FR2508971A1 - Cathodic protection of deep water borehole lining against corrosion - by immersing rows of anodes fed with DC to successive depths in water in borehole - Google Patents

Abstract

The borehole extends downwards in the earth to a deep aquifer, and is lined by a pipe or duct. The lining, and also equipment immersed in the water such as a water collection pump and pipe, are protected from corrosion by several vertical rows of anodes, each row being connected to a conductor and immersed to successive depths in the borehole. Each conductor is connected to the positive pole of a d.c. power supply, the negative poles of which are connected to the pipe forming the borehole lining. Each power supply has a potentiometer so a specific current can be fed to each row of anodes. The anodes are pref. rods or tape made of Nb, Ta, Th or Cu, each with a very thin coating of Pt. Each anode consists esp. of a helical winding of Nb tape with a Pt coating 0.015 mm thick on its outer surface. Efficient protection of the lining of boreholes which are several kilometers deep, is achieved.

Description

 Method and devices for cathodic protection against corrosion of the casing and of the equipment of a water extraction borehole from a deep aquifer.

 The subject of the present invention is a method and devices for cathodic protection against corrosion of the casing and of equipment for a water extraction borehole from a deep aquifer.

 The boreholes intended to extract water from an aquifer located at great depth include a casing, most often made of black steel, the external and internal faces of which are subject to corrosion.

 It is imperative to protect the casing and the equipment located inside the borehole from corrosion. Indeed, drilling and placing the casing represent a significant investment which must last long enough to be amortized.

In addition, the deep layers of freshwater or low-salt water that may be suitable for watering are generally surmounted by soils containing more salty aquifers, which are isolated from the borehole by the casing. If the latter were to be damaged by corrosion, the fresh water would mix with the more salty water and would become unfit for consumption or watering, hence the loss of the borehole which should be closed and the risk of loss of a water reserve.

 Finally, the corrosion of the casing causes the presence of iron in the water, which risks giving it a taste and a coloring making it unfit for consumption.

 We know the so-called cathodic protection process to protect against corrosion of pipes or other buried metallic structures. This process consists in burying in the ground electrodes of a metal more electropositive than the structure to be protected, so that these electrodes act as consumable anodes towards which the aggressive anions go while there is deposit of cations on the metal structure which serves as a cathode and which is thus protected from corrosion.

 Generally, masses of a more electropositive metal than iron are used as anodes, for example zinc masses. It is also possible to use electrodes buried in the ground and connected to the positive terminal of a DC voltage source, the negative terminal of which is connected to the structure to be protected.

 The objective of the present invention is to adapt the cathodic protection method to the particular problem of protecting the internal face of the casing over the entire submerged height thereof which is high, of the order of one or more thousands of meters, as well as the protection of equipment immersed in the water contained in the borehole.

 The protection of the external face of the casing does not present any special difficulty and it is obtained by the usual means, by burying in the ground one or more metallic masses which are maintained at a sufficiently positive potential with respect to the casing.

 On the other hand, the protection of the internal face of the casing presents particular difficulties. The anodes must be distributed over the entire height of the borehole and a sufficient current density must be ensured at the level of each anode or group of anodes to ensure good cathodic protection but not too high to avoid unnecessary power consumption . It is necessary to develop anodes which resist corrosion so that the device, which constitutes a significant investment, is durable.

 The anodes located in the borehole are placed at a variable distance from the casing which is generally telescopic and the resistance of the electrical circuit connecting the anodes to the casing varies with the resistivity of the liquid contained in the borehole. It is therefore necessary to be able to precisely adjust the potential positive of each anode or of each group of anodes as a function of the electrical characteristics of the liquid and of the surrounding lands and of the distance between anode and casing.

 The objectives according to the invention are achieved by means of a process for protecting the internal face of the casing of a borehole for extracting water from a deep aquifer and equipment immersed in the water of this borehole, according to which one descends into the water contained in the drilling several chains of superimposed anodes, each chain comprising a series of superimposed anodes which are connected on an anode conductor, which anode conductor is connected to the surface by a connecting conductor which is connected to the positive terminal of an adjustable DC voltage source, the negative terminal of which is connected to said casing and the voltage of each source is adjusted separately to obtain a determined current density at each chain of anodes .

 The anodes are rods or ribbons of niobium, thorium, tantalum or copper, coated with a very thin platinum plating.

 According to a preferred embodiment, the anodes are each composed of a niobium ribbon, having a thickness of a few tenths of a millimeter, which is coated with a platinum plating having a thickness of the order of 15 microns and is prefabricated in the factory anode chains by helically winding sections of this strip around the insulating sheath of a conductive cable, the platen plating being directed towards the outside, and by connecting one end of each section to said conductor; the conductors carrying the anodes are then wound on drums which are transported to the drilling head; a carrier cable is lowered into the borehole to which one after the other hooks superimposed anode chains, the upper end of which is connected to a conducting cable. connection that is attached to the carrying cable; and connecting the upper end of each connecting conductor to the positive terminal of an adjustable DC voltage source.

 A cathodic protection device against corrosion of the internal face of the casing of a borehole to extract water from an aquifer and equipment immersed in the borehole water comprises several superimposed anode chains immersed in the water contained in the borehole, each chain of anodes comprising a plurality of superimposed anodes, which are connected in parallel on the same anode conductor, the upper end of which is connected on a conductor for connection with the surface and the device furthermore comprises a plurality of DC voltage sources, the negative terminals of which are connected to said casing and to the equipment and the positive terminals of which are connected respectively to the upper ends of said connecting conductors by means of potentiometric devices making it possible to separately adjust the positive potential of each anode channel.

 The invention results in the protection against corrosion of a black steel casing of a water extraction borehole from an aquifer located at great depth, of the order of two hundred meters or more, and the protection of equipment immersed in well water.

 The arrangement of the anodes according to several anode chains, each having a separate and adjustable DC voltage supply, makes it possible to separately adjust the positive potential of each anode chain to take account of line voltage drops and electrical characteristics. specific to each vertical section of the well and the surrounding land, in order to obtain an optimum current density for good cathodic protection without excessive energy expenditure and a potential difference, between anodes and casing, slightly greater than the polarization potential and corrosion.

 The anodes composed of a niobium ribbon coated with a very thin platinum plating, make it possible to produce anode chains which resist corrosion well and which have a very long service life. In addition, these flexible anodes wound helically around the anode cable can be manufactured in the factory and wound on a reel which is transported to the place of drilling, which facilitates the installation of the device in the drilling.

 The arrangement of the anodes superimposed at a short distance, of the order of one to two meters, makes it possible to obtain an anode system which uniformly protects the entire internal surface of the casing. This result requires the provision of an almost continuous anode system over the entire height of the well because the protective perimeter or useful range of each anode is limited by the electrolytic coupling with the casing steel which interests a relatively small casing surface by following the small diameter of it.

 In addition, the contact resistance between the anode and the liquid leads to the use of anodes whose surface must not be less than a minimum and this objective is achieved thanks to the individual anode chains which are connected in parallel on the same conductor carrying a plurality of superimposed anodes.

 A numerical example makes it possible to specify the order of magnitude of the electrical energy expended. A 2000 m deep submerged borehole includes every two meters an anode formed by a platinum-coated niomium ribbon having a width of 5 mm and a developed length of I meter, or a plating surface of 0.005 m2. The current density used is 100 to 150 mA / m2. The DC voltage is around 20 volts. The electrical power consumed is 2 to 5 Kw, that is to say a very low power.

 The cathodic protection devices according to the invention make it possible to eliminate the risks of corrosion of the casing and to ensure a great longevity of the boreholes for drawing water from a deep sheet and an operation without the very serious risks due to a rupture of the casing. .

 The following description refers to the appended drawings which represent, without any limiting nature, exemplary embodiments of devices according to the invention.

 Figure 1 is a vertical section of a borehole used to extract water from an aquifer.

 FIG. 2 is a schematic half-section of the well according to FIG. 1.

 FIG. 3 is a vertical section of a section of device and FIG. 4 a cross section of FIG. 3.

 Figure 5 is a vertical section of a first embodiment of an anode.

 Figures 6 and 7 are a partial vertical section and a horizontal section of a second embodiment.

 Figures 8 and 9 are a vertical and cross section of an anode used in a device according to Figures 6 and 7.

 FIG. I represents a borehole 1, of axis z zl, intended for extracting water from an aquifer 2 which is located at great depth, greater than 200 m and which can reach 1000 meters or more.

 The borehole crosses layers of land which generally include more or less salt water, which must not penetrate the borehole. For this reason, it is equipped with a casing 3 (casing) which is constituted by black steel tubes, which have decreasing diameters. The section 3a of the casing, which crosses the aquifer 2, is perforated (strainer).

 FIG. 1 represents an example of a well equipped with a production casing 4 carrying at the lower end a submerged pump 5 equipped with a strainer 6. As a variant, the aquifer formation can be artesian.

 Figure 1 shows a well that is filled with water to the top. In the case where the water only rises in one part of the well, the anodes are placed in the submerged part only.

 The casing is exposed to risks of generalized corrosion by chemical attack due to the anions contained in the strip, in particular from chlorides and sulphates. In addition, the casing which is in contact with the surrounding land is exposed to risks of corrosion by the effect of geological piles due to the land of different nature crossed by the drilling. Finally, the casing is attacked by electrochemical corrosion due to the more or less regular structure of the metal, to local variations in flow, to local concentrations of CO which create on the internal surface anodic and cathodic ranges, permanent or temporary, generating electrical currents and corrosion.

 It is absolutely imperative to protect the casing from corrosion. Indeed, deep drilling represents a significant investment and it must be kept over time to amortize the expense. If it happened that the casing was pierced in line with a more salty aquifer, this would cause the water contained in the aquifer 2 to mix with the salt water, which would make it unsuitable for the uses for which it is intended. for drinking or watering where a very significant loss of water reserves.

In addition, corrosion of the casing can lead to the presence of iron in the water which colors it or which gives it a bad taste making it unfit for consumption.

 The problem to be solved is to effectively protect the casing and the pumping equipment against all forms of corrosion by ensuring both the internal and external protection of the casing in an adjustable manner to take account of the parameters specific to each drilling and which vary with the composition. of water and the nature of the surrounding land.

 Cathodic protection has already been used to protect the external surface of the casing of a well or of a borehole against corrosion by burying in the soil around the well anodes brought to a positive potential with respect to the potential of the casing.

 The subject of the invention is cathodic protection devices which at the same time provide protection against corrosion of the submerged internal surface of the casing and of the equipment (pump, strainer, casing) immersed in the borehole water.

FIG. 1 shows a cathodic protection device according to the invention which comprises, several conductors carried by a carrying cable 7, which descends in the borehole to the bottom thereof. Each of the conductors feeds an anode chain and is supplied with direct current by a separate rectifier 8l ... 8n or by any other source of direct current. The
n negative terminal of the rectifiers is connected to the top of the casing 3 and there is electrical continuity between the different sections of the casing.

 The lower end of the cable 7 includes a ballast 7a to prevent the cable from moving.

 In addition, the device comprises an anode 9 which is buried in the ground and which is constituted for example by recovery tubes or profiles made of mild steel which are brought to a positive potential with respect to the casing by a rectifier 9a or by any other direct current source.

 Figure 2 is a diagram in which there is shown, in an exploded form, the cable 7 composed of six conductors 71 each carrying at the lower end a chain of anodes 11. The anode channels are superimposed and immersed in water located in the borehole.

 FIG. 2 shows a potentiometric device 101, 102 ... 10, associated with each rectifier.

not
Figure 3 is a partial longitudinal section showing a first embodiment and arrangement of the anodes located inside the well.

 The reference 12 represents a carrying cable which is for example a synthetic fiber cable extending over the entire height of the well.

 The references 71 72 ... 7n represent the electrical cables carrying the anode chains. Each anode chain comprises, at the lower end, a first section having a length of between 50 and 150 m carrying a series of superimposed anodes 11 having an individual height of 0.50 m for example.

These anodes are distributed for example at the rate of one anode every two meters, along an anode cable 13 having for example
2 ple a section of 25 mm, which is connected by a junction box
2 waterproof whose section varies between 10 and 55 mm depending on the length.

The lower end of each anode cable has a cable head 15 which serves as ballasting.

 Figure 4 is a cross section along IV-IV of Figure 3. This figure shows the carrying cable 12 which is composed of a central core A of synthetic fibers, a first envelope B of polyethylene which protects the cable central contact of water and an external sheath C in polyvinyl chloride which effectively protects the cable jacket against nascent chlorine.

 Around the carrying cable are arranged the electrical connection cables 14 and the anode cables 13 carrying the anodes 11.

 The connection cables and the anode cables include a central core D which is a steel carrying cable, copper conductors E, a sheath F of crosslinked polyethylene which is an electrical insulating sheath resistant to water temperature and an external sheath G made of polyvinyl chloride for mechanical protection against impact and protection against nascent chlorine.

 All the cables are connected together by plastic collars 16. The central core D of steel cable of the electric cables makes them self-supporting as a safety in the event of a break in the carrying cable 12.

 FIG. 5 represents a section of anode cable 13 carrying an anode 11. The anode 11 is a niobium strip having a width of 5 mm and a thickness of 0.3 mm. This tape is coated on the outside with a platinum plating having a thickness of the order of 15 microns.

 As can be seen in FIG. 5, the ribbon making up each anode is wound in a helix around the outer sheath of the anode cable.

 Each anode has an unwound length of around one meter.

It is wound on a sheath whose external diameter is of the order of 12.5 mm with a pitch of 19 mm and occupies a height of the order of 50 cm.

 One end of the anode, for example the upper end, is connected by a ligature 17 to the copper conductors 13e of the anode cable, which are exposed by cutting the insulating sheaths 13g and 13f. The connection is placed inside a plastic box 18, which is filled with insulating resin 19. The other end of the anode is enclosed inside a junction box 18, also filled with insulating resin 19.

it is not necessary to connect the two ends of each anode, which saves connection work.

 Each anode 11 is mechanically protected against impact during the descent by a cage formed by a metallic wire mesh or by a metal wire 20 coated with a plastic sheath, for example a sheath made of polyvinyl chloride or polyamide. The wire 20 is placed at a certain distance from the anode so that it does not come into contact with the latter. Wire 20 goes from one junction box to another.

 The flexible ribbon anodes 11 are wound on the anode cables in the factory and the cables carrying the anodes can be wound on a reel and delivered to the site ready to be lowered into the borehole. Each anode chain has a variable number of anodes, but limited to around 150 to take account of line voltage drops and current distribution. Each of the anode chains involves a specific section of casing. It is supplied by a connecting cable 14 whose section varies from 2 10 to 55 mm or more, depending on the depth at which the anode chain is located. Each anode chain is supplied separately, which allows the voltage to be individually adjusted to obtain the potential difference and the desired current density on the anodes.

 The choice of the constituent material of the anodes was the subject of special research and laboratory tests.

 Niobium has good corrosion resistance. it is easy to work by metallurgical processes, in particular it is easy to manufacture in the form of very thin ribbons. It contracts a strong polarization, so that it is not necessary to coat platinum the internal face of the ribbons which is pressed against the 13g sheath. The polarization prevents the current from going out through this internal face which is therefore not corroded.

 The plate is perfectly resistant to corrosion even in an aggressive environment in the presence of hot and chlorinated water.

 Given its good resistance to corrosion, the longevity of anodes coated with platinum is very long, of several decades.

 Platinum is easy to apply in very thin plating by simple crushing. It supports high current densities with polarizations much lower than those of other metals. It is an excellent conductor of electricity.

 Despite the high price of platinum, its qualities have made it an anode coating choice. The quantity used is relatively small, of the order of 1.5 to 2 g per anode of 1 meter in length.

 it is specified that niobium could be replaced by thorium or tental, but niobium is preferable.

 An exemplary embodiment of copper rod anodes entirely covered with platinum will also be described below.

 Each chain of anodes comprises an independent supply of direct electric current constituted for example by a rectifier mounted in series with a potentiometric device allowing an independent adjustment of the voltage applied to each anode cable.

 The same rectifier can supply several anode chains in parallel, each comprising an independent potentiometer. The voltage applied at the head of each connecting conductor 14 is limited to 48 V for safety reasons.

 The voltage is adjusted to obtain a specific current density on the metal surfaces to be protected, depending on the nature of the water and the surrounding land.

 The voltage to be applied is calculated and then adjusted experimentally by playing on the potentiometers.

 The anode cables equipped with the anodes arrive on a reel on the site and the installation in the drilling is very fast.

 The carrying cable carries a ballast at its end and it is kept suspended from a pulley located above the borehole. It is gradually lowered by means of a motorized winch. During the descent, a first chain of anodes is attached to the carrying cable, which unwinds from a reel.

 When the desired length is unwound, the end of this first chain is connected to a connecting cable and the connecting cable and a second chain of anodes are continued to descend, and so on. The number of cables joined by collars to the carrying cable progressively goes from 1 to n. The carrying cable is then fixed at the head of the borehole and the electrical connection cables are each connected to a potentiometer which is itself connected to a rectifier whose negative terminal is connected to the head of the casing 3.

 The external protection of the casing is obtained by burying in the ground at a drilling distance of several hundred meters, one or more masses of scrap 9 which are connected to the positive terminal of a rectifier whose negative terminal is connected to the head. of the casing 3. The masses 9 are preferably coated with a good conductive product, for example bentonite moistened with a liquid containing conductive ions if necessary.

 The masses 9 are watered periodically or permanently to maintain the humidity of the soil and a good degree of electrical conduction.

 Metal bars can also be used as the external anode, for example ferro-silicon bars planted in the ground several hundred meters from the drilling head.

 FIG. 6 schematically represents, in vertical section, a section of a second cathodic protection device according to the invention.

 Figure 7 is a cross section along Vil-Vil of Figure 6.

 These two figures represent a device which is lowered into a deep borehole and which comprises a central carrying cable 21, for example a cable made of synthetic fibers of the polyamide type or equivalent. Around this cable, are arranged four electric cables 22 which comprise a conductor 22a, insulated by a sheath 22b.

 As a variant, the cables 22 can be of the same type as the cables 13 and 14 in FIG. 4.

 The carrying cable 21 and the conductive cabals 22 are placed inside a first cylindrical sheath 24 made of plastic.

 The anodes 25 are placed vertically outside the sheath 24 to which they are fixed by collars 26 made of plastic. The device further comprises a cable 27 which is diametrically opposite the anodes to restore symmetry. This cable 27 comprises a core of synthetic fibers surrounded by a plastic sheath.

 FIG. 8 is a longitudinal section of an anode 25 and FIG. 9 is a cross section of FIG. 8.

 The anode 25 is composed of a copper rod 25a, for example a rod 80 cm long and 1.6 cm in diameter, which is entirely coated with a very thin platinum plating 25b, for example 20 microns thick.

 We see in Figure 6 that the anodes are connected in pairs on the same conductor 224. Figure 8 shows for example the lower anode of a pair.

 The lower end is engaged in a plastic box 28 filled with insulating resin 29.

 The upper end of the rod 25a is brazed on a copper conductor 30 which is connected to a flexible anode cable 31. The solder and the electrical connection are enclosed in a plastic junction boot 32 which is filled with insulating resin 33 .

 Each anode comprises a mechanical protection against shock and untimely contact which is constituted for example by rods 34 connecting together the two junction boots, which are separated from the anode by insulating rings 35 and around which is wound, in helix, a wire 36, which is a metal wire coated with plastic. The helically wound wire can be replaced by a plastic or plastic mesh.

 FIG. 6 shows the connection between two anodes and one of the cables 22.

 An opening is made in the sheath 24 to access the cable 22 and a collar 37 is fixed thereon having a screw needle 37a for making electrical contact with the conductor 22a. The two flexible cables 31 are connected to the collar 37.

 Once the connection has been made, the opening made in the sheath and the cables 31 is wrapped in a junction box filled with insulating resin 38.

 The anodes according to FIGS. 6 to 9 are rigid anodes which must be branched to the conductors on the site as the cables are lowered into the borehole.

Claims (12)

 1. Method for cathodic protection against corrosion of the casing (3) of a borehole (1) for extracting water from a deep aquifer and of the equipment (4, 5, 6) immersed in the water of this drilling, characterized in that we descend into the water contained in the drilling several superimposed anode chains (710 72 ... 7) each chain comprising a series of superimposed anodes (11) which are connected on a anode conductor (13), which anode conductor is connected to the surface by a connecting conductor (14) which is connected to the positive terminal of an adjustable DC voltage source (81 ... 8n), of which the negative terminal is connected to said casing and the voltage of each of the sources is adjusted separately to obtain, at each anode chain, a determined current density.  2. Method according to claim 1, characterized in that said anodes are rods or ribbons of niobium, tantalum, thorium or copper, which are coated with a very thin platinum plating.  3. Method according to claim 2, characterized in that:  - Anode chains are prefabricated in the factory comprising equidistant anodes (11) composed of a niobium ribbon, having a thickness of a few tenths of a millimeter, coated on one side with a platinum plating, having a thickness of l '' order of 15 microns, by helically winding sections of this tape around the insulating sheath (13g) of a conductive cable (13e), the platinum plating being directed outwards and each section being connected to the conductor by one of its ends;  - Said conductors carrying the anodes are wound on drums which are transported to the head of a borehole;  - A carrier cable (12) is lowered into the borehole to which one after the other is hooked up on superimposed anode chains (13) the upper end of which is connected to a conductive connecting cable (14) which is hooked to the carrying cable;  - And the upper end of each connecting conductor (14) is connected to the positive terminal of an adjustable DC voltage source (81 ... 8n).  4. Device for cathodic protection against corrosion of the internal face of the casing (3) of a borehole (1) for extracting water from an aquifer (2) and submerged equipment (4, 5, 6) in the water of this borehole, characterized in that it comprises several superimposed anode chains (71 72 ... 7), immersed in the water contained in the borehole, each chain of anodes comprising a plurality of 'anodes (11) superimposed, which are connected on the same anode conductor (13), the upper end of which is connected on a conductor (14) for connection with the surface and the device also comprises a plurality of direct voltage sources (81, 82 ... 8n), the negative terminals of which are connected to said casing and to the equipment and the positive terminals of which are connected respectively to the upper ends of said connecting conductors (14) by means of devices potentiometers (101, 102 ... 10) allowing the positive potential of each chain of anodes.  5. Device according to claim 4, characterized in that the anodes are niobium or tantalum ribbons having a thickness of a few tenths of a millimeter and a width of a few millimeters, coated with a platinum plating having a thickness of the order from 10 to 20 microns.  6. Device according to claim 5, characterized in that one of the faces of said ribbon does not include platinum plating.  7. Device according to claim 6, characterized in that each anode is constituted by a strip (11) having a length of the order of one meter, which is wound in a helix around the insulating sheath (13g) of a conductor cable (13), the platinum plating being directed towards the outside and one end of said tape is connected to the conductor (13e).  8. Device according to claim 7, characterized in that the two ends of said strip are embedded in an insulating resin (19) placed inside a junction box.  9. Device according to any one of claims 7 and 8, characterized in that each anode is surrounded by a mechanical protection cage formed by a wire (20) wound in a helix or by a mesh.  10. Device according to any one of claims 4 to 9, characterized in that it comprises a central support cable made of synthetic fibers surrounded by insulating sheaths and conductive cables (13, 14) comprising a support core (13d) surrounded by conductors (13e) and insulating sheaths (13f, 13g), which conductive cables are held together by plastic collars (16).  11. Device according to claim 4, characterized in that the anodes are copper rods, coated externally with a thin platinum plating having a thickness of the order of 10 to 20 microns.  12. Device according to claim 11 characterized in that it comprises a central support cable (21) made of synthetic fibers and a plurality of conductive cables (22) composed of a central conductor (22a), wrapped in an insulating sheath (22b ), which carrier and conductor cables are wrapped in an insulating sheath (24) and it further comprises superimposed anodes (25) which are arranged outside said sheath and which are fixed to the latter by clamps made of plastic (26).