WO2017017198A1

WO2017017198A1 - Downhole tool cooling system

Info

Publication number: WO2017017198A1
Application number: PCT/EP2016/068024
Authority: WO
Inventors: Wilhelmus Hubertus Paulus Maria Heijnen; Werner Wilhelm VORHOFF
Original assignee: Shanghai Hengxu Materials Co. Ltd
Priority date: 2015-07-28
Filing date: 2016-07-28
Publication date: 2017-02-02
Also published as: GB2540788A; GB201513254D0

Abstract

A downhole tool being adapted to operate in a well bore (2) is shown, the downhole tool comprising: a housing (28) surrounded by an outside fluid, the housing having a cooling fluid outlet (26), the cooling fluid outlet being open to said outside fluid, an electronics compartment for installation of downhole tool equipment (60), a cooling system, which comprises a cooling fluid tank (30) fillable with a cooling fluid and a heat transfer device (40) for exchange of heat between the cooling fluid and the downhole tool equipment.

Description

Downhole Tool Cooling System

Specification

Field of the invention

The invention is related to a Cooling System for Downhole Tools .

Background and Summary of the invention

Well bores are used in the petroleum and natural gas industry to produce hydrocarbons (production well) or to inject fluids, for example water, CO2 and/or Nitrogen

(injection well) . Typically, such fluids are injected to stimulate, i.e. to enhance the hydrocarbon recovery.

Lately, CO2 injection has been introduced to this to reduce the C02-concentration in the atmosphere in order to defeat global warming.

Typically, a well bore is lined with a steel pipe or steel tubing, generally referred to as casing or liner, and cemented in the overburden section to reduce the risk of unwanted evacuation of fluids from the overburden and/or the reservoir into the surface environment. For completion of the reservoir section at present several options are typically used, namely open hole completion, or using a liner with several formation packers for sealing off sections of the annulus around the steel liner, or using a steel liner which is cemented in place and access to the reservoir is gained by perforating the liner and cement in a later stage of the completion, or completion of the well with a liner in open hole which has predrilled holes in the liner to gain access to the reservoir. It should be noted that the holes can also be made in a later stage of the well life. During the production or injection of fluids from a well bore in an earth formation the well bore can enlarge due to chemical reactions and/or an instability of the borehole. This may occur due to injection or production pressure changes and/or erosion which can take place e.g. in case of production from unstable geological formations such as turbidites known for their unpredictable sand face failure resulting in massive sand production leading to well failure. Furthermore, when injection processes are being used fractures can be generated resulting in undesired direct communication between the injection and production wells. On the other hand the well can collapse, for example caused by compaction, a process which happens when the pressure in the reservoir reduces, or by the use of

chemicals used to improve injectivity or productivity. The latter can cause a collapse of the annulus and therewith possibly block the access to the reservoir and, therewith, preventing injection or production. Also of importance may be a phenomenon which is called cross flow in the annulus. Cross flow in the annulus is the result of pressure

differences along the liner of the production or injection well in an un-cemented completion. The latter can lead to loss of production and/or loss of economic reserves.

The well bore and/or the casing or liner and/or the

reservoir section may, for example, be subject to

inspection e.g. in order to verify physical properties such as pressure or temperature, more general to collect

information about the status, or in order to observe defects or anomalies, in particular in order to prevent collapses of all kind of the well. As the total length from the reservoir to an access at the top end of the well bore may sum up to several hundred or even several thousand meters retrieving such data, e.g. to an extraction facility at said access, is difficult and subject to continued development.

It is particularly desirable to deliver the data related to the afore-mentioned phenomena, which is, however, difficult because of the environmental conditions, e.g within a steel pipe or steel tubing extending between the reservoir and the access.

Matter is further complicated by the fact that the Oil and Mining Industry is confronted nowadays with higher

temperature and pressure levels for many of their

production of hydrocarbons than two decades ago. This means that tools and equipment, suitable for the oil exploration and production as well as for geothermal applications, used in the past may not do the job anymore. Temperatures up to 200° Celcius or more as well as pressure levels up to 2000 MPa or more are possible. Typical electronic components which are capable of obtaining or processing such data have, for example, a tolerable temperature level in the range of up to 55° Celcius.

First tests have been undertaken with heating the

electronic components to the desired temperature level, exposing the electronic components to said temperature level for a certain amount of time and checking, whether the electronic components are still functioning properly after being exposed for the time to this temperature level. Those electronic components are then defined as ^xhigh temperature electronics' . However, this tests achieved ^xhigh temperature electronics' capable to survive only slightly higher temperature levels. And still the failure rate of these electronic components had been quite high resulting in termination of measurement or, more

complicated, in mixed up measurement values which have to be verified if they are properly. Therefore it is an object of the invention to provide a downhole tool with which electronics can survive high outer temperature levels, i.e. the conditions in a well bore, especially in the field of downhole measurement and control systems such as geosteering of drill bits and petrophysical measurement systems and tools.

Another aspect of the object of the invention is to provide a downhole tool which is simple, robust and comparatively cheap but still allows for sufficient operation time of the electronics, for example when deployed in a well bore.

Yet another aspect of the object of the invention is to improve the limitations mentioned above. The object of the invention is achieved by subject matter of the independent claims. Preferred embodiments of the invention are subject of the dependent claims.

The downhole tool which achieves the object of the

invention and which is presented here comprises a

specialized cooling system which is capable of cooling the electronic components in the extreme conditions of a well bore .

The downhole tool being adapted to operate in a well bore comprises a housing which is surrounded by an outside fluid. The outside fluid may, for example, be the well bore fluid, wherein the cooling system has the special

advantage, that it is quite independent from the outside fluid. It can operate more or less without loss of

efficiency or loss of cooling power in liquid outside fluids and also in gaseous fluids, as will be explained in detail later.

The housing of the downhole tool has a cooling fluid outlet which is open to said outside fluid. The cooling fluid outlet is used for outflux of used cooling fluid.

Therefore, the cooling system is designed such, that the cooling fluid can stream out of the cooling fluid outlet despite the environmental conditions. By way of example, the pressure level of the outside fluid can be as high as

2000 MPa. To force a gas to stream out of the cooling fluid outlet from the inside of the downhole tool to the outside, which is into the outside fluid, it can either be driven by an engine or by an excess pressure. Keeping an excess pressure in the cooling fluid over the outside fluid such allows for an engine-free driven exhaust of used cooling fluid. However, at least parts of the cooling system may therefore be constructed such, that in a beneficial manner they resist the outside fluid pressure level.

The downhole tool further comprises an electronics

compartment for installation of downhole tool equipment. The electronics compartment is advantageously sealed with respect to the outside and/or thermally insulated to reduce heat flow between the downhole tool equipment and the outside. For example, if the outside fluid is a hot well bore fluid heat flow is to be expected from the outside to the electronics compartment, wherein the heat flow can be reduced by said thermal insulation.

Further comprised is a cooling system, the cooling system being adapted to provide a heat exchange between the downhole tool equipment on the one side and the outside fluid on the other side.

The cooling system comprises a cooling fluid tank fillable with a cooling fluid. The cooling fluid tank contains, in operation mode, a high pressure fluid, either gaseous or liquid. A valve is connected to the cooling fluid tank for release of the cooling fluid. A controllable valve can be installed for the benefit of allowing adjustment of cooling intensity and/or switching on and off of the cooling system.

To explain it in a physically way first, the compressed cooling fluid which is stored in the cooling fluid tank is expanded in or by the valve when extracted from the cooling fluid tank. Taking benefit of the Joule-Thomson effect a determinable cooling effect is thereby obtained, wherein essentially the mass of the cooling fluid flowing through the valve and the pressure difference before and after the valve are to be taken into account for. Therefore, for example, when the temperature difference between the outside fluid and the downhole tool equipment is known, e.g. the excess temperature exceeding the tolerable

temperature for electronics to be used as downhole tool equipment, the needed amount of cooling fluid mass and cooling fluid initial pressure can be calculated for any desired measurement duration of the downhole tool in the bore well. More precisely the pressure difference between the outside fluid pressure level and the cooling fluid tank initial pressure level is preferably taken into account, as the outside fluid pressure level may be, as described already, quite high. The process of extracting the cooling fluid and using it for cooling the downhole tool equipment is in a preferred way self-driven, in other words, by the fluid following the pressure difference. The cooling system further comprises a heat transfer device for exchange of heat between the cooling fluid and the downhole tool equipment.

In a preferred embodiment, the cooling fluid tank is designed and prepared such, that it is fillable with a high pressure cooling fluid, for example a pressurized liquid which expands to gas when extracted out of the cooling fluid tank. Structural reinforcements of the cooling fluid tank preferably provide for enough deformation resistance, so that the high pressure cooling fluid can be filled into the cooling fluid tank even in the range of normal surface pressure. When filling the cooling fluid tank in around 1 Bar atmospheric pressure, the pressure difference between the high pressure cooling fluid and the outside fluid can be significantly higher than compared to the pressure difference between the high pressure cooling fluid and the outside fluid being the well bore fluid the downhole tool being deployed in the well bore.

In a quite simplified embodiment, the cooling fluid may in the heat transfer device surround or wash round the

downhole tool equipment and carry away thermal energy therefrom. However, in a practical way, a direct heat transfer between the downhole tool equipment and the cooling fluid turned out to be more difficult to handle. This is, e.g. in the preferred embodiment of using a cooling fluid which is or turns into the gaseous phase for exchanging heat with the downhole tool equipment, the gaseous cooling fluid may transfer a lesser amount of thermal energy in the heat transfer device and/or demands for a larger sized heat transfer device, e.g larger by a factor of up to 100 or even up to 1000 (which is at most dependent from the volumetric difference between gas and liquid) .

In a preferred embodiment of the invention, the downhole tool comprises further a secondary cooling fluid separated from the cooling fluid. The idea behind this separation is, that the cooling fluid can deliver its cooling capacity, e.g. continuously, to the secondary cooling fluid, and the secondary cooling fluid can then deliver this cooling capacity to the downhole tool equipment. In other words, the cooling fluid is released from the cooling fluid tank for exchanging thermal energy with the secondary cooling fluid, whereas the secondary cooling fluid delivers or exchanges the thermal energy (which has e.g. a negative value in the case of cooling) to (or with) the downhole tool equipment and transports further the thermal energy taken from the downhole tool equipment to circulate back to the cooling fluid. Thus, the secondary cooling fluid circulates in the downhole tool. The secondary cooling fluid is regenerated by the cooling fluid by taking over the thermal energy in the secondary cooling fluid collected from the downhole tool equipment. The cooling fluid then may be disposed of by way of releasing it to the outside fluid .

Further preferably included is a primary heat transfer device, wherein a heat exchange is provided between the cooling fluid and the secondary cooling fluid, wherein the secondary cooling fluid is provided to the heat transfer device. In other words, the cooling fluid from the cooling fluid tank is provided to the primary heat transfer device. The primary heat transfer device is advantageously designed such, that thermal energy can flow from the secondary cooling fluid to the cooling fluid, warming up the cooling fluid which after flowing out of the primary heat transfer device is disposed of.

In another preferred embodiment the secondary cooling fluid is provided in the liquid state for improvement of heat exchange between the secondary cooling fluid and the downhole tool equipment. In other words, the secondary cooling fluid is a liquid which stays liquid in the

temperature and pressure conditions to be expected in the well bore.

The cooling fluid (78) is preferably gaseous under standard atmospheric conditions and/or under the conditions in the well bore and liquefied when set under such a pressure to be established in the cooling fluid tank. By way of an example, the cooling fluid is nitrogen. The nitrogen liquid is stored in the cooling fluid tank and when released from the cooling fluid tank it expands, turns to gas and thus cools down.

The cooling fluid can also comprise a chemical material which expands or reacts in such a way that it as able to absorb thermal energy. The downhole tool can advantageously comprise a thermal insulating material. For example, the devices and

components installed or placed inside the downhole tool may be embedded in or enclosed by a thermal insulating

material. However, specifically preferred at least the downhole tool equipment is embedded in the thermal

insulating material.

The thermal insulating material can be situated all along the inside of the housing. Thereby, the thermal insulating material embeds the cooling system and the downhole tool equipment. For example, the whole "spare volume" in the downhole tool can be filled with insulating material. The cooling system and the downhole tool equipment can also be installed decoupled from an outer hull of the downhole tool, e.g. where the insulating material forms a separation layer between the outer hull and the cooling system and/or the downhole tool equipment.

Further, the downhole tool equipment comprises in an embodiment electronic systems, for example a sensor or a measurement device for measurement of downhole properties or instabilities. By way of example, the electronic systems may be normal or standard electronic components which are even not specifically designed for "high temperature conditions". Therefore, by using the cooling system, comparatively cheap electronics can for example be taken and integrated in the downhole tool.

The primary heat transfer device may comprise metal as its construction material and can additionally be covered by a copper foil .

In a particularly preferred embodiment, the primary heat transfer device comprises permeable steel. By way of example, this can be gas permeable mold steel which is a sintered, pre-hardened material with porosity in the range of 20 to 30 percent by volume, wherein a system of

interconnected micron-sized pores is dispersed throughout the material. The permeable steel shows a vastly increased surface, so that the heat transfer in the primary heat transfer device is augmented. Especially when using a cooling fluid which is in the gaseous state after expansion a large heat transfer surface in the primary heat transfer device is advantageously.

The permeable steel can be situated inside a cooling fluid side of the primary heat transfer device where the cooling fluid flows, which is particularly preferred in the case the cooling fluid is in the gaseous state.

Permeable steel may also be situated inside a secondary cooling fluid side of the primary heat transfer device where the secondary cooling fluid flows. Although it is preferred to use a secondary cooling fluid in the liquid state, and the liquid fluid can exchange more heat already with standard heat transfer devices, an increase in heat transfer surface is even so advantageously. When both sides, the cooling fluid side as well as the secondary cooling fluid side, comprise the permeable steel, the whole primary heat transfer device can be made from permeable steel, thereby simplifying production process.

The primary heat exchange device can further comprise an inner sealing housing to separate the cooling fluid side from the secondary cooling fluid side.

The permeable steel inside the cooling fluid side then is in direct contact with the inner sealing housing and/or the permeable steel inside the secondary cooling fluid side then is in direct contact with the inner sealing housing.

In a particularly preferred embodiment, an expander valve at an outlet of the cooling fluid tank is provided. In other words, the cooling fluid tank has an outlet for releasing the cooling fluid. The valve allows for control or regulation of the cooling fluid flow. The valve can be situated directly next to the cooling fluid tank, but it also possible, that the outlet bridges a spacing between the cooling fluid tank and the valve.

The expander valve may even be situated inside the primary heat transfer device, which is particularly preferred. As the maximal cooling effect occurs directly at the expander valve, the whole cooling capability of the cooling fluid can thus be used for heat exchange. Preferably, the primary heat exchange device further comprising an external pressure housing for resisting pressure differences between the outside of the primary heat exchange device and the inside of the primary heat exchange device. The external pressure housing surrounds the primary heat exchange device.

Preferably, the downhole tool is designed as an autonomous downhole tool in this respect, that it is able to operate independently, e.g. without any cables or external power supply connected to it, in the well bore. The cooling fluid tank therein can be seen as a fuel tank defining a time duration for maintaining provision of cooling energy.

Advantageously, the downhole tool is designed such, that due to the stored cooling energy in the cooling fluid tank a time duration long enough to perform the desired actions in the wellbore is obtained. In other words, the cooling fluid tank is sized such, that a calculatable amount of cooling fluid is storable in the cooling fluid tank. For example, the cooling system is particularly preferred designed such, that it is able to perform the downhole tool operation for a time duration of about 48 hours. Further exemplarily, the cooling fluid tank may be sized to hold the liquid amount corresponding to 2 to 3 cubic meters of nitrogen gas.

In addition, the autonomous downhole tool may comprise a stand-alone power supply such as a battery pack.

Particularly preferred the stand-alone power supply can be driven by the exhaust flow of cooling fluid. This can be taken e.g. between the cooling fluid tank and the expansion valve and/or at the cooling fluid outlet. The cooling fluid tank then has the additional effect of comprising a power generator for extending the lifetime of the downhole tool, e.g. by feeding said battery pack. The cooling system may be adapted such that the downhole tool is able to operate in harsh environmental conditions such as an outside fluid pressure of at least 100 Bar (10 MPa) , at least 1000 Bar (lOOMPa), at least 5000 Bar (500 MPa) and/or up to 20000 Bar (2000 MPa) , or even at an outside fluid pressure of above 20000 Bar (2000 MPa) . The cooling system is advantageously also designed to sustain an outside fluid temperature of at least 373K, at least 423K and/or up to 473K and/or even an outside fluid

temperature of above 473K.

Provided is also a cooling system for enabling operation of electronics in a downhole tool, for example a downhole tool according to any of the preceding claims, comprising: a cooling fluid tank fillable with a cooling fluid, a heat transfer device for exchange of heat between the cooling fluid and the downhole tool equipment, wherein the cooling fluid tank is connected with a cooling fluid outlet at a housing of the downhole tool, the cooling fluid outlet being open to said outside fluid for dissipation of said cooling fluid after the heat exchange. The cooling system may be designed to retrofit traditional downhole

measurement tools.

The downhole tool being adapted to operate in the well bore fluid in a well bore may comprise an elongated housing. The housing may further comprise a first chamber for installation of the downhole tool equipment. The downhole tool may be part of a multifunctional downhole tool which, for example, collects data in the well bore and/or the reservoir or which operates other functions particularly for sustaining the well bore, e.g. does cementations of an outer wall of the well bore or the like. The downhole tool can also comprise the functionality of a communication equipment in order to exchange data e.g. with a central station in the extraction facility.

The invention is described in more detail and in view of preferred embodiments hereinafter. Reference is made to the attached drawings wherein like numerals have been applied to like or similar components.

Brief Description of the Figures

It is shown in

Fig. 1 a schematic cross-sectional view of an earth

formation with a downhole tool in a well bore;

Fig. 2 another schematic cross-sectional view of an

earth formation with a downhole tool in a well bore having a horizontal section partly covered by a liner;

Fig. 3 a sideview of a downhole tool showing the cooling system;

Fig. 3a a sectional view of the primary heat exchange

device ;

Fig. 3b a sectional view of downhole tool equipment

Fig. 4 another embodiment of the primary heat exchange device . Detailed Description of the Invention

In Fig. 1 a well bore 2 is drilled in an earth formation 4 to exploit natural resources like oil or gas. The well bore 2 continuously extends from the extraction facility 9 at or near the surface 6 to a reservoir 8 of the well bore 2 situated distal from the wellhead 10 at the extraction facility 9.

A casing/liner 12 in the form of an elongated steel pipe or steel tubing is located within the well bore 2 and

extending from the wellhead 10 to an underground section of the well bore 2. The reservoir 8 and/or the casing/liner 12 are typically filled with a fluid 16, 17, 18, respectively. The fluids 16, 17, 18 are e.g. oil or gas in case of a production well or water, CO2 or nitrogen in case of an injection well. A downhole tool 20 is located within the casing or liner 12. Advantageously, the downhole tool 20 operates

autonomously having internal power storage 92 (see e.g. Fig. 2) and thus needs not be powered or wired externally. To sum up, the downhole tool 20 can be operated freely in the well bore 2 and needs not to be cable linked to the surface .

The downhole tool 20 may additionally be a movable downhole tool 20 being moved by moving means 21, generally known to the skilled person, within the casing or liner 12 to any desired position in the casing or liner 12 or even in the reservoir 8. The downhole tool 20 is equipped with a cooling system 15 according to the invention in order to perform measurements in the well bore 2 under harsh environmental conditions.

Fig. 2 shows another earth formation with a downhole tool 20 positioned in a horizontal portion of the casing/liner 12. The liner 12 in this embodiment only partly covers the well bore 2. The downhole tool 20 comprises a power supply 92.

Fig. 3 depicts a sketched sideview of a part of the

downhole tool 20. The cooling fluid tank 30 is fillable with cooling fluid 35, e.g. a high pressure liquefied nitrogen 35a.

The elongated housing 28 has a cooling fluid outlet 26 for disposal of used cooling fluid 35. The fluid outlet 26 comprises a channel portion 26a which links the primary heat exchange device 40 with the outside fluid 18.

The primary heat exchange device 40 is in the embodiment of Fig. 3 situated between the cooling fluid tank 30 and the downhole tool equipment 60. Between the primary heat exchange device 40 and the cooling fluid tank 30 lies a valve 34, which in this case simply comprises an open state and a closed state. The valve 34 therefore is a shutdown valve 34. The valve 34 is connected to the cooling fluid tank outlet 32. Via the shutdown valve 34 provision of cooling fluid 35 to the primary heat exchange device 40 is selectable . In the embodiment of Fig. 3 a choke 46 is placed behind the shutdown valve 34 to adjust the amount of cooling fluid 35 provided to the primary heat exchange device 40. The choke 46 can either be a fixed choke 46 or a variable choke 46.

In a cooling fluid side of the primary heat exchange device 40 thermal conductive plates 44 are arranged for thermal interaction with the cooling fluid 35. The cooling fluid 35 can be guided through the thermal conductive plates 44 to intensify the exchange of thermal energy. After exchanging thermal energy the cooling fluid 35 is disposed of by way of releasing it to the outside fluid 18 through the cooling fluid outlet 26. A secondary cooling fluid side of the primary heat exchange device 40 is in the embodiment of Fig. 3 constituted by heat transfer piping 42 situated at an outer side of the primary heat transfer device 40. In other words, the cooling fluid side of the primary heat exchange device 40 is surrounded by the secondary cooling fluid side.

The secondary cooling fluid 45 is transported via an elongated portion of the heat transfer piping 42 to the downhole tool equipment 60. The downhole tool equipment 60 is, in the embodiment of Fig. 3, e.g. an electronic board

60a with components 60b placed on the electronic board 60a. The downhole tool equipment 60 is embedded in a thermal conductive material 52 to transport the heat to the heat transfer body 50 situated around the electronic board 60a. While flowing through the heat transfer body 50 the

secondary cooling fluid 45 exchanges thermal energy with the downhole tool equipment 60. After passing the heat transfer body 50 the secondary cooling fluid 45 is directed to a circulation pump 54 which provides a circulation of the secondary cooling fluid 45 in the downhole tool. The secondary cooling fluid 45 is then fed again to the primary heat exchange device 40.

The excess volume inside the downhole tool is filled with thermal insulation material 70 and provides separation of the downhole tool equipment 60 from the housing 28.

Fig. 3a shows a sectional drawing of the spot marked by line A-A in Fig. 3 and shows details of the primary heat exchange device 40. The thermal conductive plates 44 are arranged concentrically in the inner region of the primary heat exchange device 40. Heat transfer piping 42 is

arranged such that it encircles the thermal conductive plates 44. Farther outwards the thermal insulation material 70 is installed surrounding the primary heat exchange device 40. Although the primary heat exchange device 40 comprises a round shape, also other shapes such as a rectangular shape is usable.

Fig. 3b shows a cross sectional view of the spot marked by line B-B in Fig. 3 and shows details of the downhole tool equipment 60 and the heat transfer body 50. The heat transfer body 50 has an upper portion 50a and a lower portion 50b which are both substantially flat. The

secondary cooling fluid 45 flows through the upper and lower portion 50a, 50b of the heat transfer body 50. A wall portion 50c links the upper and the lower portion 50a, 50b, provides an augmented heat transfer to the upper and/or lower portion 50a, 50b of the heat transfer body 50 and/or provides for an increased stability of the heat transfer body 50. It is preferred, that the secondary cooling fluid 45 does not flow through the wall portion 50c.

The electronic board 60a is situated along both sides of the wall portion 50c and preferably attached to the wall portion 50c, so that heat can be transferred quickly to the heat transfer body 50 and thus out of the downhole tool 20.

Fig. 4 shows a particularly preferred embodiment of the primary heat exchange device 40. Cooling fluid 35 is provided to a cooling fluid side 40a of the primary heat exchange device 40. Secondary cooling fluid 45 is provided to a secondary cooling fluid side 40b. The expander 36 is arranged in the cooling fluid side 40a, which is inside the primary heat exchange device 40. The primary heat exchange device 40 is filled with permeable steel 47, e.g. Gas

Permeable Mold Steel 47, in order to maximize transfer of thermal energy in the primary heat exchange device 40.

The cooling fluid side 40a is surrounded by an inner sealing thin walled metal housing 43. The outer limit of the primary heat exchange device 40 is composed by an external pressure housing 48 which surrounds the secondary cooling fluid side 40b.

It will be appreciated that the features defined herein in accordance with any aspect of the present invention or in relation to any specific embodiment of the invention may be utilized, either alone or in combination with any other feature or aspect of the invention or embodiment. In particular, the present invention is intended to cover a downhole tool configured to include any feature described herein. It will be generally appreciated that any feature disclosed herein may be an essential feature of the invention alone, even if disclosed in combination with other features, irrespective of whether disclosed in the description, the claims and/or the drawings.

It will be further appreciated that the above-described embodiments of the invention have been set forth solely by way of example and illustration of the principles thereof and that further modifications and alterations may be made therein without thereby departing from the scope of the invention .

List of reference signs:

2 Well bore

4 earth formation

6 surface

8 reservoir

9 extraction facility

10 well head

12 casing/liner

15 Cooling system

16 fluid

17 fluid

18 fluid, outside fluid

20 downhole tool

21 moving means

26 cooling fluid outlet

26a channel portion

28 housing

30 cooling fluid tank

32 cooling fluid tank outlet

34 valve

35 cooling fluid

35a liquefied nitrogen

36 expander

40 primary heat exchange device

40a cooling fluid side

40b secondary cooling fluid side

42 heat transfer piping

43 inner sealing thin walled metal housing

44 Thermal conductive plate

45 secondary cooling fluid

46 choke

48 external pressure housing heat transfer body

a upper portion

b lower portion

c wall portion

thermal conductive material circulation pump

Downhole tool equipmenta Electronic board

b Board components

thermal insulation material stand-alone power supply

Claims

What is claimed:

1. Downhole tool (20) being adapted to operate in a well bore (2), comprising:

" a housing (28) surrounded by an outside fluid (16,

17, 18), the housing (28) having a cooling fluid outlet (26), the cooling fluid outlet being open to said outside fluid (16, 17, 18),

^■ an electronics compartment for installation of

downhole tool equipment (60, 60a, 60b),

^■ a cooling system (15), which comprises a cooling

fluid tank (30) fillable with a cooling fluid (35) and a heat transfer device (50, 50a, 50b, 50c, 52) for exchange of heat between the cooling fluid (35) and the downhole tool equipment (60, 60a, 60b) .

2. Downhole tool (20) according to the preceding claim, wherein the cooling fluid tank (30) is designed and prepared such, that it is fillable with a high pressure cooling fluid (35) , for example a pressurized liquid which expands to gas when extracted out of the cooling fluid tank (30) .

3. Downhole tool according to any of the preceding claims, Comprising further:

a secondary cooling fluid (45) separated from the cooling fluid (35) ,

a primary heat transfer device (40), wherein a heat exchange is provided between the cooling fluid (35) and the secondary cooling fluid (45) ,

wherein the secondary cooling fluid (45) is provided to the heat transfer device (50, 50a, 50b, 50c, 52)

Downhole tool (20) according to the preceding claim, wherein the secondary cooling fluid (45) is provided in the liquid state for improvement of heat exchange between secondary cooling fluid (45) and the downhole tool equipment (60, 60a, 60b).

Downhole tool (20) according to any of the preceding claims ,

wherein the cooling fluid (35) is gaseous under standard atmospheric conditions and liquefied when set under such a pressure to be established in the cooling fluid tank (30), in particular where the cooling fluid (35) is nitrogen.

Downhole tool (20) according to any of the preceding claims, wherein at least the downhole tool equipment (60, 60a, 60b) is embedded in a thermal insulating material (70).

Downhole tool (20) according to the preceding claim, wherein the thermal insulating material (70) is situated all along the inside of the housing (28) thereby embedding the cooling system (15) and the downhole tool equipment (60, 60a, 60b).

Downhole tool (20) according to any of the preceding claims ,

wherein the downhole tool equipment (60, 60a, 60b) comprises electronic systems (60a, 60b), for example a sensor or a measurement device for measurement of downhole properties or instabilities.

Downhole tool (20) according to any of the claims 3 to 8,

wherein the primary heat transfer device (40) comprises permeable steel (47) .

Downhole tool (20) according to claim 9,

wherein the permeable steel (47) is situated inside a cooling fluid side (40a) of the primary heat transfer device (40) where the cooling fluid (35) flows, and/or wherein the permeable steel (47) is situated inside a secondary cooling fluid side (40b) of the primary heat transfer device (40) .

Downhole tool (20) according to claim 10,

wherein the primary heat transfer device (40) further comprises an inner sealing housing (43) to separate the cooling fluid side (40a) from the

secondary cooling fluid side (40b) ,

wherein the permeable steel (47) inside the cooling fluid side (40a) is in direct contact with the inner sealing housing (43) and/or wherein the permeable steel (47) inside the secondary cooling fluid side (40b) is in direct contact with the inner sealing housing (43) .

Downhole tool (20) according to any of the preceding claims ,

further comprising an expander valve (36, 46) at an outlet (32) of the cooling fluid tank (30),

the expander valve (36, 46), for example, being situated inside the primary heat transfer device (40)

Downhole tool (20) according to any of the claims 3 to 12,

the primary heat transfer device (40) further

comprising an external pressure housing (48) for resisting pressure differences between the outside of the primary heat transfer device (40) and the inside of the primary heat transfer device (40) .

Downhole tool (20) according to any of the preceding claims ,

wherein the downhole tool (20) is designed autonomously, e.g. without any cables connected to it, and due to the stored cooling energy in the cooling fluid tank (30) capable of performing the desired actions in the wellbore for a long enough time

duration .

15. Downhole tool (20) according to any of the preceding claims ,

wherein the cooling system (15) is adapted such that the downhole tool (20) is able to operate in harsh environmental conditions such as an outside fluid pressure of at least 100 Bar, at least 500 Bar and/or up to 2000 Bar, or even at an outside fluid pressure of above 2000 Bar and/or an outside fluid temperature of at least 373K, at least 423K and/or up to 473K and/or even an outside fluid temperature of above 473K.

16. Cooling system (15) for enabling operation of

electronics (60a, 60b) in a downhole tool (20), for example a downhole tool according to any of the

preceding claims, comprising:

^■ a cooling fluid tank (30) fillable with a cooling fluid (35),

^■ a heat transfer device (40, 50, 52) for exchange of heat between the cooling fluid (35) and the downhole tool equipment (60, 60a, 60b),

^■ wherein the cooling fluid tank (30) is connected with a cooling fluid outlet (26) at a housing

(28) of the downhole tool (20), the cooling fluid outlet (26) being open to said outside fluid (16, 17, 18) for dissipation of said cooling fluid

(35) after the heat exchange.