EP0067781A1 - Method and electrical apparatus for the enhanced recovery of oil - Google Patents

Abstract

The present invention relates to a method and device for promoting oil recovery. The device according to the invention for promoting the recovery of oil contained in a reservoir formation from a well equipped with a casing, comprises an anode which is placed at the bottom of the well, below said reservoir formation, in an electro-conductive liquid, and which is connected to the positive terminal of a direct or pulsed source of unidirectional voltage. Said casing is connected to the negative terminal of said source. The method consists in applying a unidirectional voltage for periods varying between several days and several months and alternating with OFF-period (when no voltage is applied).

Description

The subject of the invention is a method and an electrical device for enhanced oil recovery.

Many methods and devices using electrical energy have been proposed and tried to increase the mobility of hydrocarbons in their deposit and to improve the tertiary recovery of hydrocarbons.

A first category of processes use the heating produced by the Joule effect. These processes require high electrical power to sufficiently heat the deposit and they use alternating current.

The heating is used for example to carbonize oil shales in the methods described in U.S. patents 3,106,244, 3,137,347, 3,428,125 (H. PARKER).

Heating is also used to liquefy paraffins, asphalts or bitumen or to reduce viscosity.

Patents. U.S. 1,372,743 - (B. GARDNER), 3,848,671 - (L. KERN), 3,149,672 (J. ORKISZEWSKI et al).

The heating due to the passage of electric current has also been used to produce water vapor in place US patents 3,507,330, 3,547,193, 3,605,888, 3,614,986 - (W, G. Gill et Al) , 3.620.300 - (F. CROWSON), 3.547.192 - (E. CLARIDGE).

A second category of processes uses electrolysis and the resulting gases to pressurize the deposit or to combine them with hydrocarbons. Such methods are described in US Patents 1,784.2-14 - (P. WORKMAN), 3,103,975 (AW-HAUSON) 3,724,543 and 3,782,465 (CW BELL et Al), 4,037,655 (N. CARPENTER ).

A third category of processes use the action of a unidirectional electric current on a liquid contained in a porous medium similar to capillary tubes to move the liquid by a phenomenon known as electro-osmosis.

The phenomenon of electro-osmosis is used for example to avoid the invasion of the bottom of a well by the layer of salt water (Coning effect).

U.S. Patent 3,202,215 (A. STANOWIS) describes a process in which electro-osmosis is used to hold a lens of pure water in the bottom of a well to repel salt water.

Other patents describe methods for moving oil by electro-osmosis.

The FR patent. 1.268.588 (Institut Français du Pétrole) describes a process by which a high potential difference is applied between two electrodes located at two points in a deposit; having a specific meaning, for example a potential difference between 1000 and 100,000 volts.

US Patent 2,799,641 (TG BELL) describes an enhanced recovery method according to which a potential difference is applied between a first electrode of positive polarity located in the remote formation of a producing well and a second electrode of negative polarity located in the producing well and in direct contact with the reservoir formation.

The direct voltage is at least 70 Volts and the current intensity of about 10 amps. The oil is moved to the cathode. The current is interrupted periodically with a frequency of 6 to 30 cycles per minute. It is also possible to use pulses due to the discharges of a capacitor charged under a voltage of the order of 1000 to 3000 volts. You can also use a current asymmetrical AC having a frequency of the order of 1 to 10 Hz.

U.S. Patent 3,417,823 (S. R. FARIS) describes a process by which an anode and a cathode are placed in the same well - and water is moved to the cathode by electro-osmosis.

US Patent 3,642,066 (William G. GILL) describes an enhanced recovery process in which a cathode is placed in a producing well and an anode in an auxiliary well to move water from the formation to the anode and petroleum towards the cathode. A continuous but pulsed potential difference is applied between the anode and the cathode. The electrodes enter the reservoir formation

US Patents 3,724,543 and 3,782,465 (Christy W. BELL and Charles H. TITUS) describe devices according to which an anode of small area is placed in a cavity located in the middle of the formation and water is injected conductive around this anode. A continuous potential difference of several thousand volts is established between this anode and the casing of a producing well which serves as a cathode. The heated water around the anode migrates towards the cathode, under the effect of pressure and electro-osmosis, entraining the oil.

These patents show that it is known to use a unidirectional potential difference, continuous or pulsed, to promote the migration of petroleum to an electrode by an electro-osmosis effect in porous medium which can accumulate with heating and with electrolysis of the salt water contained in the formation.

The objective of the present invention is to provide an assisted recovery method using electrical energy by optimizing the balance sheet of the operation, that is to say the ratio between the additional oil which can be recovered and the electrical energy expended.

Tests have been carried out in the laboratory on samples of land in the shape of a drill core.

These rock samples were first washed and dried and then saturated with salt water. Oil is then circulated through these samples until water is irreducibly saturated. By difference, we know the oil permeating the rock.

We apply a unidirectional potential difference between the two longitudinal ends of the carrot during determined times and we measure the quantity of additional oil which is extracted from the sample either during the application of the current or after and we study how the amount of additional oil extracted from the sample, depending on parameters such as the electric field, the intensity of the current and the duration of application thereof and the rest periods alternating with periods of energization.

These tests confirmed that if one extracts petroleum only by hydrauric sweeping by creating a pressure difference between the two ends of the sample, when one does not manage any more to extract petroleum, one recovered only a small part of the oil initially impregnating the sample. (Recovery by hydraulic sweep is around 40 to 50% for most laboratory samples, and 15 to 20% in general for a deposit).

If a unidirectional potential difference is applied between the two ends of the sample for a certain time, additional oil recovery is obtained at the during a production start-up simultaneous or consecutive to the application of the electric field.

The amount of additional oil recovered is independent of the direction of production relative to the direction of application of the electric field, which makes it possible to suppose that the electric field has the effect of promoting the ability to move the oil which can then be moved in any direction. The electric field would therefore act not to entrain the petroleum towards the cathode or towards the anode. but to break certain bonds which retain the oil in the formation such as capillary forces or surface tensions and, once these bonds broken, the oil would be made more mobile.

Tests have been conducted to determine on samples how the energy balance varied, that is to say the amount of additional oil that can be recovered as a function of the time of application of a determined electric field and therefore of the electrical power consumed.

These tests have shown that the energy balance remains substantially stable during a determined period of application of a determined electric field, which varies from a few days to a few months, depending on the nature of the formation and the value of the electric field. The stability of the balance means that the quantity of additional oil that can be recovered varies in the same direction as the duration of application of the field and that the balance, that is to say the ratio between the quantity of oil recovered and the electrical energy expended remains substantially constant for periods of application of the current not exceeding an upper limit. On the other hand, for longer periods of application of the electric field, the balance sheet decreases and, whatever the relative cost of oil and electric energy, there is a threshold beyond which the economic balance sheet becomes unfavorable.

These tests show that it is therefore important, in the case of each deposit, to determine beforehand the maximum duration of application of the electric field, to avoid continuing to spend electrical energy for a result lower than the benefit. that we take out of it, and the optimum duration for which the balance sheet is the best.

It was natural to consider that when we had applied an electric field during a first period at the end of which the recovery of additional oil had become below an economic threshold, this meant that all the oil recoverable by this process was exhausted and that it was useless to start again and it is what has been done in the electrical processes known to date.

However the tests on sample showed this surprising result that if one applied again an electric field after a period of interruption of the current, having a duration ranging between a few days and a few months, according to the nature of the grounds, one obtained a new additional oil recovery with an economic balance which remains favorable for a new period of application of the electric field.

It is thus possible to repeat the cycles of application of an electric field comprising successive periods of application of a unidirectional electric field, having durations determined of the order of a few days to a few months depending on the nature of the deposit, which periods of electrical drainage are separated by periods of electrical rest having a duration of the order of a few days to a few months depending on the nature of the terrain.

The explanation of the phenomenon is poorly understood.

We can compare this phenomenon to a cycle of polarizations and depolarizations.

The tests carried out on samples have shown that the recovery of additional petroleum is sensitive, even in the case where the electric field is weak of the order of 0.1 V / m. Of course, one can use higher or lower potential gradients. A weak field applied for a long time gives a result equivalent to a stronger field, for example 1 V / m, applied for a shorter time.

.The quantity of oil recovered is therefore appreciably proportional to the electric energy supplied as long as the duration of application of the field remains less than a determined duration.

It is therefore advantageous to use a weak field in order to minimize the losses due to heating by the Joule effect.

The extraction of additional oil does not necessarily take place at the same time as the application of the electric field. It can continue after the difference in potential.

Tests have been carried out on samples by applying an alternating electric field having a frequency between a few Hz and a few KHz.

These tests have shown that, under the same conditions, one did not obtain with an alternating field an effect for improving the recovery of oil as good as that which is obtained by applying a unidirectional, continuous or pulsed electric field. .

The present invention relates to methods of enhanced recovery of petroleum contained in a reservoir formation from a well which extends to said formation.

The methods according to the invention are of the known type in which the oil is displaced by applying a unidirectional potential difference, continuous or pulsed, between two electrodes, at least one of which is placed in said well.

The tests on samples lead to proposing a process of this type in which the unidirectional potential difference is applied during periods having a duration of between a few days and a few months, which periods are separated by periods of electrical rest having a duration between a few days and a few months and these cycles of alternating periods are repeated until the quantity of oil recovered falls below a break-even point.

Preferably, the duration of the periods of application of the unidirectional electric field is substantially equal to the duration of the rest periods.

Advantageously, the lower electrode can be moved away, in order to obtain a better distribution of the electric field, within the limits of usable power.

Advantageously, it may be necessary to vary in a discontinuous manner the vertical spacing of the two electrodes as well as the power injected between these two electrodes, during the exploitation of the deposit or the application of the current. For this purpose, it will be possible, during the preliminary equipment, to deepen the well taking into account the maximum depth of the low electrode.

Advantageously, before each period of application of the potential difference, it is possible to inject into the reservoir formation, around the well, a lens of a liquid of high resistivity, for example pure water.

• - une électrode de grande longueur qui est disposée dans le fond dudit puits dans un milieu électroconducteur qui établit un bon contact électrique entre ladite électrode et le terrain encaissant situé au-dessous de ladite formation réservoir;
• - une source de tension unidirectionnelle située en surface;
• - des moyens conducteurs électriques pour relier la borne positive de ladite source à ladite électrode, lesquels moyens conducteurs sont isolés de ladite formation et dudit cuvelage;
• - des moyens pour connecter la borne négative de ladite source à l'extrémité supérieure dudit cuvelage;
• - et des moyens pour appliquer la tension unidirectionnelle délivrée par ladite source, de façon continue ou pulsée avec une fréquence de l'ordre de plusieurs pulsations à la minute, pendant des périodes déterminées de plusieurs jours à plusieurs mois alternant avec des périodes de repos de plusieurs jours à plusieurs mois.

A device according to the invention for recovering an additional quantity of oil from a well which is equipped with a metal casing extending at least up to the roof of a reservoir formation and traversing it or not, comprises:

• - a very long electrode which is disposed in the bottom of said well in an electroconductive medium which establishes good electrical contact between said electrode and the host soil situated below said reservoir formation;
• - a unidirectional voltage source located on the surface;
• - electrical conductive means for connecting the positive terminal of said source to said electrode, which conductive means are isolated from said formation and from said casing;
• - means for connecting the negative terminal of said source to the upper end of said casing;
• - And means for applying the unidirectional voltage delivered by said source, continuously or pulsed with a frequency of the order of several pulses per minute, for determined periods of several days to several months alternating with rest periods of several days to several months.

The vertical distance between the electrode and the lower end of the conductive casing is at least 40 meters.

The methods and devices according to the invention make it possible to recover an additional production of oil from a well by spending electrical energy whose cost is lower than the economic value of the additional oil recovered. Compared to known electro-osmosis methods which apply a unidirectional voltage without interruption, until the oil recoverable by this method is used up, the method and the device according to the invention make it possible to increase the total quantity of oil that the it is possible to extract from a well by alternating periods of application of a unidirectional voltage with periods of electrical rest during which the production can continue.

The methods and devices according to the invention make it possible to separate the phases of application of the electrical voltage and the phases of production. They make it possible to increase the production of a well by placing the two electrodes in the same well while influencing a large volume of the formation located around the well.

One of the advantages of the methods and devices according to the invention, in which the two electrodes are placed in the same well, resides in the fact that they make it possible to choose the distance between electrodes, which is not the case when the electrodes are placed in two different wells and to vary this distance.

The following description refers to the appended drawings which represent, without any limiting character, devices for the implementation of the methods according to the invention.

Figures 1 to 4 are vertical sections of a production well showing examples in which the two electrodes are located in the same well.

Figure 5 is a vertical section showing an example of implementation of the method using two wells.

Figure 1 shows a well 1 which is drilled in the ground and which extends to a reservoir formation 2 containing oil and water. Well 1 is, for example, a producing well. A seal 11 divides the formation into two parts, an upper part 2a which corresponds to the oil-producing layers at a determined time in the exploitation of the deposit and a lower part 2b which corresponds to layers containing water, generally salty.

The well 1 is equipped with a metal casing 3 (casing) which ends with a shoe 3a located at the roof of the reservoir formation. In this example it is a so-called overdraft well. The well 1 is normally fitted with production tubing 4, the upper end of which is connected to a production pipe 5. The casing 4 can be equipped, as shown in FIG. 1, with a pump comprising a piston 6 driven by a rod 7 which passes through a sealing gland 8.

In the example shown, the well comprises a suspended insulating column 25 having a diameter less than that of the casing, the upper end of which is attached to a sealing gasket 21 fixed to the casing. This suspended column is made of an insulating material such as a resin reinforced with fiberglass. It crosses formation 2 and has perforations 25a at the crossing of reservoir formation 2a.

The lower end of the production tubing is located above the upper end of the hanging column 25 and above the roof of the reservoir formation.

The lower end of the production casing is fitted with a non-return valve 30. A seal 31 is arranged between the valve 30 and the upper end of the suspended column which has an enlargement 32.

Production hole 1 normally descends to the wall of reservoir formation 2.

For an implementation of the method according to the invention in which the two electrodes are placed in the same well, it is advantageous to extend the drilling below the wall of the reservoir formation over a height of between 20 m and 500 m depending cases.

Figure 1 shows a well that extends below the wall. The suspended column 25 also extends below the wall and the lower end of the column 25 is located at a distance of the order of 10 m above the bottom of the well.

An electrode 9 is placed at the bottom of the well in an electroconductive medium 9a which establishes good electrical contact between the electrode 9 and the surrounding ground. The electroconductive medium can be salt water coming from layers 2b of the formation. It can also be a non-electrolyzable liquid containing conductive particles, or a metallic powder or a metallic alloy which melts at low temperature, for example an alloy of bismuth, lead, tin, cadmium and antimony.

The electrode 9 has a great length, greater than several meters, so that the current density at the start of the electrode is reduced, which makes it possible to avoid a rise in temperature by the Joule effect and to reduce corrosion.

In the embodiment according to FIG. 1, the electrode 9 is placed at the lower end of a conductive rod 33 which is coated with an insulating sheath 10, which insulates the rod over its entire length, at the exception of the lower end which is used. electrode. Preferably, the lower end is removable so. to facilitate the replacement of the electrode in the event of its wear. A gasket 34 is placed between the insulating coating 10 and the suspended column 25, at a level equal to or less than that of the gasket 11.

The conductive rod 33 is fixed to the lower end of the valve body 30 by a fixing which ensures continuity electric and which has perforations for the passage of oil. The production tubing 4 serves as a conductor for energizing the electrode 9.

The upper end of the casing 4 is connected to a terminal of a source 22 of unidirectional, direct or pulsed voltage, and the upper end of the casing 3 is connected to the other terminal of the source 22. Preferably, the casing is connected to the positive polarity terminal and the electrode 9 is an anode.

This choice of polarities allows the phenomena of electrolytic corrosion which may occur at the anode to take place on the electrode which is fixed to the casing and which is easy to remove from the well and to replace.

The casing 3 is normally a conductive metallic casing which is in electrical contact with the surrounding land.

The producer tubing 4 which serves as a conductor is isolated from the conductive casing by an insulating column 14- which delimits, with the casing 3 and with the casing 4, two coaxial annular conduits 15 and 16 which communicate by their lower end.

An insulating liquid, for example a mineral oil, can be circulated between the casing and the casing in order to extract the calories due to the heating of the casing by the Joule effect. For example, the upper ends of the annular conduits 15 and 16 are connected respectively to pipes 17 and 18 which constitute the flow and return of an insulating oil circuit which is discharged by a pump 19 and which returns to a tank oil 20.

The voltage delivered by the source 22. is a high unidirectional voltage, for example between 200 V and several thousand volts.

The unidirectional voltage can be applied continuously or pulsed with frequencies on the order of several pulses per minute.

The unidirectional voltage is applied during a first period of long duration, from several days to several months, after which the application of electric current is stopped for a second period of long duration, that is to say a period of several days to several months having for example the same duration as the previous period. During this second period, oil extraction can continue.

New cycles of application of the current and of electrical rest are then repeated until the production of oil falls below a threshold where the economic value of the oil extracted during a cycle is less than the cost of the electrical energy expended during this same cycle.

The arrangement according to FIG. 1 makes it possible to have two electrodes in the same well with a distance between electrodes which can be large, for example of the order of several hundred meters, so that the volume of the formation traversed by the lines of strength of the electric field going from the anode to the cathode is important.

The distance between the anode and the lower end of the casing serving as a cathode is greater than 40 meters in order to influence a sufficient formation volume.

It will be noted that in this example, neither the anode nor the cathode are in the reservoir formation, unlike the known methods in which it is sought to channel the oil towards one of the electrodes, generally towards the cathode, which is placed in training. The electric field is used to mobilize the oil by modifying the capillary attraction forces, after which the oil travels to the well by the usual methods, for example by hydraulic sweeping.

FIG. 1 shows a lens of a slightly conductive liquid 23 which can be injected into the reservoir formation 2 around the well from the casing 4 before the application of the electrical voltage. The liquid 23 is a liquid with high resistivity, for example pure water. It forms around the well, between the electrode and the lower end of the casing 3, an annular zone of high resistivity, so that it allows greater penetration of the electric current into the formation 2a hence the possibility of improve recovery in a larger volume of formation around a single well.

FIG. 2 represents an application of the invention in the case of a producing well 1, the equipment of which comprises a perforated casing 3 which passes through the reservoir formation 2 and whose shoe 3a is located on the wall of the formation. The homologous parts are represented by the same references in the figures 1 and 2.

In this example, the well is also deepened beyond the wall of the formation to a height of between 20 m and 500 m and it is equipped with a suspended insulating column 25 whose upper end is hooked to the lining 11 which separates the producing layers 2a of the underlying layers 2b.

The conductive casing 3 is over-perforated in a part which is located below the perforations and in the layer 2b underlying the producing layer 2a. The over-drilling is filled with a ring of insulating material 13 which penetrates into the formation 2b.

The valve housing 30. is electrically insulated from the casing and the sealing gasket 31 is at the same time an insulating gasket. All the other parts of FIG. 2 are analogous to the homologous parts of FIG. 1.

As in the case of FIG. 1, the electrode 9 is an anode which is connected to the positive terminal of a unidirectional voltage source 22, the casing 3 being connected to the negative terminal of the source 22. The anode 9 and the lower end of the casing 3 located above the insulating ring 13 are located at a distance of at least 40 meters, which can reach several hundred meters. The direct voltage delivered by the source 22 is between 200 V. and 10,000 V. The resistance equivalent to the formation volume located between the two electrodes is of the order of a few ohms and the intensity of the current is between a few hundred and a few thousand amps.

FIG. 3 represents a vertical section of another example of application of the method according to the invention. This example represents a production well with a double column. A production column 4, and an electrode column which comprises a conductor 33 surrounded by an insulating sleeve 10.

The metal casing 3 crosses the reservoir formation 2, and has perforations 27 in the crossing thereof.

The well is drilled below the shoe 3a of the casing to a height of 40 m and more, depending on the applications.

In this example, the anode 9 can be moved by raising or lowering the electrical column, during operation of the method, thus modifying the spacing of the electrodes. The drilling below the shoe will take into account at the start the lowest position of the electrode.

FIG. 4 represents an assembly similar to that of FIG. 1. The only difference resides in the fact that the metal casing 3 comprises an insulating sleeve 13 intended to electrically insulate the lower part of the casing from the upper part.

As the casing of the producing wells is already in place and generally consists of metal tubes, the casing is sectioned over a height of several meters by over-drilling which is located at a distance of the order of one or several tens of meters at - above the roof of the formation.

After having carried out the over-drilling, the over-perforated space is filled with a ring 13 of an insulating material, for example a polymerizable resin which penetrates into the surrounding grounds. This solution makes it possible to transfer the lower end of the cathode above the insulating ring 13, that is to say several tens of meters above the roof of the reservoir formation.

Another solution, in the case where the well is not already equipped with a metal casing, consists in equipping it with a casing comprising, in its lower part, an insulating section of several tens of meters, for example a resin section reinforced with glass fibers.

FIG. 5 represents another example of application of the method according to the invention, in which the two electrodes are spaced horizontally by being placed in two wells 1 and 1a. The wells 1 and 1a can be producing wells, for example wells equipped with a perforated casing 3, which passes through the reservoir formation 2 and with production tubing 4.

In the example shown, two DC voltage sources 22a, 22b are used which deliver two different or equal unidirectional voltages.

The tubing 4 and the casing 3 of one of the wells, for example the well 1a, are connected in parallel on the negative terminal of a continuous voltage source 22a.

The casing 4 of the other well carries at its lower end an electrode 9 and it is connected to the positive terminals of the two sources 22a, 22b.

The mounting of the well 1 is essentially the same as one of the assemblies described in FIGS. 1, 2, 3.

In this application, the casing 3 of the well 1 is also connected to the negative terminal of the source 22b as shown in Figure 5, but this connection could be removed as well as the source 22b.

To improve the recovery of the oil contained in formation 2, one or more wells can be used as cathode and one or more wells as anode. This technique lends itself to production by sweeping by means of a pressurized fluid or by pumping, the electrode placed in each well being able to be constituted by the casing, the casing or by an electrode suspended from a cable.

The latter solution is an extension of the devices according to FIGS. 1, 2 or 3, in which the negative terminal of the source 22b is connected, not only to the casing of a first well containing the anode, but also to the casing of several others producing wells surrounding the first well, so that recovery is improved in all wells. All the wells, both those containing the anode and the cathodes, can be producers and the water injection wells for the hydraulic sweep are dissociated from the wells used for injecting the current.

The examples which have just been described have the following common characteristics.

The cathode consists of the upper part of the conductive metal casing which is the casing normally fitted to the well.

The anode is a very long anode which is preferably located below the reservoir formation, which may necessitate to resume drilling a well to deepen it. The position of this anode may, in certain cases, be modified, during the operation of the process, in order to vary the spacing of the electrodes, which makes it possible to vary the volume of formation subjected to a field. electric field and to vary the electric field. The voltage between electrodes can be varied at the same time as they are moved.

In the case where the two electrodes are in the same well, the distance between the anode and the lower end of the cathode is at least 40 meters and can reach several hundred meters, so that the lines of force of the electric field, include a large volume of formation.

According to a variant, the rod 33 which electrically connects the body of the valve 30 to the anode 9, can be replaced by a metal chain which is suspended from the valve body and which hangs down to the bottom of the well. The length of this chain is greater than the height separating the valve body from the bottom of the well, so that part of the chain is coiled at the bottom of the well and replaces the electroconductive medium 9a. This solution makes it easy to replace the chain or part of it worn out by electrolytic corrosion. The chain is threaded through a sleeve or an insulating sheath 10 with the exception of the coiled part which serves as an electrode.

Of course, the rod 33 can also be replaced by a conductive cable.

Claims (13)

1. Electric process for the enhanced recovery of petroleum contained in a reservoir formation (2), from a well (1) which extends to said formation, according to which a unidirectional, continuous or pulsed potential difference is applied , between two electrodes, at least one of which is placed in said well, characterized in that said difference in unidirectional potential is applied for periods having a duration of between a few days and a few months, which are separated by. electrical rest periods ranging from a few days to a few months and these cycles of alternating periods are repeated until the amount of oil recovered falls below a break-even point. 2: Method according to claim 1, characterized in that the duration of the periods of application of the unidirectional electric field is substantially equal to the duration of the rest periods. 3. Method according to any one of claims 1 and 2, characterized in that one applies the unidirectional potential difference between an anode placed below the reservoir formation and a cathode placed, in the same well, au- above the tank formation. 4. Method according to claim 4, characterized in that, before a period of application of the potential difference, is injected into the reservoir formation, around the well, a lens of a liquid having a high resistivity, for example pure water. 5. Method according to claim 3, characterized in that the vertical spacing of the two electrodes is varied, as well as the power injected between these two electrodes, during the exploitation of the deposit, or during the application of the current. 6. Electric device for enhanced recovery of petroleum contained in a reservoir formation (2), from a well (1), which is equipped with a metal casing (3) extending at least up to the roof of said reservoir formation (2), characterized in that it comprises: - a very long electrode (9), which is arranged in the bottom of said well (1) in an electroconductive medium which establishes good electrical contact between said electrode and the surrounding land located below said reservoir formation; - a unidirectional voltage source (22) located on the surface; - electrical conductive means for connecting the positive terminal of said source to said electrode, which conductive means are isolated from said formation and from said casing; - means for connecting the negative terminal of said source to the upper end of said casing; - And means for applying the unidirectional voltage delivered by said source, continuously, or pulsed with a frequency of the order of several pulses per minute, for determined periods of several days to several months alternating with periods of rest from several days to several months. 7. Device according to claim 6, characterized in that the vertical distance between said electrode (9) and the lower end of the conductive casing (3) is at least 40 meters. 8. Device according to any one of claims 6 and 7, characterized in that the lower end of said casing is electrically insulated, over a height of the order of one or more tens of meters, from the upper metallic part which serves as a cathode. 9. Device according to claim 8, characterized in that said metal casing (3) is sectioned over a height of several meters by an overbore located at a distance of about. of one or more tens of meters above the roof of the formation and the over-perforated space is filled with a ring of insulating resin (1-3). 10. Device according to any one of claims 6 to 8, characterized in that said well is deepened over a height of the order of 20 m to 500 m below said formation and said electrode (9) is placed in said deepening which is filled with an electroconductive medium (9a) in the bottom thereof or at an intermediate height between the bottom of the metal casing and the bottom. 11. Device according to any one of claims 6 to 9, comprising a production casing (3) whose lower end is equipped with a non-return valve (30), characterized in that said electrode (9) is placed at the end of a rod conductive (33) whose upper end is electrically connected to the lower end of said production casing whose upper end is connected to the positive terminal of said unidirectional voltage source, which rod is located in a deepening of the well beyond of the formation wall and is coated with an insulating sheath - (10) except for a length of several meters located at the lower end which serves. anode and which is immersed in an electroconductive medium (9a) which fills the bottom of the well. 12. Device according to any one of claims 6 to 9, characterized in that said electrode (9) consists of metallic chain / which is coiled at the bottom of said deepening and which is placed inside an insulating sheath except for the coiled lower end. 13. Device according to any one of claims 5 to 12, used in an operating field comprising several other wells, equipped with a casing, arranged around a first well, in the bottom of which is placed said electrode (9 ), characterized in that the casings of all the other wells are connected to the negative terminal of a second unidirectional voltage source, the positive terminal of which is connected to said electrode (9).

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