EP2440743B1

EP2440743B1 - A wire screen for a well and method for making it

Info

Publication number: EP2440743B1
Application number: EP10763418.0A
Authority: EP
Inventors: Nicola Cempini
Original assignee: INGEGNERIE TOSCANE Srl
Current assignee: Ingegnerie Toscane Srl
Priority date: 2009-06-11
Filing date: 2010-06-11
Publication date: 2016-03-30
Anticipated expiration: 2030-06-11
Also published as: EP2440743A1; IT1400406B1; WO2010143060A1; ITPI20090073A1

Description

Field of the invention

The present invention relates to hydrogeology and to the exploitation of underground fluids, in particular, it relates to a screen structure for a well, for instance an artesian well, a geothermal well and the like.
Furthermore, the invention relates to a method for making such a screen structure.

Description of the prior art

As known, a well comprises a tubular duct, or casing, which is arranged in a wellbore that has been drilled in the ground to find a aquifer or a gas or oil reservoir or a geothermal reservoir. In particular, at the aquifer or at the reservoir, the well casing provides an extraction screen that is adapted to allow water to flow upwards through the well casing. In particular, a drainage material is arranged about the extraction screen in order to filter away from the fluid the solid particles that are present in the aquifer or in the reservoir. In this case, the extraction screen is used to retain the drainage material, and to cause the fluid to leak within the well casing.
Extraction screens are known that have a different structure, the most widespread of which are the shutter screen, the bridge slot screen and the wire wrap screen.
However, such types of screens have some drawbacks. In fact, the fluid motion in a aquifer can be assimilated to a plurality of fluid threads, which have each an indefinitely small cross section, and move along any line and according to any direction, such lines and directions defining the flow lines of the threads. Normally, in a aquifer at rest the flow lines are substantially horizontal and have only very small vertical quickly changing components, therefore the fluid can be effectively filtered in a conventional screen, for example in a wire wrap screen.
Nevertheless, if a ground water or other fluid reservoir is disturbed, as it is an extraction well, proximate to the well the flow threads deviate from the horizontal orientation.
As known, in a close proximity to the screen a horizontal speed-vector undergoes a sudden acceleration which increases its intensity from some cm/s, which is generally the aquifer speed, up to some m/s, which is the speed at the entrance of the well casing. The direction of the speed-vector changes from a horizontal direction into a vertical one. In particular, when approaching the screen, a vertical component of the fluid speed arises, which becomes greater than the horizontal speed once the fluid is close to the screen. Accordingly, the horizontal slits of the wire wrap screen oppose a resistance against the vertical component, causing a local pressure drop.
Therefore, it is noted that that a horizontal arrangement of the slits causes a local pressure drop when the fluid enters the screen. Furthermore, the screen slits are more likely to clog.
A further filter type is disclosed in US3211234A , which provides a plurality of triangular cross section slats that are spaced with respect to each other to form a tubular body and a plurality of longitudinal openings along it. The slats are bound to one another by one or more reciprocally spaced weld lines.
Unless properly sized, such solution limits the filtration performance, on the one hand, and the structural resistance, on the other hand.
Furthermore, this structure is not well-suited for to medium-depth and deep installations, since the screen would undergo radial forces that would damage the same.
A further drawback is that the slat cross section angle causes a high turbulence of the threads that cross the filter and, therefore, gives rise to high fluid pressure drops through the screen. Moreover, such turbulence can develop gas (e.g. a CO₂ gas loss) from the fluid, which in turn can give rise to a precipitation of salts that may be present as a solute in the fluid, and therefore can cause the formation of salt scale (e.g. calcium carbonate scale).
Especially in the case of geothermal wells, it is therefore a common practice to reinject a fluid amount that has been previously extracted from an extraction well back into a reinjection well, to maintain a well energy balance and to optimize the utilization of underground heat.
For instance, reinjection may be necessary when the fluid extracted from a geothermal reservoir cannot be used as such, since it carries a very high mineral content and is not well suited for a direct home or industrial use, such as in the case of low enthalpy fluid sources whose temperature ranges from 20°C to 70°C.
For this reason, the extracted fluid passes through a heat exchanger, in which a conduction/convection heat exchange takes place with a working fluid, which is then used by a heating device. The underground fluid reinjection makes it possible to extend the life of the well and makes it unnecessary to treat the extracted fluid, as required by environmental emission limitations.
Like an extraction well, a reinjection well comprises a well casing that is provided with a filtration portion, i.e. a screen adapted to reintroduce into a aquifer, or into a reservoir, the fluid that has been reinjected through the well casing.
In particular, a reinjection well, which is installed close to an extraction well, has the same construction of the extraction well, such that a circuit is obtained, which is adapted for energy exchange.
However, by reinjecting a fluid into a geothermal reservoir, scale problems may arise when a screen is arranged close to the aquifer, which may plug the reinjection wells. In particular water that is extracted from a high mineral content aquifer, in particular water with a high content of silica, gives rise to scale layers that can remarkably reduce the reinjection flow rate and therefore can drastically limit the exploitation of the well in a too short time to maintain its profitability. Therefore silica scaling must be kept under control and minimized.
Due to their construction, the well-known reinjection screens cause such flow conditions generate conditions that a local pressure drop arise at the fluid entrance into the screen. This makes the filter openings more likely to be plugged.
Furthermore, maintenance of the screens is particularly expensive, and to avoid the above mentioned drawbacks maintenance is not programmed frequently.

Summary of the invention

It is therefore a general feature of the present invention to provide a screen structure for a well which reduces the pressure drops and improves the effectiveness with respect to prior art screens.
It is another general feature of the present invention to provide a screen structure for a well that makes it possible maintainance of the filter by known systems, without damaging or breaking the structure.
It is also a feature of the present invention to provide a screen structure for a well which is structurally strong for any application, even for deep applications.
It is also a feature of the present invention to provide a screen structure for a well which is easy and cheap to be made.
It is also a feature of the present invention to provide a method for making such structure.
These and other objects are achieved through a screen structure for a well casing comprising:

a plurality of parallel slats, integral to each other, said slats arranged to define a tubular body, said slats spaced from each other such that said tubular body provides a plurality of longitudinal slit that act as passageways for a fluid, each slit defined between two adjacent slats,

a triangular cross section that has a base and two sides that extend from said base and meet each other at a vertex opposite to said base, or a trapezoidal cross section that is obtained by removing said vertex from said triangular cross section, said triangular cross section having a vertex angle set between 8 and 17 degrees.

This way, the slats define a plurality of solid portions that are spaced from each other by respective void portions, i.e. by longitudinal slits, such that, in use, the slits are arranged in a substantially vertical direction, i.e. they are arranged parallel to the longitudinal axis of the tubular body.
Advantageously, said slats are blades of a predetermined size, said blades tapered along a direction that extends from outside to inside of said tubular body, forming an extraction screen. This way, the slits that are located between two respective slats have a divergent cross section, which has a minimum width at the outer side surface of the tubular body. Therefore, if a draining granular material is arranged outside of the tubular body, the diameter of the granular material larger than the slits minimum width, the tubular body peripherally retains the draining granular material while allowing the fluid to flow through the slits. The diverging and vertical cross section of the slits makes it possible to keep the pressure drops of the fluid below a minimum value. In particular, since the fluid speed increases its vertical component while crossing the drainage material before reaching the tubular body, the vertically extending slits do not offer any resistance to the vertical speed component, which is substantially unaffected by the passage of the fluid through the screen or through the tubular body. In other words, the flow lines of the fluid, near the screen, enter the tubular body without any substantial pressure drop and without any substantial reduction of the vertical component of the speed-vector.
Alternatively, said slats are blades of a predetermined size, said blades tapered along a direction that extends from the inside to the outside of said tubular body, to provide a reinjection screen. This way, the screen is used as a filter for a fluid reinjection well. In particular, the flow lines of the fluid, near the screen, exit from the tubular body without any substantial pressure drop and without meeting any jammed passages between the slats.
In particular, the angle that is formed between said sides at said vertex, which is set between 8.5° and 17°, remarkably reduces fluid turbulence, and therefore reduces the fluid pressure drop across the screen. In this case, the high turbulence reduction that is achieved by the invention does not cause any loss of gas that might be dissolved in the fluid (e.g. a CO₂ gas), therefore it doesn't give rise to any precipitation of salts that may be present as a solute in the fluid, thus preventing the formation of salt scale (e.g. calcium carbonate scale).
This solution is also particularly advantageous for reinjection screens which are generally used in the case of high mineral content water (for example geothermal water which carries a high content of silica salts). The low turbulence, which is a feature of the invention, enormously reduces scaling on the reinjection screen.
Advantageously, said parallel slats are kept integral with respect to each other by a means selected from the group comprised of:

joint elements that are provided inside of said tubular body;
joint elements that are provided outside of said tubular body;
joint elements in which said slats are inserted.

In particular, said tubular body is a cylindrical tubular body and said joint elements are a plurality of support rings that are arranged at a distance along the axis of the tubular body. This way, the rings also provide a reinforcement of the structure to make it suitable for deep applications in which buckling loads or specially lateral loads are relevant.
In particular, said support rings are selected from the group comprised of: open support rings that are provided with open housings where said slats engage along a defined portion, or closed support rings with closed housings where said slats engage along their whole length.
Advantageously, the number of said support rings is selected according to the well depth at which said screen is used, in particular, in the case of a depth set between 0 and 50m a stiffening ring is provided at every 20 cm, in the case of a depth set between 50 m and 100 m a stiffening ring is provided every 10 cm and, finally, in the case of a depth beyond 100 m a stiffening ring is provided every 2.5 cm.
Advantageously, stiffening elements are provided arranged between said slats. This way, the construction is stronger and is adapted to bear high loads.
In particular, said stiffening elements are large sized stiffening slats that are uniformly arranged along said tubular body, said stiffening slats having a height that is shorter than or equal to the height of said slats. This way, such stiffening slats do not reduce the screen inner diameter and therefore they do not hinder maintenance of the screen.
Advantageously, end elements are provided, in particular ring flanges are provided, which are arranged on the upper face and on the lower face of said tubular body, and which abut against said slats.
According to another aspect of the invention a method for making such a screen structure for a well comprises the steps of:

prearranging a plurality of parallel slats, said slats arranged to define a tubular body, said slats spaced from one another such that said tubular body exposes a plurality of longitudinal slit passageways for a fluid, each slit defined between two adjacent slats;
fixing said parallel slats integral to one another, wherein said slats have a cross section selected from the group comprised of: a triangular cross section, said triangular cross section having a base and two sides that extend from said base and meet each other at a vertex opposite to said base, or a trapezoidal cross section obtained by removing said vertex from said triangular cross section, said triangular cross section having a vertex angle set between 8 degrees and 17 degrees.

Advantageously, said step of fixing said parallel slats integral to each other is carried out through a step selected from the group comprised of:

providing joint elements inside of said tubular body;
providing joint elements outside of said tubular body.

Preferably, said step of prearranging said slats to define a tubular body is carried out through the steps of:

prearranging at least one guide mask that is provided with a plurality of introduction holes for introducing said slats, said holes evenly spaced apart with respect to one another and creating a geometric figure that corresponds to the cross section of said tubular body;
inserting respective slats into said introduction holes.

In particular, said step of providing said joint elements is carried out starting from said slats that are bound together by means of said at least one guide mask, which is withdrawn after application of said joint elements.
Preferably, a step is provided of applying an abutment ring or an abutment flange on the upper face and on the lower face of said tubular body to cover said slats.
According to another aspect of the invention, a reinjection screen structure for a well casing, for reinjecting a fluid comprises:

a plurality of parallel slats, integrally arranged with respect to each other, said slats arranged to define a tubular body, said slats spaced from one another such that said tubular body exposes a plurality of longitudinal slit passageways for a fluid, each slit defined between two adjacent slats,

In particular, said slats have a cross section selected from the group comprised of: a triangular cross section, said triangular cross section having a base and two sides that extend from said base and meet each other at a vertex opposite to said base, or a trapezoidal cross section obtained by removing said vertex from said triangular cross section, said triangular cross section having a vertex angle set between 8 degrees and 17 degrees.

Brief description of the drawings

The invention will be now shown by the following description of an exemplary embodiment thereof, exemplifying but not limitative, with reference to the attached drawings that are shortly described hereinafter, wherein:

figure 1 shows a diagrammatical cross sectional view of a well that is equipped with the filter or screen structure according to the invention;
figure 2 is a cross sectional view of the screen structure according to the invention formed by a plurality of slats that are integrally arranged with respect to one another, to form a tubular body;
figures 2A and 2A' show a cross sectional view of a triangular cross section slat and of a trapezoidal cross section slat, respectively, of an extraction filter;
figure 3 is an elevational side view of the screen structure of Fig. 2, in which the vertical slits between the slats are highlighted;
figure 4 shows a diagrammatical view of the screen arranged within a draining layer, in which the flow lines of the fluid are shown between the aquifer and the well casing;
figure 4A shows a time sequence that points out the progressive change of the speed-vector while approaching the screen;
figure 5 shows a diagrammatical cross sectional view of an extraction well and of a corresponding reinjection well, which is equipped with the fluid reinjection screen structure, according to the invention;
figure 6 is a cross sectional view of the reinjection screen structure of Fig. 5, which comprises a plurality of slats that are integrally arranged with respect to each other, and have a decreasing cross section along a direction that extends from the inside to the outside, to form a tubular body;
figures 6A and 6A' show a cross sectional view of a triangular cross section slat and of a trapezoidal cross section slat of the reinjection screen, respectively;
figure 7 is an elevational side view of the screen structure of Fig. 5, in which the vertical slits between the slats are highlighted, as well as a stiffening ring that is located at a middle position of the reinjection screen.

Detailed description of some exemplary embodiments

With reference to Fig. 1, a well 100 for extracting a fluid, for example water, comprises a wellbore 10 that extends from a ground level 11 through an underground position 12 to reach an aquifer, i.e. a ground water reservoir 20, and also comprises an extraction duct or well casing 21 that is arranged within wellbore 10 down to aquifer or reservoir 20. In particular, about extraction duct 21 a plurality of layers of a filling material 30 is arranged. As well known, extraction duct 21 has a filtration portion or a screen 23, which is arranged proximate to aquifer 20. More in detail, screen 23 externally retains a selected drainage material 31 at aquifer 20 which is contained between an upper insulation layer 32 and a lower insulation layer 33, i.e. between two plugs, the drainage material adapted to retain impurities and particulate matter from the extracted fluid.
As shown in Figs. 2 and 3, screen 23 of the well casing, according to the invention, comprises a plurality of parallel slats 40 that are integrally arranged with respect to one another, such that a tubular body 50 is defined. In particular, slats 40 are spaced from one another such that tubular body 50 exposes a plurality of longitudinal slits 41 that act as passageways for the fluid (Fig.3). Each slit is defined between two adjacent slats 40. This way, slats 40 define a plurality of solid portions separated by respective void portions, i.e. by longitudinal slits 41, such that the void portions, in use, are arranged in a substantially vertical direction, i.e. they are arranged parallel to a longitudinal axis 51 of the tubular body, as shown in Fig. 3.
As shown in Fig. 3, slats 40 may be kept integral with respect to each other by means of joint elements that are arranged within tubular body 50 or by means of external joint elements that are arranged outside of the tubular body, in particular a plurality of support rings 70 that are spaced along axis 51 of the tubular body. Alternatively, joint elements may be provided in which slats 40 are inserted, to form a fixed joint. Advantageously, stiffening elements 80 are also provided arranged between slats 40, such that the construction is stronger and is adapted to bear high loads.
Yet advantageously, end elements, not shown, in particular ring flanges, may be arranged on the upper face and on the lower face of tubular body 50, to abut against slats 40.
In particular, slats 40 are blades of a predetermined size, said blades tapered along a direction that extends from outside to inside tubular body 50. More in detail, the blades are tapered according to a triangular cross section but alternatively, they may be tapered according to a trapezoidal cross section or to a differently shaped cross section.
In particular, as shown in Fig. 2A and 2A', slats 40 have a cross section that is selected from the group comprised of: a triangular cross section that has a base b and two sides 1 that extend from base b and meet each other at a vertex 45 opposite to base b, or a trapezoidal cross section (Fig.2A') that is obtained by removing vertex 45 from the triangular cross section (Fig.2A). The triangular cross section has a vertex 45 angle α that is set between 8 and 17 degrees. In particular, vertex angle α is set between 9° and 12°, preferably it is equal to 10°.
In other words, vertex 45 of the triangular cross section or of the trapezoidal cross section defines a height h with respect to base b. The triangular or trapezoidal cross section has a base-to-height b/h ratio set between 3/20 and 3/10.
In particular, angle α formed between sides 1 at vertex 45, which is set between 8.5° and 17°, remarkably reduces fluid turbulence and therefore reduces the fluid pressure drop across the screen. In this case, the remarkable turbulence reduction that is achieved by the invention does not cause any loss of gas that might be dissolved in the fluid (e.g. a CO₂ gas), therefore it doesn't give rise to any precipitation of salts that may be present as a solute in the fluid, thus preventing the formation of salt scale (e.g. calcium carbonate scale).
Constructionally, blades 40 may have various sizes according to the type and depth of the application for which the screen is used. For instance, base b has a length that is set between 1 mm and 20 mm, whereas external longitudinal slit has a width d2 set between 0.2 and 5 mm. The latter is responsive to the size of drainage material 31, which is arranged around screen 23. Normally, the filter may have various solid-to-void ratios, for example the ratio may be 0.25, 0.50, 0.75, 1, 1.25, 1.5, 1.75 and so on. For instance, a 3 mm base b and a 1 mm slit d2 give a 1/4 solid-to-void ratio, which means a 25% filtering capacity. On the other hand, to define the divergence of blades 40 a distance d3 is used between two vertices 45 of two adjacent blades. Such distance is preferably set between 2 mm and 5 mm, most preferably it is set between 3 mm and 4 mm.
In particular, slits 41, which are arranged between two blades 40, have their minimum cross section width at a side surface of the tubular body, as it can be seen in Fig. 3. Therefore, by arranging outside of tubular body 50 a granular material 31 of a diameter larger than the minimum width of slits 41, the tubular body peripherally retains the draining granular material and allows the fluid to flow through slits 41. Moreover, the diverging cross section of slits 41 assists possible particulate solids to flow through slits 41, which prevents the formation of a sediment that would otherwise plug the screen.
Referring now to fluid dynamics, as diagrammatically shown in Fig. 4 and 4A, a horizontal speed-vector 60 undergoes a sharp acceleration closely proximate to screen 23, and increases its intensity from some cm/s, which is the speed hat it generally has in aquifer 20, up to an intensity 65 of some m/s, which is the speed the fluid has when it passes from aquifer 20 into the well casing. More in detail, since fluid speed 60, while crossing drainage material 31 and approaching screen 23, gain a vertical component 61 (Fig. 4a), the vertically extending slits do not offer any resistance to such vertical speed component, which even before reaching the casing is far greater than the horizontal component 60, i.e. vertically extending slits 41 do not offer any resistance to the vertical speed component 61, which is substantially unaffected by the passage of the fluid through tubular body 50. In other words, the fluid flow lines, near screen 23, enter tubular body 50 without any substantial pressure drop and without any substantial reduction of vertical speed-vector component 61.
Constructionally, a method for making such a screen structure for a well comprises the steps of prearranging a plurality of slats 40 in such a way that a tubular body is defined that has a plurality of longitudinal slit passageways 41 for the fluid, wherein each slit is defined between two adjacent slats 40. A subsequent step provides fixing parallel slats 40 integral to one another, and is carried out through a step selected from the group comprised of providing joint elements inside the tubular body, or providing joint elements outside the tubular body, or providing joint elements in which said slats are inserted to form a fixed joint.
More in detail, the step of prearranging slats 40 is carried out by means of at least one guide mask that has a plurality of introduction holes, where the holes are evenly spaced apart with respect to one another and form a geometric figure that corresponds to the cross section of the tubular body. Then the slats are introduced into respective introduction holes.
The subsequent step of applying the elements is carried out starting from slats 40 that are bound together by means of said at least one guide mask which is withdrawn after providing said joint elements, for instance one or more circular rings 70, as shown in Fig. 3. Alternatively, the mask remains inserted within the screen structure and works as a stiffening element, as described above.
Finally, a step is provided of applying an abutment ring or an abutment flange on the upper face and on the lower face of the tubular body to cover slats 40.
In screen structure 23, according to the invention, blades 40 serve both as a support structure and as a filtration structure, whereas in a wire wrap screen the two functions, i.e. the support structure and the filtering line, are provided by separate means. Therefore, a screen structure is obtained which is well suited and ready for use with any well type and at any depth, and no further adaptation is necessary.
Figure 5 shows a fluid extraction well 500, for example a geothermal well, which is combined with a fluid reinjection well 600. This way, the fluid that is extracted from extraction well 500 flows through a heat exchanger 250, heats a working fluid, and is reinjected into the reservoir through reinjection well 600.
In particular, each well comprises a hole 110 that extends from a ground level 111 through an underground position 120 down to a reservoir 200, and also comprises a well casing 210 that is arranged within hole 110 down to reservoir 200. In particular, well casing 210 of reinjection well 600 provides a filtration portion or a reinjection screen 130, which is arranged proximate to aquifer 200. Even in this case, a plurality of filling material layers are provided arranged about well casing 210, which comprise a selected drainage material 310 between an upper insulation layer 320 and a lower insulation layer 330, i.e. between two plugs, in order to retain impurities and particulate matter from the extracted fluid.
In particular, as shown in Figs. 6 and 7, reinjection screen structure 130 comprises a plurality of slats 140, which are parallel and integral with respect to one another, and are arranged to define a tubular body 150. In particular, slats 140 are spaced from one another such that tubular body 150 exposes a plurality of longitudinal slit passageways 141 for the fluid (Fig.5 and 6). Each slit 141 is arranged between two adjacent slats 140. This way, slats 140 define a plurality of solid portions that are spaced by respective void portions, i.e. longitudinal slits 141, such that, in use, the void portions extend along a substantially vertical direction, i.e. they extend parallel to a longitudinal axis 151 of the tubular body, as shown in Fig. 6.
In particular, slats 140 are blades of a predetermined size and are tapered along a direction that extends from the inside to the outside of tubular body 150. More in detail, the blades are tapered according to a triangular cross section but alternatively, they may be tapered according to a trapezoidal cross section or to a differently shaped cross section. Constructionally, blades 140 may have various sizes according to type and depth of the application for which the reinjection screen 130 is used. For example, a base b' has a length that is set between 1 mm and 20 mm, whereas an inner longitudinal slit width d2' is set between 0.2 mm and 5 mm.
In particular, as shown in Fig. 6A and 6A', slats 140 have a cross section that is selected from the group comprised of: a triangular cross section that has a base b' and two sides the that extend from base b' and meet each other at a vertex 145 opposite to base b', or a trapezoidal cross section (Fig.6A') that is obtained by removing vertex 145 from the triangular cross section (Fig.6A). The triangular cross section has a vertex 145 angle α that is set between 8 degrees and 17 degrees. In particular, vertex angle α is set between 9 degrees and 12 degrees, preferably is equal to 10°.
In other words, vertex 145 of the triangular cross section or of the trapezoidal cross section defines a height h' with respect to base b'. The triangular or trapezoidal cross section has a base-to-height b'/h' ratio set between 3/20 and 3/10.
Such solution is also particularly advantageous both for an extraction screen and for a reinjection screen 130, which is normally used in case of high mineral content water (for example, a high silica content geothermal water). The low turbulence, which is a feature of the invention, enormously reduces scaling on the reinjection screen.
Normally, filter 130 may have various solid-to-void ratios, for example the ratio may be 0.25, 0.50, 0.75, 1, 1.25, 1.5, 1.75 and so on. For example, a 3 mm base b' and an 1 mm slit width d2' give a 1/4 ratio, which means a 25% filtering capacity. On the other hand, to define the divergence of blades 40 a distance d3' is used between two vertices 145 of two adjacent blades. Such distance is preferably set between 2 mm and 5 mm, most preferably it is set between 3 mm and 4 mm.
In particular, slits 141, which are arranged between two blades 140, have their minimum cross section width at inner side face 165 of the tubular body, as it can be seen in Fig. 7. Moreover, the diverging cross section of slits 141 assists possible particulate solids to flow through slits 141, which prevents the formation of a sediment that would otherwise plug the reinjection screen 130.
As shown in Fig. 7, slats 140 may be kept integral with respect to each other by means of joint elements that are arranged within tubular body 150 or by means of joint elements external that are arranged outside of the tubular body, in particular a plurality of support rings 170 that are spaced along axis 151 of the tubular body. Alternatively, joint elements may be provided in which slats 140 are inserted, to form a fixed joint. Advantageously, stiffening elements 180, which can be seen in Fig. 6 cross sectional view as well, are also provided arranged between slats 140, such that the construction is stronger and is adapted to bear high loads.
Yet advantageously, end elements, not shown, in particular ring flanges, may be arranged on the upper face and on the lower face of tubular body 150, to abut against slats 140.
The foregoing description of an embodiment will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such embodiment without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the embodiment. The means and the materials to realize the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Claims

A screen structure (23) for a well casing comprising:
- a plurality of parallel slats (40) integral to each other, said slats (40) arranged to define a tubular body (50), said slats (40) spaced from each other such that said tubular body (50) provides a plurality of longitudinal slit passageways (41) for a fluid, each slit (41) defined between two adjacent slats (40),
characterized in that said slats (40) have a cross section selected from the group comprised of: a triangular cross section that has a base and two sides that extend from said base and meet each other at a vertex opposite to said base, or a trapezoidal cross section that is obtained by removing said vertex from said triangular cross section, said triangular having a vertex angle set between 8 and 17 degrees.
A screen structure (23), according to claim 1, wherein said vertex angle is set between 9 and 12 degrees, preferably 10°.
A screen structure (23), according to claim 1, wherein said slats (40) are blades of a predetermined size, said blades (40) tapered along a direction that extends from outside to inside said tubular body (50), to provide an extraction screen.
A screen structure (23), according to claim 1, wherein said slats (40) are blades of a predetermined size, said blades (40) tapered along a direction that extends from the inside to the outside of said tubular body (50), in order to provide a reinjection screen.
A screen structure (23), according to claim 1, wherein said parallel slats (40) are kept integral with respect to each other by a means selected from the group comprised of:
- joint elements that are provided inside of said tubular body (50);

- joint elements that are provided outside of said tubular body (50);

- joint elements that are provided with holes in which said slats (40) are inserted.
A screen structure (23), according to claim 1 and 5, wherein said tubular body (50) is a cylindrical tubular body and said joint elements are a plurality of support rings that are arranged at a distance along the axis of the tubular body (50).
A screen structure (23), according to claim 1, wherein end elements are provided, in particular ring flanges are provided, which are arranged on the upper face and on the lower face of said tubular body (50), and which abut against said slats (40).
A method for making a screen structure (23) for a well comprising the steps of:
- prearranging a plurality of parallel slats (40), said slats (40) arranged to define a tubular body (50), said slats (40) spaced from one another such that said tubular body (50) exposes a plurality of longitudinal slit passageways (41) for a fluid, each slit (41) defined between two adjacent slats (40) ;

- fixing said parallel slats (40) integral to one another,
characterized in that said slats (40) have a cross section selected from the group comprised of: a triangular cross section, said triangular cross section having a base and two sides that extend from said base and meet each other at a vertex opposite to said base, or a trapezoidal cross section that is obtained by removing said vertex from said triangular cross section, said triangular cross section having a vertex angle set between 8 degrees and 17 degrees.
A method, according to claim 8, wherein said vertex angle is set between 9 degrees and 12 degrees, in particular said vertex angle is equal to 10°.
A method, according to claim 8, wherein said step of fixing said parallel slats (40) integral to each other is carried out through a step selected from the group comprised of:
- providing joint elements inside of said tubular body (50);

- providing joint elements outside of said tubular body (50);

- providing joint elements in which said slats (40) are inserted.
A method, according to claim 8, wherein said step of prearranging said slats (40) to define a tubular body (50) is carried out through the steps of:
- prearranging at least one guide mask that has a plurality of introduction holes for introducing said slats (40), said holes uniformly spaced apart with respect to one another and creating a geometric figure that corresponds to the cross section of said tubular body (50);

- inserting respective slats (40) into said introduction holes.
A method, according to claim 8, wherein said step of providing said joint elements is carried out starting from said slats (40) that are bound together by means of said at least one guide mask, which is withdrawn after application of said joint elements, in particular a successive step is provided, after said withdrawing step, wherein the application of a ring or of an abutment flange is provided on the upper face and on the lower face of said tubular body (50) to cover said slats (40).
Use of an extraction screen structure (23), according to claims 1,2, and 4 to 7, to provide a well for a fluid, such that the vertical component of the fluid speed is substantially unaffected by the passage of the fluid from outside to inside the screen structure.
Use of a screen structure (23) for a well casing, according to any one of claims from 1 to 6, to reinject a fluid, said slats (40) being blades (40) of a predetermined size tapered along a direction that extends from the inside to the outside of said tubular body..
Use of the screen structure, according to claim 14, wherein said vertex angle is set between 9 degrees and 12 degrees, preferably said vertex angle is 10°.