Abstract:
An apparatus for the in situ extraction of bitumen or very heavy oil from oil sand deposits, and applying heat energy to the deposit to reduce the viscosity of the bitumen is provided. A high-frequency generator feeds electric power to a linearly extended conductor loop at a predefined depth of an oil sand deposit, the inductance of the conductor loop is compensated in some sections or continuously. Advantageously, one of the conductors of the conductor loop may be disposed substantially in a vertical direction above the extraction pipe.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2008/060927, filed Aug. 21, 2008 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2007 040 605.5 DE filed Aug. 27, 2007. All of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to an apparatus for “in-situ” extraction of bitumen or very heavy oil from oil sands deposits as reservoir, with heat energy being applied to the reservoir to lower the viscosity of the bitumen or very heavy oil present in the oil sand, for which purpose an electric/electromagnetic heater is provided. 
     BACKGROUND OF INVENTION 
     Oil sands deposits close to the surface can be extracted in an open-cast system if necessary, with processing to separate the oil subsequently being required. However “in-situ” methods are also known in which, by introducing “solvent” or thinning agents and/or alternatively by heating up or melting the very heavy oil the deposit is made flowable while still in the reservoir. The “in-situ” methods are especially suitable for reservoirs which are not close to the surface. 
     The most widespread and widely-used “in-situ” method for extracting bitumen is the SAGD ( S team  A ssisted  G ravity  D rainage) method. In this method, steam, which can be added to the solvent, is injected at high pressure through a pipe running horizontally within the reservoir. The bitumen heated-up, melted or dissolved from the sand or rock seeps down to a second pipe located around 5 m (distance between injector and production pipe depends on reservoir geometry) through which the liquefied bitumen is extracted. In this method the steam has a number of tasks to perform, namely the introduction of heat energy for liquefaction, the removal of sand and building up the pressure in the reservoir, in order on the one hand to make the reservoir porous for the transport of bitumen (permeability) and on the other hand to make it possible to extract the bitumen without additional pumps. 
     The SAGD method starts by both pipes being heated up by steam, typically for 3 months, in order to initially liquefy the bitumen in the space between the pipes as quickly as possible. Then steam is introduced into the reservoir through the upper pipe and extraction through the lower pipe can begin. 
     A method for resistive heating up of a very heavy oil deposit is known from US 2006/0151166 A1, in which a tool with electrodes for a three-phase resistive heating of the deposit is provided for reducing the viscosity of the very heavy oil. With the applicant&#39;s older, not previously published German patent applications AZ 10 2007 008 292.6 entitled “Vorrichtung and Verfahren zur in situ-Gewinnung einer kohlenwasserstoffhaltigen Substanz unter Herabsetzung deren Viskosität aus einer unterirdischen Lagerstätte (apparatus and method for in-situ extraction of a substance containing hydrocarbons from an underground deposit while reducing its viscosity)” and AZ 10 2007 036 832.3 entitled “Vorrichtung zur in situ-Gewinnung einer kohlenwasserstoffhaltigen Substanz (apparatus and method for in-situ extraction of a substance containing hydrocarbons)” electrical/electromagnetic heating methods for an “in situ” extraction of bitumen and/or very heavy oil have already been proposed in which in particular an inductive heating of the reservoir is undertaken. 
     Using the prior art as its starting point, the object of the invention is to create an apparatus with a suitable design for electrical/electromagnetic heating of the reservoir of an oil sands deposit. 
     SUMMARY OF INVENTION 
     The object is inventively achieved by the features of the claims. Developments of the invention are specified in the subclaims. 
     The subject matter of the invention is the application in mining of a resonantly-tuned harmonic circuit for inductive heating up of an oil sands deposit referred to as a reservoir underground at a depth of up to several hundred meters in an “in-situ” oil production process. To achieve this object the inventive apparatus contains an external alternating current generator known per se for electrical power which is used to supply power to a conductor loop. The conductor loop is formed from two or more conductors which are connected electrically-conductively inside or outside the reservoir. The inductance of the conductor loop is compensated for in sections. This avoids any undesired reactive power. The ac-supplied conductor loop creates an alternating magnetic field in the reservoir through which eddy currents are stimulated in the reservoir which lead to the heating up of same. 
     Two inductive effects are to be distinguished in the invention:
         The overall inductance of the conductor loop which is primarily formed by the undesired self-inductance and must be compensated for to prevent a large voltage drop along the lines and to not demand any reactive power from the generator.   The desired mutual inductance to the reservoir, which makes possible the current flow and thereby the heating up of the reservoir.       

     The inventive apparatus makes it possible to heat up unconventional heavy oil with viscosities of e.g. 5° API to 15° API from temperatures of 10° C. ambient temperature to as much as 280° C. This enables the oil to flow in a gravitative process through the improvement of the fluidity down to the lower non-permeable boundary layer and to flow out from there by means of known drainage production pipes, in order to either be pumped by means of lifting pumps up to the surface or to be conveyed to the surface overcoming gravity through the pressure built up in the reservoir by heating and/or injection of steam. 
     In the invention the electromagnetic heating process can be combined with a steam process which is injected for an improved permeability and/or conductivity e.g. by an additional electrolytic additive. It is also possible to have the steam simulation through the production pipe undertaken at the beginning of the heating-up phase or later cyclically. 
     In a specific development a purely electromagnetic-inductive method for heating up and extracting bitumen can be provided with especially favorable arrangement of the inductors. The essential factor here is to place one of the inductors directly over the production pipe, i.e. without any significant horizontal offset. An offset cannot be entirely avoided when drilling the bore holes however. The offset should be less than 10 m in any event, preferably less than 5 m, which is viewed as negligible with the corresponding dimensions of the deposit. 
     This involves the positioning of those inductors which are decisive for an extraction method without steam, as well as the electrical connection of the conductor sections. 
     Where the invention refers exclusively to electromagnetic heating, this is also called the EMGD (Ëlectro-{umlaut over (M)}agnetic {umlaut over (D)}rainage {umlaut over (G)}ravity) method. The EMGD method involves the positioning of the inductors with individual conductor sections which are very much the decisive factor for an extraction method without steam, as well as the electrical connections of the conductor sections. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details and advantages of the invention emerge for the subsequent description of the figures of exemplary embodiments based on the drawing in conjunction with the patent claims. 
       The figures show the following schematic diagrams: 
         FIG. 1  a section through an oil sands reservoir with injection and extraction pipe, 
         FIG. 2  a perspective section from an oil sands reservoir with an electric conductor loop running horizontally in the reservoir, 
         FIG. 3  an illustration of the electrical compensation of longitudinal conductor inductances by series capacitors, 
         FIG. 4  a section through a conductor with tubular electrodes of the integrated capacitors, 
         FIG. 5  a conductor with tubular electrodes of the integrated capacitors nested within one another,  FIG. 6  a tubular electrode with integrated capacitors and an apparatus for introducing electrolyte, 
         FIGS. 7   a  and  7   b  the electrical principle of the apparatuses according to  FIG. 4  and  FIG. 5  as a conventional coaxial arrangement, 
         FIG. 8  a first embodiment of the circuit technology of a power generator for an inductive heating circuit which is suitable for use in FIG.  1 / 2 , 
         FIG. 9  a second embodiment of the circuit technology of a power generator for an inductive heating circuit with parallel connection of inverters, 
         FIG. 10  a third embodiment of the circuit technology of a power generator for an inductive heating circuit with series connection of clocked inverters. 
         FIG. 11  by combination of  FIG. 1  and  FIG. 2 , the prior art of the SAGD method with electromagnetic-inductive support, 
         FIG. 12  the electrical connection of the inductive conductor sections with two conductor sections, 
         FIG. 13  the electrical connection of the inductive conductor sections with three conductor sections with parallel connection of two conductor sections 
         FIG. 14  the electrical connection of the inductive conductor sections with three conductor sections with alternating current and 
         FIGS. 15 to 16  four variants of the new EMGD method with different arrangement of the inductors. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The same units or units that act in the same way are provided in the figures with the same or corresponding reference signs. The figures are described below in groups together in each case. 
     An oil sands deposit  100  referred to as a reservoir is shown in  FIGS. 1 and 2 , with subsequent remarks always identifying a cuboid unit  1  of length  1 , width w and height h. The length  1  can amount to several multiples of 500 m, the width w to 60 m and the height h to between 20 and 100 m. It should be noted that, starting from the surface of the earth E, a “superstructure” of size s of up to 500 m can be present. 
     For realizing the SAGD method, according to  FIG. 1  an injector pipe  101  for steam or a water/steam mixture and an extraction pipe  102  for the liquefied bitumen or oil is present in the known way in the oil sands reservoir  100  of the deposit. 
       FIG. 2  shows an arrangement for inductive heating. This can be formed by a long, i.e. a few hundred m to 1.5 km conductor loop  10  to  20  laid in the ground, with inductor conductors  10  and  20  being routed next to one another at a predetermined distance and being connected to each other as a conductor loop at the end via an element  15  or  15 ′. The element  15  is especially arranged outside the reservoir  100  and the element  15 ′ alternately inside the reservoir. At the start the conductors  10  and  20  are routed vertically or at a shallow angle through the superstructure to the reservoir  100  and supplied with electrical power by an HF generator  60  which can be accommodated in an external housing. In particular the conductors  10  and  20  run at the same depth alongside one another, but also possibly above one another. There is a lateral offset of the conductors  10  and  20 . 
     Typical spacings between the outward and return conductors  10 ,  20  are between 5 and 60 m for an external diameter of the conductors of between 10 and 50 cm (0.1 to 0.5 m). 
     An electrical twin conductor  10 ,  20  in  FIG. 2  with the typical dimensions given here has a longitudinal inductance figure of 1.0 to 2.7 μH/m. The cross capacitance figure for the dimensions given is only between 10 and 100 pF/m so that the capacitive cross currents can be initially ignored. Ripple effects are to be avoided in such cases. The ripple speed is given by the capacitance and induction figure of the conductor arrangement. The characteristic frequency of the arrangement is conditional on the loop length and the ripple propagation speed along the arrangement of the twin conductor  10 ,  20 . The loop length is thus to be selected short enough for no disruptive ripple effects to be produced here. 
     It can be shown that the simulated power loss density distribution in a plane at right angles to the conductors—as is embodied in an opposing-phase powering of the upper and lower conductor—reduces radially. 
     For an inductively-introduced heating power of 1 kW per meter of twin conductor, at 50 kHz a current amplitude of around 350 A is needed for low-resistance reservoirs with specific resistances of 30 Ω·m and around 950 A for high-resistance reservoirs with specific resistances of 500 Ω·m. The required current amplitude for 1 kW/m falls quadratically with the excitation frequency. I.e. at 100 kHz the current amplitudes fall to ¼ of the above values. 
     At an average current amplitude of 500 A at 50 kHz and a typical inductance figure of 2 μH/m the inductive voltage drop amounts to around 300 V/m. 
     With the overall lengths of the twin conductors  10 ,  20  given above the overall inductive voltage drop would add up to values &gt; 100 kV. Such high voltages must be avoided for the following reasons:
         A controlling inverter is characterized by the apparent power, i.e. the blocking voltage and current carrying capacity, so that the reduction of the reactive power demand is vital.   The electrodes would have to be insulated from the reservoir  100  to be high-voltage-proof in order to suppress a resistive current flow, which requires large insulation thicknesses and would make the electrodes and their insertion into the reservoir more expensive.   Insulation problems or dangers of flashover, especially at the current conducting points.       

     There is therefore provision to compensate for the conductor inductance L in sections by discrete or continuously embodied series capacitances C, as is shown schematically in  FIG. 3 . This type of compensation is actually known from the prior art in inductive energy transmission systems on translationally moved systems. In the current context this provides particular advantages. 
     A peculiarity of a compensation integrated into the conductor is that the frequency of the RF conductor generator must be tuned to the resonant frequency of the current loop. This means that the twin conductor  10 ,  20 , when used for heating purposes, i.e. with high current amplitudes, can only be operated at this frequency. 
     The decisive advantage in the latter mode of operation lies in the fact that an addition of the inductive voltages along the conductor is prevented. If in the example given above—i.e. 500 A, 2 μH/m, 50 kHz and 300 V/m—a capacitor C i  of 1 μF capacitance is inserted every 10 m in the outwards and return conductor, the operation of this arrangement can be carried out resonantly at 50 kHz. This limits the inductive and accordingly capacitive sum voltages occurring to 3 kV. 
     If the distance between adjacent capacitors C i  is reduced the capacitance values must conversely increase in proportion to the distance—with a reduced requirement for the dielectric strength of the capacitors in proportion to the distance in order to retain the same resonant frequency. 
       FIG. 4  shows an advantageous embodiment of capacitors integrated into the conductor with respective capacitance C where the conductor includes an insulating tube  30 , a tubular outer electrode  32 , and a tubular inner electrode  34 . The capacitance is fanned by cylinder capacitors C i  between a tubular outer electrode  32  of a section I and a tubular inner electrode  34  of the section II, between which a dielectric  33  is located. The adjacent capacitor between the sections II and III is formed in an entirely corresponding way. 
     For the dielectric of the capacitor C, as well as a high dielectric strength, a high temperature resistance is also a requirement, since the conductor is located in the inductively-heated reservoir  100 , which can reach a temperature of 250° C. for example, and the resistive losses in the conductors  10 - 20  can lead to a further heating up of the electrodes. The requirements imposed on the dielectric  33  are fulfilled by a plurality of capacitor ceramics. The dielectric  33  may also be formed from composites based on Teflon, glass fiber, and ceramic. 
     For example the group of aluminum silicate, i.e. porcelains, exhibit temperature resistances of several 100° C. and electrical flashover resistances of &gt;20 kV/mm with permittivity figures of 6. This means that the above cylinder capacitors can be realized with the required capacitance and can typically be between 1 and 2 m in length. 
     If the length is to be shorter, a nesting of the number of coaxial electrode in accordance with the principle illustrated in  FIGS. 5 and 7   b  is to be provided. Other normal capacitor designs can also be integrated into the conductor provided these the exhibit the required voltage and temperature resistance. 
     In  FIG. 4  the entire electrode is already surrounded by an insulation layer  31 . The insulation from the surrounding earth is necessary to prevent resistive currents through the earth between the adjacent sections, especially in the area of the capacitors. The insulation further prevents the resistive current flow between outward and return conductor. The requirements in respect of the dielectric strength the insulation are however reduced by comparison with the non-compensated conductor of &gt;100 kV in the above example to something over 3 kV and can therefore be met by a plurality of insulating materials. The insulation, like the dielectric of the capacitors, must have permanent resistance to higher temperatures, with ceramic insulation materials again being suitable for this purpose. In such cases the insulation thickness of the insulation layer  31  should not be selected too small since otherwise capacitive leakage currents could flow out into the surrounding earth. Insulation material thicknesses greater than 2 mm for example are sufficient in the above exemplary embodiment. 
     In detail  FIG. 5  shows that the number of tubular electrodes are connected in parallel. Advantageously the parallel connection of the capacitors can be used to increase the capacitance or to increase its dielectric strength. The electrical principle for this is shown in  FIG. 7   b.    
     In an arrangement in accordance with  FIG. 4  an introduction of an electrolyte  45  in sections can be carried out for explicitly increasing the heating effect. In  FIG. 6  the compensated electrode is expanded by an insulated inner pipe  40  with insulated outlet openings  41 ,  42  and  43 . This enables water or an electrically-conductive aqueous salt solution or another electrolyte to be introduced into the reservoir for example in order to increase the conductivity of the reservoir. 
     The introduced water can also serve to cool the conductor. If the outlet openings are replaced by valves the change in conductivity can be explicitly undertaken temporally and spatially in sections. 
     The increase in the conductivity is used to increase the inductive heating effect without having to increase the current amplitude in the conductors. 
     In  FIGS. 4 and 5  the longitudinal inductances are therefore compensated for by means of primarily concentrated cross capacitances. Instead of introducing more or fewer short capacitors as concentrated elements into the conductor, the capacitance figure that a two-wire conductor such as a coaxial conductor or multiwire conductors for example provided in any event over their entire length can be used to compensate for the longitudinal inductances. To this end the inner and outer conductor are interrupted alternately at equal distances and thereby the current flow forced over the distributed cross capacitances. Such a method is described in DE 10 2004 009 896 A1. In this document belonging to the prior art it is explained in detail how the resonant frequency can be adjusted by the distances between the conductor interruptions. 
     The latter concepts, which are illustrated with reference to  FIG. 7   a  and  FIG. 7   b , can also be used to advantage here for the conductors for inductive reservoir heating, if the conductors—as already described above—are provided with an additional outer insulation in order to suppress resistive cross currents into the surrounding earth. In detail the numbers  51  to  53  in these figures indicate the electrodes, C i  indicates the capacitances distributed via the electrodes and  54  indicates a respective interruption of the conductor. The advantage of the distributed capacitances lies in a reduced requirement for dielectric strength of the dielectric. 
     Naturally a compensated electrode with distributed capacitances in combination with an apparatus for introducing electrolyte can also be used. 
     A heating effect is not desirable in the superstructure through which the outward and return conductor to reservoir  100  are routed vertically. In the vertical area of the twin conductors  10 ,  20  which does not yet lie in the reservoir  100 , but leads down to the latter, outwards conductor  10  and return conductor  20  can be placed at a small distance of for example 1 to 3 m away from each other, whereby their magnetic fields already compensate for each other in the smaller distance from the twin conductor and the inductive heating effect is correspondingly reduced. 
     As an alternative outwards conductor  10  and return conductor  20  can be surrounded by a screening made of highly-conductive material surrounding one of the two conductors in order to avoid the inductive heating up of the surrounding earth of the superstructure. 
     In a further alternative a coaxial conductor arrangement in the vertical area of outwards and return conductor is conceivable which leads to a complete extinction of the magnetic fields in the outer area and thereby to no inductive heating up of the surrounding earth. The increased cross capacitance figure in this case can be employed to assist the embodiment of the gyrator which in accordance with the prior art converts a voltage of a voltage-injecting current converter into an alternating current. 
     In all three of the given methods a compensation of the respective inductance figure of the conductor arrangement including the screening which may be present is necessary. 
     A power generator  60  which is embodied as a high-frequency generator is shown in  FIG. 8 . The power generator  60  is a three-phase design and advantageously contains a transformational coupling and power semiconductor as its components. The actual compensated conductor loop  10 ,  20  is shown in this diagram abstracted as an inductor  95 . In particular the circuit contains a voltage-injecting converter. A current injection with load-independent fundamental mode which is able to be set by means of filter components, with a suitable choice of adaptation quadripole is produced beyond the latter. Depending on the topology of the quadripole, a different current loading of the feeding converter is produced. 
     The high-frequency generator  60  embodied as a power generator in accordance with  FIG. 7  can generate outputs of up to 2500 kW. Typically frequencies of between 5 and 20 kHz are used. 
     If necessary higher frequencies can also be employed. In such cases increased switching losses which are sometimes too high occur in the feeding current converter. To remedy this:
         A number of inverters can be connected in parallel either at resonant frequency and small individual power and high overall power. For example the reader is referred to the topology from  FIG. 9 , in which the voltage-injecting full bridge, four-quadrant setter feed a parallel-switching filter which converts the square wave output voltage into an output current and of which the fundamental mode amplitude is independent of the load impedance.   Accordingly a number of inverters can be connected in series as in  FIG. 10 .   Alternately a number of inverters can also in the same topology as in  FIG. 10  can be operated with offset clocks at low individual frequency to obtain a high-frequency (resonant frequency fr) at the transformer output.       

     As already explained, with such a generator, operation under resonant conditions is required for use according to specifications in order to achieve a reactive power compensation. If necessary the activation frequency in operation is to be suitably adjusted. 
       FIG. 8  illustrates the function of the RF generator already mentioned in conjunction with  FIG. 2 : Starting from the three-phase AC mains source  65 , a three-phase inverter  70  is activated, downstream from which is connected via a conductor with capacitor  71  a three-phase inverter  75  that generates periodic square-wave signals of suitable frequency. Inductors  95  are controlled as an output via an adaptation network  80  consisting of inductances  81  and capacitors  82 . It is possible to dispense with the adaptation network. 
     With a pure conductor loop  10 ,  15 ,  20  according to  FIG. 2 , which represents a two-pole inductor, a single-phase generator can also be used. Such generators, with 440 KW at 50 Hz, are commercially available. 
     Shown in  FIG. 9  is a corresponding circuit with three parallel-switched inverters  75 .  75 ′,  75 ″. Connected downstream here is an example of an adaptation network  85  comprising inductances  86 ,  86 ′ and  86 ″. The adaptation network  85  is followed, as in  FIG. 8 , by the inductors not shown in any greater detail here. 
     Finally the function of a series circuit of three inverters  75 ,  75 ′,  75 ″ is realized in  FIG. 10 , in which higher frequencies and powers, are achieved by offset clocking or higher voltages and thereby powers are achieved with in-phase clocking. For this the switched inverters  75 ,  75 ′,  75 ″ are connected by means of a transformer  80  to inductances  81 ,  81 ′,  81 ″ on the primary side as well as inductances  82 ,  82 ′,  82 ″ on the secondary side, so that a series circuit is produced on the secondary side. An adaptation quadripole of the inductors can again be connected upstream of the transformer. 
     The described RF generators can basically be used as described as voltage-injecting converters or accordingly as current-injecting converters in reservoirs, with or without there being support by steam. Reservoirs with lower horizontal permeability, which are insufficiently permeable to steam, can be heated up over wide areas with this method. Even if the electrical conductivity of the reservoir exhibits inhomogeneities—for example conductive areas that are insulated electrically from the rest of the reservoir, eddy currents can form in these islands and create Joulean heat. It is not effectively possible here to use vertical electrodes with resistive heating, since this requires contiguous electrically-conductive areas between the electrodes. In addition the conductance of the reservoir and permeability are related. 
     In  FIG. 11 , which basically shows a combination of  FIGS. 1 and 2  in a projection view, the following labels are used.
     0: Section of oil reservoir, is repeated multiply on both sides   1′: Horizontal well pair, with injection pipe a and production pipe b, shown in cross section   A: 1st horizontal, parallel inductor   B: 2nd horizontal, parallel inductor   4: Inductive power supply by electrical connection to the ends of the inductors (according to  FIG. 12 )   w: Reservoir width, distance from one well pair to the next (typically 50 to 200 m)   h: Reservoir height, thickness of the geological oil layer (typically 20 to 60 m)   d1: horizontal distance from A to 1 is w/2   d2: vertical distance from A and B to a: 0.1 m to 0.9*h (typically 20 m to 60 m)   

     Arranging a conductor section or the conductor loop directly above the production pipe gives the specific advantage that the bitumen in the environment above the production pipe is heated up in a comparatively short time and thereby becomes thin. The effect of this is that production begins after a comparatively short time (e.g. 6 months) which is accompanied by a relieving of the pressure of the reservoir. Typically the pressure of a reservoir is limited and dependent on the strength of the superstructure in order to prevent the vaporized water from breaking through (e.g. 12 bar at a depth of 120 m, 40 bar at a depth of 400 m, . . . ). Since the electric heating results in an increase in pressure in the reservoir, the amount of power for heating up must be controlled as a function of the pressure. This in its turn means that a higher heating power is only possible once production has started. The early extraction is made possible by arranging the inductors close to one another. Putting two inductors that are linked into a conductor loop close to one another is not possible since then the inductive heating power would be greatly reduced and the amount of power required in the cable would become too great. 
     The associated electrical circuit emerges from  FIGS. 12 to 14 . A distinction is to be made here as to whether there are two or three conductor sections. 
     In  FIG. 13  A is a first inductive conductor section and B is a second inductive conductor section, to which a converter/high-frequency generator  60  from  FIG. 2  is connected. 
       FIG. 13  shows a switching variant in which three inductors are used, with two of these carrying half of the current. In  FIG. 13  A is a first inductive conductor section, B is a second inductive conductor section and C is a third inductive conductor section, with conductor sections B and C being connected in parallel. Other combinations of the conductor sections are also possible. A converter/high-frequency generator is present. 
       FIG. 14  shows a switching variant in which three inductors are likewise used, but which are connected to an alternating current generator and therefore all have the same amount of current. In  FIG. 14  A is a first inductive conductor section, B is a second inductive conductor section and C is a third inductive conductor section. All conductor sections are connected to an alternating current converter/high-frequency generator. 
     The switching variants according to  FIGS. 12 through 14  are used to realize the arrangements of the inductors in the reservoir described below on the basis of  FIGS. 15 through 18 . In this case one inductor, for example inductive conductor section A or A′, serves as outward conductor and one inductor B or B′ as return conductor, with outward conductor and return conductor in this case carrying the same current strength with a phase offset of 180° in relation to the sectional diagrams in  FIGS. 15 and 16 . 
     As depicted in  FIG. 13 , one inductor A can also serve as the outward conductor and two inductors B and C as the return conductors. In this case the parallel-switched return conductors B, C each have the current strength with an offset of 180° in relation to the current of outward conductor A. 
     Finally one inductor can serve as an outwards conductor and more than two conductors as return conductors, with the phase offset of the currents of the outward conductor to all return conductors amounting to 180° and the sum of the return conductor currents corresponding to the outward conductor current. 
     In accordance with  FIG. 14  three inductors A, B and C can carry the same current strength and the phase offset between said conductors can be 120°. The three inductors A, B and C are fed on the input side by the alternating current generator and are connected on the output side in a star point which can lie with or outside the reservoir and corresponds to the connection element  15 . In such cases it is also possible for the three inductors A, B and C to carry unequal current strengths and to have phase offsets other than 120°. Current strengths and phase offsets are selected such that a circuit with a star point is made possible. In this case the sum of the outward currents corresponds at all times to the sum of the return currents. 
       FIG. 15  shows a first advantageous embodiment for an EMGD method. One inductor is present above the production pipe and a second inductor on the line of symmetry. The labels have been selected as follows:
     0: Section of oil reservoir, is repeated multiply on both sides   b: Production pipe, shown in cross section   A: 1st horizontal, parallel inductor   B: 2nd horizontal, parallel inductor   A′: 1st horizontal, parallel inductor of the adjacent reservoir section   4: Inductive power supply by electrical connection to the ends of the inductors (according to  FIG. 4 )   w: Reservoir width, distance from one well pair to the next (typically 50 to 200 m)   h: Reservoir height, thickness of the geological oil layer (typically 20 to 60 m)   d1: horizontal distance from A to B (w/2)   d2: vertical distance from B to b: preferably 2 m to 20 m   d3: vertical distance from A to b: preferably 10 in to 20 m   

     A further advantageous embodiment of an EMGD method is shown in  FIG. 16 . The figure shows a first inductor above the production pipe and a second inductor on the line of symmetry, but by contrast with  FIG. 15  there are two separate circuits. The labels have been selected as follows:
     0: Section of oil reservoir, is repeated multiply on both sides   b: Production pipe, shown in cross section   A: 1st horizontal, parallel inductor   B: 2nd horizontal, parallel inductor   A′: 1st horizontal parallel inductor of the adjacent reservoir section   B′: 2nd horizontal parallel inductor of the adjacent reservoir section   4: Inductive power supply by electrical connection to the ends of the inductors (according to  FIG. 13 )   w: Reservoir width, distance from one well pair to the next (typically 50 to 200 m)   h: Reservoir height, thickness of the geological oil layer (typically 20 to 60 m)   d2: horizontal distance from A to B (w/2)   d2: vertical distance from B to b: preferably 2 m to 20 m   d3: vertical distance from A to b: preferably 10 m to 20 m.   

     A third advantageous embodiment of an EMGD method is shown in  FIG. 17 . There is a first inductor above the production pipe and two inductors on the line of symmetry, with the circuit being branched. The labels have been selected as follows:
     0: Production pipe, shown in cross section   A: 1st horizontal, parallel inductor directly above the production pipe b   B: 2nd horizontal, parallel inductor on the line of symmetry to the adjacent reservoir section   C: 3rd horizontal, parallel inductor on the line of symmetry to the adjacent reservoir section 4: inductive power supply by electrical connection to the ends of the inductors (in accordance with  FIG. 13 )   5: Second inductive power supply by electrical connection to the ends of the inductors   w: Reservoir width, distance from one well pair to the next (typically 50 to 200 m)   h: Reservoir height, thickness of the geological oil layer (typically 20 to 60 m)   d2: horizontal distance from A to B (w/2)   d2: vertical distance from B to b: preferably 2 m to 20 m   d3: vertical distance from A to b: preferably 10 m to 20 m.   

     A fourth advantageous embodiment of the invention for an EMGD method is shown in  FIG. 18 . There is a first inductor above the production pipe and there are two further inductors with lateral offset, with a branched circuit again being present. The labels have been selected as follows:
     0: Section of oil reservoir, is repeated multiply on both sides   b: Production pipe, shown in cross section   A: 1st horizontal, parallel inductor directly above the production pipe b   B: 2nd horizontal, parallel inductor   B: 3rd horizontal, parallel inductor   4: Inductive power supply by electrical connection to the ends of the inductors (according to  FIG. 13  or  14 )   w: Reservoir width, distance from one well pair to the next (typically 50 to 200 m)   h: Reservoir height, thickness of the geological oil layer (typically 20 to 60 m)   d1: horizontal distance from A to C and from B to A (w/2)   d2: vertical distance from B to b: preferably 2 m to 20 m   d3: vertical distance from C and B to b: preferably 5 m to 20 m.   

     This document has described different variants which put the subject matter of the main patent application for the EMGD method in concrete terms. The following variants are viewed as especially advantageous: 
       FIG. 15  with the switching variants according to  FIG. 12 . An inductor B is located above the production pipe b, the second inductor A is located on the border of symmetry to the adjacent part reservoir. 
       FIG. 16  with two circuits switching variants according to  FIG. 12 . Two inductors A and A′ are located on the borders of symmetry to the adjacent part reservoirs. Two inductors B and B′ are located above the production pipe b as well as the production pipe of the adjacent part reservoir not shown here. 
       FIG. 17  with switching variant according to  FIG. 13  or  14 . One inductor A is located above the production pipe b, the second inductor B is located on the border of symmetry to the left-hand adjacent part reservoir. The third inductor C is located on the border of symmetry to the right-hand adjacent part reservoir. 
       FIG. 18  with switching variant according to  FIG. 13  or  14 . One inductor A is located above the production pipe, the second inductor B is located at a horizontal distance d 1  from the latter. The third inductor C is likewise located at a horizontal distance d 1 , but on the other side however.