Patent Publication Number: US-9839075-B1

Title: Downhole induction heater

Description:
FIELD OF THE INVENTION 
     The invention relates to the oil industry, in particular to induction heaters used in production wells of paraffinic, viscous and other oils for removal of paraffin deposits. 
     BACKGROUND OF THE INVENTION 
     There is known an induction heater (RF Patent 2086759), including a housing, a body and three separate induction coils (one for each phase) with three radiators. The cavity between the housing and the body is filled up with transformer oil. The disadvantages of this heater are the design complexity, inefficient way of using of eddy currents in conversion of the electric energy into heat and, accordingly, high power consumption. 
     The closest related art (analogue) to the present invention is an induction heater (RF Patent 2284407), comprising a housing, a bearing element disposed coaxially with the housing with series-connected induction coils placed on it and provided with ferrite magnetic cores. 
     Furthermore, the bearing element is made in the form of a conductive nonmagnetic rod, to the bottom of which an out coming round wire of the lower coil is attached. The upper part of the bearing element is shorted to a logging cable armor shell via a connector; the primary winding of an upper coil is connected to a central core of the cable (CCC) via a connector. The upper part of the housing is made of non-magnetic non-electrically-conductive material; the lower part of the housing is made of magnetic electrically conductive material, wherein the coil windings are wound on ferrite magnetic cores of different diameters, and the upper coil windings are wound on a ferrite magnetic core of a larger diameter, and the lower coil windings are wound on a ferrite magnetic core of a smaller diameter. Disadvantages of this heater are large power losses when operating at great depths, for instance, from 5000 meters or more, as well as a low output frequency of about 1 kHz, which reduce efficiency of the heater. 
     BRIEF SUMMARY OF THE INVENTION 
     The problem to be solved by the present invention is how to reduce losses of electric energy in power supply cables feeding downhole induction heaters and improve efficiency thereof. 
     The problem is solved and the sought result is achieved due to engineering solutions proposed herein, which are used in designing structural components of an inventive induction heater in conjunction with utilizing modern electronic components, in a limited space of a drilling pipe/oil production column determined by the width of the pipe/column. 
     As a result, the present invention allows for achieving: a reduction of losses of electric energy in power supply cables, when working at large downhole depths (5,000 m or more); and an increase of operating efficiency of the induction heater by controlling the output frequency of induction heating in a range of 60 to 200 kHz. 
     According to the present invention, there is proposed an induction heater being a component of an equipment complex for removal of paraffin deposits in drill pipelines/columns of production oil wells. The induction heater is electrically powered substantially from a standard power supply source. The induction heater is immersed into the drill pipeline. 
     The induction heater includes an inductor comprising: —an external induction coil disposed along a central longitudinal axis; the external induction coil includes a first external butt end; the external induction coil creates an external vortex magnetic field mostly heating up an inner surface of the drill pipe, thereby melting the paraffin deposits accumulated thereon; —an internal induction coil disposed along the central longitudinal axis; the internal induction coil includes a first internal butt end proximal to the first external butt end; the internal induction coil is nested inside the external induction coil; the internal induction coil creates an internal vortex magnetic field; —a contact bushing electrically connecting the first external butt end and the first internal butt end; the internal induction coil and the external induction coil, connected by the contact bushing, form a single two-layer induction coil; —a support rod mechanically securing the external induction coil and the internal induction coil; the support rod is capable of conducting heat; —a heating element converting energy substantially of the internal vortex magnetic field into heat; the heating element is tightly fitted onto the support rod and transfers heat thereto; and —a tip receiving heat from the heating element via the support rod; the tip accumulates heat and transfers heat mostly to the paraffin deposits accumulated inside the drill pipe and surrounding the tip thereby melting the paraffin deposits accumulated therein. This provides for heating the paraffin deposits from the inside (by the tip) and from the outside (by the inner surface of the drill pipe), which significantly accelerates the melting process. 
     In a preferred embodiment, the internal induction coil includes a second internal butt end located opposite to the first internal butt end; the internal induction coil further defines a spiral cutting, which forms an internal angle (preferably 72° 38′) with the central longitudinal axis; the internal angle is determined so that the internal induction coil has N complete turns (preferably N=7); the internal induction coil further includes a number of apertures (preferably of a rectangular shape) provided at the second internal butt end; the apertures serve for connection of electric cables further passed through hollow passages passing through an attachment unit, and further connecting the internal induction coil substantially with a capacitor battery; and the external induction coil further defines a spiral cutting, which forms an external angle (preferably 101° 46′) with the central longitudinal axis; and the external angle is determined so that the external induction coil has N+1 complete turns. 
     In a preferred embodiment, the induction heater includes: —a capacitor battery series-connected substantially with the internal induction coil and the external induction coil; the capacitor battery, the internal induction coil and the external induction coil substantially form an oscillatory LC-circuit; the capacitor battery includes a predetermined number of capacitors; —a transformer transmitting electric power into said oscillatory LC-circuit; and —a multiple-contact connector that (a) electrically connects in parallel the capacitors of the capacitor battery; (b) electrically connects the transformer with the capacitors of the capacitor battery; and (c) electrically connects the capacitors of the capacitor battery with the internal induction coil. Each capacitor has a number of leads; and the multiple-contact connector is made of a plurality of brass foil strips with a thickness of 0.2 mm and a width of 30 mm; the strips are furnished with a number of contact zones soldered to the leads of the capacitors; each strip is coated with an insulation layer made of high-temperature enamel, except for the contact zones; and the insulation layer individually covers each strip providing for electrical isolation between any two of the strips. 
     In a preferred embodiment, the induction heater includes: a thermistor disposed inside the tip; the thermistor measures a temperature the tip, and generates temperature feedback signals corresponding to the temperature of the tip; and electronic components, in particular: —the aforementioned capacitor battery, in conjunction with the external and internal coils forming an oscillatory LC-circuit with a resonant frequency; —a high-frequency inverter generating electric pulses characterized with a high frequency; the high-frequency inverter is powered substantially from the standard power supply source; —a high-frequency ferrite transformer receiving the electric pulses from the high-frequency inverter, transforming the electric pulses, and powering the oscillatory LC-circuit; —a current transformer measuring electric current flowing through the oscillatory LC-circuit, the current transformer generates current feedback signals corresponding to the electric current; —a unit of stabilizers providing low-voltage DC power supply; and —a microprocessor unit including a CPU and a memory pre-loaded with a control program; the microprocessor unit is powered from the unit of stabilizers; the microprocessor unit receives the temperature feedback signals and the current feedback signals; the control program, based on the temperature feedback signals and the current feedback signals, controls the high-frequency inverter essentially regulating the high frequency thereof, such that the high frequency becomes equal to the resonance frequency thereby providing a power-efficient mode of operation of the induction heater. 
     In a preferred embodiment, the internal induction coil is made of a copper-rolled tube; the external induction coil is made of a brass tube with a copper content not less than 62%; and the support rod is made of brass. 
     There is also proposed a method for control of the above-described induction heater comprising the steps of: —powering the microprocessor unit; —generating pulses of high frequency voltage by the microprocessor unit; —transmitting the pulses to the high frequency inverter; amplifying power of the pulses thereby converting the pulses into amplified pulses by the high frequency inverter; —applying the amplified pulses to the oscillatory LC-circuit; —measuring electric current in the oscillatory LC-circuit and generating current feedback signals by the current transformer; —transmitting the current feedback signals from the current transformer to the microprocessor unit; —scanning a predetermined work range of frequencies (preferably 80-200 kHz) by the control program, wherein the scanning starts with a maximum frequency of the predetermined work range and further reduces the frequency by a predetermined frequency step (preferably 300 Hz) within a predetermined time interval (preferably 2 seconds); based on the current feedback signals received by the microprocessor unit, determining amounts of electric current running in the oscillatory LC-circuit; —storing the amounts of electric current running in the oscillatory LC-circuit to the memory of the microprocessor unit; —after the scanning reaches a minimal frequency of the predetermined work range, processing the amounts of electric current stored in the memory; —determining a frequency at which the amount of electric current was maximal, wherein the frequency is equal to the resonant frequency; —continuing operation of the high frequency inverter at the resonant frequency within a predetermined pause time (preferably 10 minutes); and —restarting the scanning of the predetermined work range of frequencies by the control program. 
     The foregoing method may further comprise the additional steps of: —measuring a temperature of the tip and generating the temperature feedback signals corresponding to the temperature of the tip by the thermistor; —transmitting the temperature feedback signals from the thermistor to the microprocessor unit; when the temperature of the tip reaches a predetermined maximal temperature (preferably 105° C.), storing a corresponding value of the resonance frequency to the memory by the microprocessor unit, and further changing a frequency of the pulses of high frequency voltage by the microprocessor unit, so that an output power of the high frequency inverter is reduced by 50%; —when the temperature of the tip reaches the predetermined maximal temperature minus a predetermined hysteresis step (preferably 10° C.); and restoring the frequency of the pulses of high frequency voltage by the microprocessor unit to the corresponding value of the resonance frequency. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS OF THE INVENTION 
         FIG. 1  illustrates a general view of an induction heater assembled of three major modules: an inductor coupled with a cylindrical housing (enclosing electronic control circuit components) further coupled with a head connector (serving for connection with a power supply cable electrically feeding the induction heater), according to a preferred embodiment of the present invention. 
         FIG. 2  illustrates a sectional view of the inductor with its components, according to a preferred embodiment of the present invention. 
         FIG. 3  illustrates a schematic view of the cylindrical housing with components enclosed therein, according to a preferred embodiment of the present invention. 
         FIG. 4  illustrates a schematic view of the induction heater placed in a drill pipe clogged with paraffin deposits, according to a preferred embodiment of the present invention. 
         FIG. 5  illustrates a structural block-diagram of essential components of the induction heater connected by a power supply cable with a power supply source, according to a preferred embodiment of the present invention. 
         FIG. 6  illustrates an electric scheme of essential circuitry components for control of operation of the induction heater, according to a preferred embodiment of the present invention. 
         FIG. 7  illustrates frontal and lateral projections views of two induction coils, being major innovation components of the induction heater, according to a preferred embodiment of the present invention. 
         FIG. 8  illustrates a general view of a multiple connector, being another innovation component of the induction heater, according to a preferred embodiment of the present invention. 
     
    
    
     DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     While the invention may be susceptible to embodiment in different forms, there are described in detail herein below, specific embodiments of the present invention, with the understanding that the instant disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that as described herein. 
     An equipment complex for removal of paraffin deposits in production oil wells (specifically, in drilling pipes) includes an induction heater assembled of three modules coupled with each other, as explained below. 
     The induction heater (shown in  FIG. 1 ) is composed of a head connector  1  (the first module), designed to electrically connect an inductor (numbered  3  in to a ground power supply source (numbered  31  in  FIG. 5 ) by means of a power supply cable (numbered  26  in  FIGS. 4, 5 ) through a cable head (numbered  27  in  FIG. 4 ). 
     Yet, the induction heater (shown in  FIG. 1 ) is composed of an electronic module (the second module), including a cylindrical housing  2  that encloses electronic control circuitry components (numbered  19 - 25  in  FIGS. 3, 5, 6 ). The cylindrical housing  2  is designed to protect the electronic components from damage, when operating in aggressive environments, such as oil wells. The cylindrical housing  2  is attached to the head connector  1  (as shown in  FIG. 1 ). The cylinder shape is considered more suitable for the housing, though another type of shape can be used in particular embodiments of the invention. 
     The induction heater (shown in  FIG. 1 ) is also composed of an inductor  3  (shown in  FIGS. 1, 2, 3, 5-8 ) (the third module) designed to create powerful vortex magnetic fields, whose energy is converted into heat, further used preferably to melt paraffin deposits in drilling pipes/columns. The inductor  3  is attached to the cylindrical housing  2  (as shown in  FIG. 1 ). 
     In turn, the inductor  3  comprises:
         an attachment unit  4  ( FIG. 2 ) providing for mechanical coupling of components of the induction heater  3 , in particular, of a support rod (numbered  9  in  FIG. 2 ), and for protection of the electronic control circuitry components (numbered  19 - 25  in  FIGS. 3, 5, 6 ), disposed in the cylindrical housing  2  ( FIG. 1 ), from exposure to strong magnetic fields, high temperature of heating elements of the inductor, and downhole environment aggressive factors, such as high hydraulic pressure and borehole fluid chemicals;   an internal induction coil  5  ( FIG. 2 ), designed to generate vortex magnetic field. It is an innovative component of a resonant oscillatory LC-circuit (contour) used for frequency control of the inductor  3 . In detail, design of the coil  5  is shown in  FIG. 7  and described below;   hollow passages  6  ( FIG. 2 ) passing through the attachment unit  4 . The hollow passages  6  serve for passing cables of electrical connection of the internal induction coil  5  with electronic circuits (explained herein below in detail) arranged inside the cylindrical housing  2  ( FIG. 1 );   an upper insulating bushing  7  (Fi designed to electrically isolate the internal induction coil  5  from short circuiting to the attachment unit  4 ;   an external inductive coil  8  ( FIG. 2 ), designed to generate vortex magnetic field. It is also an innovative component of the resonant oscillatory LC-circuit used for frequency control of the inductor  3 . Design of the coil  8  is shown in  FIG. 7 . The coils  5  is nested inside the coil  8 , whereas each of the coils preferably has its own precisely calculated angle of spiral cutting, which is a novel design feature of inductor coils. Connection of the induction coils, produced in such way, makes them a single bi-layer coil ( FIG. 7 ). Connection of the coils  5  and  8  is shown in  FIG. 2  and  FIG. 6 . In detail, design of the coil  8  is described below;   a support rod  9  designed for mechanical attachment and support o the internal coil  5  and the external coil  8 . The support rod  9  also serves as an intermediate component for heat conduction from a heating element (numbered  10  in  FIG. 2 ) to a tip (numbered  13  in  FIG. 2 ). The support rod is preferably made of brass and has a substantial thermal conductivity that allows thermal energy to be effectively supplied into the tip;   a heating element  10  ( FIG. 2 ), designed to convert energy of the vortex magnetic field into heat. It&#39;s tightly fitted onto the support rod  9  ( FIG. 2 );   a contact bushing  11  ( FIG. 2 ), designed to provide electrical connection of the internal induction coil  5  and the external induction coil  8 ;   a lower insulating bushing  12  ( FIG. 2 ), designed to electrically isolate the connection of the induction coils  5  and  8  against short-circuiting them to the support rod  9  and hence to the cylindrical housing  2 ;   a tip  13  ( FIG. 2 ) designed for accumulation of heat energy received through the support rod  9  from the heating element  10  and for transfer of the heat energy into external environment, e.g. melting paraffin deposits from the inside;   a thermistor  14  ( FIG. 2 ) used for temperature control of the tip  13 ;   a cable channel  15  of the thermistor  14  ( FIG. 2 ), through which feedback signals from the thermistor  14  are transmitted to the electronic control circuit components (shown in  FIG. 5 ) disposed in the cylindrical housing  2  ( FIG. 1 );   a buffer cylinder  16  ( FIG. 2 ), designed for casting (introduction) of a high temperature compound with a significant adhesion to metals. Thereby it prevents penetration of drilling mud through the hollow passages  6  ( FIG. 2 ) to the electronic control circuitry;   an attachment sleeve  17  ( FIG. 2 ) for coupling with a semi-cylindrical brass container (numbered  18  in  FIG. 3 ), enclosing the electronic control circuitry components (numbered  19 - 25  in  FIGS. 3, 5, 6 );       

       FIGS. 3, 5 and 6  show an electronic module (the third module) of the induction heater, wherein the electronic module comprises:
         a semi-cylindrical brass container  18  designed for mechanical mounting of the electronic control circuitry components (numbered  19 - 25  in  FIGS. 3, 5, 6 ). In addition, it serves as a passive heatsink cooling power elements of a high-frequency inverter (numbered  21  in  FIGS. 3, 5, 6 ). The semi-cylinder shape is considered more suitable for the container  18 , though another type of shape can be used in particular embodiments of the invention. The semi-cylindrical brass container  18  is located inside the cylindrical housing  2 . A preferable width of cross-section of the container  18  is 38 millimeters;   a unit of stabilizers  19  (also herein called a ‘secondary pulse power supply’), designed for power supply of low-voltage DC electronic circuits of a microprocessor unit (numbered  20  in  FIGS. 3, 5, 6 ). Since the constant voltage fed directly through a power supply cable (numbered  26  in  FIGS. 4, 5 ) is excessive to power the microprocessor unit, the unit  19  converts voltage received from a ground power source (numbered  31  in  FIG. 5 ) to DC voltage of 12 V;   a microprocessor unit  20  ( FIGS. 3, 5, 6 ) controls operation of the induction heater. The microprocessor unit  20  ( FIG. 5 ) comprises a CPU and an internal non-volatile memory for loading a control program, which program, during its execution, controls operation of the inductor  3 .   a high-frequency inverter  21  ( FIGS. 3, 5, 6 ), designed to generate high power electric pulses. The unit  21  is powered by the ground power source (numbered  31  in  FIG. 5 ) via the power cable (numbered  26  in  FIG. 4 );   a high-frequency ferrite transformer  22  ( FIGS. 3, 6 ). Together with the high-frequency inverter  21 , it is designed for generating and transmitting energy into a resonant oscillatory LC-circuit (also known as an LC-contour) formed by a capacitor battery (numbered  23  in  FIGS. 3, 5, 6 ) and the induction coils  5  and  8  ( FIG. 6 );   a capacitor battery  23  ( FIGS. 3, 5, 6 .) being a major component of the resonant oscillatory LC-circuit. The capacitor battery  23  includes a predetermined number of capacitors (numbered  41  in  FIG. 8 ), each with a predetermined capacitance. The capacitor battery  23  is series-connected substantially with the induction coils  5  and  8  thereby forming the resonant oscillatory LC-circuit;   a multiple-contact connector  24  ( FIGS. 3, 8 ), designed to electrically connect the high frequency transformer  22 , the capacitor battery  23 , and the inductive coil  5  and the high-frequency ferrite transformer  22  (see  FIGS. 6 and 8 ). Design of the multiple-contact connector ( FIG. 8 ) is a novel feature in induction heaters. In detail, the design of multiple-contact connector  24  is described below;   a current transformer  25  ( FIGS. 3, 5, 6 ) designed to measure the electric current flowing through the LC-circuit formed by the capacitor battery  23  and the induction coils  5  and  8  (see  FIG. 6 ).       

     The equipment complex is preferably powered from a standard electric power supply source, such as an industrial electric power grid  30  ( FIG. 5 ). The standard electric power supply source is preferably located on the earth surface. 
     The equipment complex also includes: a power supply cable  26  (also known as ‘geo-physical cable’, or ‘logging cable’) ( FIGS. 4, 5 ) used for power transmission from the ground-based pulse power supply (numbered  31  in  FIG. 5 ); a cable head  27  ( FIG. 4 ) that connects the power supply cable  26  with the induction heater  3 ; a drill pipe (also called a ‘downhole pipe’, or a ‘well pipe’)  28  ( FIG. 4 ) used in oil production; a paraffin deposits clot  29  ( FIG. 4 ) often accumulated in the drill pipe  28 ; a ground-based pulse DC power supply unit  31  ( FIG. 5 ) converting AC voltage of the industrial grid  30  into DC voltage. 
     A best design mode of the internal induction coil  5  is depicted in  FIG. 7 . The internal induction coil  5  has a spiral cutting  33  which forms an angle of 72° 38′ with an axis  34  being a central longitudinal axis of the coil  5 . The angle is calculated so that the internal induction coil  5  has seven complete turns. At the right end of the internal induction coil  5 , there are provided six rectangular apertures  32  for cable electrical connections of the coil  5  with the capacitor battery  23 . The connection is made by six electric cables passed through the hollow passages  6  ( FIG. 2 ) of the attachment unit  4  ( FIG. 2 ). 
       FIG. 7  also shows a best design mode of the external induction coil  8  having a spiral cutting  35  that forms an angle of 101° 46′ with an axis  36  being a central longitudinal axis of the coil  8 . The angle is calculated so that the external induction coil  8  has eight full turns. 
     Yet,  FIG. 7  shows an area of electrical connection  37  of the internal induction coil  5  and the external induction coil  8  via the contact bushing  11  ( FIG. 2 ) and the formation of a single two-layer induction coil that together with the capacitor battery  23  form the resonant LC-circuit ( FIG. 6 ). 
     In addition,  FIG. 7  shows a zone of electrical connection  38  of the external induction coil  8  with the attachment unit  4 . The connection is made over the top of the upper insulating bushing  7  ( FIG. 2 ). 
     The internal induction coil  5  is preferably made of a copper-rolled tube in which the spiral cutting is made at a pre-calculated angle so that it provides for forming a spiral of eight turns ( FIG. 7 ). 
     The external coil  8  is preferably made of a brass tube with a copper content not less than 62%. Connected together through the contact bushing  11  ( FIG. 2 ), the internal coil  5  and the external coil  8  form a single two-layer induction coil (the connection circuit of the coils  5  and  8  is shown in  FIG. 6 ). 
     Vortex magnetic field of the internal coil  5  ( FIG. 2 ) effectively heats up the heating element  10  tightly fitted onto the support rod  9 . The support rod  9  is preferably made of brass, and has a substantial thermal conductivity′ that allows for thermal energy to be effectively supplied into the tip  13  ( FIG. 2 ) being in direct contact with a paraffin deposit clot  29  on an inner surface of the drill pipe  28  and effectively melts the clot from inside thereof. Melted paraffin is removed by a flow of drilling fluid. 
     It was experimentally found that heating up of paraffin deposits to a temperature above 115° C. might lead to caking of solids present in the deposits into a hardly removable substance. 
     In order to prevent this situation, the thermistor  14  is fitted at the end of the support rod  9  ( FIG. 2, 5 ). When the temperature of the tip  13  ( FIG. 2 ) reaches 105° C., the microprocessor unit  20  stores a resonance frequency value to the memory (RAM), and further changes the frequency of oscillations of the high frequency inverter so that its power output would be reduced by 50%. 
     Hysteresis of temperature adjustment is 10° C. That is, when temperature of the tip decreases down to 95° C., the microprocessor restores the resonance frequency and the induction heater  3  continues working at its maximum power. 
     During the operation, because of heating up the capacitor battery  23 , the battery&#39;s electrical capacitance is changing (effectively changing the C-component of the oscillating LC-circuit), and consequently, the resonance frequency of the LC-circuitry changes as well. For this reason, re-search (re-scanning) of the changed resonance frequency is required. 
     The control program performs such search for every 10 minutes of operation of the induction heater. 
     Experiments have shown that frequency changes are insignificant, but nonetheless they can significantly affect the thermal mode of operation of the high-frequency inverter, causing excessive heating of its power elements and thus increasing energy losses. 
     A somewhat different function is performed by the external induction coil  8  ( FIG. 4 ). Its vortex magnetic field effectively heats up metal of the drill pipe  28 , the inner surface of which has the paraffin deposit clot  29  formed thereon. Thusly, it heats up the paraffin deposit clot  29  from outside of the paraffin deposit clot. 
     As a result, the melting of paraffin takes place both inside and outside of the paraffin clot. Due to elimination of paraffin deposits in oil wells, the time and costs spent for the drill works are reduced, which improves the overall performance of the equipment complex with the inventive induction heater. 
     For comparative analysis of operation efficiency of the induction heater, it should be taken into account that the time spent on cleaning of one well from paraffin, using the induction heater, does not exceed 3-4 hours, whereas traditional methods for mechanical removal of paraffin deposits may require a few days. 
       FIG. 8  shows a portion of a longitudinal cross-section view of the multiple-contact connector  24  ( FIG. 3 ). The multiple-contact connector  24  is designed to provide:
     a) parallel connection of capacitors  41  being parts of the capacitor battery  23  ( FIG. 8 );   b) connections of the capacitor battery  23  with the high frequency ferrite transformer  22  ( FIGS. 5 and 8 ) on one side;   c) connections of the capacitor battery  23  to the internal induction coil  5  ( FIGS. 5 and 8 ) on the other side.   

     The multiple-contact connector  24  is preferably made of twelve strips  39  manufactured from brass ( FIG. 8 ) with a thickness of 0.2 mm and a width of 30 mm. The strips  39  are furnished with contact zones  42  and dielectric sleeves  43 . Each such strip is coated with an insulation layer  44  of high-temperature varnish ( FIG. 8 ), except for the contact zones  42 , which are soldered to leads of the capacitors  41 . The high-temperature enamel individually covers each strip providing for electrical isolation between any two of aforesaid strips. This is necessary to avoid a negative influence of the skin effect on the performance of the induction heater. 
     The capacitor leads are passed through a number of orifices made in the multiple-contact connector  24 . Those orifices, where the capacitor leads must not contact the material of multiple-contact connector  24 , are isolated by the dielectric sleeves  43  ( FIG. 8 ). 
     Connections with the high frequency ferrite transformer  22  ( FIG. 5 ) are arranged in a zone  40  ( FIG. 8 ). Connections to the induction coil  5  ( FIG. 5 ) are arranged in a zone  45  ( FIG. 8 ). 
     For clarity,  FIG. 8  shows only four of the twelve brass strips  39  and only three of the five capacitors  41  of the capacitor battery  23 . 
     The multiple-contact connector  24  is preferably formed of brass foil with a predetermined thickness. The thickness is predetermined such that it provides for full compensation for energy losses caused by the skin effect. The connector  24  is preferably formed of 12 foil strips, each coated with a protective layer of a high-temperature enamel. 
     Thus, the effective cross-section of the multiple-contact connector  24  provides for electric current flowing through the LC-circuit with almost no loss, and with minimal dimensions of the connector  24 . The minimal dimensions of the connector are an important feature of the invention, since they are limited by the size of semi-cylindrical container  18  (enclosing the electronic circuits components) ( FIG. 3 ). As disclosed hereinabove, a preferable width of cross-section of the container  18  is 38 millimeters. 
     Operation of a Preferred Embodiment of the Invention 
     The induction heater works as follows. The ground DC power supply unit  31 , by, means of the power supply cable  26  and the cable head  27 , electrically feeds most of the electronic components, contained in the cylindrical housing  2 , and the power components of the inductor  3 . 
     When the microprocessor unit  20  is powered and starts working, its control program (loaded into the internal non-volatile memory of the unit  20 ) begins searching for the resonant frequency of the LC-circuit. Pulses of high frequency voltage are generated by the microprocessor unit  20  and supplied to the high frequency inverter  21  which amplifies power of the pulses. Next, the voltage pulses, transmitted by the high-voltage transformer  22 , are applied to the series LC-circuit ( FIG. 5 ). 
     It is known that full resistance of a series LC-circuit at resonance is minimal and therefore, electric current flowing through the circuit reaches its maximum. Therefore, tuning of the series LC-circuit into resonance is provided by searching for a frequency at which the current in the LC-circuit is maximized. 
     In the induction heater, electric current in the LC-circuit is measured by the current transformer  25  ( FIGS. 3, 5, 6 ), and measurement signals from the transformer  25  are transmitted to the microprocessor  20  as feedback. 
     The control program scans a work range of frequencies (80-200 kHz) in order to find the maximal current value of the LC-circuit. The choice of this work frequency range is conditioned by preliminary calculations of parameters of the LC-circuit essentially for all feasible design options of the inventive induction heater. These design options account for changes of dimensions of the inductor and the induction heater as a whole, for carrying out induction heating of the drill pipes  28  of various known diameters. 
     The control program starts searching with the frequency of 200 kHz. A step of changing the frequency is 300 Hz. By reducing the signal in each frequency step, the microprocessor  20 , within 2 seconds, measures the amount of electric current flowing through the LC-circuit. The current amounts are stored into the microprocessor&#39;s memory in the form a data array. After reaching the lower limit of 80 kHz, the program proceeds to processing the data array recorded and determines the frequency at which the LC-circuit current was maximal, i.e. the resonant frequency. After determining the resonant frequency, the high-frequency inverter  21  keeps operating at its maximum power. 
     Electric currents running in the LC-circuit at resonance can reach several hundred amperes. This imposes high demands upon the design of electrical connections of the capacitor battery and the induction coils of the inductor. These connections are described hereinabove. 
     Skin effect becomes very significant at high resonance frequencies, and the effect is manifested in displacement of electrical conduction currents from the internal areas of the conductor into the external ones. Along with this, the active resistance of cables (providing electrical connections of the LC-circuit&#39;s components) increases. 
     The result is significant losses of power consumed for heating the cables, which dramatically decreases efficiency of operation of the device and makes it worthless. The problem was resolved in the present invention by using the specially designed multiple-contact connector  24  described above ( FIG. 8 ). 
     Another important feature of the induction heater is the design of the induction coils  5  and  8  described hereinabove ( FIG. 2, 7 ). 
     OPTIONS OF INDUSTRIAL APPLICABILITY 
     The inventive induction heater has a number of other useful properties. It is known that high mechanical loads operatively applied to the well pipes&#39; material can result in noticeable magnetization of some segments of the pipes. At the same time, drilling fluid is not an electrically neutral liquid and it has an ionic composition due to dissociation of salt molecules dissolved in the drilling solution (also called drilling mud). In other words, the drilling fluid is essentially electrolyte. 
     Movement of the electrolyte transporting electric charges across the magnetic fields of the magnetized sections of the well pipe causes a small potential difference in diametrically opposite points of the pipe. 
     This process is known in physics as the magnetic hydrodynamic effect. Among other reasons, this potential difference increases the corrosion rate of the well pipe, which over time makes it inoperable without costly repairs. Powerful high-frequency field of the induction heater is capable of eliminating the magnetization of well pipes, thus extending their operation lifespan. 
     Another option of use of the induction heater is the warming up of a perforation zone in production oil wells in order to remove tar impurities that reduce the efficiency of oil inflow.