EP0517597A1 - Process for the production of hydrocarbons, undeground cavity for carrying out the process and related use of this cavity - Google Patents

Abstract

In order to avoid reduction in the productivity of a well connecting with a hydrocarbon reservoir in unconsolidated sands as a consequence of the clogging of its filters, the present production process makes provision for allowing the sand to enter the well, then for ensuring that it settles in an underground cavity (10) situated not far from the bottom of the well. This process can, in particular, be used when the reservoir is situated under a bed of rock salt (2), the cavity (10) then being leached from the well itself. In general, it is expedient, to this end, for the volume of the cavity (10) to be sufficiently large for the hydrocarbons passing through the cavity (10) to be driven at a speed less than the sedimentation speed of the sands in the hydrocarbons according to their viscosity at the temperature within the cavity (10). The invention relates to such cavities as well as a related use of the latter as buffer storage in order to provide a povisional store for the hydrocarbons produced. <IMAGE>

Description

The present invention relates to a process for producing hydrocarbons which overcomes the problems associated with the arrival of sand. It also relates to an underground cavity for implementing this process as well as to an additional use of this cavity.

Some hydrocarbon deposits contain sands which, according to the terminology of Soil Mechanics, are unconsolidated and therefore liable to be entrained with the flow of oil within the production well. This is particularly troublesome, because the grains which manage to rise with the oil are gradually eroding the various installations, both bottom and surface. This can lead to damage to the well safety valves, the operation of which in the event of an incident is no longer ensured.

So far we have tried to prevent the arrival of sand. For this purpose, artificial filters are created around the production tube in line with the perforations. Most of the time, these are layers of sand and gravel, the grain size of which is chosen to stop the sands of the deposit without resulting in clogging with respect to hydrocarbons (see techniques known in the profession under the designations " gravel-pack "and" pre-pack strainer "). In practice, however, these filters do not take long to become dirty, leading to significant pressure drops. And the production of hydrocarbons is quickly reduced, or even canceled.

The object of the present invention is a process for the production of hydrocarbons from a deposit comprising free sands, which avoids such a reduction in the productivity of the well.

Contrary to known techniques, it achieves this in substance while the sand is allowed to come into the well, and then it is settled in an underground cavity located not far from the bottom of the well.

When the deposit is under a solid mass of rock salt, the cavity is advantageously produced by leaching. The production well then generally comprises a production tube which crosses the mass of salt. In this case, the production tube is preferably provided at the level of the salt mass with three levels of communication as well as two plug seats, a lower seat being below the communication levels and an upper seat being between the level intermediate communications and higher communications level.

Advantageously, in the preliminary phase of producing the cavity, the intermediate level of communications is closed while the other two levels are released, a concentric tube being arranged in the production tube so that its free end is opposite the lower communications level, the lower plug seat receiving a plug to completely block the production tube while the upper plug seat receives an annular plug to close only the ring finger between the production tube and the concentric tube, fresh water being brought by the concentric tube to come into contact with the salt at the level of the lower level of communications, dissolve the salt around the production tube and be recovered in the form of brine passing through the level of superior communications within the ring finger.

Advantageously, in the production phase, the lower level of communications is closed while the other two levels of communication are cleared, the lower plug seat does not receive a plug while the upper plug seat receives a plug to completely block the production tube, the hydrocarbons going up in the tube being thus diverted by the intermediate level of communications in the cavity to return in the tube by the higher level of communications.

Advantageously, the three levels of communication of the production tube each include a sliding jacket to close or release them as appropriate.

The invention also relates to the underground cavity used for decantation. Its volume must in fact be large enough for the hydrocarbons to pass into the cavity at a speed lower than the sedimentation speed of the sand, taking into account the viscosity of the oil at the temperature of the cavity. Typically, its volume is of the order of a few tens of thousands of cubic meters.

In general, the cavity is crossed by the production well. Consequently, the production well is preferably provided with means for bringing the hydrocarbons into the cavity at a height above its bottom such that the sand produced during the life of the well can be received over this height of the cavity. It is recommended that it be placed as close as possible to the bottom of the production well, taking into account the geophysical properties of the deposit so that its temperature is as high as possible.

When the deposit is under a solid mass of salt, the cavity is advantageously made in the salt by leaching from the production well, its depth being chosen according to preliminary reconnaissance to avoid the blocks of insoluble susceptible, by detaching as a result of leaching, disrupt the production conditions of the well.

Finally, the invention discloses an additional use of this type of cavity consisting in temporarily storing therein the oil produced by the deposit, then in recovering the stored oil, the filling and recovery operations being carried out by varying the pressure of the gas overcoming the oil (methane, nitrogen, etc.).

Advantageously, the production well accommodates an inner production tube concentric with an outer production tube, an annular being therefore formed between the tubes, a safety sleeve constituted by a thick-walled cylinder closed internally by a plug being disposed in the extension of the two tubes, axial conduits made in the wall of the cylinder connecting the ring finger above the stopper with the inner production tube under the stopper while other axial conduits made in the wall of the cylinder connecting the ring finger in below the plug with the interior production tube above the plug, the exterior production tube communicating with the top of the cavity while the interior production tube communicates with the hydrocarbons within the cavity, the gas pressure being controlled in the internal production tube using compressors and the hydrocarbons being forced into the ope recovery ration by the ring finger while their ascent into the inner production tube is prevented in the filling operation.

When the deposit is located under a rock salt mass in which the cavity is made, the internal production tube is preferably provided at the cavity with three levels of communications as well as two plug seats, a lower seat being below the communications levels and an upper seat located between the intermediate communications level and the upper communications level, only the intermediate communications level remaining clear during filling and recovery operations, a plug being placed in the plug seat higher during the filling operation to prevent the rise of hydrocarbons in the inner production tube.

Advantageously, a plug is then placed in the lower plug seat during the recovery operation so that the recovered hydrocarbons are exclusively the hydrocarbons previously stored during the filling operation.

Hydrocarbons can be produced with a low flow rate, the recovery operation taking place from time to time when the cavity has been sufficiently filled during the previous filling operation. But we can also, during the filling operation, activating the production of hydrocarbons by maintaining the gases at the lowest possible pressure compatible with the stability of the cavity.

If, finally, the well is only accessible during specified periods, the recovery operation can be carried out during these periods while the filling operation takes place in the meantime.

• la figure 1 est une coupe schématique d'un massif comportant un gisement d'hydrocarbures ainsi qu'une couche de sel gemme où des modes de réalisation avantageux du présent procédé et de la présente utilisation peuvent être mis en oeuvre ;
• les figures suivantes sont des coupes d'une cavité réalisée dans la couche de sel de la figure 1. Elles illustrent différentes phases du présent procédé :

• . le lessivage de la cavité sur la figure 2,
• . la production d'huile avec décantation du sable dans la cavité de la figure 3, ainsi que deux opérations de la présente utilisation :
• . le remplissage en huile de la cavité à des fins de stockage sur la figure 4,
• . la récupération de l'huile stockée dans la cavité sur la figure 5.

Seule la première figure respecte une échelle réaliste. Les autres au contraire sont distordues pour des raisons de lisibilité. Elles font apparaître notamment un puits de diamètre notablement exagéré par rapport à celui de la cavité. Des paires de droites légèrement inclinées sur l'horizontale permettent par ailleurs d'éviter de représenter les hauteurs de massif comprises entre ces droites (coupes imaginaires).The invention will be better understood on reading the detailed description which follows and on examining the appended drawings which represent, by way of nonlimiting example, embodiments of the present method and of the present use. In these drawings:

• Figure 1 is a schematic section of a massif comprising a hydrocarbon deposit as well as a layer of rock salt where advantageous embodiments of the present method and of the present use can be implemented;
• the following figures are sections of a cavity made in the salt layer of figure 1. They illustrate different phases of the present process:

• . the leaching of the cavity in FIG. 2,
• . the production of oil with decantation of the sand in the cavity of FIG. 3, as well as two operations of the present use:
• . filling the cavity with oil for storage purposes in FIG. 4,
• . recovery of the oil stored in the cavity in FIG. 5.

Only the first figure respects a realistic scale. The others, on the contrary, are distorted for reasons of readability. They reveal in particular a well of diameter which is considerably exaggerated compared to that of the cavity. Pairs of straight lines slightly inclined on the horizontal also make it possible to avoid representing the heights of the massif comprised between these straight lines (imaginary sections).

The type of deposit shown schematically in Figure 1 is particularly suitable for an economical implementation of the present process. It corresponds to a geological structure which is not uncommon to meet. Layer 1 produces hydrocarbons there supposed to be in the form of a liquid oil. In the context of the present invention, they could however be just as well in gaseous form. Layer 1 is also located not far from a rock salt bank 2, itself most often topped with sedimentary rocks 3. In this example, it is rather under the bank. The scale graduated in meters indicating the depths H to the left of Figure 1 is given, however, purely for information, substantial variations in dimensions can be noted from one deposit to another.

According to the present method, a cavity 10 is then produced in rock salt 2 so as to communicate with a production well 20. This well 20 is traditionally coated over the entire height of the sedimentary rocks 3 with a casing 24. The latter has a shoe 26 a little under the roof of the salt 2. It is also made integral with a production tube 21 which extends in the salt 2 and is generally cemented to the solid (see cementing 25). A priori, the location of the cavity relative to this system is indifferent. However, it is preferable that it be arranged as shown in FIG. 1. It is then in particular traversed by the tube 21 for producing the well 20, which allows the cavity to be created simply by virtue of the known technique of leaching.

FIG. 2 shows how the latter could be implemented in the context of the present method. For this, the production tube 21 is provided with at least two levels of communication with the salt 2. They advantageously correspond to sliding liners 40. These bear the reference numbers 40h and 40b and are located respectively at the top and at the bottom of the cavity. Their distance actually regulates the final height of the excavation. Indeed, using a tube 22 concentric with the production tube 21, fresh water is for example brought to the right of the jacket 40b which is left open. The water then spreads around the tube 21 and dissolves the surrounding salt. And it is brine 12 which enters through the jacket 40h (also open) within the ring finger between the tubes 21 and 22 to be recovered at the surface. A plug 52 ensures in this case the isolation of the annular from the lower end of the concentric tube 22 for supplying water, itself separated from the producing layer 1 by another plug 51b which thus protects the hydrocarbon deposit.

The dissolution results in the gradual widening of the cavity 10 while the elements of the insoluble solid mass 11 accumulate at the bottom of the cavity 10. As will be seen below, it is necessary to reckon with total volumes to be excavated of the order of a few tens of thousands of cubic meters. Thus for a cavity height of 80 m for example, its diameter is of the order of 20 m. Therefore, there is little risk that blocks of insoluble matter are released by leaching so large that they can, by falling to the bottom of the cavity, cause the rupture of the production tube 21. This type of hazard must however be present in the mind of the specialist responsible for choosing which depth to have the cavity. Preliminary reconnaissance will allow him to rule out areas unsuitable from this point of view.

This specialist will also have an interest in placing the cavity as deeply as possible. Indeed, the temperatures being higher, the viscosity of the oil is lower. This improves the settling of the sand within the cavity during the production phase, which then takes place according to the diagram in FIG. 3. The latter does inter alia appear the interest of a third level of perforations also advantageously comprising a sliding shirt 40i. This is placed between the two previous shirts 40h and 40b while a plug seat 50h is placed immediately above this new shirt 40i. This seat may for example have been used in the previous excavation phase to accommodate the annular plug 52. But now that the concentric water supply tube 22 has been removed, the seat 50h is rather provided with a complete plug 51h.

Thus, by removing the lower insulation plug 51b, closing the lower jacket 40b and opening the intermediate jacket 40i, the oil flow is established from the producing layer 1 to the surface. according to the arrows in FIG. 3. The oil therefore firstly enters the production well by passing, for example, through perforations 30 made in the production tube 21. The size of these perforations 30 is in this case chosen without particular concern for the free sand that the oil entrains. And that the latter comes into the well, is perfectly admitted.

It then rises with the oil to the plug 51h which deflects the flow of oil through the openings of the sliding jacket 40i and leads it into the cavity. From this moment, and although this does not really appear in Figure 3 because of its distorted scale, the passage section offered to the oil is significantly increased. In other words, the driving speed on the grains of sand is greatly reduced. As long as the diameter of the cavity has been chosen to be large enough, the speed of the oil can thus drop to a value lower than the speed of sedimentation of the sand. For the cavity volumes mentioned above, the speed of 0.13 m / s for example in a production tube 21 of 6 inches in diameter could be reduced below a speed of 2 m / day in the cavity. Given the low viscosity of the oil at temperatures prevailing at great depths and, moreover, in the layers of salt always hotter than their neighboring rocks, such a speed is entirely compatible with effective settling of the sand.

As the small dots 13 representing the grains of sand in FIG. 3 suggest, these then fall into the brine 12 that the oil 14 discharged at the level of the intermediate sliding jacket 40i has not made it possible to entrain out of the cavity. They gradually accumulate above the insolubles 11. This is how the distance separating the intermediate jacket 40i from the bottom of the cavity (bottom itself adjusted, as we have seen, by the dimension of the jacket 40b lower) governs the sand recovery capacity. The specialist will therefore determine this distance according to the total amount of sand likely to come throughout the life of the production well (generally around 10 years).

This is how the oil 14 which finally returns to the production tube 21 passing through the sliding sleeve 40h above is freed from almost all of the grains of sand which have come with the oil at the level of the producing layer 1. It should be noted that this result is obtained by keeping the production potential of hydrocarbons intact. The perforations 30 of the well in line with the producing layer 1 do not a priori comprise any particular filter. Their fouling with the clogging effect that this causes is therefore not to be feared.

This is how the present process turns out to be very interesting for the profitability of wells. In fact, stopping their production with a view to changing the filter systems most of the time results in considerable financial losses. This is even more marked in the offshore field, especially when the water depth (typically beyond 300 m) requires placing the wellhead at the bottom of the sea, which makes it difficult to produce economically.

But apart from this result, the creation of such an underground cavity communicating with a production well is accompanied by various advantages. These reside in particular in a use of the cavity other than the actual decantation of the sand. Although its interest could already justify the creation of a cavity, the cost of excavation is however generally too high compared to the expected benefits apart from the elimination of the sand. This is why the elements of description which follow form part of the invention only as an additional use of a cavity for the implementation of the present process for the production of hydrocarbons.

Figures 4 and 5 illustrate two typical operations of this additional use which generally means using the cavity as buffer storage. The first of these operations consists in fact in allowing the oil to rise from the producing layer 1 into the cavity 10. In an identical manner to what happens during the exploitation phase described above with regard to FIG. 3 , the oil enters through the perforations 30 of the production tube 21 and exits for example through the intermediate sliding sleeve 40i. This determines, as before, the trapping of the brine 12 located under the opening of this jacket 40i. The grains of sand 13 possibly entrained by the oil settle there. Similarly, the lower plug seat 50b remains devoid of a plug while the upper 50h seat accommodates a 51h which closes the entire production tube 21.

With regard to the sliding sleeves which have been used for leaching the cavity, if the lower 40b is still closed, the sliding sleeve 40h upper is in practice left open. Care is then taken to maintain the oil level 15 in the cavity above this 40h shirt. The well 20 is also provided with various casings which are generally found in underground storage of hydrocarbons in rock salt. A metal casing 24 extends in the salt 2 beyond its interface with the sedimentary rocks 3 and ends in a cemented shoe 26. A cementation 25 ensures the connection of this casing 24 with the ground. Inside the casing 24, two concentric production tubes are arranged according to a known method. The outer tube 23 is then advantageously separated from the casing 24 by a liquid 27 retained with the aid of an annular plug 53 in order to participate in the support of the well 20. It extends to the top of the cavity 10. As for the inner tube, this is for example the production tube 21 passing through the cavity 10 and which continues to the surface.

Still according to the known method specific to underground storage of hydrocarbons, use is finally made of a sleeve 60 adapted to take place in the extension of the concentric production tubes 21 and 23. This configuration is in particular the subject of French patent application 90-05931 filed by the company GEOSTOCK. The sleeve 60 is none other than a thick-walled cylinder. It internally receives a plug 54 while axial conduits are hollowed out in its wall. Half 61 of them is adapted to put in communication the part of the inner tube 21 located under the plug 54 with the ring finger 28 defined between the production tubes 21 and 23 and being located above the plug 54. The other half 62 of the conduits is adapted to connect this annular 28 under the stopper 54 to the part of the inner tube 21 located above the stopper 54. In other words, the sleeve 60 allows the crossings of the flows established respectively in the tube 21 of internal production and the annular 28 between the tubes 21 and 23 of production. This has in particular the advantage of allowing the use of safety valves arranged only on the inner tube 21. It suffices that one of these valves is located under the plug 54 and the other above to allow any passage to the surface to be cut off.

Thus the first operation of the annexed use envisaged here using FIG. 4 results in filling the cavity 10 with the oil 14 produced. It then consists in authorizing the concomitant evacuation of the gases 15 surmounting the oil, the ring finger 28 being isolated from the surface (by a plug not shown) while the gases are withdrawn at the surface by the tube 21 of internal production. The speed of filling is then conditioned by in particular the pressure difference between that of the oil in the producing layer 1 and that of the gases at the top of the cavity 10. For a given pressure in the deposit, the filling will in particular be all the faster that the gases will be maintained at a low pressure, the latter however having to remain sufficient to in particular ensure the stability of the cavity and, more generally, take into account the characteristics of the reservoir. We count most of the time with filling times of one to several months.

The second operation of this additional use makes it possible to recover the oil 14 stored in the cavity 10. In FIG. 5 which illustrates it more particularly, a plug 51b is shown in the lower seat 50b. In practice, the presence of this plug is not essential, especially if the oil production rate at the pressures in question is low. The plug 51b is postponed essentially to recall that during this second operation, it is not the hydrocarbon deposit which produces the oil.

This is in fact driven back thanks to gases 15 introduced under pressure. This can advantageously be achieved using compressors arranged in the inner production tube 21 according to a known method specific to hydrocarbon storage. The oil 14 then passes through the sliding jacket 40i left open and rises through the production tube 21. At the sleeve 60, it is finally deflected towards the annular 28 between the two concentric production tubes to be recovered on the surface.

Such use of the cavity as a buffer storage is particularly advantageous when the production of oil must be interrupted at the surface. This is for example the case when the pipelines 5 (cf. FIG. 1) must undergo repairs following sabotage in particular. The cavity then temporarily collects the oil, so that production at the level of the well can continue. This is a very interesting possibility when we know the difficulties posed by the reactivation of certain wells temporarily stopped.

• les puits dont le débit de production est très faible. Il suffit alors de laisser la cavité se remplir. En réduisant au minimum la pression des gaz au sommet de la cavité, on accélère d'ailleurs ce remplissage par rapport à une colonne d'huile allant jusqu'à la surface. L'huile peut ensuite être récupérée à fort débit à intervalles de temps réguliers ;
• les puits en zone arctique où les pétroliers ne peuvent pas accéder lorsque la mer est gelée. La cavité est alors à même de stocker toute la production d'huile de la période hivernale qui sera récupérée au printemps.

Other situations make this kind of buffer storage very interesting. To name just a few:

• wells with a very low production rate. Then just let the cavity fill. By minimizing the gas pressure at the top of the cavity, this filling is also accelerated with respect to an oil column going to the surface. The oil can then be recovered at a high flow rate at regular time intervals;
• wells in arctic areas where tankers cannot access when the sea is frozen. The cavity is then able to store all the production of oil from the winter period which will be recovered in the spring.

Claims (19)

1.- Method for producing hydrocarbons from a deposit comprising free sands, a production well (20) communicating with the deposit, characterized in that, in a production phase, it consists in letting the sands come with the hydrocarbons in the well (20), then passing the hydrocarbons into an underground cavity (20) so that the sands settle there and finally recovering the hydrocarbons freed from the sands. 2.- Method according to claim 1, the deposit being under a rock salt mass (2), characterized in that, in a preliminary phase of realization of the cavity (10), the cavity (10) is produced by leaching in the salt mass (2). 3.- Method according to claim 2, the production well (20) comprising a production tube (21) which passes through the salt mass (2), characterized in that the production tube (21) is provided at the massive salt (2) of three levels of communications (40b, 40i, 40h) as well as two seats (50b, 50h) of plug, a seat (50b) lower being under the levels of communications (40b, 40i, 40h ) and an upper seat (50h) located between the intermediate communications level (40i) and the upper communications level (40h). 4.- Method according to claim 3, characterized in that in the preliminary phase of realization of the cavity (10), the level of communications (40i) intermediate is closed while the other two levels (40b, 40h) are released, a concentric tube (22) being disposed in the production tube (21) so that its free end faces the lower communications level (40b), the lower plug seat (50b) receiving a plug (51b) for sealing the production tube (21) completely while the upper plug seat (50h) receives an annular plug (52) to seal only the annular between the production tube (21) and the concentric tube (22), fresh water being brought by the concentric tube (22) to come into contact with the salt (2) at the level of the lower communications level (40b), dissolve the salt (2) around the production tube (21) and be collected under brine form (12) passing through the level of communications (40h) higher within the ring finger. 5.- Method according to claim 3 or claim 4, characterized in that in the production phase, the lower level of communications (40b) is closed while the other two levels of communications (40i, 40h) are released, the lower plug seat (50b) does not receive a plug while the upper plug (50h) receives a plug (51h) to completely block the production tube (21), the hydrocarbons rising in the tube (21) thus being deflected by the level of communications (40i) intermediate in the cavity (10) for return to the tube (21) via the upper communications level (40h). 6.- Method according to any one of claims 3 to 5, characterized in that the three levels of communications (40b, 40i, 40h) of the tube (21) of production each comprise a sliding jacket for closing or releasing them according the case. 7.- Method according to any one of claims 1 to 6, characterized in that, in a filling operation, the hydrocarbons from the deposit are allowed to enter the cavity by lowering the pressure of the gases (15) surmounting them and in that that, in an operation for recovering the hydrocarbons (14) thus temporarily stored, they are driven back by increasing the pressure of said gases (15). 8.- Method according to claim 7, characterized in that the production well (20) receives a tube (21) concentric internal production of a tube (23) external production, an annular (28) therefore being formed between the tubes (21, 23), a safety sleeve (60) consisting of a thick-walled cylinder closed internally by a plug (54) being arranged in the extension of the two tubes (21, 23), axial conduits (61) formed in the wall of the cylinder connecting the annular (28) above the plug (54) with the tube (21) of internal production under the plug (54) while other axial conduits (62) formed in the wall of the cylinder connect the ring finger (28) below the plug (54) with the inner production tube (21) above the plug (54), the outer production tube (23) communicating with the top of the cavity ( 10) while the interior production tube (21) communicates with the hydrocarbons (14) within e the cavity (10), the gas pressure (15) being controlled in the inner production tube (21) using compressors and the hydrocarbons (14) being discharged in the recovery operation by the annular ( 28) while their ascent into the inner production tube (21) is prevented in the filling operation. 9.- Method according to claim 7 or claim 8, the deposit being under a rock salt mass (2) in which is formed the cavity (10), characterized in that the tube (21) of internal production is provided at the level of the cavity (10) three communication levels (40b, 40i, 40h) as well as two plug seats (50b, 50h), a lower seat (50b) being below the communication levels (40b, 40i , 40h) and an upper seat (50h) located between the level of communications (40i) intermediate and the level of communications (40h) higher, only the level of communications (40i) intermediate remaining clear during filling and recovery operations, a plug (51h) being disposed in the seat (50h) of upper plug during the filling operation to prevent the rise of hydrocarbons (14) in the tube (21) of internal production. 10.- Method according to claim 9, characterized in that a plug (51b) is disposed in the seat (50b) of the lower plug during the recovery operation so that the recovered hydrocarbons are exclusively the hydrocarbons (14) stored beforehand during the filling operation. 11.- Method according to any one of claims 7 to 10, characterized in that the hydrocarbons are produced with a low flow rate, the recovery operation taking place from time to time when the cavity has been sufficiently filled during the 'previous filling operation. 12.- Method according to claim 11, characterized in that during the filling operation, the production of hydrocarbons is activated by maintaining the gases (15) at the lowest possible pressure compatible with the stability of the cavity (10). 13.- Method according to any one of claims 7 to 10, characterized in that the well (20) is accessible only during specified periods, the recovery operation being carried out during these periods while the filling operation takes place in the meantime. 14.- underground cavity to implement a method according to any one of claims 1 to 13, characterized in that its volume is large enough for the hydrocarbons passing through the cavity (10) to be driven at a speed less than the rate of sedimentation of the sands in the hydrocarbons according to their viscosity at the temperature within the cavity (10). 15.- Cavity according to claim 14, characterized in that its volume is of the order of a few tens of thousands of cubic meters. 16.- Cavity according to claim 14 or claim 15, characterized in that it is crossed by the production well (20). 17.- Cavity according to claim 16, characterized in that the production well (20) is provided with means (40i) for bringing the hydrocarbons into the cavity (10) at a height above its bottom such that the sand produced during the life of the well (20) can be received on this height of the cavity (10). 18.- Cavity according to any one of claims 14 to 17, characterized in that it is arranged as close as possible to the bottom of the production well (20) taking into account the geophysical properties of the deposit so that its temperature is the as high as possible. 19.- Cavity according to any one of claims 14 to 18, the deposit being under a solid mass of salt (2), characterized in that it is produced in salt (2) by leaching from the well (20 ) of production, its depth being chosen according to preliminary reconnaissance to avoid the blocks of insolubles (11) likely, by detaching themselves as a result of leaching, to disturb the conditions of production of the well (20).

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