GB2549832A

GB2549832A - Geothermal power system

Info

Publication number: GB2549832A
Application number: GB1703395.2A
Authority: GB
Inventors: Henderson William
Original assignee: Individual
Current assignee: Individual
Priority date: 2016-03-08
Filing date: 2017-03-02
Publication date: 2017-11-01
Also published as: GB201703395D0

Abstract

Geothermal power system comprising borehole with outer tube and insulated inner tube arranged concentrically therein A geothermal system, which may be a closed loop system, comprises borehole 122 and outer tube or pipe 132 arranged concentrically with inner or central tube 134, which has one or more sections of insulated tubing 159, to form annular passage 137. Pump 138 injects a working fluid, which may be brine, into the annular passage and heat exchanger 172 receives heated working fluid from the inner tube. Heat exchanger 172 receives heated working fluid from the inner tube and pump 138 injects a working fluid, which may be pre-heated by the heat exchanger, into the annular passage. A portion of outer tube 132 may be formed from an expandable liner expandable within the borehole using an expansion tool. The insulated tubing may feature an insulating vacuum sealed from the working fluid with sections of the insulated tubing connected to each other by insulated couplings. The geothermal system may feature a district heating plant and an electrical generator.

Description

GEOTHERMAL POWER SYSYSTEM

The invention relates to geothermal systems for the generation of electrical power or heat, in particular geothermal systems which extract heat from the earth’s core and use the extracted heat for the generation of electrical power or to provide heating for buildings and the like.

Background of the invention

The geothermal gradient of temperatures through the earth’s crust is typically 25-40 °C per kilometre of depth in most of the world. The conductive heat flux is of the order of 0.1 MW/km2. A geothermal heat pump can extract enough heat from shallow ground anywhere in the world to provide home heating, but industrial applications need the higher temperatures of deep resources. The thermal efficiency and profitability of electricity generation is particularly sensitive to temperature. The more demanding applications receive the greatest benefit from a high natural heat flux, ideally from using a hot spring. The next best option is to drill a well into a hot aquifer. If no adequate aquifer is available, an artificial one may be built by injecting water to hydraulically fracture the bedrock. This last approach is called hot dry rock geothermal energy in Europe, and much greater potential may be available from this approach than from conventional tapping of natural aquifers. Fig. 1 shows a typical geothermal power system, with a reservoir 1, a pump house 2, a heat exchanger 3 and a turbine hall 4. Cold water is pumped down a production well 6 to fractured crystalline bedrock at a depth of 4 to 6 km. The water is returned as hot pressurised water and/or steam through the production wells 5 to the heat exchanger 3, from where it is fed to the turbine hall 4 for the generation of electricity and to a hot water supply 7 for district heating. Additional observation wells 9 may be used to monitor the process at depth. Such systems require a number of well bores, making them relatively expensive. CA2679905A1 describes a geothermal power system which uses a wellbore extending from the ground to hot rock strata at a depth of 5 to 9 km. The system has closed loop circulation to protect the water and steam circulating in the system from any direct contact with the base rock. The system has a single L-shaped bore with a casing that lines the wellbore and a sealed bottom end. The injected water passes under pressure in the annular space between the casing and the tubing held centrally in the casing, where it is heated by the rock surrounding the casing, before returning through the tubing to the surface. The system is limited by the amount of heat which is radiated through the casing and the cement which surrounds the casing. Such a system cannot achieve more than about 10 to 15% thermal efficiency.

It is an object of the present invention to overcome one or more of the above problems of the prior art.

According to a first aspect of the present invention there is provided a geothermal system comprising: a borehole comprising a first section extending from a ground surface to selected rock strata and a second section extending from the first section at an angle to the first section within the rock strata, the second section of the borehole defining a formation surface ; a first tubular member extending within the first section; a second tubular member connected to the first tubular member and extending within the second section, the second tubular member being provided with fluid passage means adapted to permit the flow of fluid from inside the first tubular member to the second section of the borehole; a third tubular member extending from the ground surface within the first tubular member, thereby forming a first annular passage between the first tubular member and the third tubular member; a fourth tubular member connected to the third tubular member and extending within the second section, the fourth tubular member extending to a distal end in the second section of the borehole within the formation surface of the second section, thereby forming a second annular passage between the fourth tubular member and the formation surface, the distal end of the fourth tubular member being open and in communication with the second annular passage; plug means to prevent the flow of fluid between the second tubular member and third tubular member beyond the fluid passage means; an apparatus for injecting a working fluid into the first annular passage between the first and third tubular members; and an apparatus for receiving heated working fluid from the third tubular member. wherein one or both of the second and fourth tubular members comprise one or more sections of insulated tubing.

The geothermal system uses a single borehole to permit the passage of fluid from the ground surface along the first annular passage between the first tubular member, typically a liner, and the third tubular member, typically a length of production tubing, and then through the fluid passage means to the second annular passage between the formation surface and the fourth tubular member, typically an extension of the length of production tubing or an extension of the liner, where it is in direct contact with the hot rock, before returning to the surface through the third tubular member, thereby maximising heat extraction from the hot rock strata and requiring only a single borehole. The insulated tubing reduces heat loss from warm fluid passing upward to cooler fluid passing downwards.

The first tubular member may be a liner.

The second tubular member may be a liner and may be connected to the first tubular member by a hanger.

The second tubular member may be spaced from the formation surface to form a third annular passage in fluid communication between the first and second annular passages.

The second tubular member may be a slotted liner.

The fluid passage means may comprise the slots of the slotted liner.

Preferably the second section is at a depth of between 1500m and 8000m, typically between 3000 and 5000 m.

Preferably the second section has a length of between 500 and 3000 m.

Preferably the plug means is a packer or packer assembly.

The first section of the borehole may further comprise a casing surrounding the first tubular member. The casing may be cemented within the borehole.

The first and/or second tubular members may be physically connected to the casing, for example by a hanger.

Preferably the apparatus for injecting a working fluid comprises one or more pumps.

The apparatus for receiving heated working fluid may comprise a heat exchanger.

The apparatus for receiving heated working fluid may comprise electricity generation apparatus.

The apparatus for receiving heated working fluid may comprise district heating plant.

The system may be a closed loop system whereby working fluid received from the third tubular member may be injected as a working fluid into the first annular passage between the first tubular member and the third tubular member;

According to a second aspect of the present invention there is provided a method of heating a working fluid comprising the steps of: injecting a working fluid into a first annular passage between a liner and a tubular member in a first section of a borehole extending from a ground surface to selected rock strata, the tubular member comprising one or more sections of insulated tubing, passing the working fluid through a fluid passage means into a second annular passage between a tubular member and a formation surface of a second section of borehole extending from the first section at an angle to the first section within a hot rock strata; passing the fluid along the second annular passage to a distal end of the tubular member; returning the working fluid to the ground surface from the distal end of the tubular member through the tubular member; and receiving the heated working fluid from the tubular member.

Preferably in the injecting step the working fluid flows to a depth of between 1500 and 8000 m.

Preferably in the step of passing the fluid along the second annular passage to a distal end of the tubular member the working fluid flows a distance of at least 500 m.

In the step of passing the fluid along the second annular passage to a distal end of the tubular member the working fluid may flow through part of the formation beyond the formation surface in addition to flowing along the annular passage.

Preferably the working fluid is water, steam or a mixture thereof. The working fluid may be seawater. The working fluid may be a synthetic-based drilling mud (SBM) or oil or a mixture thereof.

Preferably the method includes the further step of passing the heated working fluid received from the tubular member through a heat exchanger.

The method may include the further step of using the heat exchanger to pre-heat working fluid prior to injection.

The method may include the further step of using the heated working fluid received from the tubular member as the working fluid used in the injecting step.

The method may include the further step of using the heated working fluid to generate electrical power.

The method may include the further step of using the heated working fluid to provide heat to a district heating plant.

According to a third aspect of the present invention, there is provided a geothermal system comprising: a borehole extending from a borehole proximal end located at a ground surface to a borehole distal end located within a selected rock formation; a first tubular member extending within the borehole from a tubular member proximal end located at the borehole proximal end to a tubular member distal end located at the borehole distal end; a second tubular member extending from a proximal end located at the borehole proximal end to a distal end located at a portion of the borehole which receives positive geothermal heat, and within the first tubular member, thereby forming an annular passage between the first tubular member and the second tubular member, wherein the distal end of the second tubular member is open and in communication with the annular passage; an apparatus for injecting a working fluid into the annular passage; and an apparatus for receiving heated working fluid from the second tubular member, wherein the second tubular member comprises one or more sections of insulated tubing. A portion of the first tubular member located within the selected rock formation may be formed from an expandable liner which is expanded against a surface of the selected rock formation.

The portion of the first tubular member located within the selected rock formation formed from the expandable liner may be expanded against the formation by an expansion tool which is adapted to be inserted into the first tubular member to expand the portion of the first tubular member within the selected rock formation against the surface of the selected rock formation and then withdrawn from the first tubular member.

The working fluid may be conductive brine.

The first tubular member may be at a depth of between 4000 and 8000m.

The first tubular member may be at a depth of 6000m.

The apparatus for receiving heated working fluid may comprise a heat exchanger.

The apparatus for receiving heated working fluid may comprise a district heating plant.

The system may be a closed loop system whereby working fluid received from the second tubular member may be Injected as a working fluid Into the first annular passage between the first and second tubular members.

The second tubular member may comprise one or more sections of insulated tubing.

Each section of insulated tubing may comprise: an outer tubular member; and an inner tubular member, the inner tubular member being located inside the outer tubular member, thereby forming an annular space between the outer tubular member and the inner tubular member, wherein the annular space is sealed from the working fluid.

The annular space may be a vacuum.

The one or more sections of insulated tubing may be connected to each other by insulated couplings.

According to a fourth aspect of the present invention there is provided, a method of heating a working fluid using a geothermal system, the geothermal system comprising: a borehole extending from a borehole proximal end located at a ground surface to a borehole distal end located within a selected rock formation; a first tubular member extending within the borehole from a tubular member proximal end located at the borehole proximal end to a tubular member distal end located at the borehole distal end; and a second tubular member extending from a proximal end located at the borehole proximal end to a distal end located at a portion of the borehole which receives positive geothermal heat, and within the first tubular member, thereby forming an annular passage between the first tubular member and the second tubular member, wherein the distal end of the second tubular member is open and in communication with the annular passage, wherein the second tubular member comprises one or more sections of insulated tubing, wherein the method comprises the steps of: injecting a working fluid into the annular passage between the first tubular member and the second tubular member; passing the working fluid from the annular passage to the second tubular member via the distal end of the second tubular member; returning the working fluid from the second tubular member to the ground surface; and receiving the heated working fluid from the second tubular member. A portion of the first tubular member located within the selected rock formation may be formed from an expandable liner which is expanded against a surface of the selected rock formation.

In the injecting step the working fluid may flow to a depth of between 4000 and 8000m.

In the injecting step the working fluid may flow to a depth of 6000m.

The working fluid may be conductive brine.

The method may include the further step of using the heated working fluid received from the second tubular member as the working fluid used in the injecting step.

The method may include the further step of passing the heated working fluid received from the second tubular member through a heat exchanger.

The invention will now be described with reference to the drawings in which:

Fig. 1 shows a prior art geothermal energy system;

Fig. 2 shows a geothermal energy system according to one embodiment of the present invention in schematic form;

Fig. 3 shows an enlarged view of part of the geothermal energy system of Fig. 2 according to a modified embodiment of the invention;

Fig. 4 is a schematic plan view of a group of geothermal energy systems of the present invention;

Fig. 5 shows a geothermal system according to an alternative embodiment of the present invention in schematic form;

Fig. 6 shows a lower portion of an example geothermal system according to the embodiment of Fig. 5 in schematic form; and

Fig. 7 shows a lower portion of a further example geothermal system according to the embodiment of Fig. 5 in schematic form.

Referring to Figs. 2 and 3, the geothermal system 20 of the invention uses a single borehole 22 to pump working fluid from the ground 24 to the hot rock strata 26 and to convey the heated working fluid from the hot rock strata back to the ground, where the heat is extracted using a heat exchanger (not shown). The heat can be used for electrical power generation using binary cycle turbines or the like (not shown) and/or for district heating plant (not shown).

The first section 30 of the borehole 22 is drilled from the ground surface 24 using conventional drilling methods to the required depth, which may be for example between 1500 and 8000 m, typically 3000 to 5000 m in the UK, where the rock strata 26 can be at a temperature of 150 to 200°C. A conventional casing 31, not shown in Fig. 2, but illustrated in Fig. 3, is used to line the borehole, and this is cemented with cement 33 in the usual way. Typically a 9 5/8 inch OD casing may be used for the casing 31.

At the required depth a lateral second section 34 of the borehole is drilled, using known techniques. This borehole extends substantially horizontally, or at least at an angle to the first section, through the hot rock strata 26. The second section 34 of the borehole 22 typically extends for about 1000 m, although the length can vary, for example between 500 m and 2000 m. The second section 34 is not lined by a casing, except for the first portion 36, which is formed as a continuation of the first section 30, and which in practice will generally include a curved transition portion of the borehole 22, as shown in Fig. 2. In the embodiment of Fig. 3, the casing 31 does not continue into the second section 34 of the borehole. For most of its length the second section 34 defines a formation surface 38, which is effectively the side wall of the borehole 22.

The first and second sections 30, 34 may have the same diameter or different diameters. A first tubular member 32, typically in the form of a 7 inch liner, extends from the surface 24 within the borehole 22. A second tubular member 40, also typically in the form of a 7 inch liner, is connected to the lower end of the first tubular member 32 and extends into the second section 34 of the borehole 22. The second tubular member 40 typically includes a slotted liner, and is attached to the first tubular member 32 by a hanger 41 (not shown in Fig. 2). The slots 42 in the slotted liner 40 form a plurality of fluid passages 42 which hydraulically connect the interior of the first tubular member 32 with the interior volume of the unlined second section 34 of the borehole 22. The slots 42 therefore permit flow of working fluid from inside the first tubular member 32 to the second section 34 of the borehole 22. A third tubular member 50, typically a length of 5 inch production tubing, extends from the ground surface 24 within the first tubular member 32, thereby forming a first annular passage 52 between the tubing 50 and the liner 32. The third tubular member 50 continues through the second tubular member 40 into the unlined second section 34 of the borehole 22. A fourth tubular member 51 is connected to the third tubular member and extends to the distal end 54 of the borehole 22. A second annular passage 56 is thus formed between the fourth tubular member 51 and the formation surface 38.

In the embodiment of Fig. 2 the fourth tubular member 51 is a continuation of the production tubing which forms the third tubular member 50. In the embodiment of Fig. 3 the fourth tubular member 51 is a continuation of the liner which forms the second tubular member 40.

The second annular passage 56 extends from a third annular passage 57 formed between the second tubular member 40 and the formation surface 38. In operation working fluid flows from the first annular passage 52 through the slots 42 into the third annular passage 57 and from there to the second annular passage 56.

The end 58 of the fourth tubular member 51 is open and in communication with the second annular passage 54, so that working fluid flowing along the second annular passage 56 reaches the void 60 at the distal end 54 of the borehole 22 and then returns along the fourth tubular member 51 and the third tubular member 50 to the surface 24. The direction of flow is indicated in Fig. 3 by arrows A, which show the flow of working fluid from the surface 24 to the distal end 54, and by arrows B, which show the return flow of heated working fluid from distal end 54 to the surface 24. A packer 62 or other suitable plug seals between the third tubular member 50 and the second tubular member 40, to prevent the flow of fluid between the third tubular member 50 and the second tubular member 40 beyond the slots 42.

The working fluid is in contact with the hot rock strata 26 while flowing through the second annular passage 56, so that heat transfer from the rocks to the fluid is maximised. If the rock strata are fissured, the fluid will also travel through the fissures, and the contact surface area is increased further still, thereby increasing heat transfer.

The system includes pump apparatus 70 for injecting the working fluid into the first annular passage 52. The system also includes an apparatus such as a heat exchanger 72 for receiving the heated working fluid from the third tubular member 50. Valves 74 to control the flow of working fluid may optionally be provided. The apparatus above ground, such as the pump 70 and heat exchanger 72, are known from existing systems, such as that illustrated in Fig. 1 and are not further described here.

The heat exchanger 72 may be connected to an electricity generating unit (not shown), such as turbines, to use the thermal power from the working fluid to drive the turbines. The working fluid is preferably maintained in the liquid phase until it passes through the heat exchanger. Consequently the system may include one or more check manifolds and high pressure pipework to maintain the working fluid in the liquid phase.

The system of the present invention offers a number of advantages over the prior art. It requires only one borehole, instead of the two or more boreholes required in a conventional system, in which different boreholes are used for delivering fluid to the hot rock strata and bringing heated fluid to the surface. The method is simple and brings the fluid into direct contact with the geological formation, maximising thermal transfer. It does not require fracturing the formation to create a fluid path from one borehole to another. It can be used in any location where surface drilling is possible. It can be used with any formation, and is not limited to granite formation, as some “fractured formation” systems are. It requires a minimum of surface disruption, as only one borehole is required, so can be used in locations with relatively small working areas available for use. The boreholes are deep, typically 5000 m or more, so there is no possibility of interference with surface installations. Once the initial capital costs have been recovered, the continued operational costs are low. The energy required to keep the system running is only a small fraction of the energy recovered by the system from the geothermal energy source. The system is environmentally friendly, in that it is a closed loop system, which does not require discharge of working fluid.

In one embodiment, illustrated in Fig. 4, a power generation system can comprise a plurality of separate systems, each system arranged with the vertical first sections 30 of the boreholes relatively close to each other, within a site boundary 80, and the horizontal second sections 34 of the boreholes fanning out radially in plan. This enables the extraction of energy from a large area at a single location, and is particular appropriate for power stations and the like. The surface plant 82 of the power generation system can be shared between the plurality of geothermal systems.

The system can be used in marine locations, with the vertical first section 30 of the borehole being situated on land, and the horizontal second section 34 of the borehole extending out to sea under the seabed.

The system is scalable, so can be used for small scale schemes, such as for industrial users of energy for generation of electricity and heat for use onsite, and for large scale schemes, such as power stations. The residual heat from the system can be used for industrial processes, greenhouses, biomass fuel drying, social heating etc. The system can be used to provide electricity and heat for factories, hospitals and any other large users of energy.

An alternative embodiment of the geothermal system is shown in Figs 5 and 6. With reference to Fig. 5, the geothermal system 120 of the alternative embodiment, hereinafter refer to as “the system 120”, uses a single borehole 122 with a single vertical section to convey heated working fluid such as conductive brine from a portion of the borehole which receives positive geothermal heat to a ground surface 124 where heat is extracted using a heat exchanger 172. The heated working fluid can be used for power generation as previously described with reference to the geothermal system of Figs. 2 to 4.

The borehole 122 extends from a borehole proximal end 126 located at the ground surface to a borehole distal end 128 located within a selected rock formation 130.

The borehole 122 is drilled from the ground surface 124 using a conventional drilling method to within the selected rock formation 130, which may be for example between 4000 and 8000m in depth. In the depicted example, the borehole 122 is drilled to a depth of 6000m within a selected formation 130 located at a depth of 4400M.

Located within the borehole 122 is a first tubular member 132 which extends from a tubular member proximal end 133 located at the borehole proximal end 126 to a tubular member distal end 135 located at the borehole distal end 128. An upper section of the first tubular member 132 may line the borehole 122 with a tubular casing formed of conventional tubular casing which is cemented into place in the usual way.

Located within the first tubular member 132 is a second tubular member 134 which extends from a proximal end located at the ground surface to a distal end 136 located at a portion of the borehole 122 which receives positive geothermal heat. An annular passage 137 for the passage of working fluid is located between the first tubular member 132 and the second tubular member 134. The distal end 136 of the second tubular member 134 is open and in communication with the annular passage 137. The second tubular member 134 is adapted to receive heated working fluid from the annular passage 137.

The system 120 also includes a pump apparatus 138 for injecting the working fluid into the annular passage 137 and an apparatus such as the heat exchanger 172 for receiving heated working fluid from the second tubular member 134. Valves 142 to control the flow of working fluid may be provided. This above ground apparatus is well known from existing systems as with the embodiment of Figs. 2 to 4. A portion 144 of the first tubular member 132 which is located within the selected rock formation 130 is formed from an expandable liner which is expanded against a surface of the selected rock formation 130.

The expandable liner is expanded against a surface of the borehole 122 within the selected rock formation 130 by the use of an expansion tool (not shown), which is inserted inside the first tubular member 132 to the portion located inside the selected rock formation and expanded against the portion 144 such that the portion expands plastically against the surface of the rock formation within the borehole 122. The expansion tool may be for example an expansion cone. Suitable expandable liners and expansion tools are sold by Weatherford under the trade mark Metal Skin.

Fig. 6 shows an example lower portion of the system 120. In the example, the borehole 122 comprises first, second, third and fourth portions 147, 149, 151, 153 which are lined by first, second, third and fourth casing portions 148, 150, 152, 154 of the first tubular member 132, respectively.

The first portion of the borehole 147 descends from the borehole proximal end 126 to a depth of 1000m and has a diameter of 26 inches. The first casing portion 148 which lines the first portion of the borehole 147 has a diameter of 20 inches.

The second portion of the borehole 149 descends from the first portion of the borehole 147 to a depth of 3000m and has a diameter of 17.5 inches. The second casing portion 150 which lines the second portion of the borehole 149 has a diameter of 13.38 inches.

The third portion of the borehole 151 descends from the second portion of the borehole 149 to a depth of 4400m and has a diameter of 14.75 inches. The third casing portion 152 comprises a MetalSkin Monobore Openhole Liner casing system formed from expandable liner which has a post expansion outer diameter of 13.517 inches.

The fourth portion of the borehole 153 descends from the third portion of the borehole 151 to the borehole distal end 128 located at a depth of 6000m and has a diameter of 12.25 inches. The fourth casing portion 154 comprises a Hydraskin casing system formed from expandable liner which has a post expansion diameter of 12.212 inches. The first, second and third casings 148, 150, 152 are fixed into their respective portions within the borehole 122 using cement. The fourth casing portion 153 is expanded against the surface of the formation 130 in the manner described above. The third portion 152 formed of expandable liner may also be expanded outwardly with the expansion tool described above.

Each of the second, third and fourth tubular casing portions 150, 152, 154 are fixed to an interior surface of the tubular casing portion located above it by a hanger 156. Each of the casing portions 148, 150, 152, 154 is supported by a tie-back shoe 158.

The third casing portion 152 located directly above the fourth casing portion 154 located within the rock formation 130 may comprise a carbide anchor which may assist in the anchoring of the fourth casing portion 154.

The first, second and third casing portions 148, 150, 152 which are located outwith the selected formation 130 are cemented into position within the borehole 122. The diameter of the portion of the borehole 122 located within the selected rock formation 130 is smaller than the diameter of the borehole outwith the selected rock formation.

In the depicted example, only the third and fourth casing portions 152, 154 are formed from expandable liner. However, each of the tubular casing portions 148, 150, 152, 154 may be formed from expandable liner.

The dimensions of the borehole 122 and casing portions 148, 150, 152, 154 may vary from those described above, and different types of expandable liners than those described above may be used.

Fig.7 shows a further example lower portion of the system 120. In the depicted example, the second tubular member 134 comprises a plurality of sections formed from insulated tubing 159. Each section of insulated tubing 159 comprises outer and inner tubular members 160, 162. The inner tubular member 162 is located inside of the outer tubular member 160, thereby forming an annular space 164 between the outer and inner tubular members 160, 162. The annular space 164 is sealed from the working fluid. Preferably, the annular space 164 is a vacuum. The sections 159 may be coupled to each other by insulated couplings (not shown). A suitable second tubular member 134 comprising sections of insulated tubing is a VIT MODEL 970 sold by Vallourec.

With reference to Figs. 5 and 6, in use the working fluid is injected from the ground 134 surface by the pump apparatus 138 into the annular passage 137. The working fluid then flows down the annular passage 137 in the direction indicated by the arrows. When the working fluid passes through the portion of the borehole 122 which receives positive geothermal heat, the temperature of the working fluid rises. The heated working fluid then passes through the distal end of the second tubular member 134 and flows upwards to the ground surface in the direction indicated by the arrows.

Once the heated working fluid reaches the surface, it passes through the heat exchanger 172 and transfers heat to the heat exchanger. The working fluid is then directed back to the pump apparatus to be injected back into the annular passage 137. The working fluid may be pre-heated by the heat exchanger 172 prior to injection into the annular passage 137. The heated working fluid may be used for electrical generation or heating purposes as described above for the embodiment of Figs. 2 to 4.

The insulated tubing sections insulate the working fluid flowing down the annular passage 137 from heated working fluid flowing up the second tubular member 134. This reduces the amount of heat lost from heated working fluid flowing up the second tubular member 134 to working fluid flowing down the annular passage 137.

In the examples of the invention described above, the distal end 136 of the second tubular member 134 is located within the selected rock formation 130. However, for the avoidance of any doubt it should be understood that the distal end 136 of the second tubular member may be located at a different portion of the borehole 122 which receives positive geothermal heat and still achieve the results of the invention described herein.

The second tubular member 134 may include a slotted liner provided with slots attached to the first tubular member by a slotted liner hanger. The slots may form a plurality of fluid passages which hydraulically connect the annular passage 137 with the interior volume of the second tubular member 134. The slots may permit flow of working fluid from inside the annular passage 137 to the second tubular member 134.

The borehole 122 may include an inclined portion, and the borehole proximal end 126 may not be positioned directly above the borehole distal end 128.

Prior systems generally include a gap or cement layer between the first tubular member and the selected rock formation.

As the portion of the first tubular member 132 located within the selected rock formation is expanded against the formation surface, the system 120 allows for more effective thermal transfer from the selected rock formation to the working fluid than prior systems as there is no gap or cement layer between the first tubular member and the selected rock formation.

Prior systems drilled to a depth of 6000m require a telescoping borehole with a very large diameter at the borehole proximal end to ensure that the borehole is of a sufficient diameter to provide a sufficient flow of working fluid within the selected rock formation.

As the first tubular member of the system 120 may comprise tubular casing portions which may be formed from expandable liner, the diameter of the borehole may be much smaller than prior systems and still provide a sufficient flow of working fluid within the selected rock formation.

Claims

1. A geothermal system comprising: a borehole extending from a borehole proximal end located at a ground surface to a borehole distal end located within a selected rock formation; a first tubular member extending within the borehole from a tubular member proximal end located at the borehole proximal end to a tubular member distal end located at the borehole distal end; a second tubular member extending from a proximal end located at the borehole proximal end to a distal end located at a portion of the borehole which receives positive geothermal heat, and within the first tubular member, thereby forming an annular passage between the first tubular member and the second tubular member, wherein the distal end of the second tubular member is open and in communication with the annular passage; an apparatus for injecting a working fluid into the annular passage; and an apparatus for receiving heated working fluid from the second tubular member, wherein the second tubular member comprises one or more sections of insulated tubing.

2. The geothermal system according to claim 1, wherein a portion of the first tubular member located within the selected rock formation is formed from an expandable liner which is expanded against a surface of the selected rock formation.

3. The geothermal system according to claim 2, wherein the portion of the first tubular member located within the selected rock formation formed from the expandable liner is expanded against the formation by an expansion tool which is adapted to be inserted into the first tubular member to expand the portion of the first tubular member within the selected rock formation against the surface of the selected rock formation and then withdrawn from the first tubular member.

4. The geothermal system according to any of claims 1 to 3, wherein the working fluid is conductive brine.

5. The geothermal system according to any of claims 1 to 4, wherein the first tubular member is at a depth of between 4000 and 8000m.

6. The geothermal system according to any preceding claim, wherein the first tubular member is at a depth of 6000m.

7. The geothermal system according to any preceding claim, wherein the apparatus for receiving heated working fluid comprises a heat exchanger.

8. The geothermal system according to any preceding claim, wherein the apparatus for receiving heated working fluid comprises a district heating plant.

9. The geothermal system according to any preceding claim, wherein the system is a closed loop system whereby working fluid received from the second tubular member may be injected as a working fluid into the first annular passage between the first and second tubular members.

10. The geothermal system according to any preceding claim, wherein the apparatus for receiving heated working fluid comprises electricity generation apparatus.

11. The geothermal system according to any preceding claim, wherein each section of insulated tubing comprises: an outer tubular member; and an inner tubular member, the inner tubular member being located inside the outer tubular member, thereby forming an annular space between the outer tubular member and the inner tubular member, wherein the annular space is sealed from the working fluid.

12. The geothermal system according to claim 11, wherein the annular space is a vacuum.

13. The geothermal system according to any of claims 10 to 12, wherein the one or more sections of insulated tubing are connected to each other by insulated couplings.

14. A power generation system comprising a plurality of geothermal systems according to any preceding claim, wherein each borehole is arranged to extend from within a defined area at the ground surface.

15. A method of heating a working fluid using a geothermal system, the geothermal system comprising: a borehole extending from a borehole proximal end located at a ground surface to a borehole distal end located within a selected rock formation; a first tubular member extending within the borehole from a tubular member proximal end located at the borehole proximal end to a tubular member distal end located at the borehole distal end; and a second tubular member extending from a proximal end located at the borehole proximal end to a distal end located at a portion of the borehole which receives positive geothermal heat, and within the first tubular member, thereby forming an annular passage between the first tubular member and the second tubular member, wherein the distal end of the second tubular member is open and in communication with the annular passage, wherein the second tubular member comprises one or more sections of insulated tubing, wherein the method comprises the steps of: injecting a working fluid into the annular passage between the first tubular member and the second tubular member; passing the working fluid from the annular passage to the second tubular member via the distal end of the second tubular member; returning the working fluid from the second tubular member to the ground surface; and receiving the heated working fluid from the second tubular member.

16. The method according to claim 15, wherein a portion of the first tubular member located within the selected rock formation is formed from an expandable liner which is expanded against a surface of the selected rock formation,

17. The method according to claim 15 or 16, wherein in the injecting step the working fluid flows to a depth of between 4000 and 8000m.

18. The method according to any of claims 15 to 17, wherein in the injecting step the working fluid flows to a depth of 6000m.

19. The method according to any of claims 15 to 18, wherein the working fluid is conductive brine.

20. The method according to any of claims 15 to 19, wherein the method includes the further step of using the heated working fluid received from the second tubular member as the working fluid used in the injecting step.

21. The method according to any of claims 15 to 20, wherein the method includes the further step of passing the heated working fluid received from the second tubular member through a heat exchanger.

22. The method according to claim 21, wherein the method includes the further step of using the heat exchanger to pre-heat working fluid prior to injection.