Coaxially Arranged Mode Converters

Abstract

The present invention relates to a device for generating a disturbance in the differential mode of propagation of an RF signal transmitted along a coaxial transmission line.

Claims

1. A device for generating a disturbance in the differential mode of propagation of an RF signal transmitted along a coaxial transmission line, the coaxial transmission line including an external conductor and an internal conductor which are separated by a layer of dielectric material, the device including: a first conductor; a second conductor; connection means which are suitable for forming an electrical connection between the device and the coaxial transmission line such that said first conductor of said device forms an electrical connection between the external conductor of the coaxial transmission line upstream of said device and the external conductor of the coaxial transmission line downstream of said device, and said second conductor of said device forms an electrical connection between the internal conductor of the coaxial transmission line upstream of said device and the internal conductor of the coaxial transmission line downstream of said device; wherein, in the presence of an RF signal along the coaxial transmission line, a disturbance in the differential mode of propagation of the signal along the coaxial transmission line is generated, inducing a current in the external conductor of the coaxial transmission line and an electromagnetic field in the area surrounding the coaxial transmission line.

2. The device according to claim 1, in which said first conductor includes at least one inductive element.

3. The device according to claim 1, in which said first conductor includes at least one capacitive element.

4. The device according to claim 1, in which said second conductor includes at least one inductive element.

5. The device according to claim 1, wherein said second conductor includes at least one capacitive element.

6. The device according to claim 1, used in a system for facilitating the extraction of hydrocarbons by RF heating of high-viscosity hydrocarbons in situ by means of an antenna comprising a coaxial array of mode converters.

7. An array of devices according to claim 1, comprising an antenna used in a system for facilitating the extraction of hydrocarbons by RF heating of high-viscosity oils in situ.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Reference will now be made to a series of drawings to facilitate the description of some preferred embodiments of the present invention:

[0039] FIG. 1 shows a mode converter according to an embodiment of the present invention;

[0040] FIG. 2 shows some alternative embodiments of a mode converter;

[0041] FIG. 3 shows a mode converter according to an embodiment of the present invention with an example of connection interfaces with the coaxial line.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0042] According to an embodiment of the present invention, the device includes electrical constructions which may be used as mode converters for the formation of the RF antenna in the well. A system for heating the wells by means of a coaxial antenna to which the (one or more) devices according to the present invention may be applied is for example described in the patent application filed, in parallel with the present one, by the same applicant.

[0043] The system operates by applying power in the order of 100-1000 kW at frequencies in the range of 0.1-10 MHz. An embodiment according to these parameters may be advantageous in achieving moderate heating along a drain in the order of several hundred metres in length, such as 1000 m or more. An embodiment of this kind may increase the productivity of a heavy oil well to a significant extent, at the same time ensuring a limited expenditure of energy per barrel of oil produced. In an embodiment of this kind, the increase in temperature may be 50° C. at the well, 28° C. five metres away from the well in the radial direction, 13° C. ten metres away and 10° C. fifteen metres away. In a further embodiment, the system operates at frequencies of 0.1 -10 MHz and is used to recover heavy oils.

[0044] The system may furthermore be suitable, by way of the design of the array parameters, for different reservoirs and for achieving the desired distribution of RF radiation along the well.

[0045] The system is thus characterised by the ability to irradiate along the drain at the frequencies concerned in controlled manner.

[0046] Particularly advantageous is the configuration in which irradiation is uniform, or rather the power irradiated from each mode converter is constant along the drain.

[0047] According to a possible configuration of the system for heating by means of RF radiation output by a coaxial antenna equipped with mode converters, the system includes an RF generator, a well perforator, a coaxial RF connection, and one or more (e.g. a coaxial array of) mode converters according to a preferred embodiment of the present invention. The RF generator is advantageously installed at the surface and operates within the range of frequencies of 0.1-10 MHz. In some embodiments, the generator may deliver power <1 MW to achieve moderate heating, if this is sufficient to reduce the viscosity of the heavy oils to a significant extent. In other embodiments, the power may be >=1 MW, if there is a requirement to reach high temperatures over a distance of several metres from the well in order to mobilise the hydrocarbon.

[0048] There are various ways to construct a high-power RF generator in the range of frequencies concerned. The transmitter may take the form of an array of solid state amplifiers, of vacuum tubes or of hybrid solutions combining the two.

[0049] The transmitter may also comprise an inverter. The generator may also incorporate an impedance adapter unit which adapts the output from the transmitter to the load in order to maximise the transfer of power to the medium. The generator output is connected to the well head by means of a coaxial cable.

[0050] The wellhead perforator according to the system described in the above-mentioned parallel patent application to the present one is the part of the system that enables the signal to be transmitted from the surface to the inside of the well by way of a construction integrated in the equipment at the well head. The two ends of the perforator are connected to the coaxial cable coming from the generator and the coaxial cable installed inside the well for the transmission of power to the base of the well.

[0051] The wellhead perforator is normally coaxial in construction or has a two-wire construction. Any electrical construction which gives limited insertion loss and return loss values may be used to form the perforator.

[0052] The coaxial transmission line at the base of the well is the construction allowing the signal to be transported to the base of the well, or to the antenna input. Different types of construction may be used to form the coaxial cable.

[0053] The coaxial cable must ensure characteristics that are appropriate for the distance over which power is to be transferred, in respect of both peak power and average power, and low attenuation of the signal, in order to be able to transfer the desired power to the base of the well continuously and to supply a high level of energy efficiency.

[0054] These characteristics improve as the diameter of the cable increases. To this end, the coaxial cable must be dimensioned with sections of external conductor (braid) and internal conductor (core) large enough to transfer the power over the desired distance. The characteristics of the coaxial cable also depend on the dielectric material separating the internal conductor from the external one. The use of materials with low dielectric losses enables the distance over which the cable can transfer power and the efficiency to be increased. Materials that can be used to form a cable suitable for the application are for example PTFE (polytetrafluoroethylene) and expanded PTFE, which have low losses. Other dielectric materials may also advantageously be used to form the coaxial cable. The antenna comprising a coaxial array of mode converters has a length compatible with that of the drain, or with a relevant proportion of the drain (e.g. 30%, 50% or 70%). The length of the antenna thus depends on the length of the drain and may thus vary with the type of well and reservoir. For horizontal wells, a typical drain length may be 1000 m. Lengths of drain and substantial sections of bore hole may also be found in vertical or slant wells that intersect very thick reservoirs (for example drain lengths of 100 m in vertical wells).

[0055] In such contexts, the antenna comprising the array of mode converters may be designed and used to heat the reservoir over the entire extent of the drain of the vertical or slant well.

[0056] The mode converters are electrical constructions which are connected to one another along the coaxial cable. The particular construction of the mode converters has the function of disturbing the differential mode of propagation of the RF signal along the cable. Disturbance of the propagation mode sets up a common mode. This produces currents that flow outside the coaxial cable in a coaxial section that is centred on the point where the mode converter is installed. An emf is associated with such external currents in the surrounding area, and this heats the geological formation. This mechanism transfers a proportion of the power transferred along the coaxial cable to the outside.

[0057] The use of an array of mode converters positioned along the coaxial line allows a considerable proportion or all of the power supplied to the coaxial cable to be transferred. The mode converters may be of the inductive type. Inductance may be brought about by the geometric structure of one of the two conductors or both the conductors. Inductance may be brought about by combining the geometric structure of the conductors with the use of materials of high magnetic susceptibility.

[0058] As an alternative, the converters may be of the capacitive type. Capacitance may be brought about by the geometric structure of one of the two conductors or both the conductors. Capacitance may be brought about by combining the geometric structure of the conductors with the use of materials of high dielectric permittivity.

[0059] The converters may also be of the inductive-capacitive type. Converters of this kind are characterised by combinations of constructions described above.

[0060] The inductance and/or capacitance values brought about by a mode converter are selected at the design stage of the antenna and depend on the electromagnetic characteristics of the reservoir, the electromagnetic characteristics of the fluids inside the well and any antenna coverings, and the efficiency of radiation sought for the particular mode converter.

[0061] In the case of a plurality of converters forming an array, the individual mode converters may have different structural characteristics from one another. In particular, the mode converters positioned at the beginning of the array must be designed to supply low radiation efficiency, that is to say to irradiate a limited proportion of the power that is input, and allow a substantial proportion of the power to be transmitted downstream. The mode converters positioned at the end of the array, by contrast, must supply a high radiation efficiency to irradiate a substantial proportion of the remaining power.

[0062] As illustrated in FIG. 1, the mode converter has at least two conductors: the first conductor connects the braid of the coaxial cable upstream of the device to the braid of the coaxial cable downstream of the device, and the second conductor connects the core of the coaxial cable upstream of the device to the core of the coaxial cable downstream of the device. The geometric shape adopted by these two conductors is such that inductive and/or capacitive elements are created along the transmission line. FIG. 1 shows an embodiment in which each of the two conductors creates four different elements, two inductive and two capacitive (for the external conductor these are C1, C2, L1 and L2; for the internal conductor these elements are C3, C4, L3 and L4). As shown in the figure, such elements may be connected to one another in series and/or in parallel in order to bring about equivalent inductance and capacitance values as desired for the application. The construction shown in FIG. 1 is an exemplary embodiment in which a plurality of inductive and capacitive elements are used within a single mode converter. In practice, a mode converter may advantageously be formed using only some of the inductive and capacitive elements shown in FIG. 1.

[0063] FIG. 2 shows some exemplary embodiments of mode converters derived from that shown in FIG. 1, where only some elements are selected.

[0064] In particular, FIG. 2a shows a mode converter of the inductive-capacitive type in which the external conductor is wound to form a coil structure which creates an inductance parameter, and the internal conductor is interrupted by a pair of plates which create a capacitance parameter; FIG. 2b shows a mode converter of the inductive-capacitive type in which the external conductor is interrupted by a pair of plates which create a capacitance parameter, and the internal conductor is wound to form a coil structure which creates an inductance parameter. FIG. 2c, by contrast, shows a mode converter of the inductive type in which the external conductor is wound to form a coil structure which creates an inductance parameter, and the internal conductor forms a direct link from the core of the coaxial cable upstream to the core of the coaxial cable downstream. FIG. 2d, by contrast, shows a mode converter of the inductive type in which the external conductor is wound to form a coil structure which creates an inductance parameter, and the internal conductor, like the external one, is also wound to form a coil structure which creates an inductance parameter; finally, FIG. 2e shows a mode converter of the inductive type in which the external conductor is wound to form a coil that is coaxial in relation to the internal conductor and in which, unlike the structures above, coils are positioned laterally in relation to the internal conductor.

[0065] As illustrated in FIG. 3, the mode converter 100 has, according to a preferred embodiment of the present invention, at least two conductors 103 and 105. The mode converter is joined into a coaxial transmission line (also called the antenna) that is connected to a generator and suitable for transmitting the signal along the drain, the coaxial line including an external conductor (also called the braid) and an internal conductor (also called the core) which are separated by a layer of dielectric material. The first conductor 103 of the mode converter connects the braid of the coaxial section upstream of the line to the braid of the coaxial section downstream of the line. The second conductor 105 connects the core of the coaxial section upstream of the line to the core of the coaxial section downstream of the line.

[0066] The mode converter may be connected to the coaxial cable by means of appropriate connectors, which may be of the coaxial or two-wire type. According to a preferred embodiment, as illustrated in FIG. 3, a connector 107 of the coaxial type ensures there is a connection between the mode converter 100 and the coaxial transmission line. The converter shown in FIG. 3 is of the inductive type, in which a central conductor 105 connects the core of the coaxial section upstream to the core of the coaxial section downstream and a coil conductor 103 of the coaxial type relative to the central conductor connects the braid of the coaxial section upstream to the braid of the coaxial section downstream.