GB2579642A

GB2579642A - Storing and extracting thermal energy in a hydrocarbon well

Info

Publication number: GB2579642A
Application number: GB1820047.7A
Authority: GB
Inventors: William Hamilton David
Original assignee: Rigon Energy Ltd
Current assignee: Rigon Energy Ltd
Priority date: 2018-12-10
Filing date: 2018-12-10
Publication date: 2020-07-01
Also published as: GB2579683B; GB201904307D0; GB2579683A; GB201820047D0

Abstract

A system to transfer heat to or from a gas or oil borehole comprises a heat exchanger 530 located within the borehole and used to transfer heat to or from the walls / sides of the borehole 330 whilst maintaining a fluid path for oil or gas along the borehole outside the heat exchanger. The heat exchanger may comprise of concentric pipes (340, 350, fig 2) spaced (380) from the borehole wall by resilient brackets 390 by applying a biasing force to the wall. The concentric pipes may be biased into contact with the wall (fig 3) or may comprise parallel pipes (340, 350, fig 4) for respective flow and return heat transfer fluid and a parallel tube (370) for hydrocarbon. The brackets may comprise of collars 500 clamped to the heat exchanger and bow springs which may have a flattened center portion 520. The springs may be made from copper, copper alloy, aluminium or a flexible material containing graphene or carbon fiber. The heat exchanger is used in an active well to store thermal energy that may be extracted at another time – for example store solar heat in the summer to be extracted and used in the winter.

Description

Storing and extracting thermal energy in a hydrocarbon well

Field of the Invention

[0001] The present invention relates to systems and methods for storing and extracting thermal energy in a hydrocarbon well.

Background of the Invention

[0002] The idea of storing heat underground is known. Seasonal thermal energy storage (STES) is the storage of heat or cold for periods of up to several months. The thermal energy can be collected whenever it is available and be used whenever needed, such as in the opposing season. For example, heat from solar collectors or waste heat from air conditioning equipment can be gathered in hot months for space heating use when needed, including during winter months. In some cases, solar heat is simply collected through the ground itself during summer months, and extracted by ground source heat pumps during the winter months to provide thermal energy, for example for domestic heating. Alternatively, waste heat can be actively stored underground and retrieved when required. This often requires heat pumps, in order to increase the temperature difference between the stored heat and the heat load. Ground source heat pumps for space heating purposes tend to use shallow boreholes, in the uppermost 100 meters of the subsurface.

[0003] Geothermal heat can be extracted from deep boreholes, when the underground rock structures have a raised temperature due to geological activity.

In some cases, these rely on pumping water into hot dry rocks, and then extracting the heated water, in other cases a heat exchanger is used to transfer heat between a working fluid and the rock structure. Conventional borehole heat exchangers rely on pipes in direct contact with the ground, often using a lined borehole as a part of the pipes. The wellbore itself is used as the fluid conduit, or a liner is cemented into a hole to provide close thermal contact between the fluid and the ground. Australian patent application AU201110069A4 discloses a geothermal well with a depth of 1,000 to 4,000 meters using the principle of thermosiphon to circulate water down an inner steel casing and up the well which is also lined with a steel casing. The heat source driving the thermosiphon is a geothermal heat reservoir.

[0004] Rob Westaway, in "Repurposing of disused gas wells for subsurface heat storage: preliminary analysis concerning U.K. issues", Quarterly Journal of Engineering Geology and Hydrology, August 18th 2016, discusses development of shale gas wells and the reuse of the resulting subsurface infrastructure after gas production has ceased. It is shown that this infrastructure might be repurposed for Borehole Thermal Energy Storage (BTES).

[0005] The Westaway paper assumes that the selected lateral shale well is disused before the heat storage and heat exchange commences. The essential feature of the proposed solution is to install within each well a liner so it acts as a heat exchanger; in summer, hot water -containing waste heat -is circulated into the well, thus heating the surrounding rock mass (Fig. 4(a)); in winter, heat stored within this rock mass is extracted and used for heat supply (Fig. 4(b)). Westaway notes an advantage of deep wells, that the ambient temperature at 2 to 3 km depths is many tens of °C higher than at the Earth's surface. This can make the heat injection and extraction more efficient, as heat pump coefficients of performance (COP) depends on the temperature difference between the heat source and the load.

[0006] In the Westaway paper, a problem acknowledged is that any disused shale gas well will have been fracked, so the casing of its lateral will have been perforated, probably at many points, to permit outflow of fracking fluid; the fracture network extending from each perforation will have dimensions of hundreds of metres. It is proposed by Westaway to use sliplining' of the well, i.e. the insertion of an outer liner, whose external diameter is slightly less than the internal diameter of the well casing, which contains an inner liner to create the heat exchanger. This would only be possible once the well has finished production, as the slipliner would make the well gas tight, and the entire borehole would be used for heat exchanger fluid flow, therefore gas could no longer flow up the borehole from the gas bearing rocks.

[0007] The concept disclosed by Westaway is applicable to the reuse of any well as a heat store, but is focused on shale gas wells, where the geological structure is impermeable which precludes the production of thermal water from geothermal sources. Shale gas wells also have extensive horizontal bores in a single geological layer, giving access to a large thermal mass from a single point, for use as an energy store.

[0008] Figure 1 shows a typical layout of a known shale gas production well 100. Wellhead area 110 includes various wellhead equipment, and wastewater ponds 120. Vertical borehole 140 is lined with casing 150, and passes through various geological layers, for example the geological layers illustrated may be a shallow aquifer 160, impermeable layer (aquiclude) 170, deep aquifer or any other permeable or impermeable layer 180, second impermeable layer 190 and gas bearing formation 200, but the actual geological structure will vary. When the gas bearing formation is relatively impermeable to gas flow, as is usually the case with shale gas deposits, the well may have a horizontal bore 210 and the gas bearing formation may be hydraulically fractured to ease the flow of gas into the well. Fractures 220 are shown as cracks in the rock structure, these are sometimes held open by the use of a proppant, which is injected into the well at high pressure along with fracking fluid to form the fractures. Arrows 230 indicate the flow of natural gas from the gas bearing structure into the bore and up to the surface. At the surface there is a wellhead. During drilling, this is used to support the drilling equipment, and during production this may support a "tree" to provide valves and connections for downhole equipment.

[0009] Fracking, or hydraulic fracturing, is a process for extracting oil or gas by drilling horizontally into shales containing hydrocarbon reserves. Typically, the wellbore is lined with a casing, and cemented to the surrounding rock. The wellbore may connect one or more lateral bores to a vertical bore. During the fracturing stage, a perforating gun is used to perforate the well lining to permit the fracking fluid to be pumped to fracture surround rock. Working from the toe of the lateral wellbore the fracking is done in stages, with the well-being plugged between each stage. Post fracking the fracked stage is plugged and each successive stage is done in the same way. Typically all the plugs remain in place until the wellbore heel is reached.

At that point all the plugs in the lateral wellbore are drilled through and pent up gas and remaining fracking fluid released to the surface. Multiple lateral bores may be drilled from a single vertical bore, if the local geology permits it.

[0010] Like coal seam natural gas, shale natural gas production declines progressively over the first few years, but there is a long tail of declining production for 20 years or more. Having created a well by drilling, the well continues to require maintenance for many years. Eventually, when the gas output is insufficient to justify the cost of maintenance, the well is normally capped and abandoned.

[0011] There would be a clear technical advantage if the heat storage could be contemporaneous with the development of a gas well, or start before the main flow from a well has ceased for example after five years from well inception. Such a process would enhance the energy production of a well throughout its life, and enable the well to act as a low carbon energy source both during and after its value as a hydrocarbon well is exhausted. Subject to prevailing regulations, re-use of a well for heat storage can make it possible to defer capping and abandonment of a well for many years, allowing the capture of residual hydrocarbons that would not otherwise be practical.

Summary of the Invention

[0012] A problem to be solved is enabling the use of a hydrocarbon well as a thermal energy store contemporaneously with gas or oil production.

[0013] According to a first aspect of the present invention, there is provided a support for a borehole heat exchanger comprising support means to support a heat exchanger; positioning means to position the heat exchanger within a borehole; heat transfer means to provide a thermally conductive path between the heat exchanger and the sides of the borehole, wherein the positioning means are operable to permit the heat exchanger to be inserted into the borehole, position the heat exchanger in the borehole after insertion, and maintain a fluid path for oil or gas to pass along the length of the borehole outside the heat exchanger.

[0014] Preferably, the positioning means and the heat transfer means are provided by one or more thermally conductive members. More preferably, the thermally conductive members comprise heat transfer surfaces that are in contact with the heat exchanger and the sides of the borehole.

[0015] The thermally conductive members may comprise thermal insulation on at least one surface of the thermally conductive members, except for the heat transfer surfaces.

[0016] Advantageously the heat transfer means may be formed of copper, copper alloy, or aluminium. The heat transfer means may comprise a graphene or carbon fiber element, preferably as a composite structure with the thermally conductive axis of the carbon fiber or graphene aligned with the direction of required heat transfer.

[0017] A second aspect of the invention provides a system for storing heat in a borehole of an active oil or gas well, comprising: a first tubular heat exchanger, installed in the borehole; pumping means to pump fluid through the heat exchanger; a second heat exchanger, outside the borehole, operable to transfer heat between the fluid and either a heat source or a heat load.

[0018] The system of may advantageously have the second heat exchanger coupled to a heat pump.

[0019] The system can also include sensing means to sense a parameter indicating the rate of heat transfer, and/or control means, to control the speed of fluid flow through the heat exchanger.

[0020] A third aspect of the invention provides a method of using a borehole heat exchanger in a borehole of an active oil or gas well, the method comprising: inserting a heat exchanger into the borehole; pumping fluid through the heat exchanger, wherein the heat exchanger does not fill the borehole so as to allow gas or oil to flow along the length of the borehole, and the heat exchanger is configured to exchange heat between the fluid and the ground through which the borehole passes.

[0021] Another aspect of the invention provides a borehole thermal energy store, comprising: a borehole in use for gas or oil production; a heat exchanger in 25 the borehole at least one thermal conduction path between the heat exchanger and the sides of the borehole.

[0022] When a gas well is in operation it is not possible to convert it into a heat exchanger using the borehole thermal energy storage methods described in the background section. In the present invention an inner pipe or pipes containing the heat transfer fluid and providing heat exchanger elements are maintained in thermal contact with surrounding rock. The heat exchanger is supported with brackets or fixings that interface with the well lining to ensure good heat transfer while permitting hydrocarbon flow.

In a hydrocarbon well.

[0023] One advantageous use of the invention is the storage of heat in an underground rock formation from which oil and/or gas is being removed, whilst oil/gas extraction continues. The invention is suitable for storage and extraction of heat in the ground surrounding any type of fluid hydrocarbon well, particularly gas wells, and coal bed methane wells.

[0024] In a well that requires hydraulic fracturing, the insertion of the heat exchanger can be made after the fracking stage has been completed in a lateral bore, and all the plugs removed. During well completion, a tubular heat exchanger can be inserted into the well using the same wellhead mechanism, such as wireline equipment, used during the installation of production piping and packing. In a well that has been hydraulically fractured, such as a shale gas well, the insertion of the heat exchanger into the well enables the use of the fractured section of the borehole for heat extraction without requiring re-lining of the well to convert the borehole itself into a heat exchanger tube. This greatly simplifies the use of a fractured well for heat extraction and heat storage, and also provides the means to use the well for heat extraction and storage while fluid hydrocarbons continue to be extracted. As "fracking" wells are generally deep, and have significant horizontal sections in a geological structure, they provide a larger thermal mass for the purpose of energy storage than other types of borehole. The well may have a higher ambient ground temperature allowing heat transfer to take place at a higher temperature than shallow boreholes. The higher temperature of the extracted "high grade" heat enables the heat to be used for a wider range of applications, including process heat or thermal power generation. This invention could also have application to a Coal Bed Methane (CBM) development. Such a development would be much closer to the surface and the ambient temperature of CBM laterals would be lower but still above the typical temperature of domestic ground heat installation.

[0025] The gas producing life of a CBM well is much shorter at 5 to 7 years and potentially heat storage in a live a CBM lateral cluster could be attractive, and most of the same advantages as apply to shale gas laterals would also apply to CBM laterals.

[0026] The tubular heat exchanger comprises a flow pipe, carrying fluid at a first temperature into the well or borehole, and a return pipe carrying fluid at a second temperature after it has exchanged heat with the underground rock formation. One portion of the heat exchanger pipework is in thermal contact with the rock formation where the heat exchange takes place. Another portion of pipework carries the fluid down the well to reach the thermal store rock formation, portions of the pipework may be insulated to prevent undesirable heat exchange with other fluids in the well, or rock formations that may be at a different temperature to the thermal store.

[0027] The tubular heat exchanger can advantageously comprise an inner insulated pipe within an outer pipe. The outer pipe will be in thermal and mechanical contact with a support or bracket, which provides a path for heat exchange with the borehole liner, and through the borehole liner to the ground or surrounding rock.

The outer pipe may also be in direct thermal contact with the borehole liner itself.

[0028] The space between the outer pipe and the well casing is sufficient to accommodate the emanating gas. The tubular heat exchanger and the casings are fixed apart by regularly spaced brackets or supports with cross sections designed to anchor the inner water pipe but also to conductively transmit or extract heat between the rock and the heat exchanger, for example for storing captured heat during summer and the reverse in winter.

[0029] Alternatively the heat exchanger can comprise parallel flow and return pipework. A portion of the return pipework will preferably be insulated, so that the returning fluid does not exchange heat with the fluid at a different temperature travelling down the flow pipework.

The support bracket [0030] A support for a borehole heat exchanger comprises positioning means to position a heat exchanger within a borehole, the positioning means being operable to permit the heat exchanger to be inserted into the borehole; position the heat exchanger in the borehole after insertion; provide a thermally conductive path between the heat exchanger and the sides of the borehole; and maintain a fluid path for oil or gas to pass along the length of the borehole outside the heat exchanger.

[0031] The thermally conductive path comprises one or more members made from material with a high thermal conductivity, preferably over 100 W/m K, such as a metal, preferably copper, copper alloys or aluminum, which thermally connects heat exchange surfaces that are in contact with the heat exchanger and the borehole walls, or casing. The members may also provide the positioning means. Because the members are made from a thermally conductive material, the cross-sectional area of the members can provide sufficient thermal conductivity while still allowing sufficient free area to permit the flow of hydrocarbons in the well. The heat exchange surfaces of each member that are in contact with the borehole walls are preferably configured to provide at least 1cm2 of contact area, preferably over 10 cm2, more preferably over 30cm2 of contact area with the borehole walls for each member. The heat exchange surfaces of each member that are in contact with the heat exchanger are preferably configured to provide a heat transfer surface to more than 10% of the heat exchanger surface, preferably more than 50%, more preferably greater than 75% of the heat exchanger surface.

[0032] The bracket, in one embodiment, can take a form similar to a spring centralizer, as are commonly used in oil and gas wellbore installations and generally serve to center a pipe or casing within a wellbore or previous casing string. Conventional spring centralizers are typically characterized by a pair of opposed stop collars or stop rings with a number of outwardly-bowed springs extending longitudinally there between to contact the wellbore sidewalls and exert a centering force on a pipe or casing segment. When the bracket takes the form of a centralizer, the bowed springs are arranged so as to have a high contact area as described above with the casing wall or the sides of the borehole, in order to increase the thermal contact. The stop collars or stop rings are also arranged to provide a large contact area with the heat exchanger pipework, for example by providing a long stop collar. This is in contrast to a conventional centralizer, where the springs are designed to minimize friction on the casing walls.

[0033] In order to maximize the heat transfer between the supported heat exchanger and the casing walls, the bracket in the present disclosure is made from a material with high thermal conductivity. The thermal conductivity is preferably greater than 100 W/mK, more preferably higher than 150 W/mK. The bracket is preferably made from copper, or an alloy of copper such as bronze or brass, selected to be resistant to corrosion from the environment within the borehole.

Aluminum may also be used, having a reasonably high thermal conductivity and good resistance to corrosion, as well as being light and strong. In a preferred solution the heat transfer means may comprise graphene or carbon fiber, preferably as part of a composite structure with the more thermally conductive axis of the carbon fiber or graphene aligned with the direction of required heat transfer.

[0034] The contact area of the heat transfer surfaces bracket required to effectively transfer heat from the rock formation to the heat exchanger will vary depending on the geological structure of the rock formation that is to be used for thermal energy storage and the desired heat storage.

[0035] In general the bracket or support will have one or more clamps, to hold the necessary pipes, for example, the heat exchanger flow and return pipes, or the outer pipe of a concentric heat exchanger, and optionally a production pipe, if it is not desired to allow the produced hydrocarbons, e.g. gas, to flow up the outside of the heat exchanger pipework within the wellbore. The bracket will have one or more resilient extending members, to press against the walls of the wellbore, or the wellbore casing, so as to support the pipes in their desired positions, and to provide a thermal path between the pipes and the wellbore walls. Where the members make contact with the wellbore walls, the members will extend in a curved profile so as to make contact with a large area of the wellbore wall, to increase the heat transfer surface available.

[0036] If the wellbore is used for hydrocarbon production flow, the curved profile will have a small thickness, such as less than 15mm, preferably less than 5mm, so as to prevent restriction to the flow of hydrocarbons, e.g. natural gas, up the wellbore.

[0037] When the heat exchanger is formed from concentric tubes, the bracket preferably has a clamp configured to position the outer pipe in contact with one wall of the borehole, or borehole casing, while members extending from the bracket provide a resilient bias against the sides of the borehole to keep the pipe in this position.

[0038] When the heat exchanger comprises parallel tubes, the insulated return tube may be held by the bracket in the centre of the borehole, while uninsulated flow pipe may be positioned against one side of the borehole to increase the thermal transfer.

Optimization of hydrocarbon and heat storage [0039] A heat storage and heat transfer calculation is used to determine the total heat transfer area required based on the known or estimated thermal diffusivity and other thermal properties of the rock. Based on at least: * the temperature of the heat source providing the heat to be stored, * the thermal diffusivity and thermal conductivity of the rock structure, * the minimum and maximum desired temperatures of the thermal store, * the temperature required by the end use of the stored heat, * the coefficient of performance (COP) of any heat pumps to be employed, * the available length and diameter of the borehole, and * the thermal resistance between the heat exchanger and the borehole, including the thermal resistance of the bracket, the seasonal heat flow between the heat exchanger pipework and the ground can be estimated using iterative techniques, algorithms or mathematical solutions such 20 as those discussed by Westaway (2016).

[0040] When heat pumps are employed, energy is used to transfer heat from a source at a low temperature to a load with a higher temperature. In the borehole thermal energy storage application described herein, a heat pump may be used at first to extract heat from a heat source, transfer the heat at a higher temperature into the borehole heat exchanger, so as to raise the temperature of the rock formation in the gas or oil bearing structure of the well. The coefficient of performance of a heat pump used in this arrangement can be as high as 8 or more, meaning that one unit of energy used to drive the heat pump will transfer 8 units of thermal energy into the ground. When the heat is required for an end use, the heat pump is reversed, and heat is extracted from the rock formation, and transferred at a higher temperature to the load.

[0041] The heat source can be a source of waste heat, such as an industrial process. The heat source may also be a solar collector. The heat source may be the rejected heat from a cooling plant.

[0042] An ideal application for this process is building heating and air conditioning, where during summer months a cooling medium is required at around 10° Celcius (C), and in winter months a heating medium is required above 50°C. The thermal store in the rock formation can be maintained in between 20 and 40°C, providing the ideal conditions for a heat pump.

[0043] In other applications, the ambient ground temperature may be in the range 70°C to 100°C, and the invention can provide heat storage for process heat at up to 80°C above or below the ambient ground temperature. Where the temperature of the working fluid exceeds its boiling point at standard pressure, the heat exchanger will need to be pressurized so as to prevent the liquid boiling.

[0044] Geothermal activity in the ground surrounding the heat exchanger may be identified by survey before the drilling of the well, or by measurement during operation of the well. Additional heat produced by local geothermal activity can allow more heat to be extracted from the ground than was originally stored. In some cases, the geothermal energy may provide all the heat extracted from the well.

[0045] The energy required to pump fluid through the heat exchanger can be calculated based on the desired heat transfer rate, the length and cross sectional area of the heat exchanger pipework and any additional flow restrictions imposed by the mechanical requirements of the installation. When the temperature of the thermal store is greater than the flow temperature of the heat exchanger fluid, buoyancy may provide a thermosiphon that may at times drive the fluid flow sufficiently to meet the desired heat load. Control of the fluid pump may be optionally provided in all embodiments of the invention to both control the rate of heat transfer to the energy store, and to minimize the energy required for pumping the fluid by taking advantage of the thermosiphon when available. For example, the pump may be provided with a variable frequency drive and means to measure the fluid flow rate so that the electrical energy provided to the pump is the minimum required to maintain the required heat transfer rate. Sensing means may be provided to sense a parameter indicating the rate of heat transfer such as the rate of fluid flow, the temperature difference between flow and return, or a heat meter.

[0046] The rate of hydrocarbon flow, e.g. gas flow can be estimated from the structure of the rock formation, the depth of the well and the area available for production flow in the borehole. This is generally difficult to estimate, and will change over the lifetime of the well. Depending on the preferences of the well operator, maximum output of hydrocarbons may be desired during an initial period of operation of the well. In this case, the introduction of the heat exchanger tubing may be deferred to maximize the available borehole space for gas flow. Once flow has decreased due to exhaustion of the hydrocarbon in the immediate vicinity of the bore and the fractures, the heat exchanger tubing and brackets can be introduced using well head equipment, which provides an opportunity for borehole maintenance to be carried out at the same time, such as dewatering.

[0047] Engineering calculations can therefore be made to estimate the energy return from the thermal store, based on the borehole dimensions, the available heat sources and heat loads. The length and diameter of the tubular heat exchanger is chosen in order to provide the required heat transfer surface within the borehole to optimize the ratio between the thermal energy to be stored and the required energy to pump the fluid through the heat exchanger, in view of the constraints of the borehole parameters.

[0048] Another aspect of the fracking process is that there is an outflow of fracking water mixed with gas for some time after the fracturing. This could limit the scope for heat storage during this initial period, as the heat exchanger would be exchanging heat mainly with the outflow water. This may however beneficially provide a high rate of transfer of residual heat or geothermal energy during this phase. Therefore it can be beneficial to include in the calculations an estimate of the thermal energy available in this first time period, and if there is a heat load that can make use of this initial heat, to install the heat exchanger immediately after fracturing and before the well begins production to take advantage of this energy.

[0049] During a first time period, heat might be extracted from the fracking water outflow which may have residual heat, or geothermal heat. During a second time period, heat can be stored and extracted in the ground while the well is producing gas. In a third period, when the well is no longer operational for gas production, the heat storage and extraction can continue after the well is capped. A system with the specific features described herein can operate effectively during two or three of these periods.

[0050] Other aspects are as set out in the claims herein which are

incorporated into the description by reference.

Brief Description of the Drawings

[0051] For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: [0052] Figure 1 shows a typical horizontal gas well, as described in the

background section above.

[0053] Figure 2 is a schematic diagram of a concentric tubular heat exchanger.

[0054] Figure 3 is a schematic diagram of a concentric tubular heat exchanger positioned in contact with a borehole casing.

[0055] Figure 4 is a schematic diagram of a parallel tubular heat exchanger positioned in contact with a borehole casing with additional heat transfer 15 surface.

[0056] Figure 5 is a schematic diagram of a concentric tubular heat exchanger positioned by a centralizer type bracket in a borehole.

Detailed Description of the drawings

[0057] There will now be described by way of example a specific mode contemplated by the inventor. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the description.

[0058] Figure 2 shows a concentric arrangement of a tubular heat exchanger in a borehole. The heat exchanger comprises an inner pipe 350 within an outer pipe 340. There is preferably insulation 360 between the inner and outer pipe. The outer pipe will be in thermal and mechanical contact with a support or bracket (not shown), which provides a path for heat exchange with the borehole wall or liner (330), and through the borehole liner to the ground. The outer pipe may also be in direct thermal contact with the borehole liner itself. Preferably the outer pipe carries the flow of fluid from the plant above ground to the heat exchanger, and inner pipe carries the return fluid from the heat exchanger to the plant above ground, the insulation layer if present will assist in maintaining the return fluid at a temperature closer to that of the rock structure by reducing heat exchange between the flow and return.

[0059] The space 380 between the outer pipe and the well casing can be sufficient to accommodate the emanating gas.. The tubular heat exchanger and the casings are fixed apart by regularly spaced brackets or supports with cross sections designed to anchor the inner water pipe but also to conductively transmit or extract heat between the rock and the heat exchanger, for example for storing captured heat during summer and the reverse in winter.

[0060] A bracket to support the heat exchanger comprises resilient members 390, which position the pipes by applying a biasing force to the well casing 330. These resilient members provide positioning means to the pipe, which may be held in a clamp, not shown, which is connected to the supporting means. In this example, the tubular heat exchanger is shown positioned approximately in the centre of the wellbore. The resilient members can also be thermally conductive, by for example being made of a thermally conductive material such as metal or carbon fiber, so that they provide both supporting means and a heat path between the well casing and the heat exchanger.

[0061] In figure 3, the same heat exchanger as described in figure 2 is shown, using like numerals for similar features. In this example, the resilient members 390 are configured to position the heat exchanger against the wellbore casing. The heat exchanger is attached to the resilient members by a clamp, not shown, which may also have an extended profile to engage with the curved wall of the wellbore to increase the heat exchange surface area between the well casing and the heat exchanger. The resilient members may also provide an additional heat transfer path.

[0062] In the alternative arrangement shown in Figure 4, the heat exchanger comprises parallel flow 340 and return 350 pipework. In this example the return pipework is insulated by a layer of insulation 360. A portion of the return pipework will preferably be insulated, so that the returning fluid does not exchange heat with the fluid at a different temperature travelling down the flow pipework. Optionally, a production tube 370 may also be fitted to contain the hydrocarbons from the well.

By providing a production tube in parallel with the heat exchanger pipework, the support brackets provided to maintain the thermal coupling between the heat exchanger and the borehole casing can be arranged to support all three pipes within the bore, without risking the flow path becoming blocked by debris or mud. The production tube may extend only as far as the fractured area of the well.

[0063] A bracket to support the heat exchanger comprises resilient members 390, which position the pipes by applying a biasing force to the well casing 330. These resilient members provide positioning means to the heat exchanger, which may be held in a clamp, not shown, which is connected to the supporting means. In this example, the flow pipe 340 is shown positioned close to the side of the wellbore. The resilient members can also be thermally conductive, by for example being made of a thermally conductive metal, so that they provide both supporting means and a heat path between the well casing and the heat exchanger.

[0064] In figure 4, the bracket may further comprise a curved profile piece 400, which is made from thermally conductive material and forms an extended the heat transfer surface to reduce the thermal resistance between the well bore casing and the heat exchanger, so as to reduce the thermal resistance between the rock formation and the heat transfer fluid used in the heat exchanger.

[0065] Figure 5 shows a tubular heat exchanger 530 supported by brackets in a wellbore casing 330. In this example, the brackets are similar in design to a centralizer, as used to position tubing or strings, as they are sometimes referred to in the well drilling industry, in a wellbore. A first embodiment of the bracket comprises collars 500 which may act as clamps for the tubular heat exchanger and bow springs 390 which are biased against the casing 330. Both collars 500 and springs 390 are preferably made of a thermally conductive material, for example copper, copper alloy or aluminium. An alternative material is a flexible material containing graphene or carbon fiber. The second embodiment shown in Figure 5 has been further adapted to improve the heat transfer characteristics by extending the collars 510 to increase the heat transfer surface between the collar and the heat exchanger. Springs 510 are formed with a flatter center portion that engages with the borehole casing, to provide a larger heat transfer surface. The center portion will preferably also have a wider, curved profile so as to increase the heat transfer surface with the casing. More preferably, the center portions of each spring will, when placed in position and under compression to provide a bias against the casing, form an almost complete annular ring with only small gaps between the springs to allow for insertion. For example, the incomplete annular ring may be in contact with the casing for more than 80% of the circumferential distance.

[0066] The choice of materials for the heat exchanger pipework will depend on the conditions identified in the borehole. Steel pipes may be used. A heat-and corrosion-resistant polymer such as polytetrafluoroethylene or fluorinated ethylene propylene can be used.

[0067] The pipes can be inserted into the well using wireline equipment, for example during the insertion of production tubing.

[0068] In one embodiment, a metal production string, or production tube is attached to polymer heat exchanger flow and return pipes using any of the supports or brackets as described in this application, and the complete assembly is inserted into the well using a wireline machine as is used in the gas and oil drilling industry.

Claims

Claims 1. A support for a borehole heat exchanger comprising: support means configured to support a heat exchanger; positioning means to position the heat exchanger within a borehole; heat transfer means to provide a thermally conductive path between the heat exchanger and the sides of the borehole, wherein the positioning means are operable to: enable the heat exchanger to be inserted into the borehole; position the heat exchanger in the borehole after insertion; and maintain a fluid path for oil or gas to pass along the length of the borehole outside the heat exchanger.
2. The support of claim 1, wherein the positioning means and the heat transfer means are provided by one or more thermally conductive members.
3. The support of claim 2, wherein the thermally conductive members comprise heat transfer surfaces that are operable to be placed in contact with the heat exchanger and the sides of the borehole.
4. The support of claim 3, wherein the thermally conductive members comprise thermal insulation on at least one surface of the thermally conductive members, excluding the heat transfer surfaces.
5. The support of any previous claim, wherein the support means comprise clamping means to hold at least two parallel heat exchanger pipes.
6. The support of any previous claim wherein the support is configured to bias a pipe of the heat exchanger in thermal contact with a portion of the wall of the borehole.
7. The support of any previous claim wherein the conductive member is curved circumferentially with respect to the wall of the borehole.
8. The support of any previous claim wherein the heat transfer means comprise a material selected from graphene, carbon fiber,. copper, copper alloy, or aluminium.
9. A system for extracting heat from a borehole of an active oil or gas well, comprising: a first tubular heat exchanger, installed in the borehole; pumping means to pump fluid through the heat exchanger; a second heat exchanger, outside the borehole, operable to transfer heat between the fluid and a heat load.
10. The system of claim 9, further suitable for storing heat in the borehole of an active oil or gas well, wherein the second heat exchanger is further operable to transfer heat between the fluid and a heat source.
11. The system of claims 9 or 10, further comprising the support of claim 1 for supporting the tubular heat exchanger in the borehole.
12. The system of any of claims 9 to 11, wherein the second heat exchanger is coupled to a heat pump.
13. The system of any of claims 9 to 12, further comprising: sensing means to sense a parameter indicative of the rate of heat transfer; control means, to control the speed of fluid flow through the heat exchanger.
14. A method of using a borehole heat exchanger in a borehole of an active oil or gas well, the method comprising: inserting a heat exchanger into the borehole; pumping fluid through the heat exchanger, wherein the heat exchanger does not fill the borehole so as to allow gas or oil to flow along the length of the borehole, and the heat exchanger is configured to exchange heat between the fluid and the ground through which the borehole passes.
15. The method of claim 14, wherein the heat exchanger is supported in 3 0 the borehole by the support of any of claims 1 to 8.
16. A borehole thermal energy store, comprising: a borehole in use for gas or oil production; a heat exchanger in the borehole at least one thermal conduction path between the heat exchanger and the sides of the borehole.
17. The borehole thermal energy store of claim 16, further comprising the support of any of claims 1 to 8 for supporting the tubular heat exchanger in the borehole.