EP2633153B1 - Process for the in situ extraction of bitumen or ultraheavy oil from oil sand deposits as reservoir - Google Patents

Description

The present invention relates to a method of "in situ" production of bitumen or heavy oil from oil sands deposits as a reservoir. The reservoir is inductively heated via at least one electrical current-carrying conductor to achieve a reduction in the viscosity of the bitumen or heavy oil. A fluid is introduced into the reservoir via the perforation in the fluid guide via at least one perforated fluid guide which surrounds or comprises the at least one conductor at least in sections.

For the extraction of hydrocarbons such as heavy oils or bitumen from storage sites with oil sands or oil shale deposits, hereinafter referred to as reservoir, open pit methods or "in-situ" methods can be used. CA 2 304 938 A1 discloses an in-situ method whereby a slotted conveyor tube extending through a reservoir is inductively heated. A solvent is passed through the production tubing, vaporized by the heat, and injected into the reservoir for oil mobilization.

Another "in situ" method is the SAGD (Steam Assisted Gravity Drainage) method. In this case, water vapor is introduced through a tube under high pressure into the soil through a pipe running horizontally within the reservoir. The steam heats the heavy oil or bitumen in the reservoir, making it flowable. The heated, flowable heavy oil or bitumen seeps over gravity to a second, for example, about 5 m deeper pipe through which it is pumped or promoted. Alternatively or supportively, the reservoir can be heated inductively, for example by an insulated, current-carrying conductor loop, which induces currents in the environment of the reservoir in its environment. The induced currents are mainly carried by the ionic conductivity in liquids. The amount of magnetic flux density around the conductor of the conductor loop decreases approximately inversely proportional to the distance to the conductor. With homogeneous electrical conductivity of the surrounding soil this leads approximately a decrease in the heating power density around the conductor. The heating power density around the conductor is inversely proportional to the square of the distance from the conductor. Thus, the highest heat density occurs in the immediate vicinity of the insulated conductor. This initially leads to a strong warming of the soil in the immediate vicinity of the conductor, which also leads by heat conduction to a correspondingly high temperature T _{L of} the conductor itself. This happens even if the ohmic losses in the conductor itself are very small.

The conductor for inductive heating of the reservoir, which is also called inductor and eg from the DE 102007040605 B3 is known, consists of a number of materials. In particular, capacitive compensation dielectrics are used in the conductor to minimize electrical losses in the conductor itself. For insulating the electrically conductive material, in particular with respect to the surrounding soil, insulation material is used, which consists for example of PFA, PTFE and / or PEEK or includes this or contains. The dielectric and the insulating material are usually thermally stable up to a maximum of 150 ° C, even over long periods such as hours, days, months and years away.

In order to be able to reliably deliver heavy oil and / or bitumen for extended periods of time, with the assistance of inductive heating of the reservoir via at least one conductor loop with insulated electrical conductor, the temperature of the conductor must be below a critical temperature of e.g. 150 ° C are kept. Only in this way can it be ensured that the insulation and the dielectric are thermally stable over time and that the conductor is not damaged by high temperatures. This is difficult especially in view of the high electrical voltages of greater than 10 kV and heat outputs in the range of MW.

Object of the present invention is therefore to provide a method in which the temperature of a conductor for inductive heating of the soil of a reservoir does not exceed a critical value.

The stated object is achieved with respect to the method for the "in situ" promotion of bitumen or heavy oil from oil sands deposits as a reservoir with the features of claim 1.

Advantageous embodiments of the inventive method for "in situ" promotion of bitumen or heavy oil from oil sands deposits as a reservoir will be apparent from the associated dependent claims. In this case, the features of the main claim with features of the subclaims and features of the subclaims can be combined with each other.

The inventive method for "in situ" promotion of bitumen or heavy oil from oil sands reservoirs as a reservoir comprises that the reservoir is inductively heated via at least one electric current-carrying conductor to reduce the viscosity of the bitumen or heavy oil, and that at least one perforated fluid guide which at least partially surrounds or comprises the at least one conductor, a fluid is introduced into the reservoir via the perforation in the fluid guide. The fluid reduces electrical conductivity in the reservoir, at least in the vicinity of the fluid guide and / or the conductor. To include the conductors at least in sections, inter alia, is understood to mean that the conductor and the fluid guide itself are arranged adjacent to each other, and e.g. are surrounded by a common insulation against the soil, which has the same and / or the same perforation as the fluid guide or is at least partially permeable to fluids.

By reducing the electrical conductivity in the vicinity of the fluid guide, the current induced by the current-carrying conductor in the vicinity of the fluid guide or of the conductor is reduced. As a result, the inductively generated heat output in the vicinity of the conductor or the fluid guide is reduced and the temperature of the conductor or the fluid guide, in particular by heat conduction of inductively generated heat in the immediate vicinity, is limited.

As the fluid, water of low conductivity, as the conductivity of water in the reservoir, can be introduced into the reservoir. The amount of water introduced and / or its conductivity should be determined depending on the value at which the temperature T _{L is to be} limited, and in particular depending on the current / voltage used for induction through the conductor.

Alternatively or additionally, gas can be introduced into the reservoir as fluid. Air as a gas is particularly inexpensive and easy to use. However, carbon dioxide and / or nitrogen may also be used as the gas, or the gas may comprise carbon dioxide and / or nitrogen.

As a fluid, a solution of chemical substances in the reservoir can be introduced, the chemical substances react to a sparingly soluble salt in the reservoir and thereby lead to precipitation of ions in the reservoir. It is advantageous if a chemical analysis is carried out before introducing the solution into the reservoir. At least one fluid, in particular water, may be used from the reservoir to determine ions in the fluid withdrawn from the reservoir and to select the chemicals in the solution depending on the particular ions. Concentration determinations can also help to create the correct composition of the solution, with which the temperature of the direct environment of the conductor and thus of the conductor can be kept at a predetermined value or below a limit even with a certain energization of the conductors. The concentration and type of ions in the solution should cause the solution to precipitate ions in the reservoir, for example in the form of a sparingly soluble salt, and thus the total ion concentration of the freely mobile, charged and thus inductively through the current Ladder movable ions is reduced to a value which leads to a predetermined temperature T _L in its immediate vicinity with a given structure and energization of the conductor. By reducing the electrical conductivity in the reservoir, the inductive heating is reduced via the current-carrying conductor.

The temperature T in the direct or indirect environment of the conductor and / or the fluid guide can be limited to a maximum value, in particular to a value less than 150 ° C. In this case, a previously described fluid introduction into the reservoir or a combination of the types of fluid introduction described above can be used. The temperature may be limited to a maximum value at which components of a device for "in situ" production of bitumen or heavy oil from oil sands deposits as a reservoir, particularly insulation materials of the conductor, dielectrics between conductor components and / or materials of the fluid conduit, are temperature stable , At a temperature of less than 150 ° C, materials such as dielectrics and insulating materials, e.g. PFA, PTFE and / or PEEK are usually thermally stable and will not be thermally damaged over time. This avoids damage to the conductor caused by high temperatures in its direct environment by keeping the temperatures below a limit.

The fluid can reduce the electrical conductivity in the vicinity of the fluid guide, in particular in the region of 3 m around the fluid guide. This may be sufficient to reduce the heat transported through the environment by thermal conductivity to the conductor so as to prevent that the temperature T _{L of} the conductor does not exceed a critical limit for inductive heating of the conductor environment.

The electrical conductor may be of an alternating current with a current in the range of more than 100 A, in particular 270 A, and / or with a frequency in the range of 10 kHz to 100 kHz, in particular 75 kHz, are flowed through, whereby in particular the soil of the reservoir in the vicinity of the electrical conductor is heated by induced currents in the ground. In this case, a heating power in the range of several MW can be generated at voltages across the electrical conductor in the range of greater than 10 KV. Other values are also possible, in particular depending on the embodiment of the conductor, the nature of the soil, the heavy oil or bitumen to be transported and other parameters involved in oil production via inductive heating.

Preferred embodiments of the invention with advantageous developments according to the features of the dependent claims are explained in more detail with reference to the figures, but without being limited thereto.

Figur 1

ein Schnitt durch ein Ölsand-Reservoir 100 mit Injektions- 101 und Förderrohr 102,

Figur 2

ein perspektivischer Ausschnitt aus einem ÖlsandReservoir 1 mit einer horizontal im Reservoir verlaufenden elektrischen Leiterschleife 2,

Figur 3

ein perforierter, rohrförmiger Leiter 3 mit integrierten Kondensatoren und einer Vorrichtung zur Elektrolyteinbringung.

It is shown in the figures:

FIG. 1

a section through an oil sand reservoir 100 with injection 101 and delivery tube 102,

FIG. 2

a perspective section of an oil sand reservoir 1 with a horizontally running in the reservoir electrical conductor loop 2,

FIG. 3

a perforated tubular conductor 3 with integrated capacitors and an electrolyte injection device.

In the Figures 1 and 2 is a designated as a reservoir oil sands deposit 100 shown, wherein for the further considerations always a cuboid unit 1 with the length l, the width w and the height h is taken out. The length l may for example be up to some 500 m, the width w 60 to 100 m and the height h about 20 to 100 m. It has to be taken into account that starting from the earth's surface E there can be an overburden of thickness s up to 500 m.

When implementing the SAGD method is according to FIG. 1 In a known manner in the oil sands reservoir 100 of the deposit, an injection pipe 101 for steam or water / steam mixture and a conveyor pipe 102 for the liquefied bitumen or oil present.

In FIG. 2 a known arrangement for inductive heating is shown. This can be formed by a long, ie some 100 m to 1.5 km, laid in the ground conductor loop 10 to 20, wherein the forward conductor 10 and return conductor 20 side by side, ie at the same depth, are guided and at the end via an element 15 within or partially outside the reservoir 100 are interconnected. Initially, the conductors 10 and 20 are led down vertically or at a shallow angle and are powered by an RF generator 60 which may be housed in an external housing. In particular, the conductors 10 and 20 run at the same depth next to each other, but possibly also one above the other.

Typical distances between the return and return conductors 10, 20 are 5 to 60 m with an outer diameter of the conductors of 10 to 50 cm.

An electric double line 10, 20 in FIG. 2 with the typical dimensions mentioned above has a longitudinal inductivity of 1.0 to 2.7 μH / m. The transverse capacitance is only 10 to 100 pF / m with the dimensions mentioned, so that the capacitive cross currents can initially be neglected. At the same time wave effects should be avoided. The shaft speed is determined by the capacity and inductance coating of the conductor arrangement given. The characteristic frequency of the arrangement is due to the loop length and the wave propagation speed along the arrangement of the double line 10, 20. The loop length is therefore to be chosen so short that no disturbing wave effects result here.

It can be shown that the simulated power loss density distribution in a plane perpendicular to the conductors - as it forms in opposite-phase energization of the upper and lower conductor - decreases radially.

For an inductively introduced heating power of 1 kW per meter of double cable, a current amplitude of about 350 A for low-resistance reservoirs with resistivities of 30 Ω · m and about 950 A for high-resistance reservoirs with resistivities of 500 Ω · m is required at 50 kHz. The required current amplitude for 1 kW / m falls quadratically with the excitation frequency. i.e. at 100 kHz, the current amplitudes fall to 1/4 of the above values.

With an average current amplitude of 500 A at 50 kHz and a typical inductance of 2 μH / m, the inductive voltage drop is about 300 V / m.

In order to limit the total inductive voltage drop across a conductor loop 2 to values of less than 100 kV in total, and in order to avoid insulation problems, the line inductance L is compensated in sections by discrete or continuous series capacitances C. The peculiarity of compensation integrated in the line is that the frequency of the HF line generator must be matched to the resonance frequency of the current loop. This means that the double line 10, 20 suitable for heating purposes, ie with high current amplitudes, only at this frequency can be operated.

As a result, an addition of the inductive voltages along the line is prevented. If in the above example - ie 500 A, 2 μH / m, 50 kHz and 300 V / m - for example, every 10 m each a capacitor C _{i introduced} in the return conductor of 1 uF capacitance, the operation of this arrangement can at 50 kHz resonant done. Thus, the occurring inductive and correspondingly capacitive sum voltages are limited to 3 kV.

If the distance between adjacent capacitors C _{i is} reduced, the capacitance values must increase in inverse proportion to the distance-proportional to the distance of the reduced voltage-resistance requirement of the capacitors-to obtain the same resonant frequency.

A beneficial, in Fig. 3 illustrated and known from the prior art embodiment with built-in line 2 capacitances provides that the capacitance of cylindrical capacitors C _i between a tubular outer electrode 32 of a section I and a tubular inner electrode 34 of the section II is formed, between which a dielectric 33 is located. Likewise, the adjacent capacitor is formed between sections II and III.

For the dielectric of the capacitor C in addition to a high dielectric strength continue to demand a high temperature resistance, since the conductor in the inductively heated reservoir 100, which can reach a temperature of 250 ° C, for example, and the resistive losses in the conductors 10, 20th can lead to a further heating of the electrodes. The requirements for the dielectric 33 are met by a large number of capacitor ceramics.

For example, the group of aluminum silicates, i. Porcelains, temperature resistances of several 100 ° C and electrical breakdown strengths of> 20 kV / mm at Permittivitätszahlen of 6 on. Thus, the above cylinder capacitors can be realized with the required capacity and have a length of, for example, 1 to 2 m.

If the overall length should be shorter, a nesting of several coaxial electrodes is to be provided. This is not shown in the figures for the sake of simplicity. Other common capacitor designs can be integrated into the line, as long as they have the required voltage and temperature resistance.

In the FIG. 3 the entire electrode is already surrounded by insulation. The insulation against the surrounding soil is necessary to prevent resistive currents through the soil between the adjacent sections, in particular in the region of the capacitors. The insulation also prevents the resistive current flow between the return and return conductors. However, the requirements with respect to the dielectric strength to the insulation are compared to the uncompensated line of> 100 kV dropped in the above example, slightly above 3 kV and thus meet by a variety of insulating materials. Like the dielectric of the capacitors, the insulation must withstand higher temperatures permanently, which in turn offers ceramic insulating materials. The insulation layer thickness must not be too low, otherwise capacitive leakage currents could flow into the surrounding soil. Insulation thickness greater z. B. 2 mm are sufficient in the above embodiment.

It can also be connected in parallel several raw-shaped electrodes. Advantageously, the parallel connection of the capacitors can be used to increase the capacitance or to increase its dielectric strength.

In an arrangement according to FIG. 3 According to the state of the art, an electrolytic introduction into the ground may be made in sections for a targeted increase in the heating effect. For this purpose, the line 2 next to the compensated electrode on an insulated inner tube 40 with insulated outlet openings 41, 42 and 43, which is also referred to as perforated hereinafter. As a result, for example, water or an electrically conductive aqueous salt solution or other electrolytes may be introduced into the reservoir in order to increase the conductivity of the reservoir.

Furthermore, the introduced water can be used to cool the conductor. If the outlet openings are replaced by valves, the change in conductivity can take place temporally and spatially in sections.

Increasing the conductivity serves to increase the inductive heating effect without having to increase the current amplitude in the conductors.

In Fig. 3 compensation of the longitudinal inductance takes place by means of predominantly concentrated transverse capacitances. Instead of introducing more or less short capacitors as concentrated elements in the line, and the capacity coating can be a two-wire line such. As a coaxial line or multi-wire cables anyway over their entire length provide for the compensation of the longitudinal inductances are used. For this purpose, the inner and outer conductors are alternately interrupted at equal intervals, thus forcing the flow of current through the distributed transverse capacitances. The advantage of the distributed capacitances lies in a reduced requirement for the dielectric strength of the dielectric.

Of course, a compensated electrode with distributed capacitances in combination with a device for electrolyte introduction can be used.

In the deck structure, are guided vertically through the forward and return conductors to the reservoir 100, a heating effect is undesirable: In the vertical region of the double conductor 10, 20, which is not yet in the reservoir 100, but leads down to this can Hinleiter 10 and return conductor 20 in placed at a small distance of, for example, 1 to 3 m, whereby their magnetic fields already compensate at a smaller distance from the double line and the inductive heating effect is reduced accordingly.

Alternatively, conductors 10 and return conductors 20 may be surrounded by a shield of high conductivity material surrounding both conductors to prevent inductive heating of the surrounding soil of the cover structure.

In a further alternative, a coaxial conductor arrangement in the vertical region of the forward and return conductors is conceivable, which leads to complete extinction of the magnetic fields in the outer region and thus to no inductive heating of the surrounding soil. The increased cross-capacitance coating can be used to implement a gyrator, which converts a voltage of a voltage impressing converter into an alternating current according to the prior art, with help.

For all three mentioned methods, a compensation of the respective inductance coating of the conductor arrangement including the possibly existing shielding is necessary.

The power generator 60 in FIG Fig. 2 is designed as a high-frequency generator. It can generate power up to 2500 kW. Typically, frequencies between 5 and 20 kHz are used. However, higher frequencies can also be used. The power generator 60 is three-phase and advantageously includes a transformer coupling and power semiconductors as components. In particular, the circuit includes a voltage imprinting inverter. A current injection with load-independent fundamental oscillation, which is adjustable by means of filter components, results in this case with a suitable choice of Anpassvierpols behind this. Depending on the topology of the quadripole, a different current load of the feeding inverter results.

As already mentioned, in such a generator for the intended use an operation under resonance conditions is required in order to achieve a reactive power compensation. If necessary, the drive frequency is suitably adjusted during operation.

In a conductor loop 10, 15, 20 according to FIG. 2 , which represents a bipolar inductor, a single-phase generator can also be used. Such generators with, for example, 440 KW at 50 KHz are commercially available.

As described above and known from the prior art, a targeted increase in the heating effect can take place via the introduction of electrolyte into the ground. For example, water or an electrically conductive aqueous salt solution or other electrolytes may be introduced into the reservoir to increase the conductivity of the reservoir.

The problem here, as well as without the introduction of electrolyte, is the heating of the conductor 3 or the conductor loop 10, 20, 15 by heat conduction from the ground to the conductor 3. As mentioned above, the materials such as insulator and dielectric are stable only up to certain temperatures, depending on the choice of material. Therefore, at high heat outputs, the conductivity of the surroundings of the conductors 3 can be selectively reduced according to the invention as an alternative or in addition (in terms of time) by introducing a fluid into the ground. The fluids used include water, and / or gases such as air, nitrogen, carbon dioxide, and / or solutions of chemical substances which are too sparingly soluble in the reservoir Salts react and thereby lead to precipitation of ions in the reservoir.

As a result, in the case of inductive heating of the ground via the conductor 3, the heating power in the immediate vicinity of the conductor 3 can be reduced. The fluid reduces the conductivity in the environment, eg up to 3 m around the conductor 3. The decrease in conductivity is particularly strong in the immediate vicinity of the conductor 3, where most of the heating power occurs by induction. The induced ion currents in the soil around the conductor 3 are reduced by the fluid or the reduction of the conductivity in the ground. In more distant regions of the conductor 3, where the heating power is lower by induction, there is less, if any, reduction in conductivity by the fluid. Although this is also introduced in more remote areas of the soil by eg diffusion, but to a much lesser extent than in the immediate vicinity of the conductor 3 around. As a result, the lower heating power, which occurs in remote from the conductor 3 areas of the soil, not further reduced or only slightly reduced. In the immediate vicinity, decreasing with the distance from the conductor 3, the conductivity, so that the induced heating power and thus the heating is reduced. The lower amount of heat in the vicinity of the conductor 3 leads to a lower heat conduction to the conductor 3 and thus to a lower heating of the conductor 3 itself. The temperature T _{L of} the conductor 3 can thus be limited to a maximum value at which the individual materials of the conductor 3 are not thermally damaged and long-term stability.

By the method according to the invention described above, the heating power in the vicinity of the electrical conductor 3 is made uniform. In the immediate vicinity, the heat output is reduced at high induced field strengths around the conductor 3 by reducing the conductivity, while farther away the conductivity is not or only slightly changed and thus the heating power remains substantially the same. With the same electrical power for the induction via the conductor 3, the conductor 3 is heated less strongly, with the advantages described above. As a result, the power can be further increased as long as the critical temperature value at the conductor 3 is not reached, in which materials such as insulation or dielectric are damaged. It is thus achieved that a better heating of the soil takes place away from the conductor 3 by way of more induction, and thus better liquefaction or fluidization of the heavy oil or bitumen. By reducing the conductivity in the vicinity of the conductor 3 less heating in the immediate vicinity of the conductor 3 is achieved at the same time and thus a lower heat transfer to the conductor 3 out, which has a lower heating of the conductor 3 itself result. Thus, with the inventive method without damaging the conductor 3 in a larger environment of the conductor 3 heavy oil or bitumen can be liquefied and a flow rate can be increased.

The invention is not limited to the previously described embodiment of the method. Combinations of prior art methods with the method according to the invention are also possible. For example, e.g. a time sequential introduction of electrolyte to increase the conductivity, followed by an introduction of fluid to reduce the conductivity in the vicinity of the conductor 3 possible. Thereby, e.g. a first promotion of heavy oil and bitumen in the immediate vicinity of the conductor 3 with less induction and heating power done. Subsequently, after introduction of the fluid to reduce the conductivity, 3 heavy oil or bitumen can be liquefied more remote from the line 3 and thus promoted at higher power without damaging the line.

A repeated, alternating introduction of electrolyte to increase the conductivity and of fluid to reduce the conductivity is possible. This can temporarily a Cooling of the line 3 can be achieved. A pulsewise, repeated introduction of only fluid to reduce the conductivity is possible.

An introduction of electrolyte to increase the conductivity, followed by an introduction of fluid to reduce the conductivity in the vicinity of the conductor 3 may also be advantageous if in remote areas of the conductor 3 good delivery of heavy oil or bitumen is to be achieved. Thus, to increase conductivity, the electrolyte may be introduced into more remote regions, e.g. by high pressure and / or diffusion, and in the immediate vicinity of the conductor 3, the subsequently introduced fluid to reduce the conductivity displace the electrolyte to increase the conductivity. As a result, the induced heating power is increased in more remote areas, while in the immediate vicinity of the conductor 3 and the conductor 3 itself, the heating is reduced. It may be of advantage that a liquid electrolyte conducts heat better than e.g. a gas. Thus, the lower induction can be compensated away from the conductor by a better conductivity and immediately in the vicinity of the conductor 3, a gas can lead to a reduced heat transfer to the conductor 3 out.

The above-described inventive methods, even or in combination, lead to improved heavy oil or bitumen production, by high usable electrical power and associated induction with reduced risk of thermal damage to the conductor 3 or the materials of its components, e.g. Dielectric and / or insulation.

Claims (11)

1. Process for the "in situ" extraction of bitumen or ultraheavy oil from oil-sand deposits (100) as a reservoir (1), wherein the ground of the reservoir (1) is inductively heated by means of at least one electrical current-passing conductor (3) by currents induced in the ground in the area surrounding the electrical conductor (3) to reduce the viscosity of the bitumen or ultraheavy oil, and wherein at least one perforated fluid guide (30), which surrounds or encloses the at least one conductor (3) at least in certain portions, is used to introduce a fluid (45) into the reservoir (1) via the perforation (41, 42, 43) in the fluid guide (30), characterized in that the fluid (45) reduces an electrical conductivity in the reservoir (1) in the area surrounding the fluid guide (30).
2. Process according to Claim 1, characterized in that water with a lower conductivity than the conductivity of water located in the reservoir (1) is introduced into the reservoir (1) as the fluid (45).
3. Process according to one of the preceding claims, characterized in that gas is introduced into the reservoir (1) as the fluid (45).
4. Process according to Claim 3, characterized in that air is used as the gas or the gas comprises air.
5. Process according to Claim 3, characterized in that carbon dioxide and/or nitrogen is used as the gas or the gas comprises carbon dioxide and/or nitrogen.
6. Process according to one of the preceding claims, characterized in that a solution of chemical substances, the chemical substances of which react to form a scarcely soluble salt in the reservoir (1) and thereby lead to a precipitation of ions in the reservoir (1), is introduced into the reservoir (1) as the fluid (45).
7. Process according to Claim 6, characterized in that a chemical analysis of at least one fluid from the reservoir (1) is used in order to determine ions and the chemical substances in the solution (45) are selected on the basis of the ions determined, in order then, with the solution (45), to precipitate ions in the reservoir (1).
8. Process according to one of the preceding claims, characterized in that, by reducing the electrical conductivity in the reservoir (1), the inductive heating by means of the current-passing conductor (3) is reduced.
9. Process according to Claim 8, characterized in that the temperature T in the area directly or indirectly surrounding the conductor (3) and/or the fluid guide (30) is restricted to a maximum value.
10. Process according to Claim 9, characterized in that the temperature T is restricted to a maximum value at which components of an apparatus for the "in situ" extraction of bitumen or ultraheavy oil from oil-sand deposits (100) as a reservoir (1) are thermally stable.
11. Process according to one of the preceding claims, characterized in that the electrical conductor (3) is passed through by an alternating current with a current intensity in the range of over 100 A and/or with a frequency in the range from 10 kHz to 100 kHz.

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