Geothermal wells and tubing equipment 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 relates generally to geothermal wells, and tubing equipment for such geothermal wells. More specifically, but not exclusively, the present invention relates to such geothermal wells, tubing equipment for use in geothermal wells, and methods of geothermal well start-up.

[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 hot sedimentary aquifer. Hot fluid from the aquifer, otherwise known as the "working fluid," is brought to the surface through a production well so that heat can be extracted. The working fluid is then pumped back to the aquifer to recharge the reservoir. This is generally, but not always, done via a closed-loop system.

[0003] In some cases, the production well includes an insulated tubing string received within the cased wellbore. During normal operation, fluid is pumped down the insulated tubing string, heated, and pumped up the annular space between the tubing string and the cased wellbore back toward the surface. The insulated tubing string is provided to reduce heat loss as the working fluid is conveyed up the annular space.

[0004] During the initial stages of well operation, the bottom hole temperature is very high and can exceed 400 degrees Celsius. Following the start-up phase, the bottom hole temperature decreases over time to a steady state temperature, for example 150 degrees Celsius. This is shown in Figure 1.

[0005] Thus, a large temperature difference exists between the working fluid pumped down the insulated tubing string and the working fluid conveyed in the annular space during start-up.

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

[0007] It is possible to increase resistance to mechanical stress and fracture by 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 start-up phase of a geothermal well. Summary of the Invention [Problem to be solved by the invention]

[0008] Accordingly, it is a first, non-exclusive object of the present invention to provide a tubing apparatus that overcomes or at least mitigates the known problems of the prior art. In particular, it is a non-exclusive object of the present invention to provide a tubing apparatus that mitigates the above-mentioned problems during start-up of a geothermal well. [Means for solving the problem]

[0009] Accordingly, one aspect of the invention provides a tubing apparatus for use in a geothermal well, the apparatus comprising an insulated tubing string delineating a conduit configured to transport a working fluid between a surface location and a subsurface location, and a cooling string configured to regulate a temperature of the working fluid in or around the conduit and / or of the working fluid, in use, as it is transported towards the surface location.

[0010] By providing a cooling string that regulates the temperature of the working fluid, it is possible to reduce the temperature difference between the fluid being conveyed to the surface location within or around the conduit, typically the fluid within the insulated tubing string or the annular space surrounding the insulated tubing string, and the fluid around or within the conduit, respectively, during start-up.

[0011] The tubing apparatus may further comprise a casing string. The insulated tubing string may be disposed within the casing string, for example, such that an annular space is defined therebetween.

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

[0013] Another aspect of the invention provides a geothermal well comprising a wellbore receiving therein an insulated tubing string delineating a conduit configured to transport a working fluid between a surface location and a subsurface location, and a cooling string configured to, in use, regulate a temperature of the working fluid as it is transported towards the surface location.

[0014] The tubing apparatus or geothermal well may be operable or configured, in use, to transport a working fluid from a subsurface location to a surface location, for example, through an insulated tubing string. The tubing apparatus may be operable or configured, in use, to transport a working fluid from a surface location to a subsurface location, for example, through an annular space between an insulated tubing string and a casing string or wellbore.

[0015] Another aspect of the invention provides a tubing apparatus for use in a geothermal well, the apparatus comprising an insulated tubing string disposed within a casing string, the apparatus operable or configured, in use, to convey a working fluid from a subsurface location to a surface location through the insulated tubing string and to convey a working fluid from the surface location to a subsurface location through the annular space between the insulated tubing string and the casing string.

[0016] Another aspect of the invention provides a geothermal well comprising an insulated tubing string disposed within the wellbore, the geothermal well being operable or configured, in use, to convey a working fluid from a subsurface location to a surface location through the insulated tubing string and to convey a working fluid from the surface location to a subsurface location through an annular space between the insulated tubing string and the wellbore.

[0017] The inventors observed that in conventional flow, the temperature of the working fluid is reduced via conduction through the casing of the well as it is transported up the annulus. 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 formations. Meanwhile, heat is retained within the fluid as it is pumped back up the insulated tubing string.

[0018] The cooling string may be received within the insulated tubing string. The cooling string may be removably received within the insulated tubing string, such that it is removable after start-up. This configuration is useful when working fluid is conveyed up the insulated tubing string, since in such a counter-flow configuration temperature differences can become more abrupt.

[0019] Alternatively, the cooling string may be disposed outside the insulated tubing string and / or between the insulated tubing string and the casing string. The cooling string may be removably received between the insulated tubing string and the casing string, such that it can be removed after start-up. This configuration is useful when the working fluid is conveyed up the annular space.

[0020] The insulated tubing string may comprise a pipe-in-pipe insulated tubing string. The pipe-in-pipe insulated tubing string may comprise a vacuum or an inert gas between the pipes that make up the pipe-in-pipe insulated tubing string. The insulated tubing string or the pipe-in-pipe insulated tubing string may comprise a vacuum insulated tubing string. The insulated tubing string or the pipe-in-pipe insulated tubing string may comprise an insulated tubing string that includes an inert gas, such as argon.

[0021] The insulated tubing string may be disposed substantially concentrically with the casing string and / or the wellbore. The insulated tubing string may be longer than the casing string and / or may extend beyond the casing string, for example, into the wellbore and / or to a bottom hole location, when in use.

[0022] The cooling string may be configured to inject or convey a fluid, e.g. a cooling fluid, into the conduits, in use, for example to regulate the temperature of the working fluid. The cooling fluid may be configured to reduce 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 arrangement may comprise a bypass circuit or a cooling circuit or a bypass cooling circuit that may be configured to cool the portion of the working fluid. The cooling string may be supplied by a bypass circuit or a cooling circuit or a bypass cooling circuit.

[0023] By injecting the coolant directly into the working fluid, its temperature can be more easily controlled.

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

[0025] The cooling string may have a smaller flow area than the insulated tubing string and / or the annular space. The cooling string may have a smaller diameter than the radial width of the insulated 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 intent of a lower flow rate of the cooling fluid compared to the working fluid.

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

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

[0028] The cooling string may be held in place, for example relative to the insulated 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 relative to the insulated tubing string. The cooling string may be attached or held in place by multiple brackets or supports. The brackets or supports are spaced apart along the cooling string and / or spaced apart along the insulated tubing string.

[0029] The brackets or supports may be connected, attached or secured to the insulating tubing string. Alternatively, the brackets or supports may be connected, attached or secured to the cooling string.

[0030] The cooling string may be shorter than the insulated tubing string, and the lower or free end of the cooling string may be positioned to be shorter than the lower or free end of the insulated tubing string, such that the cooling fluid is introduced upstream of the area of ​​highest temperature differential.

[0031] The cooling string may be removable from the insulated tubing string. The cooling string may be provided on a coiled tubing arrangement, so that the cooling string can be removed after the start-up phase. Alternatively, the cooling string may be permanently installed within the insulated tubing string.

[0032] The lower end of the insulated tubing string may be open and / or may be positioned to be in an open-hole portion of the wellbore, which may be an uncased portion of the wellbore.

[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 be configured to control the flow rate and / or pressure of the cooling fluid. This allows the temperature of the working fluid to be gradually and dynamically adjusted.

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

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

[0036] The tubing apparatus or control assembly may comprise one or more flow sensors. The one or more flow sensors may be configured to measure a flow rate through the annular space, the insulated tubing string, and / or the cooling string, in use. The one or more flow sensors may include a first flow sensor configured to measure a flow rate through the annular space, in use. The one or more flow sensors may include a second flow sensor configured to measure a flow rate through the cooling string, in use. The one or more flow sensors may include a third flow sensor configured to measure a flow rate through the insulated tubing string, in use. This allows for closed loop feedback of the relative flow rates of the working fluid and the cooling fluid.

[0037] The tubing apparatus or control assembly may comprise one or more pressure sensors. The one or more pressure sensors may be configured to measure pressure in the annular space, the insulated tubing string, and / or the cooling string, in use. The one or more pressure sensors may include a first pressure sensor configured to measure pressure in the annular space, in use. The one or more pressure sensors may include a second pressure sensor configured to measure pressure in the cooling string, in use. The one or more pressure sensors may include a third pressure sensor configured to measure pressure in the insulated tubing string, in use. This allows for closed loop feedback of the relative pressures of the working fluid and the cooling fluid.

[0038] The tubing apparatus or control assembly may include one or more flow control valves. The one or more flow control valves may be configured to control the flow of fluid through the annular space and / or the insulated tubing string. The one or more flow control valves may be configured to control the flow of fluid through the cooling string. The 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 the insulated tubing string. The 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 the relative flow rates of the working fluid and the cooling fluid to be gradually and dynamically adjusted.

[0039] The tubing apparatus or control assembly may include one or more temperature sensors. The one or more temperature sensors may be located at or near a surface location in the insulated tubing string. The one or more temperature sensors may be configured to measure a temperature of the working fluid in the insulated tubing string. The one or more temperature sensors may be located at or near a lower or free end of the cooling string. This allows the relative flow rates of the working fluid and the cooling fluid to be adjusted in a closed loop to achieve a required temperature in the working fluid.

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

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

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

[0043] The method may include conveying the working fluid from a surface location to a subsurface location, e.g., through an insulated tubing string. The method may include conveying the working fluid from a subsurface location to a surface location, e.g., through an annular space between an insulated tubing string and a casing string or a wellbore. The method may include conveying the working fluid from a subsurface location to a surface location, e.g., through an insulated tubing string. The method may include conveying the working fluid from a surface location to a subsurface location, e.g., through an annular space between an insulated tubing string and a casing string or a wellbore.

[0044] Another aspect of the invention provides a method of operating a geothermal well, the method including conveying a working fluid from a subsurface location to a surface location through an insulated tubing string, and conveying the working fluid from the surface location to a subsurface location through an annular space between the insulated tubing string and a casing string or wellbore.

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

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

[0047] Injecting the cooling fluid through the cooling string may include injecting the working fluid. The method may include cooling a portion of the working fluid, e.g., before injecting it through the cooling string. The method may include bypassing a portion of the working fluid, e.g., through a bypass circuit or a cooling circuit or a bypass cooling circuit, before injecting it through the cooling string. The working fluid may include water or other fluids, e.g., supercritical fluids.

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

[0049] For the avoidance of doubt, any feature described herein applies equally to any aspect of the invention, for example, a tubing apparatus may include any one or more features of a method related to a tubing apparatus and / or a method may include any one or more features or steps related to one or more features of a tubing apparatus or a cooling string.

[0050] A further aspect of the invention provides a computer program element comprising computer readable program code means for causing a processor to execute procedures for performing one or more steps of the methods described above.

[0051] A still further aspect of the invention provides a computer program element embodied on a computer readable medium.

[0052] A still further aspect of the invention provides a computer readable medium having stored thereon a program configured to cause a computer to execute procedures for performing one or more steps of the method described above.

[0053] A still further aspect of the invention provides a control means or a control system or a controller comprising a computer program element or a computer readable medium as hereinbefore described.

[0054] For purposes of this disclosure, notwithstanding the above, 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 a 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 multiple control units or controllers that operate collectively to provide the necessary control functions. A set of instructions may be provided that, when executed, causes said controller or control unit or control module to perform the control techniques described herein (including the methods described herein).

[0055] The set of instructions may be embedded in one or more electronic processors, or alternatively may be 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 is one or more processors that are the same as the first controller. However, other configurations are also useful. It will be understood, therefore, that the present invention is not intended to be limited to any particular configuration. In any case, the set of instructions described herein may be embedded in a computer-readable storage medium (e.g., a non-transitory storage medium), which may comprise any mechanism for storing information in a form readable by a machine or electronic processor / computing device. This includes, but is not limited to, a magnetic storage medium (e.g., a floppy diskette), an optical storage medium (e.g., a CD-ROM), a magneto-optical storage medium, a read-only memory (ROM), a random access memory (RAM), an erasable programmable memory (e.g., an EPROM and an EEPROM), a flash memory, or an electrical or other type of medium for storing such information / instructions.

[0056] Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples, and alternatives described in the preceding paragraphs, claims, and / or the following description and drawings, particularly in their individual features, may be obtained independently or in any combination, i.e., all embodiments and / or features of any embodiment may be combined in any manner and / or combination, except where such features are incompatible.

[0057] For the avoidance of doubt, terms such as "may," "and / or," "eg, for example," and the like, as used herein, should be construed as open-ended, such that the features so recited need not be present. Indeed, any combination of optional features, whether or not they are explicitly claimed, is expressly contemplated without departing from the scope of the present invention. Applicants reserve the right to modify any originally filed claims or to submit new claims accordingly, including the right to amend any originally filed claims to depend on and / or incorporate features of any other claim, even if not originally so claimed.

[0058] Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0059] [Figure 1] 1 is a graph showing an example of downhole temperature decrease during a typical start-up phase. [Diagram 2] 1 is a schematic diagram of a geothermal well tubing system according to the prior art. [Diagram 3] 1 is a schematic diagram of a geothermal well tubing system according to an embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram of a geothermal well incorporating the tubing apparatus of FIG. [Diagram 5] 3 is a graph showing the temperature of the working fluid as it flows along the tubing arrangement of FIG. 2 during the start-up phase, i.e., without the use of a cooling string. [Figure 6] 3 is a graph showing the temperature of the working fluid as it flows along the tubing apparatus of FIG. 2 when the well is in steady state. [Figure 7] FIG. 2 is a schematic diagram of a geothermal well tubing system according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060] 2, a tubing apparatus 1 for a geothermal well according to the prior art is shown. The tubing apparatus 1 includes a vacuum insulated tubing string 2. The vacuum insulated tubing string 2 is received within an intermediate casing string 3. The intermediate casing string 3 is in turn received within a surface casing string 4. The vacuum insulated tubing string 2 and the casing strings 3, 4 together delineate a wellbore.

[0061] Vacuum insulated tubing string 2, intermediate casing string 3, and surface casing string 4 each extend from a surface location A into an earth formation F, which in this example is in the form of a high temperature sedimentary aquifer. Intermediate casing string 3 includes a casing shoe 30 at its lower end. Similarly, 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 disposed at the surface location A and connected to a source of working fluid, and a second open end 21 disposed within the openbore (uncased) portion of the well 5. The second open end 21 of the vacuum insulated tubing string 2 is disposed deeper than the casing shoes 30, 40 of the intermediate casing string 3 and the surface casing string 4, respectively. The vacuum insulated tubing string 2 delineates an insulated conduit 22 extending between the surface location A and the formation F.

[0063] An annular space 6 is depicted between the vacuum insulated tubing string 2 and the intermediate casing string 3 or open hole portion of the well 5. The annular space 6 extends between a surface location A and a bottom hole location B.

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

[0065] The working fluid then travels back along the annular space 6 towards the surface location A. As the working fluid passes along the annular space 6 it is heated by heat from the surrounding formation F. Heat is then extracted from the working fluid upon reaching the surface location A.

[0066] According to one aspect of the invention, the circulation may be reversed. This involves pumping fluid into the annular space 6 between the vacuum insulated tubing string 2 and the intermediate casing string 3. The working fluid is then conveyed up through conduit 22 from the bottom hole location B to the surface location 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 wellbore by exposure to the surrounding formations, while heat is retained in the fluid as it is pumped back up the insulated tubing string.

[0068] However, the start-up phase needs to be carried out for an extended period of time to avoid large temperature differences between the fluid pumped down the annular space 6 and the working fluid transported up the vacuum insulated tubing string 2.

[0069] 3, there is shown a tubing apparatus 101 for geothermal wells incorporating another aspect of the present invention. Tubing apparatus 101 is similar to tubing apparatus 1. Like features are indicated with like reference numbers with the addition of "100." These will not be described further here.

[0070] The tubing arrangement 101 according to the present invention differs from the tubing arrangement 1 of Figure 2 in that it is provided with a cooling string 107. Such a cooling string is disposed within the vacuum insulated tubing string 102 in the embodiment shown in Figure 3. The cooling string 107 is disposed adjacent to the sidewall of the vacuum insulated tubing string 102, thereby minimizing its effect on the flow of the working fluid through the vacuum insulated tubing string 102.

[0071] Cooling string 107 has a first open end 170 connected to a cooling fluid source located at surface location A and a second open end 171 located within conduit 122 and shorter than second open end 121 of vacuum insulated tubing string 102. Cooling string 107 is configured to inject cooling fluid, e.g., water or working fluid passing through a bypass cooling circuit, into conduit 122 to regulate the temperature of the working fluid in conduit 122.

[0072] 4, a geothermal well 100 is shown incorporating a tubing apparatus 101. The geothermal well 100 also includes a wellhead control assembly 110 including a controller 111, a first flow sensor 112 configured to measure a flow rate through the annular space 106, a second flow sensor 113 configured to measure a flow rate through the cooling string 107, and a third flow sensor 114 configured to measure a 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 at a surface of the vacuum insulated tubing string 102 and proximate to the open end 171 of the cooling string 107. The temperature sensor 115 is configured to measure a 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, 114, the temperature sensor 115, and the flow control valves (not shown) are operatively connected to the controller 111. This may be via a wired or wireless connection.

[0075] In use, during normal operation, the geothermal well 100 functions essentially in the same manner as described above with respect to the geothermal well with tubing apparatus 1 of Figure 2. However, the cooling string 107 has a particular purpose in connection with the operation of the geothermal well 100, particularly during the start-up phase.

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

[0077] FIG. 5 shows the temperature of the working fluid in the tubing apparatus 1 during the start-up phase. The temperature of the formation F along the well depth is shown by the first line Fa. The temperature of the working fluid flowing down the annular space 6 is shown by the second line 6a. The temperature of the working fluid flowing up the vacuum insulated tubing string 2 is shown by the third line 2a. As is evident from this figure, at the earth's 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 near the bottom hole temperature. Due to the insulating properties of the vacuum insulated tubing string 2, there is little temperature drop in the working fluid as it travels toward the surface location A.

[0079] Figure 6 shows the temperature of the working fluid in the tubing apparatus 1 during steady state. Similar to the graph of Figure 5, a first line Fb represents the temperature of the formation F along the well depth, a second line 6b represents the temperature of the working fluid flowing down the annular space 6, and a third line 2b represents the temperature of the working fluid flowing up the vacuum insulated tubing string 2. At this steady state, the temperature difference at the 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 performed very slowly, as those skilled in the art will appreciate, both of which have substantial cost implications.

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

[0082] The present invention thus reduces the mechanical stresses on the vacuum insulated tubing string 102. This is accomplished by introducing cooling fluid from the cooling string 107 in areas where the working fluid would otherwise be at or near its highest temperature. The present invention thereby avoids the need for larger, heavier, and more expensive vacuum insulated tubing strings, as well as the need for a slower start-up phase.

[0083] The cooling fluid may be, for example, any working fluid available near the surface location A that has a lower temperature than the working fluid traveling up to the surface location A that was heated while passing through the bottom of the geothermal well.

[0084] 7, there is shown a tubing apparatus 201 according to another embodiment. Tubing apparatus 201 is similar to tubing apparatus 3. Like features are indicated with like reference numbers increased by "100". These will not be described further here.

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

[0086] The tubing apparatus 201 is configured to pump a working fluid down a conduit 222 from a surface location A. The working fluid is then exposed to the formation F and passes back up the annular space 206 towards the surface location A. In this example, a cooling fluid is injected into the working fluid via a cooling string 207, which reduces the temperature of the working fluid as it passes along the annular space 206.

[0087] 3, the working fluid has a lower temperature when it reaches the surface location 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 appreciate that several variations to the above-described embodiments are envisioned without departing from the scope of the present invention. It will also be appreciated by those skilled in the art that combinations of any number of the features described above and / or illustrated in the accompanying drawings provide distinct advantages over the prior art and therefore are within the scope of the invention described herein.

Claims

1. 1. A geothermal well comprising: a wellbore receiving therein an insulated tubing string delineating a conduit configured to transport a working fluid between a surface location and a subsurface location; and a cooling string configured, in use, to regulate a temperature of the working fluid as it is transported towards the surface location.

2. 2. The geothermal well of claim 1, wherein the cooling string is received within the conduit, and the geothermal well is configured, in use, to transport the working fluid from the surface location to the subsurface location through an annular space between the wellbore and the insulated tubing string, and from the subsurface location to the surface location through the insulated tubing string.

3. 3. The geothermal well of claim 2, wherein the cooling string is configured to inject a cooling fluid into the conduit, in use, to regulate a temperature of the working fluid.

4. The geothermal well of claim 3 , wherein the cooling string has a smaller flow area than the insulated tubing string.

5. 5. The geothermal well of claim 3 or 4, wherein the cooling string has a smaller diameter than the insulated tubing string.

6. 6. The geothermal well of claim 3, wherein the cooling string is disposed adjacent to a wall of the insulated tubing string.

7. 7. The geothermal well according to claim 3, wherein a lower end of the cooling string is positioned shorter than a lower end of the insulating tubing string.

8. 8. The geothermal well of claim 3, wherein the cooling string is detachable from the insulated tubing string.

9. A geothermal well according to any one of claims 1 to 8, wherein the well is a closed loop geothermal well.

10. 10. A tubing apparatus for use in a geothermal well as claimed in any one of claims 1 to 9, comprising: an insulated tubing string delineating a conduit configured to transport a working fluid between a surface location and a subsurface location; and a cooling string configured, in use, to regulate a temperature of the working fluid as it is transported towards the surface location.

11. 11. The tubing apparatus of claim 10, further comprising a casing string, the insulated tubing string being disposed within the casing string such that an annular space is defined therebetween, and the cooling string being received within the conduit or within the annular space.

12. 12. The tubing apparatus of claim 11, wherein the cooling string is received within the conduit, and the tubing apparatus is configured, in use, to transport the working fluid from the surface location to the subsurface location through the annular space and from the subsurface location to the surface location through the insulated tubing string.

13. The tubing apparatus according to any one of claims 10 to 12, further comprising a controller configured to control the flow rate of the working fluid and / or the cooling fluid.

14. A wellhead control assembly for use with a tubing apparatus described in any one of claims 10 to 12, comprising a controller and one or more flow valves, the controller configured to control the flow rate and / or pressure of a working fluid and the flow rate and / or pressure of a cooling fluid through the tubing apparatus.

15. 1. A method of starting up a geothermal well, comprising: circulating a working fluid between a surface location and a subsurface location through an insulated tubing string; and adjusting a temperature of the working fluid as it is circulated toward the surface using a cooling string.

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