ITCO20110023A1 - METHOD OF PREPARATION OF GEOTHERMAL FIELDS - Google Patents

Description

Description of the patent application for industrial invention entitled:

METHOD OF PREPARATION OF GEOTHERMAL FIELDS

FIELD OF THE INVENTION

The present invention relates to a method of preparing geothermal fields. The present invention therefore finds application in the geothermal sector.

STATE OF THE TECHNIQUE

Typically, for geothermal applications, the earth (particularly the subsoil) is used as it is; the only modification that is made, according to certain known geothermal technologies, is the drilling of wells. Furthermore, it is common practice to carry out geological prospecting to define the physical, chemical and energy properties of a geographical area that is to be used as a geothermal field.

SUMMARY

The Applicant thought that it was very important to modify the subsoil of a geographical area in such a way as to make it more effective for geothermal applications.

The general purpose of the present invention is to provide a method for carrying out this modification.

This object is achieved by the innovative method having the technical characteristics set out in the appended claims which form an integral part of the present invention.

Furthermore, the Applicant has conceived an innovative material (which will be outlined and described below) which can be used advantageously for this innovative method.

In general, the innovative method of preparing a geothermal field includes the phases of:

A) build at least one well that reaches hot rock layers of the geothermal field;

B) creating fractures in said hot rock layers by means of explosive charges inside said one or each well;

C) infiltrate said fractures with material with high thermal conductivity in such a way as to fill them;

said material being able to fix itself in said fractures;

typically, prospecting wells can be used advantageously for such infiltrations.

It should be noted that the hot rock layers are found in depth, typically at a depth ranging between 500 m and 3500 m.

Typically, the innovative method includes a final step of making a thermo-extractor well for the insertion of a heat extraction system that reaches hot rock layers of the geothermal field.

The innovative method can further comprise, after said step C, the step of: D) filling and sealing said one or each well.

The innovative method can further comprise, after said phase D, the phase of: E) building a further well for a heat extraction system that reaches hot rock layers of the geothermal field;

said further well being in the vicinity of said one or each well.

Advantageously, the plurality of infiltration wells is located around said further well, in particular in an area in the shape of a circular crown, to form one or more closed lines.

Said further well can be a guide well; in this case, the innovative method further includes, after said phase E, the phase of:

F) to create a thermo-extractor well for the insertion of a heat extraction system that reaches hot rocky layers of the geothermal field; said thermo-extractor well being substantially concentric to said guide well.

The innovative method can further comprise, after said phase E and before said phase F, the phases of:

creating fractures in said hot rock layers by means of explosive charges inside said further well;

infiltrate said fractures with material with high thermal conductivity in such a way as to fill them;

said material being able to fix itself in said fractures.

Said explosive charges for said phase B can be placed in a plurality of points along said one or each well.

Said step C can comprise a plurality of successive infiltration operations also carried out at different depths.

Said material is advantageously a nanofluid comprising metal nanoparticles; more particularly, the nanoparticles can be, in totality (or almost totality) or in large part, metallic.

In general, the innovative material for thermally conductive infiltrations is a nano fluid (a nanofluid is a material consisting of a mixture of a host fluid and suspended nanoparticles) comprising:

- a suspension liquid,

- a first plurality of nanoparticles of metallic material suitable for providing thermal conductivity.

Said suspension liquid can be water or an aqueous solution; preferably it is water.

Said nanoparticles of said first plurality can be of copper and / or aluminum and / or brass and / or iron.

The innovative material may further include:

- a second plurality of nanoparticles of metallic material capable of providing a binding perfusion effect thereof.

Said nanoparticles of said second plurality can be tin and / or lead and / or zinc and / or a metal alloy of tin and / or lead and / or zinc.

Said metal material of said nanoparticles of said second plurality can have a predetermined melting temperature.

Said nanoparticles of said first plurality can have different and predetermined granulometry.

The innovative material may further include:

- chemical binder material.

Said binder material can be a preferably epoxy resin.

The following description refers to an advantageous situation in which the innovative method (outlined above) and the innovative material (outlined above) are combined with each other; however, this is not indispensable for the purposes of the present invention.

LIST OF FIGURES

In the drawings attached here,

Fig. 1 shows (schematically from the side) a group of prospecting wells in which explosions are carried out at various depths,

Fig. 2 shows (schematically from above), a group of prospecting wells, a guide well and a primary well

Fig. 3 shows (schematically from the side) a group of prospecting wells through which infiltrations are carried out in fractures of hot rock layers at various depths,

Fig. 4 shows a geothermal heat extraction system, e

Fig. 5 shows the bulb of the system of Fig.4.

DETAILED DESCRIPTION PREMISES

Both this description and these drawings are to be considered for illustrative purposes only and therefore not for limitation purposes; therefore, the present invention can be implemented according to other and different embodiments.

The method described here is intended to improve the thermal conductivity (at the macro level) of deep geothermal soils.

In the exploitation of geothermal energy, there is the problem of maximizing the heat exchange surface between the hot rocks found in depth and the means of transporting heat.

The method described here can be used with great advantage in the preparation of geothermal wells, also called "primary wells", suitable for receiving a system such as the one object of the Italian patent application n ° C02011A000005 of the same Applicant partially incorporated at the end of the present description. However, the present invention is not limited to such use and therefore applies to other types of geothermal wells.

CONFORMATION OF THE STRUCTURE OF A GEOTHERMAL FIELD

The structure of a geothermal field looks like a set of permeable and fractured rock layers that are exploited, in traditional geothermal energy production methods, as a lamellar system into which water at high pressure is injected. The water is injected into special boreholes drilled up to the hot fractured rock layers where it converts to steam due to the high heat present in the rocks.

The resulting steam is collected through other nearby boreholes to be used in the production of electricity through the use of turbines.

In the energy production system described in the Italian patent application n ° C02011A000005, no use is made of water injected directly into the geothermal layers, but a closed circuit is used in which an exchange fluid circulates.

The fluid enters the descending circuit, reaches the heat extraction system where it heats up and then rises to the surface where it transfers the heat to the steam circuit to then descend again in depth for the new cycle.

In order to increase the exposed surface of the heat extraction system in contact with hot rocks, the method described here is proposed.

For the sake of clarity, the method is presented in operational phases that could also vary according to the characteristics of the geothermal ground to be exploited.

PHASE 1: PREPARATION PROCEDURE OF THE GEOTHERMAL FIELD In order to understand the crystalline structure and the chemical-physical characteristics of deep geothermal rocks, prospecting (for example by means of radar, sonar, microwave, ...) and drilling at the project depth are carried out. the chemical-physical checks and the sampling (core boring) of rock; in this way a mapping of the geothermal field can be obtained. PHASE 2: IMPROVED GEOTHERMAL FIELD PROJECT

The information provided by geological prospecting and coring are used to define the map of the physical-chemical-energetic properties of the geothermal field. This map provides precise information on the characteristics of resistance, permeability, natural billing, possible thermal flows and all the data needed to obtain a complete and precise picture of the geothermal field on which to operate.

This map allows the project of the improvement intervention which will consist in increasing the natural deep billing and the permeability to fluids to allow the injection of an infiltration material with high thermal conductivity.

PHASE 3: IMPROVEMENT OF THE PERMEABILITY OF THE DEEP ROCKY LAYERS

High explosive charges will be inserted into the boreholes used for prospecting (Fig. 1). These fillers will be sized according to natural strength, permeability and billing parameters. The charges will also be arranged and made to shine (Fig. 1) at variable depths in order to obtain an optimal distribution of the stresses and obtain a uniform billing of the geological structure.

Great attention will be paid to the level of permeability in relation to the entire area surrounding each "primary well", ie a well capable of receiving a system for extracting heat from hot rocks. In Fig. 2, a "primary well" is schematically seen PP in the center of which there is a “guide well” PG (of course, the drilling of the PP well is carried out after the drilling of the PG well and therefore the PG well is “absorbed” by the PP well); around the PP well there is a series of PR “prospecting wells” which will be used for the fracturing of the hot and deep rock layers and for the infiltration of infiltrating material with high thermal conductivity; the wells PR is located around the well PP, in particular in an area in the shape of a circular crown (internal radius R1 and external radius R2), and form a closed line; the wells PR of Fig. 2 are located on approximately the same concentric circumference to the well PP.

Once the billing and permeability level required by the project has been obtained, the next step is the filling of the fractures through the use of infiltration material with high thermal conductivity.

This material will be injected into very high pressure prospecting drilling and through the use of vibration compaction systems forced to fill every fracture of the deep geothermal field.

PHASE 4: FILLING OF THE FRACTURES OF THE DEEP ROCKY LAYERS

Deep fractured rock layers must be saturated with high thermal conductivity material by a high pressure injection process.

The injection of the material is carried out through the wells previously drilled for prospecting (Fig. 3). The process involves the preventive insertion of an iron pipe similar to that used as a drilling rod. The material with high thermal conductivity at high pressure is then pumped into this pipe.

The injection of the material is carried out in successive phases, leaving the previous injection to rest in order to allow the nanoparticles to settle in a progressive and stable way; during the rest phase the pressure is maintained in order to guarantee the infiltration of the material, its compaction and its cohesion; after each injection carried out at a certain altitude, once the maximum saturation level for that altitude has been reached, the tube is raised to a higher altitude in order to proceed with a new homogeneous injection.

If the high conductivity material is a nanofluid, the concentration of nanoparticles is progressively increased between one injection and the next.

The density of the injected fluid gradually changes from very low in the first injections (about 1 to 10 percent) to very high (80 percent and more) in the following and final ones; the low density allows the infiltration of the material into the smallest cracks and pores of the rock; the higher density fills the larger cracks; the gradualness of the density ensures that even the smallest porosity and the thinnest fracture are conveniently filled by the material.

Wanting to maintain the fluidity of the nanofluid during injection until even the smallest spaces of the rock fractures are filled, it is possible to keep the suspension fluid (typically water) in liquid form, that is to prevent it from passing to the state of vapor or gas. .

This is possible, for example and advantageously, by injecting (in the area where the nanofluid is injected into the rock fractures) compressed nitrogen through a special tube equipped, at one end, with an expansion nozzle; nitrogen is injected under high pressure and, passing through the nozzle, it expands violently, cooling suddenly; this causes a local cooling of the rock for a short time (for example a few minutes), which allows the suspension fluid of the nanofluid to be kept liquid.

In order to obtain a better constipation of the nanoparticles, a vibration will be applied to the metal injection pipe, once the material flow is terminated.

The vibration frequency and application time most suitable for obtaining the best level of constipation can be established in the laboratory according to the particle size and composition of the nanoparticles.

The injection process will be repeated according to the so-called “rejection” method, ie until each well will be able to receive material.

PHASE 5: SEALING THE PROSPECTION WELLS

Once the injection phase is completed, the pipes will be emptied, removed from the prospecting wells and then filled with cementitious concrete until they are completely sealed.

PHASE 6: DRILLING THE GUIDE WELL

Once the preparation process is completed, the guide well of the primary geothermal well will be drilled.

The drilling of this guide well (or of several guide wells close together) will allow to repeat the operation of fracturing and filling the fractures also in reaction to the guide well, therefore precisely in the points where the primary well will be found and, consequently, the heat extraction system.

Through the preparation of this guide well it will be possible to obtain a reticular distribution level of the material with extremely high thermal conductivity.

PHASE 7: DRILLING THE PRIMARY (GEOTHERMAL) WELL

Once the preparation of the guide well has been completed and the adjacent geothermal zone has been saturated, the primary large-diameter drilling is carried out.

Once the primary drilling is completed, a heat extraction system can be installed and all the volume not occupied by the system itself can be saturated with material with high thermal conductivity (advantageously of the types described in the following paragraph) until a monolithic block is reached with the system in the center.

FILLING MATERIAL WITH HIGH THERMAL CONDUCTIVITY In order to obtain a uniform and denser filling of the fractures of the hot and deep rock layers, a non-cementitious “metallic concrete” will be used; it is a nano fluid based on metal nanoparticles.

Traditional concrete - otherwise called "cementitious mineral conglomerate" is made up of inert materials, typically sand and gravel of varying sizes, and a material (usually hydraulic cement or hydraulic lime) that acts as an adhesive and binder for inert materials.

The variable size conformation has the precise function of obtaining the best possible filling of all the empty spaces that form between the various inert materials. In the same way, the “metallic nano-concrete” has precisely studied particle size characteristics (in particular differentiated particle size), which guarantee the best possible filling and the largest contact surface between nano granules.

In concrete for construction, the binder used is hydraulic cement: in contact with water, the hydraulic cement reacts and solidifies, generating a strong adhesive action.

In the "metal nanocecrete" the binder is made up of other metal nanoparticles which, once the operating temperature found in the hot and deep rocky layer is reached, softens and binds the first metal nanoparticles also through the action of the pressure exerted during the injection phase . The water present in the "metallic nano-concrete" evaporates due to the effect of heat. While the presence of the binder is fundamental in cementitious concrete, it is not essential in the "metallic nanoceccrete", since the friction exerted by the first metallic nanoparticles and the great ability they have to agglomerate indissolubly (given the extreme irregularity of shape) .

Given the variable particle size of the first metal nanoparticles, given the absence of cementitious material as a binder, given the possible presence of a binder made up of second metal nanoparticles designed to melt at temperatures that will be found in depth, this material can be "non-metallic nanoceccrete. - cement ".

To fill the fractures obtained from the fracturing of the geothermal soil in depth (magmatic, metamorphic rock, ...) with a material that guarantees a better thermal flow, a metallic material is used that has a thermal conductivity higher than that of the surrounding environment, that is, of the rocks. The injected material will therefore be metallic and highly thermally conductive.

Among the metals with the highest degree of thermal conductivity there is copper with 390W / m-K (Watts per meter / Kelvin). Other cheaper materials although not as efficient are aluminum (236W / m-K), some copper alloys such as brass (111 W / m-K), iron (80 W / m-K).

To obtain the best possible heat transmission within the fractures in geothermal rocks, it is also necessary to achieve the highest possible density of the injected material.

For this purpose we intend to use a "nanofluid" or a fluid composed of a suspension liquid (water) and highly thermally conductive metal nanoparticles.

Nanoparticles get their name from their extremely small size.

One nanometer equals one billionth of a meter, or one millionth of a millimeter. In order to obtain the maximum level of density, a multi-granular nanoconglomerate, otherwise known as "nanocecrete", will be adopted.

The multigranular material is composed of particles of different diameters.

The diameters of the particles can be selected on the basis of a precise calculation that establishes the "granulometric curve" which guarantees the best possible density level.

The nano-concrete contains a binder consisting of metal nanoparticles, which will melt exactly at the operating temperature of the geothermal layers.

Once it reaches the depth, in fact, the binding material (i.e. the second nanoparticles) melts by infiltrating the voids left free by the nano particles (i.e. the first nanoparticles). The high injection pressure allows to reach a high freedom of circulation of the binder between the voids, once the melting point is reached.

Once in operation, the temperature of the nano-concrete will tend to drop as a result, among other things, of the transit of the heat collected and, consequently, the binder will return to the solid state, adhering to the nanoparticles and giving solidity and anti-washout capacity to the structure.

Suitable materials for the binder (i.e. the second nanoparticles) are, for example, lead and zinc.

It must also be said that the presence of the binder material is not necessary in those applications where it is not necessary to maintain a form factor; for example in "dry" geothermal fields, where there is no circulation of water vapor between the rocks, the agglomeration of nano particles and their adhesion to the rock walls is sufficient to keep the infiltration of material in shape after the injection pressure it's over.

It is also possible to fill with a metallic nano fluid having metal particles of identical diameter, that is "monogranular". This solution can be adopted where the cost factor is relevant. A monogranular material is, in fact, less expensive because the production process does not foresee the addition of smaller particles and does not require a proportional mixing phase according to a granulometric curve.

A conglomerate composed of elements of different diameters, on the other hand, (i.e. with a diameter equal to the space left by the elements of the higher grade), tends to become compacted by filling the voids close to 99.9%, obtaining a higher average density level and a more large contact surface between the individual nanogranules.

It should be clarified that the method described above can be implemented with materials with high thermal conductivity of various kinds and not only with those just described. For example, a metal-based material (containing metal particles or nano particles) could be used but with a chemical binder, for example a preferably epoxy resin. In resinous nanoconglomerates the binder maintains its liquidity for a certain period of time which allows application; once hardened, the material assumes characteristics of structural solidity typical of the materials that compose it.

In integrally metal nanoconglomerates, the binder material, consisting of metal nanoparticles, contained in the aqueous suspension melts into the liquid state, and infiltrates the voids present among the other metal nanoparticles; once cooled, the binder material returns solid ensuring structural stability to the application and trapping the metal particles.

GEOTHERMAL HEAT EXTRACTION SYSTEM

Previously we talked about a "primary well", that is a thermo-extractor well for the insertion of a heat extraction system that reaches hot rock layers that are located in depth in correspondence with the geothermal field.

A possible and advantageous embodiment of such a heat extraction system will be described below with reference to Fig.4 and Fig.5.

In Fig.4, we see a well 1 deriving from a drilling (cylindrical and substantially vertical) in which an embodiment example of a heat extraction system 2 is inserted. On the surface, on the ground, there are a set 3 of machines connected to the system 2 and suitable for transforming the heat extracted from the system 2 first into mechanical energy and then into electrical energy; as will become clearer below, set 3 corresponds to the transformation section of a geothermal plant. It is noted that, in this figure, system 2 does not exactly reach the lower end, ie the bottom, of well 1 and does not exactly reach the upper end of well 1; this is not particularly relevant here.

It is good to immediately clarify that Fig. 4 is not to scale (in particular the width has been increased compared to the height) and is very simplified; this was done to facilitate the legibility of the drawing.

At the bottom of the well 1, outside it, there is a layer 4 of very hot rocks, for example at a temperature of 300-450 ° C.

At the bottom of the well 1, inside it, there is a bulb 5 of the system 2 which is cylindrical and hollow; the bulb 5 is, as far as possible, in contact with the hot rocks of the layer 4; the bulb 5 connected to a first conduit 6 and to a second conduit 7; both ducts 6 and 7 are located inside the well 1, in particular substantially parallel, and are surrounded by thermal insulation means that thermally insulate them from each other and from the ground (in the superficial part of the well 1) and from rocks (in the deep part of the well 1).

The thermal insulation means essentially correspond to two long layers 81 of gravel or sand. However, in addition, three short layers 82 of sealing material have been provided: one close to the bulb 5, one close to the surface of the well 1 and an intermediate one; in other words, the sealing layers alternate with the insulating layers; of course, the number and size of the layers depends on the particular embodiment of the system, even if it is advantageous to provide at least one sealing layer on the surface and / or a sealing layer on the bottom. As already said, Fig.4 shows a simplified example of embodiment; more realistically, 100 meters of thermal insulation means comprise for example a dozen layers of sealing material with a thickness of 1-3 meters which are therefore spaced apart by layers of insulating material with a thickness of 9-7 meters; the last section of the well, for example the last 100 meters, is advantageously filled entirely with sealing material; a sufficiently sealing and sufficiently cheap material is concrete.

The whole of the duct 6, the bulb 5 and the duct 7 constitutes a closed hydraulic circuit suitable for the circulation of a fluid for heat transport; this fluid is introduced into the duct 6 on the surface at a low temperature (for example 150-300 ° C), goes down along the duct 6, enters the bulb 5; it circulates in the bulb 5 and then heats up to a temperature close to that of the layer 4 (for example 300-450 ° C), exits the bulb 5, rises along the duct 7 and exits the duct 7 on the surface at a high temperature; if the thermal insulation means are well made, the fluid will not lose much temperature from when it leaves the bulb 5 to when it leaves the duct 7 on the surface; this loss could be, for example, 10-20 ° C. In Fig.4, the bulb 5 and the insulating means 81 and 82 are perfectly cylindrical, aligned and of the same diameter, and occupy exactly the cylindrical space of the well 1; in other words, the diameter of the well, the diameter of the bulb 5 and the diameter of the insulation means 81 and 82 are the same.

Of course, in reality this ideal situation does not occur. Well 1 derives from drilling which typically takes place partly in the ground and partly in rocks and therefore cannot be perfectly cylindrical; among other things, if the well crosses layers of soil it will probably be necessary to excavate much more than the well and create a lining, typically made in the form of a lining of the well wall, for example in concrete and / or metal plate, to contain the soil and allow the continuation of drilling and the insertion of a system 2. The bulb 5, in order to descend along the well 1, will typically have a diameter a little less than that of the well, for example up to 20% less ; in light of this consideration, it is to be envisaged that between the bulb 5 and the internal wall of the well 1, in the space that remains, a material with high thermal conductivity (advantageously of the types described in the previous paragraph) is placed in order to obtain a good thermal efficiency and yield of the system 2. The insulation means 81 and 82, as they will typically be made, will completely fill the well 1 and therefore, although not perfectly cylindrical, they will have dimensions, in particular diameter, almost equal to those of the well. This solution has been studied for high enthalpy geothermal; therefore, well I reaches very hot rock layers, for example at a temperature of 300-450 ° C; strata of this type are generally found at a depth varying between 500 m and 3500 m, depending on the geographical area; the length of the thermal insulation means will therefore be approximately equal to that of the well minus the length of the bulb which, as will be explained better later, will typically be 10-100 m. The bulb is made in such a way as to absorb a lot of heat from the surrounding rocks and effectively transfer it to the fluid that circulates inside it; details of the bulb 5 of Fig.4 are shown in Fig.5 and will be described in the following with reference to this figure.

The bulb 5 is substantially cylindrical; in particular, it has a cap at the top to facilitate the conveyance of the hot fluid into the rising duct 7; it is hollow and has an internal cavity 9 suitable for circulating the fluid by transporting heat; the cavity 9 is in communication with the fluid descent conduit 6 and with the fluid ascent conduit 7; the duct 6 is connected to a central duct 10 adapted to conduct the (cold) fluid in an internal area 11 of the bulb 5 to its lower end; the duct 10 (together with the perimeter walls of the bulb 5) therefore defines in the cavity 9 an annular area 12 between the extreme lower area 11 and an extreme upper area 13 (in the example of Fig.5 inside the shell); in the zone 12 the rising of the fluid and its heating of the fluid take place due to the effect of the contact with the perimeter walls of the bulb 5; as will become clearer from the following, various types of fluid motion occur in zone 12, including natural convective motions and induced turbulent motions.

In the zone 11, a passive rotating scrambler 14 is mounted. It is basically divided into two parts; the central part is an impeller designed to receive the fluid coming from the duct 10 and to make the entire mixer 14 rotate due to the pressure exerted by the fluid on its vanes; the peripheral part serves to impart a swirling and turbulent motion to the fluid; in particular, the peripheral part includes a plurality of perforated cups mounted on a perimeter band of the central part - the shape may seem that of an anemometer, but the effect on the fluid is quite different. The fluid coming from the duct 10 passes through the central part of the remixer 14 and, in this way, causes its rotation; after passing the remixer 14, the fluid comes into contact with the lower internal wall of the bulb 5 and reverses its motion, but due to a particular conformation of this wall it is also radially deviated and therefore rises in correspondence with the peripheral part of the remixer 14; in this upward motion, the fluid is intercepted by the perforated rotating elements of the peripheral part of the remixer 14 which impart a swirling component to the motion; moreover, by using perforated elements, turbulent and convective motions are also created (due to the "Venturi effect"). The mixer 14 is called "passive" as it does not need any type of motor to operate, but simply exploits the kinetic energy of the fluid to be remixed.

It is evident that there would be heat transfer between the perimeter walls of the bulb 5 and the fluid that rises in the intermediate annular area 12 even if a scrambler was not used in the extreme lower area 11; however, thanks to such a device, the efficiency of the thermal transfer is greatly increased since uniform heating of all the fluid exiting from the duct 10 is favored. The ducts 6 and 7 both terminate at the upper wall of the bulb 5, which in the case of the example of Fig. 5 has the shape of a cap; therefore, in this way, the connection of these ducts and the bulb can be made without the use of components that protrude radially with respect to the bulb; it follows that the diameter of the well depends only on the diameter of the bulb (and of course on the type of soil and rocks in which the drilling is carried out) and not on the size of other elements.

Preferably, the axis of the duct 6 coincides with the axis of the bulb 5 in such a way as to obtain the greatest possible fall power.

The bulb to be used for the present invention typically has a diameter between 1 m and 2 m. As regards the length of the bulb, there is a greater variability and typically between 10 m and 100 m; in fact, this depends on the temperature of the rocks of the rocky layer with which it is brought into contact, on the thickness of this rocky layer, on the quantity of heat that is desired to extract from this rocky layer. In a relative sense, the bulb typically has a length to diameter ratio between 10 and 100. The diameter of the internal duct is typically between 25 cm and 60 cm. The bulb is advantageously made of metallic material, in particular copper, copper alloys or steel; the bulb can also be made from several superimposed layers of material.

In order to protect the bulb from corrosive agents present in depth, it is advantageous that it is externally covered with a protective layer; the material of this protective layer will typically be compact and resistant ceramic material, such as "stoneware", in particular "full body porcelain stoneware"; the thickness of such a protective layer can be 1-5 cm and depends on the conditions at the bottom of the well. This protection layer can also be reinforced with metal mesh, possibly steel, single or double, to increase its resistance, for example in the case of large bulbs and / or subjected to particularly high temperatures.

The two ducts 6 and 7 can have different diameters to compensate for the pressure drops introduced by the bulb and the expansion of the fluid caused by its heating. To regulate the circulation of the fluid in the circuit of the system 2, a pumping device, possibly electronically controlled, is typically required in such a way as to have at the outlet of the duct 7 a pressure and a speed that fall within predetermined intervals.

The speed in the bulb, and more generally in the circuit, will typically be in the range from 0.5 m / s to 10 m / s, therefore relatively high. The pressure in the circuit, and in the bulb in particular, will typically be very high; taking into account that for every 100 m of well a hydrostatic pressure of about 10 atm is created, in the case of a well of 3000 m there will be a pressure of at least 300 atm in the bulb.

Other measures can be adopted for the efficiency of the thermal transfer, which can be used alternatively or advantageously in combination with a lower remixer.

A first measure that can be used successfully to favor the thermal transfer from the perimeter walls of the bulb to the fluid that circulates inside it are elements, in particular fins or blades, protruding towards the inside of the bulb and able to increase the transfer surface. thermal with fluid; these elements, typically made of good thermal conducting material, can be integrated into the perimeter walls of the bulb or simply be placed on them in such a way as to rapidly transmit heat by conduction to internal areas of the bulb cavity where fluid flows.

These protrusions can be advantageously made by means of a section of cylindrical tube inserted inside the bulb 5 in such a way as to be in contact with the perimeter walls of the bulb 5; to the inner side of the tube are joined a plurality of lamellae arranged radially and which extend up to a predetermined distance from the axis of the tube; this distance is a function of the diameter of the internal duct 10 (there is no contact between the lamellae and the pipe); to increase the density of the lamellae (and therefore the heat exchange surface), these are of different lengths; in particular, a short and a long lamella can be alternated; in this way, a lamellar element is formed; tube and lamellae are typically made of good heat conducting material, preferably copper.

The lamellae or fins of lamellar elements can have surfaces of different types: smooth, grooved, embossed (ie containing a plurality of side-by-side projections in particular in the shape of a hemisphere or semi-ellipsoid), with "microcraters" (ie containing a plurality of side-by-side recesses in detail in the shape of a hemisphere or semi-ellipsoid) and mixed; the different surfaces have different heat exchange yields and construction costs; mixed surfaces (and therefore very varied) are the most efficient because they expose a greater surface area but they are also the most expensive because they require the presence of all the mechanical processes in a single product. If a "microcrater" surface is used, an increase of 2/3 of the exposed surface is obtained with the same flat surface. A "microcrater" surface, for example with a diameter of 0.1-0.01 mm, can be advantageously carried out by high-speed projection of micro-atomized flows of acid solutions.

Claims (9)

CLAIMS 1. Method for preparing a geothermal field comprising the steps of: A) making at least one well that reaches hot rock layers of the geothermal field; B) create fractures in said hot rock layers by means of explosive charges within said one or each well; C) infiltrate said fractures with material with high thermal conductivity in such a way as to fill them; said material being able to fix itself in said fractures. Preparation method according to claim 1, further comprising, after said step C, the step of: D) fill and seal said one or each well. Preparation method according to claim 2, further comprising, after said step D, the step of: E) building an additional well for a heat extraction system that reaches hot rock layers of the geothermal field; said further well being in the vicinity of said one or each well. 4. Preparation method according to claim 3, wherein said plurality of wells are located around said further well, in particular in an area shaped like a circular crown, to form one or more closed lines. 5. Preparation method according to claim 3 or 4, wherein said further well is a guide well, and further comprising, after said step E, the step of: F) to create a thermo-extractor well for the insertion of a heat extraction system that reaches hot rocky layers of the geothermal field; said thermo-extractor well being substantially concentric to said guide well. 6. Preparation method according to claim 5, further comprising, after said step E and before said step F, the steps of: creating fractures in said hot rock layers by means of explosive charges inside said further well; infiltrate said fractures with material with high thermal conductivity in such a way as to fill them; said material being able to fix itself in said fractures. Method of preparation according to any one of the preceding claims, wherein said explosive charges for said phase B are placed in a plurality of points along said one or each well. Preparation method according to any one of the preceding claims, wherein said step C comprises a plurality of successive infiltration operations also carried out at different depths. Method of preparation according to any one of the preceding claims, wherein said material is a nanofluid comprising metal nanoparticles.