Radio Frequency (RF) System For The Recovery Of Hydrocarbons

Abstract

The present invention relates to a system for facilitating the extraction of hydrocarbons, in particular extraction by RF heating of high-viscosity hydrocarbons in situ by means of an antenna comprising a coaxial array of mode converters.

Claims

1. System for heating highly viscous hydrocarbons in a reservoir comprising at least one drain, the system being characterized in that it comprises: a radio-frequency generator adapted to generate an electromagnetic signal; a coaxial transmission line, connected to said generator and adapted to transmit the signal along said drain, said coaxial transmission line comprising an outer conductor and an inner conductor separated by a layer of dielectric material; at least a mode converter positioned along said coaxial transmission line inside the well, in which said at least one mode converter interrupts said coaxial transmission line and comprises a first and a second conductor, said first conductor realizing an electrical connection between said outer conductor of said coaxial transmission line upstream of said mode converter and said outer conductor of said coaxial transmission line downstream of said mode converter, said second conductor realizing an electrical connection between said inner conductor of said coaxial transmission line upstream of said mode converter and said inner conductor of said coaxial transmission line downstream of said mode converter; said at least one mode converter being arranged, in the presence of an RF signal along said coaxial transmission line to disturb the differential mode of signal propagation along said coaxial transmission line and to induce a current on said outer conductor of said coaxial transmission line and an electromagnetic field in the surrounding space that causes a heating of the hydrocarbons within said reservoir.

2. The apparatus according to claim 1 comprising a plurality of said mode converters distributed along said coaxial transmission line inside said well, in which said plurality of mode converters interrupts said coaxial transmission line.

3. The apparatus according to claim 2, wherein said plurality of mode converters comprises an array of said mode converters arranged at regular intervals along said coaxial transmission line.

4. The apparatus according to claim 2, wherein at least one of said plurality of mode converters is an inductive type, in which the perturbation of the differential mode signal propagation along said coaxial transmission line is caused by at least one inductive element.

5. The apparatus according to claim 2, wherein at least one of said plurality of mode converters is of capacitive type, in which the perturbation of the differential mode signal propagation along said coaxial transmission line is caused by at least one capacitive element.

6. The apparatus according to claim 2, wherein at least one of said plurality of mode converters is capacitive and inductive, wherein the perturbation of the differential mode signal propagation along said coaxial transmission line is caused by at least one capacitive element and by at least one inductive element.

7. The apparatus according to claim 1, wherein the electromagnetic signal generated by said radio-frequency (RF) generator has a frequency between 0.1 and 10 MHz.

8. The apparatus according to claim 7, wherein the RF signal has a frequency of between 0.5 and 5 MHz.

9. The apparatus according to claim 3 in which said plurality of mode converters are dimensioned from the electrical point of view and positioned along said array in order to obtain a distributed/controlled radiation along said array itself.

10. A method for the extraction of hydrocarbons comprising the step of heating the hydrocarbons within said reservoir and said drain through an apparatus according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Reference will now be made to a series of drawings to facilitate the description of some preferred embodiments of the present invention:

[0035] FIG. 1 shows a system for heating high-viscosity hydrocarbons in a drain according to a preferred embodiment of the present invention;

[0036] FIG. 2 shows the mechanism of electromagnetic mode conversion according to a preferred embodiment of the present invention;

[0037] FIG. 3 shows a mode converter according to an embodiment of the present invention;

[0038] FIG. 4 shows some alternative embodiments of a mode converter;

[0039] FIG. 5 shows possible embodiments for the end of the antenna that may be used in the system according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0040] In a preferred embodiment of the present invention, 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 of the invention of this kind 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.

[0041] In a further preferred embodiment of the present invention, the system operating at frequencies of between 0.1 and 10 MHz is used for the recovery of heavy oils.

[0042] The system to which the present invention relates may 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.

[0043] Furthermore, the system to which the present invention relates allows RF lines of limited section to be obtained, which is an advantageous aspect when installing the antenna directly in producing wells of standard dimensions without the need for additional, dedicated wells.

[0044] The system to which the present invention relates is thus characterised by the ability to irradiate along the drain at the frequencies concerned in controlled manner. Particularly advantageous is the configuration in which irradiation is uniform, or rather the power irradiated from each mode converter is constant along the drain.

[0045] According to a preferred embodiment of the present invention, the system as illustrated in FIG. 1 includes an RF generator 101, a well perforator 103, a coaxial RF connector 105 and the coaxial array of mode converters 107 that comprise the antenna system 100. The RF generator 101 is advantageously installed on 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.

[0046] 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.

[0047] 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.

[0048] The wellhead perforator 103 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.

[0049] In an embodiment of the invention, the wellhead perforator is coaxial in construction. In another embodiment, the perforator has a two-wire construction.

[0050] Any electrical construction which gives limited insertion loss and return loss values may be used to form the perforator.

[0051] The coaxial transmission line 105 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.

[0052] 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.

[0053] 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 types of dielectric materials may also advantageously be used to form the coaxial cable.

[0054] The antenna of the coaxial array of mode converters has a length compatible with that of the drian, or with a relevant proportion of the drain (e.g. 30%, 50% or 70%).

[0055] 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. Substantial lengths of bore hole may also be found in vertical or slant wells that intersect very thick reservoirs (for example drain lengths of 100 m).

[0056] In such contexts, the antenna of 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. The mode converters 107 are electrical constructions which are connected to one another along the coaxial cable 105. 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 transported 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. FIG. 2 shows an illustration of the mechanism for converting the electromagnetic mode, which is the operating principle underlying the antenna. The figure shows how the discontinuity in the transmission line (resulting from the presence of the mode converter) changes the distribution of currents along the line itself and produces common-mode currents outside the line.

[0058] An array of interconnected mode converters on a coaxial line forms the antenna installed in the section of drain.

[0059] The mode converters have at least two conductors. The first conductor 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 connects the core of the coaxial section upstream of the line to the core of the coaxial section downstream of the line.

[0060] Favourably, the geometry of the conductors in the mode converters is selected in order to create inductive and/or capacitive elements. Elements of this kind disturb the differential mode of propagation of the signal along the coaxial cable and allow a common mode to be set up. The latter induces currents in the external braid of the coaxial cable and an electromagnetic field in the surrounding area.

[0061] The electromagnetic field, of frequency f, heats the surrounding medium by means of inductive or dielectric heating mechanisms or a combination of the two.

[0062] In an embodiment of the invention, the currents that flow in the external braid induce a magnetic field in the surrounding area and in particular inside the reservoir. Variation in the magnetic field over time in turn induces an electrical field inside the reservoir, which produces eddy currents of J=sE, where J is the current density, s is the electrical conductivity of the reservoir and E is the induced electrical current. The power dissipated per unit of volume inside the geological medium is q=0.5 s E.sup.2. This procedure forms the basis for the RF heating by an antenna installed in the well.

[0063] The mode converters are elements connected to a coaxial cable on both sides by means of appropriate connectors, which may be coaxial or two-wire in type. 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.

[0064] 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.

[0065] The converters may be of the inductive-capacitive type. Converters of this kind are characterised by combinations of the constructions described above. FIG. 3 shows the general electrical layouts relating to the mode converters. The figure shows that various combinations of inductive and capacitive elements are possible. Either of the two conductors comprising the mode converter (internal and external) may include one or more inductive elements and/or one or more capacitive elements connected in series and/or in parallel. Another possibility is for the internal conductor or the external conductor to form a direct connection.

[0066] FIG. 4 shows specific embodiments of inductive, capacitive and inductive-capacitive mode converters. In particular, FIG. 4a 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 in which the internal conductor is interrupted by a pair of plates which create a capacitance parameter; FIG. 4b 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. 4c, 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. 4d, 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. 4e 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, unlike the structures above, in which coils are positioned laterally in relation to the internal conductor.

[0067] Positioning a mode converter on the coaxial line produces a discontinuity on the transmission line which causes a proportion of the power to be irradiated within the medium surrounding the antenna. The electromagnetic behaviour of a mode converter may be described by way of two fundamental parameters: the efficiency of radiation (proportion of power irradiated in relation to the power input to the mode converter) and the return loss (proportion of power reflected in relation to the power input).

[0068] The values of such parameters in a specific mode converter depend on various variables, in particular the values of inductance and/or capacitance brought about by a mode converter, the frequency and the electromagnetic characteristics (dielectric permittivity and electrical conductivity) of the reservoir, the electromagnetic characteristics of the fluids inside the well, and any antenna coverings. It follows that the design of the array and the mode converters, or rather the selection of the distance between mode converters along the coaxial array, the constructional type of converter and the relative values of inductance and/or capacitance as a function of the frequency range and the electromagnetic characteristics of the surrounding medium, is one of the major aspects in constructing the system to which the present invention relates.

[0069] In particular, the mode converters used to form an array generally have constructional characteristics that differ from one another. 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.

[0070] 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.

[0071] The end of the antenna (corresponding to the base of the well) may be formed in various ways. It may be a short circuit or an open circuit to return the remaining, non-irradiated power from the mode converters and to allow it to be irradiated as it returns along the antenna, or an antenna of the resonant type, such as a coaxial monopole to irradiate the remaining non-irradiated power from the array of mode converters.

[0072] FIG. 5 shows possible embodiments of the antenna end, in particular an open circuit, a short circuit and an antenna of the monopole type produced from the coaxial cable. The well may be an open bore hole within the reservoir, or it may advantageously be lined with a tube of non-conductive material (material such as glass fibre, PTFE or other thermoplastic materials, ceramics or systems of non-conductive materials of another type) to allow irradiation from the antenna installed within it.

[0073] The system to which the present invention relates may advantageously be formed by adapting the antenna to reservoirs having different properties or heterogeneous properties along the drain by the selection of the electrical parameters and the positioning of each mode converter along the array.

[0074] In one aspect of the present invention, the individual mode converters may be designed to control the profile of irradiation along the drain.

[0075] For example, digital simulations carried out on electromagnetic antenna modelling instruments show that, by establishing inductance values in the range from a few tenths to a few tens of microhenrys, it is possible to obtain a range of radiation efficiencies to result in homogeneous heating over a drain 1000 m long. For example, in a resistivity range within the reservoir of 50-200 ohm metres (a resistivity range which is typical of geological formations composed of rock matrices in which there is a high saturation of hydrocarbons and limited water saturation), it is possible to achieve a range of radiation efficiencies of between 1% and 3% (which is required for the construction of an array of 100 elements and a total antenna length of 1000 m) with a frequency of 1 MHz using mode converters of the inductive type (with a coil connecting the braid sections of the coaxial cable) that are characterised by inductance values of between approximately 0.5 uH and 10 uH. Such inductance values may be obtained by forming coils of a diameter that is compatible with the installation in the well and having a number of turns of between 8 and 32. Mode converters of this type may have a length in the order of 40-60 cm.

[0076] Moreover, with inductance values of this kind, little power is returned from each mode converter (for the first converters in the array, with efficiencies in the order of 1%, the return loss is around −24 dB, and for converters at the end of the array, with efficiencies in the order of 30% or more, the return loss is −10 dB) and this allows a target in the order of −15 dB of total return loss for the antenna to be achieved, a value which is sufficient for the application (equivalent to a transfer of power to the formation of 97% and of power returned towards the generator of 3%).

[0077] This exemplary embodiment shows the possibility of achieving distributed RF heating that gives high levels of performance. Moreover, electrical preconditions of this kind enable mode converters to be constructed whereof the section of the construction is limited to values compatible with their installation in drain of production wells. Purely by way of example, a diameter of 6 cm (equivalent to 2.4 inches) may be compatible with installation in the production well. This is because a production well could have a bore hole diameter of 8.5 inches and a liner having an internal diameter in the order of 5 inches. Thus, the exemplary embodiment allows the antenna to be installed in the well while leaving space for a possible antenna covering and for the flow of oil to the surface.

[0078] Installation of the RF system in the production well allows the effectiveness of thermal stimulation to be maximised while concentrating the heat close to the productive well and reducing the number of wells which have to be perforated in the production field. In another aspect of the present invention, it is possible to minimise the ohmic losses along the drain by utilising the coaxial transport line (most efficient transmission line in the range of frequencies concerned) in the antenna section as well. This may be achieved by using a low-attenuation coaxial cable to form the array of mode converters, such as the coaxial cable used for the RF connection between the surface and the antenna input. Measurements of reflection over a range of frequencies may be carried out on the RF line installed in the well by connecting the line to a spectrum analyser. Reflection measurements at the surface are dependent on the return of the corresponding signal from each mode converter. The information obtained from reflectometry may thus be utilised to monitor the radiation characteristics of the antenna and the surrounding medium and to optimise the operating frequency.

[0079] The system to which the present invention relates may advantageously be applied to the thermal recovery of an individual well or of separate wells (heater and producer) and may be combined with other advanced recovery methods (IOR/EOR, improved oil recovery/enhanced oil recovery).