 95B are made of long strips of 8 cm wide, 0.08 mm thick Kapton spirally wound around a plastic rod, forming an assembly with a diameter of 3.8 cm, and a length of 8 cm. The spacing between the plates is approximately 0.38 mm, i.e., about 4 times the thermal penetration depths. The spacing, for example, may be maintained using monofilament nylon fish line glued to the sheet, aligned along the direction of acoustic oscillation.

[0072] The resonator of the cooling device of FIG. 9B, containing helium at a pressure of 1 MPa, resonates at about 500 Hz. The resonance frequency depends on the temperature of the cold side and the geometry of the resonator. A driver delivers 13 W of acoustic power to the resonator has been shown to have an electric-to-acoustic power-conversion efficiency of 20%. A thermoacoustic refrigerator, as shown in FIG. 9B, is disclosed by Tijani et al. (Cryogen 2002, 42, 59, 66).

[0073] The fluid within a penetration depth of a plate is primarily responsible for the cooling. The fluid within a penetration depth of the resonator surface on the other hand tends to dissipate the cooling effect. Fluid in the volume is mainly responsible for storing energy. Thus, a spherical geometry minimizes the dissipation and maximizes the stored energy because of a favorable volume-to-surface-area ratio, i.e., a high resonance quality factor.

[0074] FIG. 10 shows a generalized schematic of a system for heat removal using a thermoacoustic cooler in accordance with embodiments of the invention. As shown, a thermoacoustic cooler 102 functions as a heat pump, removing heat from the cold reservoir cartridge 103 to the mud flow (hot reservoir) 101. Note that the thermoacoustic cooler 102 may be in direct contact with the object to be cooled. Alternatively, the thermoacoustic cooler 102 may be placed at a distance from the object to be cooled and a heat pipe 105 is used to conduct heat there-between. One of ordinary skill in the art would appreciate that the heat pipe 105 may be any heat transport mechanism known in the art, including circulating fluid. Therefore, the term "heat pipe" as used herein is intended to include any suitable heat transport mechanism, which may or may not physically comprise a "pipe" structure. In this manner, the heat removed from the object to be cooled (the cold cartridge 103) is effectively "pumped" to the other end (the hot end) of the thermoacoustic cooler and dissipated into the environment, such as the mud flow 101.

[0075] Some aspects of the invention relate to methods for producing a downhole tool having a cooling system in accordance with the invention. The downhole tool may be any downhole tool used in oil and gas exploration, completion, or production. It may be a wireline tool, a measurement-while-drilling (MWD) tool, a logging-while-drilling (LWD) tool, or a logging-while-tripping (LWT) tool. In addition, a cooling system of the invention may be used in a permanent installation to protect heat sensitive electronics or sensors disposed downhole or embedded in the formations.

[0076] FIG. 11 shows a process for producing a downhole tool in accordance with one embodiment of the invention. As shown, the process 110 includes disposing an insulating chamber in a downhole tool (step 112). The insulating chamber may be a Dewar flask or a chamber made of an insulating material suitable for downhole use. In some embodiments, the insulating chamber may be formed by a cutout on the insulating tool body. Then, electronics that need to function at relative low temperatures are placed into the insulating chamber (step 114). Alternatively, the electronics or sensors may be placed in the insulating chamber before the latter is placed in the downhole tool. Then, a thermoacoustic cooler is disposed in the downhole tool (step 116). Note that the relative order of placement of the thermoacoustic cooler and the insulating chamber is not important, i.e., the thermoacoustic cooler may be placed in the tool before the insulating chamber. Preferably, the thermoacoustic cooler is placed proximate the insulating chamber. However, if space limitations do not permit placement of the Thermoacoustic cooler proximate the insulating chamber, the thermoacoustic cooler may be placed at a distance from the insulating chamber and a heat pipe or other heat transport device may be used to conduct heat from the insulating chamber to the thermoacoustic cooler.

[0077] While the above description uses a few thermoacoustic coolers to illustrate embodiments of the invention, one of ordinary skill in the art would appreciate that other types of thermoacoustic coolers may also be used.

[0078] In the above description, a "refrigerator" and a "cooler" both refer to a device that is capable of cooling a temperature below that of the surrounding environment. The "refrigerant" as described above includes "fluid" and "gas." It is noted that the term "fluid" refers to a substance that is capable of flowing, and thus can include a liquid, a gas, or a mixture thereof.

[0079] The energy source used to power the thermoacoustic cooler may be selected from a surface electrical source, a downhole battery, and a downhole power generator. The downhole power generator may generate electricity through, e.g., the Seebeck effect, which is a reverse process of the TEC or Peltier effect and can generate electricity directly from temperature differences. Alternatively, the power generator may comprise a conventional electric power generator. The generator may be powered by burning hydrocarbons such as fossil fuels.

[0080] In accordance with some embodiments of the invention, instead of generating electricity, the power generator generates heat to directly power a heat-driven thermoacoustic refrigerator by burning hydrocarbons. Alternatively, a power generator is used to generate acoustic waves directly, e.g., through burning hydrocarbons, or through hydraulic power.

[0081] Advantages of the present invention include improved cooling/refrigeration techniques for downhole tools. It will be appreciated by those skilled in the art that embodiments of the invention are not limited to any particular type of downhole tool. Thus, the invention may be implemented with any tool or instrument adapted for subsurface disposal, including wireline tools, LWD/MWD/LWT tools, coiled tubing tools, casing drilling tools, and with long-term/permanently disposed tubulars used in reservoir monitoring. A cooling system in accordance with embodiments of the invention can keep the downhole electronics at significantly lower temperatures, enabling these electronics to perform better and to have longer service lives. A cooling system in accordance with embodiments of the invention uses a thermoacoustic cooler that has minimal moving parts that ensure smooth and quiet operation.

[0082] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be envisioned that do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention shall be limited only by the attached claims.

Claims

1. A cooling system for a downhole tool, comprising: an insulating chamber disposed in the downhole tool, wherein the insulating chamber is adapted to house an object to be cooled; a thermoacoustic cooler disposed in the downhole tool, wherein the thermoacoustic cooler has a cold end configured to remove heat from the insulating chamber and a hot end configured to dissipate heat; and an energy source for generating an acoustic wave in the thermoacoustic cooler.

2. The cooling system of claim 1, wherein the thermoacoustic cooler comprises: a resonator containing a refrigerant; and a plurality of plates forming a stack to exchange heat with the refrigerant.

3. The cooling system of claim 1, wherein the thermoacoustic cooler is a pulse-tube refrigerator.

4. The cooling system of claim 2, wherein the resonator has a substantially cylindrical shape and the plurality of plates are stacked along an axial direction of the resonator.

5. The cooling system of claim 2, wherein the resonator comprises a spherical-shaped portion connected to a tube-shaped portion.

6. The cooling system of claim 2, wherein the resonator comprises a tube structure forming a loop having a length that is an integer multiple of a wavelength of the acoustic wave.

7. The cooling system of claim 2, wherein the refrigerant is one selected from the group consisting of helium, argon, a mixture of argon and helium, and liquid sodium.

8. The cooling system of claim 1, wherein the energy source comprises a loudspeaker.

9. The cooling system of claim 1, wherein the energy source comprises a thermal generator.

10. The cooling system of claim 1, further comprising a heat pipe disposed between the cold end of the thermoacoustic cooler and the insulating chamber, wherein the heat pipe is adapted to conduct heat from the insulating chamber to the cold end of the thermoacoustic cooler.

11. A method for constructing a downhole tool, comprising: disposing a to-be-cooled object in an insulating chamber in the downhole tool; and disposing a thermoacoustic cooler in the downhole tool proximate the insulating chamber such that the thermoacoustic cooler is configured to remove heat from the insulating chamber.

12. The method of claim 11, wherein the thermoacoustic cooler comprises: a resonator containing a refrigerant; and a plurality of plates forming a stack to exchange heat with the refrigerant.

13. The method of claim 12, wherein the refrigerant is one selected from the group consisting of helium, argon, a mixture of argon and helium, and liquid sodium.

14. The method of claim 12, wherein the resonator has a substantially cylindrical shape and the plurality of plates are stacked along an axial direction of the resonator.

15. The method of claim 12, wherein the resonator comprises a spherical-shaped portion connected to a tube-shaped portion.

16. The method of claim 12, wherein the resonator comprises a tube structure forming a loop having a length that is an integer multiple of a wavelength of the acoustic wave.

17. A method for cooling a portion of a downhole tool, comprising: providing a thermoacoustic cooler in the downhole tool proximate the portion to be cooled; and energizing the thermoacoustic cooler to generate an acoustic wave such that heat is removed from the portion to be cooled.

18. The method of claim 17, wherein the energizing is by supplying electrical power from a source selected from the group consisting of a surface electrical source, a downhole battery, and a downhole power generator.