Geothermal wells and tubing devices therefor

Abstract

The present invention relates to a tubing apparatus (101, 201) for use in a geothermal well (100). The tubing apparatus (101, 201) includes a vacuum insulated tubing string (102, 202) delineating a conduit (122, 222) and a cooling string (107, 207) received within the conduit (122, 222). The conduit (122, 222) is configured to transport a working fluid between a surface location (A) and a subsurface location (B). The cooling string (107, 207) is configured to inject a cooling fluid into the conduit (122, 222) to regulate a temperature of the working fluid.

Description

Technical Field

[0001] The present invention generally relates to geothermal wells and tubing devices for such geothermal wells. More specifically, but not exclusively, the present invention relates to such geothermal wells, tubing devices for use in geothermal wells, and methods of starting geothermal wells.

[0002] Geothermal wells and their associated production systems are well known in the art. The basic operation of a geothermal well requires a deep borehole that connects to a high-temperature reservoir aquifer. The high-temperature fluid from the aquifer, also known as the "working fluid," is carried to the surface through the production well where heat can be extracted. The working fluid is then pumped back into the aquifer and refilled into the reservoir. This is generally done through a closed-loop system, but not always.

[0003] In some cases, the production well includes a heat-insulated tubing string received within a cased borehole. During normal operation, fluid is pumped down the heat-insulated tubing string, heated, and rises through the annular space between the tubing string and the cased borehole and is returned towards the surface. The heat-insulated tubing string is provided to reduce heat loss when the working fluid is transported above the annular space.

[0004] At the initial stage of the operation of the borehole, the bottom hole temperature is very high and may exceed 400 degrees Celsius. Following the startup phase, the bottom hole temperature decreases over time and reaches a steady temperature, for example 150 degrees Celsius. This is shown in FIG. 1.

[0005] Thus, there is a large temperature difference between the working fluid pumped down the heat-insulated tubing string and the working fluid transported through the annular space during startup.

[0006] Furthermore, due to the insulating properties of the insulated tubing string, the working fluid experiences a very slight temperature drop as it rises. Consequently, a large temperature difference can occur between the inner and outer diameters of the insulated tubing string. This can cause mechanical stress and, in some cases, lead to the failure of the insulated tubing string.

[0007] It is possible to increase resistance to mechanical stress and failure through design. However, this results in larger, heavier, and more expensive insulated tubing strings. It is also possible to reduce temperature fluctuations by slowing down the starting phase of the geothermal well. [Overview of the project] [Problems that the invention aims to solve]

[0008] Therefore, a first non-exclusive object of the present invention is to provide a tubing device that overcomes, or at least mitigates, known problems of the prior art. In particular, a non-exclusive object of the present invention is to provide a tubing device that mitigates the aforementioned problems during the starting of a geothermal well. [Means for solving the problem]

[0009] Accordingly, one aspect of the present invention provides a tubing device for use in geothermal wells. The device comprises an insulating tubing string that forms a conduit configured to transport a working fluid between a surface position and an underground position, and a cooling string configured to regulate the temperature of the working fluid inside or around the conduit, and / or, during use, as the working fluid is transported toward the surface position.

[0010] By providing a cooling string to regulate the temperature of the working fluid, it is possible to reduce the temperature difference between the fluid being transported towards the ground level inside or around the conduit, typically within an insulated tubing string or the annular space surrounding the insulated tubing string, and the fluid around or inside the conduit, respectively, during startup.

[0011] The tubing device may further include a casing string. The insulated tubing string may be arranged within the casing string, for example, such that an annular space is drawn between them.

[0012] The tubing device may be received in a geothermal well, for example, as described herein.

[0013] Another aspect of the present invention provides a geothermal well. The geothermal well comprises a well receiving an insulated tubing string inside which a conduit is configured to transport a working fluid between a surface location and an underground location, and a cooling string configured to regulate the temperature of the working fluid as it is transported toward the surface location during use.

[0014] A tubing device or geothermal well may be operable or configured to transport the working fluid from an underground location to a surface location, for example, through an insulated tubing string, during use. A tubing device may be operable or configured to transport the working fluid from a surface location to an underground location, for example, through an insulated tubing string and an annular space between the casing string or the well.

[0015] Another aspect of the present invention provides a tubing device for use in geothermal wells. The device comprises an insulated tubing string arranged within a casing string. The device is operable or configured to, in use, transport a working fluid from an underground position to a surface position through the insulated tubing string, and also transport the working fluid from the surface position to an underground position through an annular space between the insulated tubing string and the casing string.

[0016] Another aspect of the present invention provides a geothermal well comprising an insulated tubing string positioned within the well. The geothermal well is operable or configured to, in use, transport a working fluid from an underground position to a surface position through the insulated tubing string, and also transport a working fluid from a surface position to an underground position through an annular space between the insulated tubing string and the well.

[0017] The inventors observed that in conventional flows, the working fluid temperature decreases via conduction through the well casing as the working fluid is transported upward through the annular space. By reversing the direction of flow, the working fluid is heated as it is pumped down the well because it is exposed to the surrounding geological formations. Conversely, the heat is retained in the fluid as it is returned upward through the adiabatic tubing string.

[0018] The cooling string may be housed within an insulated tubing string. The cooling string may be removably housed within the insulated tubing string, making it removable, for example, after startup. This configuration is useful when the working fluid is transported up the insulated tubing string, because such a reverse flow configuration can result in more abrupt temperature differences.

[0019] Alternatively, the cooling string may be positioned outside the insulating tubing string and / or between the insulating tubing string and the casing string. The cooling string may be removably received between the insulating tubing string and the casing string, and may be removable, for example, after startup. This configuration is useful when the working fluid is transported up the annular space.

[0020] An insulated tubing string may include a pipe-in-pipe insulated tubing string. A pipe-in-pipe insulated tubing string may have a vacuum or an inert gas between the multiple pipes constituting the pipe-in-pipe insulated tubing string. An insulated tubing string or a pipe-in-pipe insulated tubing string may include a vacuum insulated tubing string. An insulated tubing string or a pipe-in-pipe insulated tubing string may include an insulated tubing string containing an inert gas, such as argon.

[0021] The insulated tubing string may be arranged substantially concentrically with the casing string and / or the well. The insulated tubing string may be longer than the casing string and / or may extend beyond the casing string, for example, into the well and / or down to the bottom of the well, when in use.

[0022] The cooling string may be configured to inject or transport a fluid, such as a cooling fluid, into the conduit during use, for example, to regulate the temperature of the working fluid. The cooling fluid may be configured to lower the temperature of the working fluid. The lower end of the cooling string is preferably open or free. The cooling fluid may comprise a cooled portion of the working fluid. The tubing device may include a bypass circuit or cooling circuit or bypass cooling circuit that can be configured to cool a portion of the working fluid. The cooling string may be supplied by a bypass circuit or cooling circuit or bypass cooling circuit.

[0023] The temperature can be more easily controlled by directly injecting the coolant into the working fluid.

[0024] Alternatively, the lower end of the cooling string may be closed. In that case, the cooling string may have one or more holes or perforations on its sidewall, or may include a cooling circuit formed by a pair of concentric tubes.

[0025] The cooling string may have a flow area smaller than that of the heat-insulating tubing string and / or the annular space. The cooling string may have a diameter smaller than the radial width of the heat-insulating tubing string and / or the annular space. This minimizes the impact of the cooling string on the flow of the working fluid and reflects the intention of making the flow rate of the cooling fluid lower compared to the working fluid.

[0026] The heat-insulating tubing string may have a substantially circular cross-sectional shape. The cooling string may have a substantially circular cross-sectional shape. Alternatively, the heat-insulating tubing string and / or the cooling string may have a non-circular shape, such as a substantially elliptical cross-sectional shape. The cross-sectional shape of the heat-insulating tubing string may be the same, similar, or different from that of the cooling string. The shape may be optimized to minimize the impact on the flow of the working fluid.

[0027] The cooling string may be arranged or positioned adjacent to the inner wall of the heat-insulating tubing string. Alternatively, the cooling string may be arranged concentrically with the heat-insulating cooling string. This arrangement also reduces the impact on the flow of the working fluid.

[0028] The cooling string may be held at a predetermined position, for example, with respect to the heat-insulating tubing string, by one or more brackets or supports. The apparatus may include one or more brackets or supports configured to attach or support the cooling string with respect to the heat-insulating tubing string. The cooling string may be attached or held at a predetermined position by a plurality of brackets or supports. The brackets or supports are arranged at intervals along the cooling string and / or at intervals along the heat-insulating tubing string.

[0029] The bracket or support may be connected, attached, or fixed to the heat-insulating tubing string. Alternatively, the bracket or support may be connected, attached, or fixed to the cooling string.

[0030] The cooling string may be shorter than the heat-insulating tubing string. The lower end or free end of the cooling string may be arranged to be positioned shorter than the lower end or free end of the heat-insulating tubing string. As a result, the cooling fluid is introduced upstream within the region where the temperature difference is highest.

[0031] The cooling string may be removable from the heat-insulating tubing string. The cooling string may be provided on a coiled tubing arrangement. Thus, the cooling string can be removed after the startup phase. Alternatively, the cooling string may be permanently installed within the heat-insulating tubing string.

[0032] The lower end of the heat-insulating tubing string may be open and / or may be arranged to be located at the open hole portion of the shaft. The open hole portion may be the uncased portion of the shaft

[0033] The tubing apparatus may include a controller. The controller may be configured to control the flow rate and / or pressure of the working fluid. The controller may also be configured to control the flow rate and / or pressure of the cooling fluid. This allows the temperature of the working fluid to be adjusted gradually and dynamically.

[0034] Another aspect of the present invention provides a controller. The controller may be configured to control the flow rate and / or pressure of a working fluid. The controller may be configured to control the flow rate and / or pressure of a cooling fluid.

[0035] Another aspect of the present invention provides a wellhead control assembly. The wellhead control assembly may include a controller. The controller may be configured to control the flow rate and / or pressure of the working fluid, and / or the flow rate and / or pressure of the cooling fluid, through the tubing device of the geothermal well. The tubing device may be one of the tubing devices described above. The wellhead control assembly may include a tubing device such as, for example, one described herein.

[0036] The tubing device or control assembly may include one or more flow sensors. One or more flow sensors may be configured to measure the flow rate through the annular space, the adiabatic tubing string, and / or the cooling string during use. One or more flow sensors may include a first flow sensor configured to measure the flow rate through the annular space during use. One or more flow sensors may include a second flow sensor configured to measure the flow rate through the cooling string during use. One or more flow sensors may include a third flow sensor configured to measure the flow rate through the adiabatic tubing string during use. This enables closed-loop feedback of the relative flow rates of the working fluid and the cooling fluid.

[0037] The tubing device or control assembly may include one or more pressure sensors. One or more pressure sensors may be configured to measure pressure in the annular space, the adiabatic tubing string, and / or the cooling string during use. One or more pressure sensors may include a first pressure sensor configured to measure pressure in the annular space during use. One or more pressure sensors may include a second pressure sensor configured to measure pressure in the cooling string during use. One or more pressure sensors may include a third pressure sensor configured to measure pressure in the adiabatic tubing string during use. This enables closed-loop feedback of the relative pressure of the working fluid and the cooling fluid.

[0038] The tubing device or control assembly may include one or more flow control valves. One or more flow control valves may be configured to control the flow of fluid through the annular space and / or adiabatic tubing string. One or more flow control valves may be configured to control the flow of fluid through the cooling string. One or more flow control valves may include one or more first flow control valves for controlling the flow of fluid through the annular space and / or adiabatic tubing string. One or more flow control valves may include a second flow control valve for controlling the flow of fluid through the cooling string. This allows for gradual and dynamic adjustment of the relative flow rates of the working fluid and the cooling fluid.

[0039] The tubing device or control assembly may include one or more temperature sensors. These one or more temperature sensors may be positioned at or near the ground level in the adiabatic tubing string. These one or more temperature sensors may be configured to measure the temperature of the working fluid within the adiabatic tubing string. These one or more temperature sensors may be positioned at or near the lower or free end of the cooling string. This allows for closed-loop regulation of the relative flow rates of the working fluid and cooling fluid to achieve the desired temperature in the working fluid.

[0040] Another aspect of the present invention provides a geothermal well. The geothermal well may include a tubing device as described above. The geothermal well may include a controller as described above. The geothermal well may include a control assembly as described above.

[0041] In some embodiments, the geothermal well is a closed-loop geothermal well.

[0042] Another aspect of the present invention provides a method for operating or starting a geothermal well. The method includes the steps of circulating a working fluid between a surface location and a subsurface location via an insulated tubing string, and regulating the temperature of the working fluid as it is circulated toward the surface using a cooling string.

[0043] The method may include the step of transporting the working fluid from a surface location to an underground location, for example, through an insulated tubing string. The method may include the step of transporting the working fluid from an underground location to a surface location, for example, through an annular space between an insulated tubing string and a casing string or well. The method may include the step of transporting the working fluid from an underground location to a surface location, for example, through an insulated tubing string. The method may include the step of transporting the working fluid from a surface location to an underground location, for example, through an annular space between an insulated tubing string and a casing string or well.

[0044] Another aspect of the present invention provides a method for operating a geothermal well. The method includes the steps of transporting a working fluid from an underground location to a surface location through an insulated tubing string, and transporting the working fluid from the surface location to an underground location through an annular space between the insulated tubing string and a casing string or well.

[0045] The method may include, for example, the step of injecting a cooling fluid through a cooling string and / or into the working fluid to regulate the temperature of the working fluid as it circulates toward the surface. The cooling fluid may be injected to lower the temperature of the working fluid.

[0046] The flow rate of the working fluid circulating between the surface and the bottom of the shaft, and / or the flow rate of the cooling fluid injected into the working fluid, may be controlled, for example, to manage the temperature of the working fluid received back to the surface.

[0047] The step of injecting a cooling fluid through a cooling string may include the step of injecting a working fluid. The method may include the step of cooling a portion of the working fluid, for example, before injecting it through a cooling string. The method may also include the step of bypassing a portion of the working fluid, for example, through a bypass circuit or a cooling circuit or a bypass cooling circuit, before injecting it through a cooling string. The working fluid may include water or other fluids, such as a supercritical fluid.

[0048] The method may include the step of removing the cooling string when the temperature of the working fluid received at the ground surface falls below a predetermined threshold.

[0049] To avoid any doubt, any features described herein apply equally to any aspect of the present invention. For example, a tubing device may include any one or more features of a method relating to a tubing device, and / or a method may include any one or more features or steps relating to one or more features of a tubing device or a cooling string.

[0050] A further aspect of the present invention provides a computer program element including computer-readable program code means for causing a processor to perform a procedure for carrying out one or more steps of the above-described method.

[0051] A further aspect of the present invention provides computer program elements embodied on a computer-readable medium.

[0052] A further aspect of the present invention provides a computer-readable medium on which a program is stored. The program is configured to cause a computer to perform a procedure for carrying out one or more steps of the method described above.

[0053] Further aspects of the present invention provide control means, control systems, or controllers that include the aforementioned computer program elements or computer-readable media.

[0054] For the purposes of this disclosure, notwithstanding the foregoing, it should be understood that any controller, control unit, and / or control module described herein may each comprise a control unit or computing device having one or more electronic processors. A controller may comprise a single control unit or electronic controller. Alternatively, different functions of the controller of a system or device may be embodied or hosted in different control units or controllers or control modules. As used herein, the terms “control unit” and “controller” are understood to include both a single control unit or controller and a group of control units or controllers that operate collectively to provide the required control functions. A set of instructions may be provided that, when executed, causes the aforementioned controller or control unit or control module to perform the control techniques described herein (including those described herein).

[0055] The instruction set may be embedded in one or more electronic processors, or alternatively, provided as software executed by one or more electronic processors. For example, the first controller may be implemented in software executed on one or more electronic processors, and one or more other controllers may be implemented in software executed on one or more electronic processors. Optionally, there may be one or more processors, the same as the first controller. However, other configurations are also useful. It will be understood that the present invention is not intended to be limited to any particular configuration. In any case, the instruction set described herein may be embedded in a computer-readable storage medium (e.g., a non-temporary storage medium) which may have any mechanism for storing information in a form readable by a machine or electronic processor / computer. This includes, but is not limited to, magnetic storage media (e.g., floppy diskettes), optical storage media (e.g., CD-ROMs), magneto-optical storage media, read-only memory (ROM), random access memory (RAM), erasable programmable memory (e.g., EPROM and EEPROM), flash memory, or electrical or other types of media for storing such information / instructions.

[0056] Within the scope of this application, the various aspects, embodiments, examples, and substitutes described in the preceding paragraphs, claims, and / or the following description, and in particular in the drawings, in terms of their individual features, are expressly intended to be obtained independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, except in cases where such features are incompatible.

[0057] To avoid any doubt, terms such as “may,” “and / or,” and “e.g., for example” used herein should be interpreted as non-limiting, meaning that the features described in this way do not necessarily have to exist. In fact, any combination of optional features, whether or not they are expressly claimed, is expressly assumed without departing from the scope of the invention. The applicant reserves the right to modify the initially filed claims or to file new claims accordingly, including the right to amend the initially filed claims to be dependent on and / or to incorporate features of any other claims, even if they were not originally claimed so.

[0058] Herein, embodiments of the present invention will be described as examples with reference to the accompanying drawings. [Brief explanation of the drawing]

[0059] [Figure 1] This graph shows an example of the decrease in downhole temperature during a typical startup phase. [Figure 2] This is a schematic diagram of a conventional geothermal well tubing system. [Figure 3] This is a schematic diagram of a geothermal well tubing device according to an embodiment of the present invention. [Figure 4] Figure 3 is a schematic diagram of a geothermal well with the tubing device attached. [Figure 5] This graph shows the temperature of the working fluid during the startup phase, i.e., when flowing along the tubing apparatus in Figure 2, without using a cooling string. [Figure 6] This graph shows the working fluid temperature when the working fluid flows along the tubing apparatus shown in Figure 2, while the well is in a steady state. [Figure 7] This is a schematic diagram of a geothermal well tubing device according to another embodiment of the present invention. [Modes for carrying out the invention]

[0060] Referring now to Figure 2, a conventional tubing apparatus 1 for a geothermal well is shown. The tubing apparatus 1 includes a vacuum-insulated tubing string 2. The vacuum-insulated tubing string 2 is housed within an intermediate casing string 3. The intermediate casing string 3 is then housed within a surface casing string 4. The vacuum-insulated tubing string 2 and the casing strings 3 and 4 together form the well.

[0061] Each of the vacuum-insulated tubing string 2, intermediate casing string 3, and surface casing string 4 extends from surface position A into geological formation F, which in this embodiment is in the form of a high-temperature sedimentary aquifer. The intermediate casing string 3 includes a casing shoe 30 at its lower end. Similarly, the casing string 4 also includes a casing shoe 40 at its lower end.

[0062] The vacuum-insulated tubing string 2 has a first open end 20 located at ground level A and connected to a working fluid source, and a second open end 21 located within the (uncased) portion of the open hole in the well 5. The second open end 21 of the vacuum-insulated tubing string 2 is located deeper than the casing shoes 30 and 40 of the intermediate casing string 3 and the ground-level casing string 4, respectively. The vacuum-insulated tubing string 2 forms an insulated conduit 22 extending between ground level A and geological formation F.

[0063] The annular space 6 is located between the vacuum-insulated tubing string 2 and the intermediate casing string 3 or open section of the well 5. The annular space 6 extends between the ground surface A and the bottom of the well B.

[0064] During use, the working fluid is pumped down the conduit 22 from the surface position A during normal operation of the geothermal well. The fluid exits the vacuum-insulated tubing string 2 at the second open end 21, where it is exposed to the geological formation F.

[0065] The working fluid then returns along the annular space 6 towards the surface position A. As the working fluid passes along the annular space 6, it is heated by the heat of the surrounding geological layer F. The heat is then extracted from the working fluid when it reaches the surface position A.

[0066] According to one aspect of the present invention, circulation may be reversed. This involves pressurizing the fluid into the annular space 6 between the vacuum-insulated tubing string 2 and the intermediate casing string 3. The working fluid is then transported upward through the conduit 22 from the bottom of the tunnel B to the surface A.

[0067] As mentioned above, reversing the direction of flow improves the efficiency of geothermal wells. More specifically, the working fluid is heated as it is pumped down the well by exposure to the surrounding geological formations. Conversely, the heat is retained in the fluid as it is returned up the adiabatic tubing string.

[0068] However, in order to avoid a large temperature difference between the fluid being pumped down the annular space 6 and the working fluid being transported up the vacuum-insulated tubing string 2, the start-up phase needs to be performed over a long period of time.

[0069] Referring now to Figure 3, a tubing device 101 for a geothermal well is shown, which combines another aspect of the present invention. Tubing device 101 is similar to tubing device 1. Similar features are indicated by similar reference numbers with "100" added. These are not described further here.

[0070] Unlike the tubing device 1 in Figure 2, the tubing device 101 according to the present invention is provided with a cooling string 107. In the embodiment shown in Figure 3, such a cooling string is located within a vacuum-insulated tubing string 102. The cooling string 107 is located adjacent to the side wall of the vacuum-insulated tubing string 102, thereby minimizing its influence on the flow of the working fluid through the vacuum-insulated tubing string 102.

[0071] The cooling string 107 has a first open end 170 connected to a cooling fluid source located at ground level A, and a second open end 171 located inside the conduit 122 and shorter than the second open end 121 of the vacuum-insulated tubing string 102. The cooling string 107 is configured to inject a cooling fluid, such as water or working fluid, into the conduit 122, passing through a bypass cooling circuit, in order to regulate the temperature of the working fluid in the conduit 122.

[0072] Referring now to Figure 4, a geothermal well 100 is shown to which the tubing device 101 is attached. The geothermal well 100 also includes a wellhead control assembly 110 including a controller 111, a first flow sensor 112 configured to measure the flow rate through the annular space 106, a second flow sensor 113 configured to measure the flow rate through the cooling string 107, and a third flow sensor 114 configured to measure the flow rate through the vacuum-insulated tubing string 102.

[0073] The head control assembly 110 also includes a temperature sensor 115 and a flow control valve (not shown). The temperature sensor 115 is located on the surface of the vacuum-insulated tubing string 102 and is close to the open end 171 of the cooling string 107. The temperature sensor 115 is configured to measure the temperature of the working fluid. The flow control valve (not shown) is configured to control the flow of fluid through the annular space 106, through the vacuum-insulated tubing string 102, and through the cooling string 107.

[0074] Each of the flow sensors 112, 113, and 114, the temperature sensor 115, and the flow control valve (not shown) are operably connected to the controller 111. This may be via a wired or wireless connection.

[0075] During normal operation, the geothermal well 100 functions essentially the same as described above with respect to a geothermal well having the tubing device 1 shown in Figure 2. However, the cooling string 107 has a specific purpose in relation to the operation of the geothermal well 100, particularly during the start-up phase.

[0076] More specifically, during the startup phase of the geothermal well 100, the working fluid circulates down the annular space 106 and up the conduit 122 back to surface position A due to the aforementioned reverse circulation of the working fluid. During the reverse circulation, the working fluid enters the vacuum-insulated tubing string 102 at the second open end 121.

[0077] Figure 5 shows the temperature of the working fluid in the tubing apparatus 1 during the starting phase. The temperature of the formation F along the depth of the well is indicated by the first line Fa. The temperature of the working fluid flowing down the annular space 6 is indicated by the second line 6a. The temperature of the working fluid flowing up the vacuum-insulated tubing string 2 is indicated by the third line 2a. As is clear from this figure, at the surface, there is a temperature difference of more than 240°C between the fluid temperature in the vacuum-insulated tubing string 2 and the fluid temperature in the annular space 6 surrounding the vacuum-insulated tubing string 2.

[0078] The fluid entering the vacuum-insulated tubing string 2 at the second open end 21 is at or close to the bottom of the tunnel. Due to the adiabatic properties of the vacuum-insulated tubing string 2, there is almost no temperature drop in the working fluid as it moves toward the ground surface A.

[0079] Figure 6 shows the temperature of the working fluid in the tubing device 1 during the steady state. Similar to the graph in Figure 5, the first line Fb represents the temperature of the formation F along the depth of the well, the second line 6b represents the temperature of the working fluid flowing down the annular space 6, and the third line 2b represents the temperature of the working fluid flowing up the vacuum-insulated tubing string 2. In this steady state, the temperature difference at the ground surface is approximately 40°C.

[0080] Therefore, the vacuum-insulated tubing string 2 must be designed to withstand very high temperature differences only during the start-up phase, or the start-up phase must be carried out very slowly. Those skilled in the art will understand that both of these have a substantial impact on cost.

[0081] To overcome this problem, a cooling fluid is injected into the working fluid through the cooling string 107 as it passes along the conduit 122, thereby lowering its temperature. The resulting working fluid has a lower temperature when it reaches ground level A. This provides a temperature difference between the working fluid in the vacuum-insulated tubing string 102 and the working fluid surrounding the vacuum-insulated tubing string 2 in the annular space 106. Ideally, the cooling fluid is injected during the startup phase to maintain a predetermined temperature difference between the working fluid in the vacuum-insulated tubing string 102 and the working fluid in the surrounding annular space 106.

[0082] Therefore, the present invention reduces the mechanical stress on the vacuum-insulated tubing string 102. This is achieved by introducing a cooling fluid from the cooling string 107 in the region where the working fluid would otherwise be at or near its highest temperature. The present invention thereby avoids the need for a larger, heavier, and more expensive vacuum-insulated tubing string, and also avoids the need to slow down the start-up phase.

[0083] The cooling fluid is, for example, some working fluid available near surface level A. This is a working fluid available near surface level A that has a lower temperature than the working fluid that rises to surface level A and travels through the bottom of the geothermal well while being heated.

[0084] Referring now to Figure 7, a tubing apparatus 201 according to another embodiment is shown. Tubing apparatus 201 is similar to tubing apparatus 3. Similar features are indicated by similar reference numbers with "100" added. These will not be described further here.

[0085] The tubing apparatus 201 according to this embodiment differs from the tubing apparatus 101 of Figure 3 in that the cooling string 207 is located in the annular space 206 between the vacuum-insulated tubing string 202 and the intermediate casing string 203 or open portion of the well 205. The cooling string 207 is positioned adjacent to the casing string 203, thereby minimizing its influence on the flow of the working fluid through the annular space 206.

[0086] The tubing device 201 is configured to pump the working fluid down the conduit 222 from the ground surface A. The working fluid is then exposed to the geological formation F and returned to the ground surface A by ascending the annular space 206. In this embodiment, a cooling fluid is injected into the working fluid via a cooling string 207. This lowers the temperature of the working fluid as it passes along the annular space 206.

[0087] Similar to the tubing apparatus 101 in Figure 3, the working fluid has a lower temperature when it reaches ground level A due to the cooling fluid introduced by the cooling string 207. This reduces the temperature difference between the working fluid in the vacuum-insulated tubing string 202 and the working fluid surrounding it in the annular space 206.

[0088] Those skilled in the art will understand that several modifications to the embodiments described above are conceivable without departing from the scope of the present invention. It will also be understood by those skilled in the art that any number of combinations of the features described above and / or the features shown in the accompanying drawings provide distinct advantages over the prior art and therefore fall within the scope of the present invention as described herein.

Claims

1. A tubing device for use in geothermal wells, An insulated tubing string is formed to represent a conduit configured to transport a working fluid between a surface location and an underground location, A casing string, wherein the insulating tubing string is arranged within the casing string such that an annular space is drawn between the casing string, A cooling string received within the conduit or the annular space, configured to regulate the temperature of the working fluid when the working fluid is transported toward the ground surface during use, and A tubing device equipped with the following features.

2. A tubing device according to claim 1, wherein the cooling string is received in the conduit, and the tubing device is configured to transport the working fluid from the ground surface to the underground position through the annular space and from the underground position to the ground surface position through the insulating tubing string when in use.

3. A tubing device according to claim 2, wherein the cooling string is configured to inject a cooling fluid into the conduit in order to regulate the temperature of the working fluid during use.

4. A tubing apparatus according to claim 3, wherein the cooling string has a smaller flow area than the insulating tubing string.

5. A tubing device according to claim 3, wherein the cooling string has a smaller diameter than the insulating tubing string.

6. A tubing device according to claim 3, wherein the cooling string is arranged adjacent to the wall of the insulating tubing string.

7. A tubing device according to claim 3, wherein the lower end of the cooling string is positioned shorter than the lower end of the insulating tubing string.

8. A tubing device according to claim 3, wherein the cooling string is detachable from the insulating tubing string.

9. A tubing apparatus according to claim 1, further comprising a controller configured to control the flow rate of a working fluid and / or cooling fluid.

10. A geothermal well comprising a well housing the tubing device described in claim 1.

11. A geothermal well according to claim 10, wherein the geothermal well is a closed-loop geothermal well.

12. A wellhead control assembly for use with the tubing apparatus described in claim 1, comprising a controller and one or more flow valves, wherein the controller is configured to control the flow rate and / or pressure of the working fluid and / or cooling fluid through the tubing apparatus.

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