WO2015132404A1

WO2015132404A1 - Geothermal plant using hot dry rock fissured zone

Info

Publication number: WO2015132404A1
Application number: PCT/EP2015/054777
Authority: WO
Inventors: Ben Laenen
Original assignee: Vito
Priority date: 2014-03-07
Filing date: 2015-03-06
Publication date: 2015-09-11

Abstract

A plant for exploiting geothermal energy by circulating a fluid such as water through a fissured geological formation at least 700 m, or 1000, 3000 or 4000 m below the earth's surface, comprising at least one supply hole (2) leading from the surface down to said geological formation, at least one return hole (4) for the transport of heated fluid such as water from said geological formation to the surface, and a heat absorbing arrangement connecting the supply and return holes (2,4), said heat absorbing arrangement comprising a series of hydraulically parallel or quasi-parallel heat transfer surfaces (10) in the geological formation across which heat is transferred from said geological formation to said fluid such as water.

Description

GEOTHERMAL PLANT USING HOT DRY ROCK FISSURED ZONE

The present invention relates to a plant for exploiting geothermal energy and to a method of generating such a plant using a geological formation at depth.

Background

WO 96/23181 discloses an attempt to utilize abandoned offshore oil wells in extracting thermal energy, which in turn is supposed to be converted to electric power and supplied to a consumer. Here, two 3000 m deep wells are used for the supply and return holes, respectively, the wells being interconnected at their lower ends by a generally horizontally drilled loop which is 1000 m long and has a diameter of 21 .5 cm. 700 m³/h of water is circulated through the loop with an inlet temperature of 20 DEG C. The publication simply assumes that the water will return at a temperature of 90 DEG C, which is the temperature of the formation where the connecting loop is situated, and thus provide 40 MW of thermal power. This assumption is considered to be inaccurate. Using the disclosed method referred to above, it has been found that the return water temperature would be just a few degrees above the supply temperature and that the loop would have to be more than 60 times longer in order to provide 40 MW.

Summary of the invention

The present invention relates to a plant for exploiting geothermal energy by circulating a fluid such as water through a geological formation at least 700 m, or 1000, 3000 or 4000 m below the earth's surface, comprising at least one supply hole leading from the surface down to said geological formation, at least one return hole for the transport of heated fluid such as water from said geological formation to the surface, and a heat absorbing arrangement connecting the supply and return holes, said heat absorbing arrangement comprising a series of hydraulically parallel or quasi-parallel heat transfer surfaces in the geological formation across which heat is transferred from said geological formation to said fluid such as water. Such geothermal energy plants in general which generate fissures in the rock to gain access to underground heat reservoirs may induce earthquakes and embodiments of the present invention are designed to limit the earthquake risk. Moreover, a geothermal plant in accordance with embodiments of the present invention limits the risks of not being able to create hydraulic communication between the supply and return holes through the hot rock mass.

The present invention also relates to a method creating a fissured geological formation for use with a plant for exploiting geothermal energy by circulating a fluid such as water through the fissured geological formation at least 700 m, or 1000, 3000 or 4000 m below the earth's surface, the method comprising:

drilling at least one supply hole leading from the surface down to said geological formation, forming a first hole from the bottom of the supply hole which is at an angle to the supply bore hole , optionally substantially horizontal; drilling at least one return hole for the transport of heated fluid such as water from said geological formation to the surface, the return hole being optionally less deep than the supply hole, forming a second hole from the bottom of the return hole which is at an angle to the supply bore hole, optionally substantially horizontal and which is separated from the first hole by a distance in the horizontal and vertical directions; generating fissured zones in said geological formation between the first and second holes to form a series of hydraulically parallel or quasi-parallel heat transfer surfaces which allow heat to be transferred from said fissured geological formation to said fluid such as water circulating between the supply and return hole.

The present invention applies for example to exploitation of geothermal energy from hot dry rock (HDR) formations. To compensate for the low thermal conductivities of such formations, the present invention extracts thermal energy via a very large heat transfer surface made available in the geological formation and related to the series of many hydraulically parallel or quasi -parallel heat transfer surfaces.

In accordance with embodiments of the present invention such very large heat transfer surface areas are created by fissured zones between the angled preferably horizontal sections of the supply and return holes which are separated from each other by a distance in the horizontal and vertical directions such as 200 to 1000 m or 250 to 800 or 300 to 750 m. The fissured zones can be generated for example, either by expanding existing fractures, by blasting the rock between the angled, e.g. horizontal first and second holes using explosives, or by establishing fissures between the angled or horizontal sections of the supply and return holes which are separated from each other by a distance in the horizontal and vertical directions such as 200 to 1000 m or 250 to 800 or 300 to 750 m, through cooling and heating and/or by by use of hydraulic pressure on the rock of which the latter is preferred. To avoid unpredictable flow conditions for the circulating fluid due to differences in hydraulic resistance between hydraulically parallel heat transfer surfaces the present invention can make use of a multi-stage process to create the hydraulically parallel heat transfer surfaces. Further, flow metering can be used in order to determine the flow resistance in fissures that intersect the different sections of the holes.

In a geothermal plant according to embodiments of the present invention a large volume of hot rock is located in close proximity to the heat transfer surfaces. A geothermal plant according to embodiments of the present invention, e.g. designed for heating a fluid such as water and to generate hot water, preferably has at least 20.000 m³ of rock located within 10 metres of each heat transfer surface for every kW the plant is to deliver. Consequently, in one aspect of the invention, a plant is provided for exploiting geothermal energy of the kind defined in the introductory paragraph above, the plant being characterized in that it has a given nominal power in MW defined as the heat to be absorbed from the fissured formation by said arrangement per second, in that said multiple and hydraulically parallel or quasi-parallel heat transfer surfaces comprise at least one drilled heat absorbing hole, and in that the rock volume of said formation, is at least about 15.000.000 m³, preferably at least 20.000.000 m³, multiplied by said nominal power.

These numbers represent a much larger mass of rock than contemplated by any prior art plant with an economically viable output.

The inventors have found that the most efficient way of establishing heat extraction from a sufficiently large volume of rock is to create a series of hydraulically parallel or quasi-parallel heat transfer surfaces at depth in hot rock. The term "hydraulically parallel" means that fluid flows exist in parallel although the geometrical shape of these interfaces is not necessarily

mathematically parallel. The present invention is based partly on the realisation that rock tens of metres away from a heat transfer surface will not contribute much heat energy due to the poor thermal conductivity of rock. Hence from a heat transfer point of view a large number of relatively closely spaced hydraulically parallel or quasi -parallel heat transfer interfaces was much more efficient.

In accordance with embodiments of the present invention, holes for supply and return of the fluid would normally exceed 3km in depth, more preferably 5 km in depth and most preferably more than 6km. Further in accordance with embodiments of the present invention multiple quasi-parallel or hydraulically parallel heat transfer interfaces are created at this depth in dry rock, in order to make available a sufficient volume of hot rock to supply the desired heat through the required lifetime of the plant.

Thus, according to a second aspect, the invention provides a plant for exploiting geothermal energy by circulating fluid such as water through a geological formation at least 700 m, or more than 1000m, 3000 m, 4000m below the earth surface, comprising the geological formulation with fissured zones as mentioned above. The minimal depth range is defined by the fact that the invention is based on the creation of a series of hydraulically parallel subvertical fractures using hydraulic techniques. Hydraulically formed fissures are formed in the direction perpendicular to the least stress. Based on experience, horizontal fractures will occur at depths less than approximately 600 to 700 m because the earth's overburden at these depths provides the least principal stress. If pressure is applied under these relatively shallow conditions, the fissures are most likely to be created along a horizontal plane, because it will be easier to part the rock in this direction than in any other. As depth increases beyond 700 m, overburden stress tends to become the dominant stress. Since hydraulically induced fractures are formed in the direction perpendicular to the least stress, the resulting fracture at depths greater than 700 m tend to be oriented in the vertical direction.

According to a further aspect of the present invention, a plant for exploiting geothermal energy of the type described above is characterized in that said heat absorbing arrangement comprises a plurality of hydraulically parallel or quasi-parallel heat transfer interfaces arranged in a parallel flow relationship from the angled or horizontal section of the supply hole to the angled or horizontal section of the return hole and located at depth. Since the rock temperature increases with increasing depth, letting the fluid flow through the hydraulically parallel or quasi-parallel heat transfer surfaces at the maximum depth will permit the highest temperature increase in the fluid used to extract the heat from the hot rock such as water and hence the maximum removal of heat energy.

Preferably the distance between adjacent heat interface layers providing hydraulically parallel flow is about 15 m such as 5 to 25m, preferably at least 10 m. On the other hand, the spacing should be less than about 50 m to limit the physical extent of the plant. A plant in accordance with the invention may have just a single supply hole and a single return hole. However, the plant may have a plurality of supply holes arranged, most preferably circumferentially equi- spaced, around a common return hole. In one particular embodiment for example three supply holes may be arranged around a single return hole. It should be noted that the return hole may be a single drilled hole or a cluster of closely spaced, smaller diameter holes, which exhibit substantially the same heat and pressure loss characteristics as a larger diameter single hole.

Preferably the upper ends of the supply hole and the return hole may be arranged close to one another, with the holes optionally diverting downwards so as to introduce a substantial spacing between the ends of the supply and return holes. Preferably this spacing is about 500 to 1000 m. Such a plant

arrangement allows for a compact plant construction at the surface but at the same time permitting the necessary size of heat transfer interfaces at depth.

Generally holes are drilled vertically into the geological formation until hard rock is met that allows easy diversion of the drilling direction. Preferably deviation starts at least 100 m, more preferably 500 m above the anticipated depth of the (semi)horizontal section of the holes, the actual start point being determined by the technically achievable build-up angle of the drilling technique used under de local geological conditions. The hole which will ultimately serve as the supply hole is extended vertically for an additional distance 500 to 2000 m. Generally, at depths of kilometres where most HDR formations exist, the planes along which such formations fracture are directionally orientated and aligned in an approximately vertical plane. Some such formations have been studied to the point that the compass direction of the vertical plane along which the formation is most likely to fracture is already known. If such is not known or as an added measure, a compass directed core sample may be taken from the bottom of at least one vertical well (which can be either the supply hole or the return hole) and the core and its vacated vault analyzed for granular orientation and tectonic stress, which in conjunction with other geophysical data available on the formation, allows the direction of the plane along which a vertical fracture is most likely to occur to be determined. Other alternative methods may be employed to determine the direction of the fracture plane, such as geophysical logs, installing fibre optics to measure deformation of the casing, pressure leak- off tests or creating a test fracture whose direction may be determined by the injection of radioactive tracers.

After the compass direction is determined for the most likely fracture plane for the formation, one or more further holes are drilled in a direction approximately perpendicular to the compass direction of such planes. Although it is preferred to achieve perpendicularity between the first and second angled or substantially horizontal holes and the fracture plane, absolute perpendicularity is not essential. The first and second angled or substantially horizontal holes may intersect the anticipated fracture planes at an angle that departs from

perpendicular by up to about 45 degrees. The term approximately

perpendicular is intended to encompass such a variance. The angle of deviation from vertical of the fissures may range from as little as 0 degrees to as much as 60 degrees, such as 30 degrees to about 45 degrees. The exact arrangement of the holes is a tradeoff determined by the temperature gradient of the formation and the drilling cost of the operation. Since generally it is preferred to extend the first and second angled or substantially horizontal holes through the HDR formation until a temperature of at least about 125°C is reached in the circulated fluid when in operation, the amount of additional drilling would be a function of the temperature gradient of the formation. The minimum distance the first and second angled or substantially horizontal holes are extended through the HDR formation must be sufficient to accommodate the multiplicity of hydraulically parallel heat transfer surfaces that will subsequently be induced along the first and second angled or substantially horizontal holes. This minimum distance is a function of the number of heat transfer surfaces desired times the spacing between the heat transfer surfaces.

Seals may be placed in one or more parts of the first or second bottom hole that is intersected by a heat transfer surface are arranged to be sealed off in case the flow resistance of the heat transfer surface is lower compared with other heat transfer interfaces.

In operation the fluid flows in the first and second bottom sections in the same absolute direction. This means that for example in one bottom section the flow is towards the end of this section whereas in the other bottom section it flows away from the end of the section. The hydraulic circuit of any hydraulically parallel or quasi-parallel heat transfer surface includes a length of the supply bore, the relevant interface and a length of the return bore. If an interface is chosen closer to the return bore then the length of the return bore reduces but the length of supply bore increases by the same amount. Thus the hydraulic circuits of all of the hydraulically parallel or quasi-parallel heat transfer surfaces are the same. This allows spreading of the flow in hydraulically parallel or quasi-parallel heat surfaces. Generating fracture zones in a rock mass includes a process of:

sealing off part of the first and second holes ,

increasing pressure within the sealed off section by pumping a fluid in the sealed section until the opening or breaking pressure is reached and the rock gives away,

at that time, injecting proppant along with the fluid in order to keep the created fissures open once the pressure has been reduced,

reducing the pressure in the sealed section by allowing the fluid to flow out, and repeating the process several times until the entire length of the horizontal first and second holes is completed or until a heat exchange area of a fractured rock mass of at least 15.000.000 m³ is created.

Brief description of the drawings

For better understanding of the invention it will be described with reference to the exemplifying embodiments shown in the appended drawings, wherein:

FIG. 1 is a schematic side view of a geothermal plant according to an

embodiment of the present invention, FIG. 2 is a schematic plan view of geological formation with the heat transfer interfaces of the plant of FIG. 1 ,

Description of the embodiments

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated. Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Figures 1 and 2 are schematic and show elements at different depths as if the rock in between were transparent. The geothermal plant illustrated in FIGS. 1 and 2 has a series of hydraulically parallel or quasi-parallel heat transfer surfaces 10 located in a geological formation below the earth surface. The heat transfer surfaces are located between horizontal bottom sections 6, 8 described as first and second horizontal bottom sections of supply and return holes 2, 4, respectively, the horizontal bottom sections 6,8 of supply and return holes 2, 4, are separated from each other by a distance in the horizontal directions ("X" and "Z") and vertical ("Y") direction such as 200 to 1000 m or 250 to 800 or 300 to 750m. The heat transfer surfaces 10 are therefore a structure which extends in all three orthogonal directions Χ,Υ,Ζ. The heat transfer surfaces 10 are shown schematically as parallel, flat sheets however in practice the exact shape of these surfaces will be determined by how the rock fissures. The fissured zones are selected so that parallel flow paths are generated which improves heat transfer.

The plant comprises a supply hole 2 with an internal diameter of at least 15,0 cm, for example 15,0 cm or 19,0 cm or 21 ,2 cm or 31 ,3 cm running from an injection well head 16 and a return hole 4 with an internal diameter of 15,0 cm, for example 15,0 cm or 19,0 cm or 21 ,2 cm or 31 ,3 cm running from a production well head 18. The supply hole 2 is formed deeper than the return hole 4 by a distance of, e.g. 250 m, for example of 250 to 500m. However the return hole could also be formed deeper than the supply hole. Substantially horizontal bottom sections 6, 8 are formed at the bottom of the supply and return holes 2,4 respectively. The bottom sections 6,8 of the supply and return holes 2,4 are interconnected by the series of hydraulically parallel or quasi- parallel heat transfer surfaces 10, the spacing of these surfaces being determined by the way enclosing the rock fractures. Preferably, the fissured zone is established in this area to provide flow communication between the supply and return holes. As shown in Figure 2, the well heads 1 6, 18 are located at corners across a diagonal of the fracture zone with the hydraulically parallel heat transfer interfaces 10.

The bore holes 2,4 are drilled substantially vertically into the geological formation until a desired hard rock formation is met that allows the safe build-up of the deviation from vertical of the bottom sections and preferably 100 m, more preferably 500 m above the anticipated depth of the (semi)horizontal bottom sections: the actual start point of the deviation being determined by the technically achievable build-up angle of the drilling technique used under the local geological conditions. The hole which will ultimately serve as the supply hole is extended vertically for an additional distance D such as 200 to 1500 m, or 250 to 2000 m, or 300 m to 3000 m, depending on the rock volume that is needed to achieve the anticipated nominal power. Generally, at depths of kilometres where most HDR formations exist, the planes along which such formations fracture are directionally orientated and aligned in an approximately vertical plane. Although some such formations have been studied to the point that the compass direction of the vertical plane along which the formation is most likely to fracture is already known, if such is not known or as an added measure, a compass directed core sample may be taken from the bottom of at least one vertical well and the core and its vacated vault analysed for granular orientation and tectonic stress, which in conjunction with other geophysical data available on the formation, allows the direction of the plane along which a vertical fracture is most likely to occur to be determined. Other alternative methods may be employed to determine the direction of the fracture plane, such as geophysical logs, installing fibre optics to measure deformation of the casing, pressure leak-off tests or creating a test fracture whose direction may be determined by the injection of radioactive tracers .

After the compass direction of the most likely fracture plane for the formation is determined, one or more further bore holes are drilled in a direction

approximately perpendicular to the compass direction of such planes. Although it is preferred to achieve perpendicularity between the deviated bore holes and the anticipated fracture plane, absolute perpendicularity is not essential. The deviated wells may intersect the anticipated fracture planes at an angle that departs from perpendicular by up to about 45 degrees. The term approximately perpendicular is intended to encompass such a variance. The minimum distance the deviated first and second bottom sections 6,8 are extended through the HDR formation should be sufficient to accommodate the multiplicity of quasi-parallel heat transfer surfaces that will subsequently be induced along the first and second bottom sections 6,8. This minimum distance is a function of the number of heat transfer surfaces desired times the spacing between the heat transfer surfaces.

In operation the fluid flows in the first and second bottom sections 6, 8 in the same absolute direction. This means that for example in the bottom section 8 the flow is towards the end of this section whereas in the bottom section 6 it flows away from the end of the section as shown in Figure 1 . The hydraulic circuit of any hydraulically parallel or quasi-parallel heat transfer surface 10 includes a length of bore 2, the relevant interface 10 and a length of bore 4. If an interface 10 is chosen closer to the bore 4 then the length of bore 4 reduces but the length of bore 2 increases by the same amount. Thus the hydraulic circuits of all of the hydraulically parallel or quasi-parallel heat transfer surfaces 10 are the same. This allows spreading of the flow in hydraulically parallel or quasi-parallel heat transfer surfaces 10in a controlled manner. The upper parts of the supply and return holes 2, 4 can be provided with one or more blind casings to seal the holes against the surrounding groundwater layers in this area. Selection of the depth, size and strength of the casing sections should be done based on local geological conditions, integrity of the hole and regulation. Each hole is drilled in one or more sections of different diameters. All sections, except the last one, are completed by installing a blind casing and cementing in order to create a stable bore hole that is properly sealed from the surrounding formations before the next section is being drilled. As a consequence, the diameter of the successive sections gradually decrease. As such, the minimal internal diameter of the final section should be taken into account when selecting the diameters of the other bore hole sections. The length of each section is defined by the depth range that can be drilled in a safe and environmentally sound manner taking into account local geological conditions, integrity of the bore hole and regulation.

On the surface, the supply and return holes 2, 4 are connected by line 12 to one side of a housing 14 having a separating heat exchanger. A production pump, such as an electric submersible pump or line shaft pump, is installed in the vertical part of the return hole 4. An auxiliary circulation pump can be located (not shown) between the separating heat exchanger and the well head of the supply hole 1 6.

Preferably, the other side of the separating heat exchanger is in flow

communication with various heat consuming appliances exemplified by a radiator, a warm air heater and a hot water tank, a district heating system and/or an electric power generator system.

The method of creating a series of hydraulically parallel or quasi-parallel heat transfer interfaces located in a geological formation below the earth surface, e.g. at a depth of up to 6 km is as follows. The method is designed to reduce the risk of seismic events. After the horizontal bottom section of the hole has been drilled, geophysical tools are run in order to define the shape of the horizontal section, to locate any pre-existing fractures and to identify the strength of the formation. This information is used along with stress

measurements to define the operational parameters for a multi stage process to enhance or create artificial fissures, such as the opening or breaking pressure and the pressure build-up to be applied and amounts of fluid and proppant needed to be pumped down to open or create the fissures and to keep them open. Once the operational parameters are known, part of the horizontal bottom section is sealed off, e.g., by using an open hole packer or cement, and the pressure within the sealed off section is increased by pumping a fluid such as water in the sealed section until the opening or breaking pressure is reached and the rock gives away. At that time, proppant, such as graded sand or a man- made ceramic material, is injected along with the fluid in order to keep the created fissures open once the pressure has been reduced. Finally, the pressure in the sealed section is reduced by allowing the fluid to flow out. This process is repeated several times until the entire length of the horizontal bottom section is completed or until a sufficiently large heat exchange area is created within a zone of fractured rock, the minimal size of the fractured rock mass being at least 15.000.000 m³, preferably at least 20.000.000 m³ per MW nominal power with a preferred fracture spacing of about 15 m, such as 5 m to 25 m, and less than 50 m. After hydraulically parallel or quasi-parallel heat transfer interfaces have been created in the supply and return holes, a flow test is performed by injecting a fluid, such as water, in the supply hole and producing the fluid through the return hole. During this test, the speed of the fluid along at least the horizontal bottom sections is measured, e.g., by running a flow meter or installing flow meters along at least the horizontal sections of the bore holes, in order to define the flow resistance of the hydraulically parallel or quasi-parallel heat transfer surfaces that intersect the bore holes. In places where a heat transfer surface with a low flow resistance intersects a bore hole, the speed of the fluid will change abruptly. In order to avoid undesirable cooling of the produced fluid due to short-circuiting, the heat transfer surfaces should have a similar flow resistance: preferably, the flow resistance of the 10 heat transfer surfaces with the lowest flow resistance should differ less than a factor of 10, preferable less than a factor of 5, preferable less than a factor of 2. In case the flow resistance of one or more of the heat transfer surfaces is too low, e.g. such that a predominant amount of fluid flows through this one or more heat transfer surfaces in comparison to the remainder, the part of the bore hole that is intersected by the heat transfer surface is sealed off, e.g., by using an open hole packer or cementing, and a blockant, such as cement, clay pills or self- hardening material, is injected in order to block the heat transfer surface. After the blockant is injected, the seal is removed and bore hole cleaned in order to remove any blockant left in the bore hole.

The present invention has the advantage that there is no need to construct an underground heat exchanger through a large number of drill holes. In accordance embodiments of the present invention fissuring of rock is used to create multiple heat exchange surfaces in an economic and safe manner, i.e. with a reduction of earthquake danger. It will be understood that the invention is not limited in any way by the

exemplifying embodiments described above, but may be varied and modified in a number of ways without departing from the spirit of the invention and scope of the appended claims.

Claims

1 . A plant for exploiting geothermal energy by circulating a fluid through a geological formation, comprising: at least one supply hole leading from the surface down to said geological formation, at least one return hole for the transport of heated fluid from said geological formation to the surface, and a heat absorbing arrangement connecting the supply and return holes, said heat absorbing arrangement comprising a series of hydraulically parallel or quasi-parallel heat transfer surfaces in the geological formation across which heat is transferred from said geological formation to said fluid, further comprising: a first bottom hole from the bottom of the supply hole which extends away from the supply hole; a second bottom hole from the bottom of the return hole which extends away from the return hole and which is separated from the first bottom hole by a distance in the horizontal and vertical directions (Χ,Υ,Ζ), the hydraulically parallel or quasi-parallel heat transfer surfaces being fluidly connected to the first and second bottom holes.

2. A plant according to claim 1 , wherein seals in one or more parts of the first or second bottom hole that is intersected by a heat transfer surface are arranged to be sealed off in case the flow resistance of the heat transfer surface is lower compared with other heat transfer interfaces.

3. A plant according to any previous claim, wherein the distance is 200 to 1000m.

4. A plant according to any previous claim wherein the geological formation is at a depth of at least 700 m optionally greater than 4 km below the earth's surface.

5. A plant according to any previous claim wherein the fluid is water.

6. A plant according to any previous claim wherein the first and second bottom holes extend in a direction approximately perpendicular to the compass direction of fracture planes of the geological formation.

7. A plant according to any previous claim wherein the first and second bottom holes extend in a horizontal direction.

8. A plant according to any previous claim wherein the distance between adjacent heat interface layers providing hydraulically parallel flow is 10 to 25m.

9. A plant according to any previous claim adapted to flow the fluid through the first and second bottom holes in the same direction.

10. A method of forming a geological formation for use with a plant for exploiting geothermal energy by circulating a fluid through the geological formation below the earth surface, the method comprising: drilling at least one supply hole leading from the surface down to said geological formation, forming a first hole from the bottom of the supply hole which extends away from the supply hole; drilling at least one return hole for the transport of heated fluid from said geological formation to the surface, forming a second hole from the bottom of the return hole which extends away from the return hole and which is separated from the first hole by a distance in the horizontal and vertical directions (Χ,Υ,Ζ); generating fracture zones in said geological formation between the first and second holes to form a series of hydraulically parallel or quasi-parallel heat transfer surfaces which allow heat to be transferred from said geological formation to said fluid when circulating between the supply and return holes.

1 1 . The method of claim 10, wherein generating fracture zones includes a process of:

sealing off part of the first and second holes ,

12. Method according to claim 10 or 1 1 wherein a flow test is performed by injecting a fluid in the supply hole and producing the fluid through the return hole,

measuring the speed of the fluid flow along at least the first and second holes in order to define the flow resistance of the hydraulically parallel or quasi-parallel heat transfer surfaces that intersect the first and second holes, and in case the flow resistance of one or more of the heat transfer surfaces is lower than for other hydraulically parallel or quasi-parallel heat transfer surfaces, the part of the firsr or second hole that is intersected by the heat transfer surface is sealed off.

13. Method according to any of the claims 10 to 12, wherein the distance is 200 to 1000m.

14. Method according to any of the claims 10 to 13 wherein the geological formation is at a depth of at least 700 m optionally greater than 4 km below the earth's surface.

15. Method according to any of the claims 10 to 14 wherein the fluid is water.

1 6. Method according to any of the claims 10 to 15 wherein the first and second bottom holes are formed to extend in a direction approximately perpendicular to the compass direction of fracture planes of the geologiocal formation.

17. Method according to any of the claims 10 to 1 6 wherein the first and second bottom holes are formed to extend in a horizontal direction.

18. Method according to any of the claims 10 to 17 wherein the distance between adjacent heat interface layers providing hydraulically parallel flow is 10 to 25m.

19. Use of the system according to any of the claims 1 to 9 or the method according to any of the claims 10 to 18 for the production of electricity, distribution of heat in a district heating scheme, or provision of heat to commercial or private buildings or for industrial processes.