WELL

Field of the Invention [0001] The present invention relates to a method and apparatus for storing and / or 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 195, 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 151 and the gas bearing formation may be hydraulically fractured to ease the flow of gas into the well. Fractures 152 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 153 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] A shale gas well may comprise an upright or substantially vertical borehole which passes from the ground surface down to a layer of shale, and then passes through a “kick off’ point where the bore curves around a bend and then runs off at an angle to the upright/vertical bore, and along the layer of shale. Individual wells can extend over 2,000m horizontally underground and wells may be drilled in clusters so that at the surface there are a plurality of wells, perhaps as many as 40 wells in a cluster, and underground, the wells extend off laterally in different directions to provide a large fracking area and hydrocarbon collection area.

[0010] If a shale layer is thick enough, there may be two networks of lateral bores each of up to 40 lateral bores per network, with the networks at different levels within the shale layer, with both networks using the same shaft/vertical extraction well to the surface. Several lateral bores can emanate from a single vertical bore, and a shale well may comprise a plurality of upright or vertical bores, each upright or vertical bore leading to one or a plurality of lateral bores.

[0011] 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. [0012] 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.

[0013] 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

[0014] A problem to be solved is enabling the use of a hydrocarbon well as a thermal energy store contemporaneously with gas or oil production.

[0015] According to a first aspect there is provided a heat storage and/or extraction apparatus for a hydrocarbon well, said heat storage and extraction apparatus comprising an underground heat exchanger and a positioning device; said underground heat exchanger comprising: a first tube for carrying a heat exchange fluid in a first direction; and a second tube for carrying said heat exchange fluid in a second direction; said positioning device comprising means for positioning said first and / or second tubes within a substantially circular bore of a borehole; wherein said positioning device comprises a plurality of apertures through which a flow of fluid hydrocarbon may pass.

[0016] Said first and second tubes form a closed circuit for containing said heat exchange fluid. [0017] As viewed along a main length axis of said positioning device, said positioning device preferably resides within a circle drawn perpendicular to said main length axis of said positioning device; said circle touching an outermost periphery of said positioning device; and wherein said heat exchanger resides within said circle; and at all positions along the length of the positioning device in said main axial length direction, there are provided one or a plurality of apertures or voids within the area bounded between said circle and said underground heat exchanger.

[0018] Preferably said positioning device resides within a circular boundary extending over a full length of said positioning device; said cylindrical boundary being drawn around an outermost part of said positioning device and touching an outermost part of said positioning device; there being one or a plurality of passages through said positioning device between said positioning device and said cylindrical boundary. [0019] Preferably said positioning device comprises one or more attachment means for attaching to said underground heat exchanger.

[0020] Preferably said attachment means fits around an outside of said underground heat exchanger such that said plurality of apertures are arranged around said underground heat exchanger.

[0021] Said attachment means may comprise a substantially circular clamp, bracket or ring which fits around said underground heat exchanger.

[0022] Preferably said positioning means comprises one or a plurality of arms extending outwardly from said attachment means in a direction radially outwardly of a main central axis of said positioning device. [0023] Said attachment member may have a central aperture therethrough; and said positioning means may comprise at least one arm member which extends outwardly in a direction perpendicular to a main central length axis of said attachment member.

[0024] One or a plurality of arms may bound one or a plurality of apertures located between said arms, when viewed in a direction parallel to a main length axis of said positioning device.

[0025] Preferably said positioning device comprises; one one or more substantially part cylindrical inner surfaces for abutting a said first and/or second tube; one or more substantially part cylindrical outer surfaces for abutting an internal surface of a said borehole; and a connecting portion between said one or more inner substantially part cylindrical surfaces and said one or more substantially part cylindrical outer surfaces, for creating a thermal bridge between said inner and outer surfaces.

[0026] Said attachment means may be configured to position said heat exchanger tubes a position which is off centre from a main central axis of said support device.

[0027] Said positioning means may further comprise a curved hear exchange plate extending along a main axial length direction of said positioning device.

[0028] Said curved heat exchange plate may be curved around a focal line which extends in a same direction as a main axial length direction of said positioningdevice.

[0029] Preferably said first or second tube comprises a thermal insulation layer.

[0030] Said attachment means may comprise first and second attachment collars spaced apart from each other along a direction parallel to a main central axis of said positioningdevice; and said positioning means comprises a plurality of arms which extend between said first and second collars; said plurality of arm members extending outwardly in a direction radially outwardly of a main central axis of said positioning device. [0031] Preferably a first said attachment collar is attached to said underground heat exchanger at a fixed position along said underground heat exchanger; and a second said attachment collar is slidably attached to said 10 underground heat exchanger.

[0032] Said plurality of arms may comprise a flatter centre portion that engages with the borehole and wherein said flatter centre portion has a curved profile when viewed in a direction along a main central axis of said positioning device.

[0033] Said positioning means may comprise one or a plurality of curved heat transfer surfaces.

[0034] Said positioning device may comprise material selected from the set: graphene; carbon fiber; copper; copper alloy; and/or aluminium.

[0035] Preferably a flow circuit of said heat exchange fluid comprises a closed circuit such that a flow of heat exchange fluid in said underground heat exchanger is physically isolated from a flow of hydrocarbon fluid outside said underground heat exchanger.

[0036] The heat storage and extraction apparatus preferably comprises a first well head heat exchanger comprising means for transferring heat energy to said heat exchange fluid; and means for extracting heat energy from said heat exchange fluid. [0037] The positioning means may be resiliently biased so as to self locate the positioning device in a bore hole, aligning the first and second tubes in positions relative to each other and relative to the cylindrical inner surface of the borehole.

[0038] A method of storing and/or extracting heat from a hydrocarbon well, at the same time as extracting hydrocarbon fluid from said hydrocarbon well, said method comprising: providing an underground heat exchanger in a borehole of said well; wherein said heat exchanger does is positioned so as to not completely fill a cross section of said borehole taken in a direction perpendicular to a main axial length of said borehole at a position immediately adjacent said cross section, so as to allow gas or oil to flow along a length of said heat exchanger through a plurality of apertures between said heat exchanger and an inner wall of said borehole; and wherein said heat exchanger is configured to exchange heat between the heat exchange fluid and said inner wall of said borehole; said underground heat exchanger comprising a first tube for carrying a heat exchange fluid in a first direction; and a second tube for carrying said heat exchange fluid in a second direction; wherein a closed circuit is formed by said first tube and said second tube; and pumping said heat exchange fluid through said heat exchanger; and extracting said hydrocarbon fluid from said borehole.

[0039] Said method may further comprise insertiing said heat exchanger in to said borehole after fracking of said well, and after plugs have been removed from said well.

[0040] Said positioning device provides a thermal bridge between an inner wall of said borehole and said underground heat exchanger. [0041] A borehole thermal energy store, comprising: a borehole in use for fluid hydrocarbon production; an underground heat exchanger located in said borehole; said underground heat exchanger comprising: a first tube for carrying a heat exchange fluid in a first direction; and a second tube for carrying said heat exchange fluid in a second direction; said first and second tubes forming a closed circuit for containing said heat exchange fluid; a plurality of positioning devices extending along a length of said borehole; each said positioning device comprising means for positioning said first and / or second tubes within a substantially circular bore of a borehole; each said positioning device comprising one or more attachment means for attaching to said underground heat exchanger and one or more positioning arms for positioning said attachment means within said borehole; wherein, as viewed in a direction axially along a main length of said borehole, each said positioning device comprises one or a plurality of apertures between said positioning device and a nominal circle drawn around the outer extremities of said positioning device through which a flow of fluid hydrocarbon may pass.

In a hydrocarbon well

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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. [0047] 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.

[0048] 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

[0049] 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.

[0050] In order to provide passage of heat exchange fluid along the borehole, the support bracket has one or more apertures or voids as viewed in the axial direction, along a main length axis of the support bracket which coincides with a main length direction of the local part of the bore in which it is installed, of sufficient cross-sectional area to allow a first flow pipe or conduit carrying heat exchange fluid in a direction into the well, and a second pipe or conduit for carrying heat exchange fluid in a return direction out of the well. Additionally, the support bracket comprises one or more apertures or voids as viewed in the main axial direction of the support bracket, which allows passage of hydrocarbon fluids in the main axial direction along the support bracket to allow contemporaneous extraction of the hydrocarbon fluids at the same time that the heat exchange fluid is being pumped along the first (flow) pipe and along the second (return) pipe. There are provided apertures or voids as viewed in a direction along a main axial length of the support bracket, enabling fluid to flow through the apertures and around the components of the support bracket.

[0051] 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 1cm ² of contact area, preferably over 10 cm ² , more preferably over 30cm ² 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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

[0059] 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 as those discussed by Westaway (2016).

[0060] 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.

[0061] 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.

[0062] 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. [0063] 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.

[0064] 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. [0065] 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. [0066] 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.

[0067] 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.

[0068] 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. [0069] 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.

[0070] In the aspects disclosed herein, there is provided:

• Extraction of underground heat from an active or dormant hydrocarbon well using an independent heat exchange circuit which is separate from the hydrocarbon fluid flow.

• Storage of heat in an active or dormant hydrocarbon well.

• Extraction of underground heat from a hydrocarbon fluid flow, independently of any heat which may be injected into or stored in the hydrocarbon well at the same time as extracting heat from hydrocarbon fluid flow.

• Storage of heat underground and/or extraction of heat underground from the heat exchange fluid circuit is independent of whether any hydrocarbon fluid is being extracted and whether any heat is being extracted from the extracted hydrocarbon fluid.

[0071] The above ground / well head first heat exchanger which is operable to extract heat from the well via a heat exchange fluid circuit, or in a heat storage mode, to inject heated heat exchange fluid into the well storage underground can operate independently of the above ground / well head located second heat exchanger which extracts heat from hydrocarbon fluid extracted from the well. [0072] Other aspects are as set out in the claims herein which are incorporated into the description by reference. Brief Description of the Drawings

[0073] 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:

[0074] Figure 1 shows a typical horizontal gas well, as described in the background section above;

[0075] Figure 2 shows schematically in cut away view a hydrocarbon well having a single borehole, and including a wellhead heat recovery system and a wellhead heat storage system; [0076] Figure 3 is a schematic diagram of a concentric tubular heat exchanger in which the heat exchanger is positioned substantially centrally in a bore, according to a first embodiment;

[0077] Figure 4 is a schematic diagram of a concentric tubular heat exchanger positioned offset from a central position in a bore so that the heat exchanger is in contact with a borehole casing, according to a second embodiment;

[0078] Figure 5 is a schematic diagram of a parallel tubular heat exchanger positioned in contact with a borehole casing with additional heat transfer surface according to a third embodiment; and

[0079] Figure 6 is a schematic diagram of a (i) a concentric tubular heat exchanger positioned by a centralizer type bracket in a borehole according to a fourth embodiment; and (ii) a further tubular heat exchanger and a fifth support device according to a fifth embodiment. Detailed Description

[0080] 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.

[0081] In the embodiments described herein, the heat exchange fluid may comprise any suitable fluid, either liquid or gas or combination of liquid and gas which provides efficient heat transfer and storage characteristics and flow characteristics for absorbing heat from the surrounding borehole liner. In the best mode, the heat exchange fluid may comprise water, and may include additives to improve the flow characteristics through the borehole, and to improve the flow and lubrication characteristics of the water to reduce pump wear and maintenance.

[0082] In this specification, the term “hydrocarbon fluid” is used to describe either oil or gas, or a combination of oil and gas. Examples include natural gas, methane, and hydrocarbon oil.

[0083] In this specification, the term “well” is used to describe one or a plurality of individual boreholes drilled into the earth, which are co - located in a common area to extract hydrocarbon fluid from one or more layers of hydrocarbon fluid bearing rock. Each individual borehole has an upright or vertical portion and extends downwardly into the earth to reach a layer of hydrocarbon containing rock, for example shale. In a well drilled into shale deposits, each individual borehole may have an upright or vertical portion and a laterally extending portion which extends along the layer of hydrocarbon bearing shale, with a bend or curved portion of the borehole between the upright and laterally extending portions.

[0084] In this specification, the term “bore” is used to describe a borehole, either vertical or upright part of a borehole, or a lateral or horizontal part of the borehole, and a curved transition part of the borehole, between a vertical or upright portion, and a lateral portion of the borehole, unless in the context, the upright/vertical, lateral horizontal or curved connecting portion of the borehole is specified.

[0085] In this specification, where the term “above ground” is used, it will be understood that in the case of an offshore hydrocarbon rig any apparatus described herein as being above ground may also be located on a marine oil rig and the term aboveground is also generically used to encompass and include above a sea surface.

[0086] Figure 2 shows schematically in cut away view a hydrocarbon well, 200 for example a shale gas well comprising a vertical or upright borehole 201, a curved connection bore portion 202, and a laterally extending bore portion 203. A wellhead at the surface of the ground comprises a drilling rig or derrick 204 which is used initially to drill down the borehole and insert a borehole liner. Once the well is drilled, the wellhead comprises a ’’Christmas tree” , being a set of valves and ports through which gas or oil may be extracted, and via which an exchange fluid may be sent into the borehole and recovered from the borehole. On the ground surface there is preferably a heat storage and recovery facility 205 comprising pumps for pumping heat exchange fluid; a reservoir for storing heat exchange fluid; a first heat exchanger for extracting heat from a heat exchange fluid; a heat source facility 206, for example an array of solar panels through which a further second heat exchange fluid may pass in order to be heated by the energy of the sun. [0087] The first above ground heat exchanger may be located at or near the wellhead and comprises a heat exchange means for transferring heat from an above ground heat source into the heat exchange fluid for pumping down the well and also includes a second heat exchange means for extracting heat from heat exchange fluid recovered from the underground well.

[0088] Using the first above ground heat exchanger, heat can either be pumped into the heat exchange fluid for storage in the well underground, or heat can be extracted from the heat exchange fluid which has been pumped back from the well.

[0089] The second aboveground heat exchanger, located at or near the wellhead comprises a heat exchange means for extracting heat energy from the hydrocarbon fluid flow extracted from the well. Using the second heat exchanger at the wellhead heat can be recovered from the hydrocarbon fluid flow independently of operation of the first heat exchanger, so that he can be extracted from the hydrocarbon fluid flow at the wellhead at the same time that heat is being injected into the well using the first above ground wellhead heat exchanger.

[0090] Using the first in the example shown, the well is a fracking well in which the lateral bore 203 extends laterally into a layer of shale gas 207. As described herein above, in relation to the prior art, in a fracking well a borehole liner may either be predrilled with a plurality of holes through which fracking fluid may be pumped into the surrounding sedimentary rock layer, or alternatively a borehole liner may be perforated using a fracking gun to create holes, once the borehole liner has been fitted.

[0091] In either case, the fracking well has a productive lifetime during which fracking fluid is injected into the borehole, and permeates out of the holes in the lateral wellbore 203. This causes fractures in the surrounding rock layer 207 which allows gas and/or oil to seep into the lateral well borehole 203 wherefrom it can be returned to the surface via the upright wellbore 201.

[0092] Whilst gas and/or oil are still being extracted from the well, according to specific methods and embodiments of the invention described herein, heat can be extracted from the well and/or stored in the well for extraction later using a tubular heat exchange apparatus as described herein, which extends down and along the borehole.

[0093] By injecting a heat exchange fluid into the heat exchanger in the well, at a lower temperature than the underground temperature, heat from the rock or sediment layer transfers through the sides of the borehole to the in borehole heat exchanger and heats up the heat exchange fluid which is then returned to the surface, whereupon heat can be extracted by the above ground surface heat exchanger apparatus.

[0094] Conversely, where heat energy is available above ground, for example from a solar array the well hole heat exchange fluid can be heated in the above ground heat exchanger to a temperature above the underground temperature, and when pumped down into the well may be stored underground by heating up the rock or sediment layer immediately surrounding the borehole. Heat stored underground during summer or hot periods of weather may later on be extracted during winter or cold periods.

[0095] Using the apparatus as herein described, heat extraction and/or heat storage in the well may take place at the same time that oil and/or gas are being extracted from the well. Therefore after the primary productive period of oil or gas extraction, using apparatus described herein a hydrocarbon well can continue to be used for the dual purpose of extracting any remaining hydrocarbons in the well, and for ground heat extraction and/or storage of heat underground. [0096] Figure 3 herein 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 a layer of insulation 360 between the inner and outer pipe, for example a tubular circular cylindrical insulation layer, so that heat transfer between the inner pipe and the outer pipe is restricted. The inner pipe can be used as a return flow pipe to return relatively higher temperature heat exchange fluid, in which case the insulation prevents heat loss to the surrounding inward flow of heat exchange fluid into the wellbore via the outer pipe. Conversely, if the flow direction is reversed so that the inner pipe is used for inward flow of heat exchange fluid, the insulation layer maintains a temperature gradient between the inward flow of heat exchange fluid and the outward flow of heat exchange fluid. [0097] To extract heat from the well heat exchange fluid at a relatively lower temperature is pumped in an inward or flow direction, the inward flow heat exchange fluid being at a lower temperature than the return flow heat exchange fluid which is heated up by the surrounding sides of the borehole. [0098] To store heat energy, heat exchange fluid at a relatively higher temperature than the sides of the borehole is pumped inwardly and successively cools towards the distal end of the inward heat exchange flow pipe. The relatively cooler outflow or return flow of heat exchange fluid is returned via the other one of the pipes, and as the first and second pipes are thermally insulated from each other, a temperature gradient is maintained between the inward flow of heat exchange fluid and the return flow of heat exchange fluid.

[0099] A positioning device locates the heat exchanger substantially coaxially centrally within the borehole so that heat exchange fluid flows within the heat exchanger and is isolated from and separated from a flow of hydrocarbon fluids which can flow in the substantially circular cylindrical annular gap between the heat exchanger and the internal circular cylindrical wall of the borehole. The outer pipe 340 of the heat exchanger will be in thermal and mechanical contact with a support or bracket 390, which provides a path for heat exchange with the borehole wall or liner (330), and through the borehole liner to the surrounding rock.

[0100] Preferably the outer pipe carries the flow of heat exchange fluid from the plant above ground into the well, down the vertical or upright borehole, and laterally along any substantially horizontal boreholes, and the inner (return) pipe carries the return heat exchange fluid from the well 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 relatively cooler flow path of the heat exchange fluid and the relatively warmer return path of the heat exchange fluid. However, it is possible that the flow direction to be reversed so that the inner pipe carries heat exchange fluid underground into the well, and the outer pipe returns the heat exchange fluid in the opposite direction back to the ground surface or well head.

[0101] The nominally annular cylindrical space or void 380 between the outer pipe and the well casing can be of sufficient cross-sectional area as viewed in the main direction of the borehole to accommodate the emanating hydrocarbon gas or fluid. The tubular heat exchanger and the casings are fixed apart by regularly spaced brackets or supports 390 which have cross sections designed to anchor the inner 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.

[0102] A bracket or support device to support the heat exchanger comprises one or a plurality of resilient arm members 390, which position the pipes by applying a biasing force to the well casing 330, and may comprise a clamp which attached to the heat exchanger. These resilient members provide positioning means to the pipe, which may be held in a clamp, 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 are 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.

[0103] The plurality of resilient members 390 are arranged radially around the outer pipe of the heat exchanger and act as spacers to centrally locate the heat exchanger with respect to the inner wall of the borehole. The one or a plurality of arms extend outwardly from said attachment means in a direction radially outwardly of a main central axis of the support device

[0104] As shown in figure 3, when viewed in a direction along a main axial length of the borehole at a position locally to the positioning device, said positioning device resides within an area bounded by the outer perimeter circle drawn at the an inner wall of the borehole and the outer wall of the heat exchanger. The positioning device does not fully occupy the cross-sectional area between the inner wall of the borehole at the outer wall of the heat exchanger, there being the voids or apertures which allow passage of fluid hydrocarbons via passages through the positioning device, and to flow in a direction along the main axial length of the borehole. As viewed in a direction axially along a main length of said borehole and along a main length axis of said positioning device, each said positioning device comprises one or a plurality of apertures between said positioning device and a nominal circle drawn around the outer extremities of the positioning device.

[0105] The positioning device provides a thermal bridge to transmit heat between the inner wall of the borehole and the outer wall of the heat exchanger so that a temperature gradient exists between the inner walls of the borehole and the outer wall of the heat exchanger. The positioning device comprises one or more substantially full or part cylindrical inner surfaces for abutting the outer heat exchange tube 340, and one or more substantially part cylindrical outer surfaces for abutting an internal surface of the borehole. The one or plurality of support brackets 390 form a connecting portions between said one or more inner substantially part cylindrical surfaces and said one or more substantially part cylindrical outer surfaces, for creating a thermal bridge between said inner and outer surfaces and thereby transferring heat between the inner surface of the borehole wall and the outer surface of the outer heat exchange tube.

[0106] In figure 4 herein, the same heat exchanger as described in figure 3 is shown, using like numerals for similar features, but with an alternative second support device which positions the heat exchanger off centre to the main central axis of the borehole, and offset to a main central axis of the second positioning and support device. In this embodiment, a plurality of resilient arm members 490 of the positioning device are configured to position the heat exchanger against the inside of the wellbore casing. The heat exchanger is attached to the resilient arm members of the support device by one or a plurality of clamp or ring members. The clamp or ring members may also have an extended curved outer profile to engage with the curved inner wall of the wellbore to increase the heat exchange surface area between the well casing and the heat exchanger. The resilient arm members may also provide an additional heat transfer path.

[0107] The resilient biased arm members 490 in figure 4 extend outwardly from the clamp or ring connecting member which connects them to the heat exchange outer pipe. As viewed in figure 4 in a direction along a main length axis of the support device, the support device comprises a ring or clamp member which fits around the heat exchanger; and a plurality of resiliently biased arms which urge between the ring or clamp member and the inside surface of the borehole, so as to urge the heat exchanger into close contact with the side of the borehole. Optionally there is provided a curved plate which extends along a main axial length of the heat exchanger and is fitted on the outside of the heat exchanger between the heat exchanger and the inner wall of the central bore of the borehole. The curved plate may comprise a part circular cylindrical plate having a part circular cylindrical outer surface which makes good thermal contact with the inner wall of the borehole liner 430. There are a plurality of gaps, apertures or voids between the resilient arm members which allows for the passage of hydrocarbon fluid in a direction along the borehole and in a direction along a main central axis of the support device.

[0108] The plurality of resiliently biased locating arms 490 act to locate a main aperture of the support device, through which the heat exchange pipes pass, so that the centre of said aperture is offset relative to a main central axis of the support device and the main central axis of the borehole, urging the heat exchange pipes and the ring or collar member of the support device towards the side of the borehole.

[0109] The positioning devices each reside within an area bounded by the borehole liner 430 when viewed in a direction along a main length direction of the borehole at any position along the length of the positioning device. The positioning device resides in an area within said circle, and between the circle and the heat exchanger. The positioning device does not fully occupy the cross-sectional area between the circle and the outside of the heat exchanger, there being one or more passages, voids or apertures between the positioning arms 490 of the positioning device, which allow passage of fluid hydrocarbons through the positioning device, and to flow in a direction along the main axial length of the borehole. As viewed in a direction axially along a main length of said borehole, each said positioning device comprises one or a plurality of apertures between the positioning arms, so that hydrocarbon fluid can flow through an area within the nominal circle drawn around the outer extremities of the positioning device, and around the positioning device. [0110] In the alternative arrangement shown in Figure 5, the heat exchanger comprises parallel flow 501 and return 502 pipes, the first pipe 501 extending substantially in parallel and side-by-side with the second pipe 502. In this example the return pipework 502 is insulated by an outer layer of thermal insulation 503. At least some ot the length 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 504 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 devices 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 of the borehole, 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.

[0111] The third embodiment support device comprises a plurality of resilient members 505, 506 which position the pipes by applying a biasing force to the inner surface of the well casing; and one or a plurality of clamp, collar or ring members which connect to the heat exchange pipes and optionally, to the production pipe 504. These resilient members 505, 506 provide positioning means to the heat exchanger, which may be held in a clamp, attached to the resilient members. In this example, the flow pipe 501 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.

[0112] In figure 5, the positioning device or bracket may further comprise a curved profile piece 507, which is made from thermally conductive material and forms an extended 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. The curved heat exchange plate 507 preferably comprises a section of part circular cylindrical plate. The curved heat exchange plate 507 may be curved around a focal line which extends in a same direction as, and runs parallel to a main axial length direction of said support device. The curved heat exchange plate 507 is fixed to the plurality of clamp, collar or ring members which fit around the first and second heat exchanger tubes so that there is efficient heat transfer between the heat exchange tubes and exchange plate 507. The resilient arm members 505, 506 act to urge the heat exchange plate 507 in a direction towards the inner surface of the surrounding borehole, and in a direction away from a main central axis of the support device to ensure enhanced thermal contact between heat exchange plate 507 and the inner surface of the borehole.

[0113] In the axial direction along the main length axis of the support device, there may be a plurality of clamp or ring portions around each heat exchange tube of the heat exchanger. The support device as a whole may be connected at a fixed length position to either the first or second pipe of the heat exchanger or to both pipes of the heat exchanger by one or more of the clamp or ring members which fit around the first and/or second pipe of the heat exchanger.

[0114] As viewed in Figure 5, in the axial direction along the main length axis of the support device, there may be a plurality of clamp or ring portions around each heat exchange tube of the heat exchanger. The support device as a whole may be connected at a fixed length position to either the first or second pipe of the heat exchanger or to both pipes of the heat exchanger by one or more of the clamp or ring members which fit around the first and/or second pipe of the heat exchanger.

[0115] In the direction along a main axial length of the support device, the support device has a plurality of apertures or voids extending between the first and second heat exchange pipes and between the outer heat exchange plate and the resilient arm members 505, 506 which allows passage of hydrocarbon fluid along the borehole. In the example shown in figure 5, hydrocarbon fluids are contained within a production pipe 504, but where the production pipe is not present, the hydrocarbon fluids flow along the outer surfaces of the first and second heat exchange pipes and substantially in parallel to those pipes.

[0116] Each of the first clamp or ring portions connecting the first or flow pipe 501 may comprise a circular cylindrical surface which fits around the first, flow pipe 501 , which in addition to securing the first flow pipe to the resilient arm members 505, 506 and/or to the curved plate 507, provide a thermal bridge between the first flow pipe 501 and the resilient arm members and/or curved plate 507. Similarly, there may be one or more second rings or clamps connecting to the second or return heat exchange pipe 502. The second rings or clamps may be themselves connected either to the first rings or clamps, to the resilient arm members 505, 506 and/or to the curved heat exchange plate 507 for supporting the second or return pipe 502. The second rings or clamps may also comprise substantially cylindrical circular surfaces for contacting around the outside of the second, or return heat exchange pipe 502.

[0117] Each positioning device reside within an area bounded by an outer perimeter circle 530 when viewed in a direction along a main length direction of the borehole at any position along the length of the positioning device. The positioning device resides in an area within said circle between the circle and the heat exchanger. The positioning device does not fully occupy the cross-sectional area between the circle and the outside of the heat exchanger, there being one or more passages, voids or apertures between the positioning arms 505 and 506 of the positioning device, sufficient to allow passage of fluid hydrocarbons through the positioning device in a direction along the main axial length of the borehole. As viewed in a direction axially along a main length of said borehole, each said positioning device comprises one or a plurality of apertures, passages or voids between the positioning arms, so that hydrocarbon fluid can flow through an area within the nominal circle drawn around the outer extremities of the positioning device, and between the positioning device and the circular cylindrical bore of the borehole over the axial length of the positioning device.

[0118] Figure 6 herein shows a tubular heat exchanger 630 supported by a plurality of support devices in a wellbore casing 640. In this example, the support devices 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 fourth embodiment of the support device or support bracket shown on the left in figure 6, comprises a plurality of collars 600 which may act as clamps to clamp around the tubular heat exchanger and a plurality of bow springs or resiliently biased arms 690 which extend between adjacent clamps in the main axial direction of the support device, and which are biased against the casing 640 of the borehole. Each collar 600 comprises a substantially annular ring which surrounds the outer pipe of the heat exchanger 630 and through a central aperture of said collar, the heat exchanger passes. The individual bow spring members extend radially outwards with respect to a main central axis through the centre of the collars, so that there are a plurality of passages between the individual bow members through which hydrocarbon fluid may flow, so that when viewed in a direction along the main central axis of the support device, there is a central aperture through which the heat exchanger passes, including inner and outer heat exchange pipes for flow and return of the heat exchange fluid, and a plurality of apertures in the substantially annular cylindrical cavity or space between the outer surface of the outer heat exchange pipe and the inner surface of the wellbore casing 640 which allows passage of hydrocarbon fluid through and past the support device.

[0119] In the example fourth embodiment support device, there are preferably first and second collars 600 spaced apart from each other in a main axial length direction, with a plurality of said bow members extending therebetween, said bow members extending both radially outwardly of the main central axis of the support device and along a main central axis of the support device in an arc or curve, each bow member having a first end connected to a said first collar, and a second end connected to a said second collar.

[0120] In a borehole, typically in some wells the internal diameter of the borehole may become successively smaller the deeper into the ground the borehole is drilled, so that the internal passage of the borehole becomes progressively narrower the further away from the wellhead, and the nearer to the most distant part of the borehole. Nearer the surface, the borehole may start off as for example a drilled diameter of around 50 cm, a diameter which is maintained for the first 30m or so of depth. There may then be a further 60m to 200m of depth at a smaller diameter, of the order of 40cm, followed by a further 600m or so at a third internal diameter of the order of 27.5 cm, which then progresses to a fourth section of internal diameter of the order 19cm extending for up to a further 2,500m, this generally being the kick off point or curved section which transitions from vertical to lateral, followed by a fifth section of a successively smaller internal diameter of the order of 11.5 to 14 cm which may extend for up to a further 2,000m laterally, this being the fracking portion.

[0121] Ideally, for heat extraction, the heat exchanger will pass as deeply as possible into the borehole because temperature rises with increasing depth into the earth. The outer dimensions of the resilient members of the support device are designed to contact the inner wall of the borehole at the depth at which that particular support device will reside when the heat exchanger is inserted into the borehole to the maximum depth at which the heat exchanger is designed to operate.

[0122] In use, either the first or second collar may be secured to the outer pipe of the heat exchanger so that it cannot slide along the heat exchanger in an axial direction of the heat exchanger, so that when the heat exchanger is pushed through the borehole, the support device is pushed with the heat exchanger along the borehole. The other one of the collars may be slidable with respect to the outer pipe of the heat exchanger so that the effective length of the support device may vary depending upon the internal diameter of the borehole, with the outwardly extending resilient members varying their maximum outward distance from the central axis of the support member as they are constrained by the internal diameter of the borehole, whilst one collar is at a fixed position relative to the heat exchanger, and the other collar is slidable along the length of the heat exchanger.

[0123] Both collars 600 and spring arms 690 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.

[0124] A fifth embodiment support device shown on the right hand side of Figure 6 has been further adapted to improve the heat transfer characteristics by extending the collars 610 in their length direction parallel to the main axial length of the support device, to increase the heat transfer surface area between the support device and the heat exchanger. Plurality of resilient spring members 620 are formed with a flatter center portion that engages with the borehole casing, to provide a larger heat transfer surface. The substantially flattened regions of the resilient members may have a curved profile when viewed in a direction along the main central axis of the support device, the curved profile being of a similar radius of curvature as the radius of curvature of the inner wall of the borehole, in order to allow a greater contact between the outer surface area of the wall contacting portions of the support device and the inner wall of the borehole. The curved profile of the resilient arm members may curve about a focal line which extends in a same direction as a main axial length direction of said support device. The center portion of the wall contacting resilient members will preferably also have a wider, curved profile, extending over a wider radial angle, 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, from 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.

[0125] As with the fourth embodiment support device, in the fifth embodiment support device, hydrocarbon fluid can pass through the voids and apertures between the outwardly extending arms of the support device in the nominally annular tubular region between the outer surface of the heat exchanger and the inner surface of the borehole.

[0126] Like the fourth embodiment herein before described, the fifth embodiment support device comprises a plurality of clamp members, in the form of annular tubular collars or rings 610 which extends around the heat exchanger, and are spaced apart from each other in a main central axial direction of the support device; the plurality of clamp members being connected to each other by a plurality of resiliently biased arms which extend both radially outwardly with respect to the main central axis of the support device, and in a direction along the main length of the support device, with a plurality of voids, apertures or gaps between the support arms which allows for the free passage of hydrocarbon fluid in the nominally annular tubular region between the heat exchanger and the inner surface of the borehole.

[0127] As viewed in a direction along a main central axis of the support device, the support device comprises a circular cylindrical tubular ring 610, and a plurality of radially extending arms extending from the outer surface of the tubular ring 610. As viewed in the opposite direction, the fifth embodiment support device will be identical or near identical, to the view from the opposite direction.

[0128] 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.

[0129] The pipes can be inserted into the well using wireline equipment, for example during the insertion of production tubing.

[0130] 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.

[0131] In the embodiments described herein, the resilient arm members which extend outwardly of the clamp members of the support device may be manufactured from spring steel or like material and may be either bolted or welded to the collar or ring members which attach to the heat exchange pipes.

[0132] In each of the above embodiments, the heat exchanger comprises a first, flow direction heat exchange pipe and a second, return direction heat exchange pipe, where the direction of flow of heat exchange fluid in the first pipe is substantially opposite to the direction of flow of heat exchange fluid in the second heat exchange pipe. At a distal end of the first and second heat exchange pipes, the heat exchange fluid reverses in flow direction so that the heat exchange fluid flows down the flow pipe into the borehole, to a distal end of flow heat exchange pipe, and then returns via the return heat exchange pipe. Optionally, in embodiments where the first and second heat exchange pipes are arranged concentrically to each other and/or coaxially to each other, and where the second, return pipe is inside the first, flow pipe there may be provided a length region along the first flow pipe, which is perforated so that fluid flows from the first flow pipe into the second return pipe over an extended length distance of the two heat exchange pipes within the borehole. [0133] When extracting heat from a well, the heat exchange fluid pumped into the flow pipe may be relatively cooler than the temperature of the rock surrounding the borehole so that the temperature gradient is from the relatively warmer rock into the relatively cooler heat exchange fluid, thereby heating up the heat exchange fluid. The warmed heat exchange fluid is then returned to the well surface via the second return heat exchange pipe, where heated energy can be extracted from the relatively warmed returned heat exchange fluid. In this manner, heat can be extracted from a hydrocarbon well, whilst at the same time allowing extraction of hydrocarbons from the well.

[0134] On the other hand, if there is a supply of relatively warmer heat exchange fluid which is warmer than the underground temperature in the well, for example if the heat exchange fluid can be heated up by a solar panel array located at the wellhead, the well can be used to store heat by pumping the relatively hotter supply of heat exchange fluid down the flow pipe, which then transfers heat from the relatively warmer flow of heat exchange fluid via the outer walls of the first flow pipe, through the positioning device and through the liner of the borehole into the surrounding rock. The return heat exchange fluid is relatively cooler than the flow heat exchange fluid, it’s heat having been transferred into the well and stored underground.

[0135] At the wellhead, switching between heat extraction from underground and storing heat underground involves opening a bidirectional valve at the wellhead between a relatively cooler heat exchange fluid flow which circulates around the heat exchanger at the wellhead, or a relatively warmer heat exchange fluid flow which flows around a heat generator, for example a solar array adjacent the wellhead. By switching between and above well head heat recovery circuit for recovering underground heat, and an above wellhead heat generator circuit, e.g. passing through an array of solar heated panels, heat can either be extracted from, or pumped in to the underground well for underground storage, and in either case the flow of hydrocarbon fluids can still be extracted from the well.

[0136] In each of the above described embodiments, 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 of the heat exchanger.

[0137] The invention is suitable to be used in wells which the borehole sleeve comprises a sliding tubular sleeve system. The invention is also suitable for use where the borehole sleeve is pre drilled, before inserting into the borehole, rather than being perforated with a perforating gun after insertion into the borehole.

[0138] In each of the above embodiments, the flow path of the heat exchange fluid is physically isolated from the flow path of hydrocarbon fluid by the walls of the heat exchange tubes, so that there is no mixing of heat exchange fluid and hydrocarbon fluid. The first heat exchange tube and the second heat exchange tube form an underground closed circuit for containing the heat exchange fluid.

[0139] In each of the above embodiments, the primary objective in a heat extraction mode is to transfer heat between the heat exchange fluid and the wall of the borehole, rather than transfer heat between the heat exchange fluid and any hydrocarbon fluids flowing in the well, so that heat energy can be extracted from the rock surrounding the borehole. Alternatively, in a heat storage mode, the object is to transfer heat between the heat exchange fluid and the rock surrounding the borehole so that the surrounding rock increases in temperature thereby storing heat underground. [0140] Where the borehole has a static or near static flow of hydrocarbon fluid, the hydrocarbon fluid will also act to transfer heat between the wall of the borehole (and hence to the volume of rock around the borehole) and the underground heat exchanger, and the heat exchange paths between the underground heat exchanger and the rock are via the arms of the positioning device and via any hydrocarbon fluid between the underground heat exchanger and the borehole wall. When there is a flow of hydrocarbon fluid out of the well, the hydrocarbon fluids still acts as a thermal bridge between the underground heat exchanger and the wall of the borehole, but may be less effective in the heat storage mode, as any he transferred from the underground heat exchanger to the hydrocarbon fluid may simply be transported by the hydrocarbon fluid to the surface where it may be recovered using a separate hydrocarbon fluid heat exchanger. [0141] Consequently, in a heat extraction mode, the apparatus may operate acceptably whether hydrocarbon fluid is flowing or not, but in a heat storage mode where heat generated at the head of the well, for example by a solar panel array or other above ground/above surface heat source is being stored underground better efficiency may be obtained at relatively lower hydrocarbon fluid flow rates, because the heat transferred from the underground heat exchanger to the hydrocarbon fluid is not being returned to the surface via the hydrocarbon fluid flow to such a great extent as where the hydrocarbon fluid flow is greater. [0142] The invention is suitable for use in wells comprising one or a plurality of vertical or upright bores, each vertical or upright bore leading to one or a plurality of lateral bores.