Patent Application
20190048696
The embodiments described herein relate to the field of heating hydrocarbon formations, and in particular to antenna modules for electromagnetically heating hydrocarbon formations.
Electromagnetic (EM) heating can be used for enhanced recovery of hydrocarbons from underground reservoirs. Similar to traditional steam-based technologies, the application of EM energy to heat hydrocarbon formations can reduce viscosity and mobilize bitumen and heavy oil within the hydrocarbon formation for production. However, the use of EM heating can require less fresh water than traditional steam-based technologies. As well, the heat transfer with EM heating can be more efficient than that of traditional steam-based technologies, leading to lower capital and operational expenses. The lower cost of EM heating provides the potential to unlock oil reservoirs that would otherwise be unviable or uneconomical for production with steam-based technologies such as shallow formations, thin formations, formations with thick shale layers, and mine-face accessible hydrocarbon formations, for example. Hydrocarbon formations can include heavy oil formations, oil sands, tar sands, carbonate formations, sale oil formations, and other hydrocarbon bearing formations.
EM heating of hydrocarbon formations can be achieved by using an EM radiator, or antenna, or applicator, positioned inside an underground reservoir to radiate EM energy to the hydrocarbon formation. The antenna is typically operated resonantly. The antenna can receive EM power generated by an EM wave generator, or radio frequency (RF) generator.
As the hydrocarbon formation is heated, the characteristics of the hydrocarbon formation, and in particular, the impedance of the hydrocarbon formation, can change. In order to maintain efficient power transfer to the hydrocarbon formation, dynamic or static impedance matching networks can be used between the antenna and the RF generator to limit the reflection of EM power from the antenna back to the RF generator. As well, the RF generator can be adjusted to limit the reflection of EM power from the antenna back to the RF generator. Such operational adjustments and impedance matching networks increase operational, equipment, and design costs.
According to one aspect, there is provided a radio frequency antenna module in a radio frequency antenna for delivering electromagnetic energy generated by a generator into a hydrocarbon formation, the antenna module comprising: a conductive member; at least one conductive sheath with a first and second end surrounding at least one portion of the conductive member; at least one electrical coupler electrically coupled to the conductive member and the at least one conductive sheath for receiving the electrical energy; and an electrically insulating seal inserted at the first and second end of each of the at least one conductive sheath between the conductive member and the conductive sheath to maintain an enclosed cavity defined by the conductive member, the conductive sheath and the electrically insulating seal for electrically separating the conductive member and the conductive sheath.
In at least one embodiment, the electromagnetic energy radiates with a frequency between 1 kHz and 100 MHz.
In at least one embodiment, the conductive member comprises a first and second connector located at a first member end and a second member end, respectively, such that a plurality of irradiating modules are connectable to form at least one module chain.
In at least one embodiment, the first and second connector are electrically conductive such that each of the at least one module chain comprises a contiguous conductive member.
In at least one embodiment, the at least one module chain comprises a plurality of chains such that a first module chain set is configured to radiate independently of another module chain set.
In at least one embodiment, the first module chain set radiates at a first target frequency and the other module chain set radiates at a second target frequency.
In at least one embodiment, the conductive member is a pipe and each of the first and second connector provides a sealed connection that prohibits flow of fluids from the hydrocarbon formation into the pipe.
In at least one embodiment, the at least one conductive sheath comprises an inner conducting surface and an outer conducting surface; and for each of the at least one conductive sheath, a segment of coaxial transmission line having an inner and outer conductor is defined by that conductive sheath and a corresponding surrounded portion of the conductive member such that the outer conductor comprises the inner conducting surface of that conductive sheath and the inner conductor comprises a corresponding portion of the conductive member surrounded by that conductive sheath.
In at least one embodiment, a first sheath has a diameter that is different from at least one other conductive sheath.
In at least one embodiment, the conductive member has at least one surrounded conductive member portion and at least one exposed conductive member portion, and the antenna module further comprises: at least one segment of an inner single-conductor transmission line defined by the at least one exposed conductive member portion; and at least one segment of an outer single-conductor transmission line defined by the outer conductive surface of the at least one conductive sheath.
In at least one embodiment, the conductive member is a pipe comprising at least one feed transmission line that delivers the electromagnetic energy to the antenna module; and the at least one electrical coupler comprises at least two feed connectors located between two ends of the segment of coaxial transmission line such that each feed connector is connected to i) a feed transmission line at a first feed connector port;
and ii) at least one of a) the inside of the hollow pipe and b) the inner conducting surface of at least one conductive sheath at a second port.
In at least one embodiment, the at least one feed connector a plurality of feed connectors are azimuthally arranged around an inner surface of the pipe.
In at least one embodiment, the at least one feed connector comprises a plurality of feed connectors that are arranged axially along an inner surface of the hollow pipe.
In at least one embodiment, the at least one feed connector is located near one end of the segment of coaxial transmission line.
In at least one embodiment, the segment of coaxial transmission line has an electrical length that is substantially one half of a wavelength of the electromagnetic energy oscillating at a target frequency such that a perfect electric conductor boundary condition is defined in a plane that is situated at a mid-point of the segment of coaxial transmission line and oriented transversely relative to a longitudinal axis defined the conductive member.
In at least one embodiment, the at least one feed connector is located near a midpoint of the segment of coaxial transmission line.
In at least one embodiment, the segment of coaxial transmission line has an electrical length that is substantially one half of a wavelength of the electromagnetic energy oscillating at a target frequency such that a perfect magnetic conductor boundary condition is defined in a plane that is situated at a mid-point of the segment of coaxial transmission line and oriented transversely relative to a longitudinal axis defined the conductive member.
In at least one embodiment, the segment of coaxial transmission line has an electrical length that is substantially an odd multiple of one half of a wavelength the electromagnetic energy oscillating at a target frequency.
In at least one embodiment, the enclosed cavity comprises at least one dielectric material to separate the inner and outer conductor of the segment of coaxial transmission line.
In at least one embodiment, the electromagnetic energy generates electromagnetic heating to produce at least one evaporated zone in the hydrocarbon formation surrounding the antenna module to define a second coaxial transmission line comprising: a second inner conductor defined by portions of the inner single-conductor transmission line and the outer single-conductor transmission line; and a second outer conductor comprising an outer boundary separating the evaporated zone and the hydrocarbon formation.
In at least one embodiment, the seal is configured with at least one of the following properties: i) prohibits flow of fluids from the hydrocarbon formation into the enclosed cavity; ii) chemically inert; and iii) electrically insulating.
In at least one embodiment, the seal is toroidal in shape with a rectangular cross-section and further comprises concentric inner and outer structural rings, the inner structural ring located proximally to the conductive member and the outer structural ring located proximally to the conductive sheath.
In at least one embodiment, the inner and outer structural rings have an electrical loss tangent of less than 0.01.
In at least one embodiment, the conductive member has a diameter that varies along its length such that the diameter is larger at the at least one exposed conductive portion relative to the at least one surrounded conductive member portion to produce a flared conductive member.
According to one aspect, there is provided method for electromagnetic heating of a hydrocarbon formation comprising: deploying at least one antenna module into the hydrocarbon formation; operating at least one electromagnetic wave generator to generate at least one electromagnetic wave having at least one target frequency; electrically coupling the at least one antenna module to the at least one electromagnetic wave generator; and delivering at least one electromagnetic wave to the hydrocarbon formation, the electromagnetic wave corresponding to electromagnetic energy used to generate heat within the hydrocarbon formation.
In at least one embodiment, the electrically coupling comprises coupling a first electromagnetic wave generator to a first set of antenna modules and coupling a second electromagnetic wave generator to a second set of antenna modules so that the first set of antenna modules irradiates the hydrocarbon formation independently relative to the second set of antenna modules.
In at least one embodiment, the method further comprises configuring the first electromagnetic wave generator to generate a first electromagnetic wave at a first frequency; and configuring the second electromagnetic wave generator to generate a second electromagnetic wave a second frequency, wherein the first frequency is different from the second frequency.
In at least one embodiment, the deploying the at least one antenna module comprises connecting a plurality of antenna modules to form at least one module chain and deploying the at least one module chain into the hydrocarbon formation, wherein each antenna module in the plurality of antenna modules is connectable to another antenna module using a first and second electrically conductive connector located at a respective first member end and a second member end of the conductive member so that each module chain comprises a contiguous conductive member.
In at least one embodiment, the method further comprises determining a length of the at least one conductive sheath based on at least one of i) the at least one target frequency; ii) an outer diameter of the conducting member the at least one antenna module; iii) an outer diameter of the at last one conductive sheath; iv) material occupying the enclosed cavity; and v) electrical characteristics of the hydrocarbon formation.
Further aspects and advantages of the embodiments described herein will appear from the following description taken together with the accompanying drawings.
For a better understanding of the embodiments described herein and to show more clearly how they may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings which show at least one exemplary embodiment, and in which:
The skilled person in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the applicants' teachings in anyway. Also, it will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.
It will be appreciated that numerous specific details are set forth in order to provide a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the embodiments described herein. Furthermore, this description is not to be considered as limiting the scope of the embodiments described herein in any way, but rather as merely describing the implementation of the various embodiments described herein.
It should be noted that terms of degree such as “substantially”, “about” and “approximately” when used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree should be construed as including a deviation of the modified term if this deviation would not negate the meaning of the term it modifies.
In addition, as used herein, the wording “and/or” is intended to represent an inclusive-or. That is, “X and/or Y” is intended to mean X or Y or both, for example. As a further example, “X, Y, and/or Z” is intended to mean X or Y or Z or any combination thereof.
It should be noted that the term “coupled” used herein indicates that two elements can be directly coupled to one another or coupled to one another through one or more intermediate elements.
The electromagnetic (EM) heating of hydrocarbon formations such as heavy oil formations can be an attractive Enhanced Oil Recovery (EOR) technology for reasons that include the potential for unlocking heavy oil reservoirs which would generally not be economically produced using more traditional steam-based technology (e.g. shallow formations, thin formations, formations with thick shale layers, etc.); lower greenhouse gas emissions and significant reduction or complete elimination of the need for fresh water can reduce environmental impact; and improved efficiency from an energy balance point of view compared to the steam-based technologies as EM heating creates and maintains smaller amounts of steam inside the heavy oil reservoir.
An EM radiator in EM EOR solutions generally have a form of a single or multiple linear or loop radiators positioned inside a heavy oil reservoir. A radiator can be sometimes referred to as an antenna or applicator. The EM power can be generated on the surface from a power source, for example, an AC or DC power source. The EM generator is often referred to as a Radio Frequency (RF) generator. The EM generator generates power in the radio frequency range, typically between 100 kHz and 100 MHz. However in some cases the EM generator can be configured to generate in other frequency ranges such as from 50 kHz to 50 MHz or 1 kHz to 100 MHz. The generated power can then be transferred to the EM radiator using a feed transmission line such as a single conductor cable or a multiple conductor cable such as a coaxial cable.
The EM radiator radiates the EM power into the formation using a radiator such as a resonant antenna. The resonant antenna's operating frequency depends on the EM properties of the formation around the antenna and the antenna's length. This means that antennas designed for different formations and different well lengths may use different operating frequencies, requiring different impedance matching circuits and EM generators. Therefore, current EM heating systems are generally custom designed for each well, increasing the cost of the system. Moreover, as the EM properties of the formation change during the heating process, the antenna electrical characteristics may also change and require some form of variable impedance matching. For example, a system of dynamic or static matching networks can be required in-situ between the transmission line delivering EM power and the antenna to improve the efficiency of the heating system. Alternatively, such a system for impedance may be installed on the surface between the EM generator and the transmission line to limit the reflection of the EM power from the antenna back to the generator.
Additionally, most existing EM heating applications propose complex antenna systems that may require at least the following: isolation of conductor sections; an electrically lossless casing; designs using machined surfaces, for example grooves or slots; the use of exotic materials such as ferrites; or special deployment techniques. Such considerations often increase the cost of manufacture and maintenance making such systems generally expensive to operate and maintain. Furthermore, such systems may be less mechanically robust and may increase the possibility of equipment failure during deployment of operation underground. While travelling wave antennas may address some of the identified issues with respect to antenna design, they are typically excited from a single terminal. As a result, heating is concentrated close to that excitation terminal, which can increase the time required to heat the hydrocarbon formation or reservoir and reduces heating uniformity. In most systems, there is no way of increasing or decreasing power at a specific segment along the well. This may result in regions of excessive heating or insufficient heating, which increases operating cost.
Described herein is a radio frequency antenna which can be used in EM EOR processes as the radiator of EM energy into a heavy-oil formation. The antenna may also be used to heat bitumen and other hydrocarbon bearing formations or for environmental remediation. In particular, the described antenna comprises an excitation module with an electrically separated conductive sheath surrounding a portion of a conductive main member to define a coaxial line for guiding EM energy along the excitation module for deposition into the hydrocarbon formation. Each excitation module may operate as a travelling wave antenna that uses a single conductor for radiating or delivering the EM energy into the hydrocarbon formation. The structure of each excitation module can further function as an impedance matching circuit. Several excitation modules may be combined together in a modular manner to form larger irradiating structures. As such, the same excitation modules may be deployable into various types of hydrocarbon formations, into wells of various lengths and excitable at various frequencies. The antenna described herein can also be used with a distributed modular in-situ RF generator as described in U.S. patent application Ser. No. 14/508,423, or with a conventional RF generator located on the surface.
Referring to
The conductive member 102 may be used to provide structural integrity to the module 100 and thus to the antenna and for providing the energy transfer into the hydrocarbon formation or reservoir by guiding an electromagnetic wave such as a travelling wave along its exterior.
In the present embodiment, as shown in
The main pipe 102 can be fabricated using any appropriate conductive material including, but not limited to, aluminum, stainless steel, and carbon steel. It can also be built using composite materials, and may have a surface that's corrugated or cladded with other metals to achieve certain advantages. For example, in some embodiments, the main pipe 102 may be a carbon steel pipe cladded with aluminum. Such a pipe has higher mechanical strength than an aluminum pipe of the same dimensions, but can exhibit electrical conductivity of an aluminum pipe.
The cross-section of the main pipe 102 along a transverse axis can be, but is not limited to, circular as shown in
In some embodiments, the interior 116 of main pipe 102 may carry one or more AC or DC cables, control electronics, and other components of a distributed RF generator (not shown) provided with the main pipe. In some embodiments, feed transmission lines 120 can be used to carry RF power generated by RF power generation points located away from the main pipe 102 to electrical couplers located inside the main pipe 102. For example, the electrical couplers comprise of one or more module feed connectors 122 connected to the conductive sheath 108 as shown in
RF power generation points may be located at the surface, underground, or a combination of both. For example, if a surface RF generator is used, the RF power generation point is the surface RF generator itself and the feed transmission line 120, such as a coaxial cable, can carry the RF power from the surface to the module feed connectors 122. On the other hand, if an in-situ distributed RF generator is used, the RF power generation points may be located inside the main pipe 102, proximate to the module feed connectors 122. Therefore, a short section of the feed transmission line 120 such as coaxial cable may be needed to connect the power generation point and the module feed connector 122.
Referring still to
The extended antenna comprising a number of excitation modules 100 connected end-to-end can be regarded as a module chain 400 as shown in
The module chain 400 can be connected to the RF generator to receive power to radiate EM energy into the hydrocarbon formation. In some embodiments, several of such module chains 400 may be deployed into the hydrocarbon formation. Where several module chains 400 are used, a group of module chains 400 may be formed. In some cases each module chain 400 may share the same RF generator or obtain EM energy from its own dedicated RF generator. In the latter case, a module chain in the group of module chains 400 can be operable to radiate independently of another module chain within the group. In yet other embodiments, each RF generator may operate at a different target frequency. Such configurations can be used to obtain the desired heating of the hydrocarbon formation.
It may be noted that while such configurations are described with respect to module chains, the same configurations may be applicable to individual excitation modules 100 (or combination of module chains and individual modules) being deployed into the hydrocarbon formation. For example, a number of excitation modules 100 can be individually deployed into a hydrocarbon formation to provide EM heating. A generator can be connected to one or a group of excitation modules 100 allowing the one module or group of modules 100 to irradiate EM waves into the hydrocarbon formation. In other cases several RF generators may be used to deliver EM energy to several corresponding excitation modules 100 or several groups of excitation modules 100. Each excitation module 100 or group of modules 100 can irradiate independently of each other. The frequency of irradiation may also vary depending on the configuration of the respective RF generator so that each excitation module 100 or group of modules 100 irradiate at a different frequency.
In some embodiments, it may be preferable for the first and second pipe connectors 104 and 106 to be sealed to impede or prohibit mixing of fluids such as the liquid or gaseous compounds located in the hydrocarbon formation and inside of the main pipe 102. This separation may be particularly relevant when different liquids or gasses, or the same liquid or gas, but of different purities, are located inside and outside of the main pipe 102.
In other embodiments, it may be preferable for the first and second pipe connectors 104 and 106 not be sealed. These types of connectors would generally be less expensive and easier to fabricate. Such connectors can be used in situations where the same liquid and/or gas are present inside and outside of the main pipe 102 so that their mixing would not interfere with operation of any components interior to the main pipe 116 via mechanical, chemical or electrical means.
The conductive sheath 108 with an inner conducting surface 130 and outer conducting surface 132 may be provided to surround or enclose a portion of the length of the main pipe 102 to operate as a waveguide structure to provide EM excitation and impedance matching to a variety of electrical environments, as will be described in further detail subsequently. Each section of main pipe 102 of the excitation module 100 has at least one conductive sheath 108. In some embodiments, the excitation module 100 can have two, three or more conductive sheaths distributed along its length. In other embodiments in which multiple conductive sheaths are present, the diameters of the sheaths may vary in size so that the diameter of one conductive sheath is different from the diameter of another conductive sheath.
According to one embodiment, the conductive sheath 108 as shown in
The inner diameter of the conductive sheath 108 is larger than the outer diameter of the main pipe 102. The conductive sheath 108 may be concentric to the main pipe 102. However in some embodiments, the conductive sheath 108 does not surround or enclose the main pipe 102 concentrically. It may be noted that a cavity 124 can be created between the conductive sheath 108 and the surrounded portion of main pipe 102. The existence of the cavity allows for electrically separating the conductive sheath 108 and the main pipe 102. Both ends of the conductive sheath 108 further define apertures, i.e. they are not electrically connected to the main pipe 102 using a metal or any other electrically conducting material. As shown in
Portions of the main pipe 102 surrounded by the conductive sheath 108, together with the material(s) provided in the cavity 124 therebetween, may define a length or segment of coaxial transmission line with an aperture at either end. In some embodiments, a dielectric material can be provided to fill the cavity 124. In other cases, several types of dielectric materials may be used. The dielectric material can be introduced to provide structural support for the excitation module 100 and/or to control the electrical properties such as the electrical length. Such dielectric material can include fluids (e.g. pressurized fluids), or one or more solid dielectric materials, or surface structures such as corrugations, or combinations thereof. Dielectric materials can be, but are not limited to ceramics such as alumina, zirconia, titanium dioxide, etc.; glass; quartz; or synthetic polymers such as PEEK, Teflon, polyethylene (PE), etc.; structural ceramics or other composite materials.
As shown in
Impedance matching between the excitation module 100 and the surrounding hydrocarbon formation generally depends upon the electrical length of the coaxial transmission line defined by the conductive sheath 108 and the diameter of the main pipe 102. The physical length of the conductive sheath 108 can thus affect the electrical length. Selection of the physical length may depend upon the operating frequency, outer diameter of the main pipe 102, outer diameter of the conductive sheath 108, electrical parameters of the surrounding medium (e.g. the hydrocarbon formation) and the material provided in the cavity 124 between the main pipe and the conductive sheath. Details with respect to selecting the length of the conductive sheath shall be described in detail subsequently.
Electrical couplers can be used as a bridge for transferring EM energy from the feed transmission line 120 inside the main pipe 102 to the conductive sheath 108 as shown in
Generally, each module has at least one feed connector located along the length coaxial transmission line (i.e. between two ends of the conductive sheath 108). In some embodiments, the feed connector can be provided at one end of the length of coaxial transmission line. In other embodiments, the excitation module 100 can have one, two, three or more feed connectors 122 depending on the number of feed transmission lines or feed transmission line conductors. The feed connectors 122 can be distributed azimuthally or radially around the main pipe 102 as shown in
In some embodiments, multiple feed connectors may be connected to a single feed transmission line, as shown
The end portions of the coaxial transmission line defined by the main pipe 102 and conductive sheath 108, which interfaces with the hydrocarbon formation or reservoir, is preferably sealed structurally to separate the formation form the cavity 124. During use, if water, clay, drilling mud or other types of electrically conductive materials from the hydrocarbon formation or well reach the feed connector inside the cavity 124, a number of outcomes may arise, including i) formation of a short-circuit or near short-circuit of the feed point; ii) cause physical damage to the feed connector 122 by chemical or mechanical means; iii) modification of electrical properties or physical damage to electrical components on the interior 116 of the main pipe 102 (such as feed transmission lines, in-situ RF generators, etc.) by chemical or mechanical means.
These identified scenarios, alone or in combination, may cause a whole or significant part of the EM energy intended to be delivered to the hydrocarbon formation to be reflected by the feed connector back toward the RF generator or excessively heat the excitation module 100 or antenna. To avoid such undesirable outcomes, seals 110, as shown in
The seal is preferably made of materials which are electrically insulating, lossless or have low loss (loss tangent less than 0.01) at the frequency of operation; capable of withstanding high temperatures, such as 100° C., 200° C., 250° C., 300° C., or 500° C.; prohibits flow of fluids from the hydrocarbon formation and do not react chemically with the materials existing inside the well or formation, such as hydrocarbons, water, natural gas, drilling mud, etc. Suitable materials include, but not limited to, ceramics such as alumina, zirconia, titanium dioxide; synthetic polymers including, but not limited to, PEEK, Ultem™, Teflon™, Polyethilene and various elastomers (e.g. silicone). In some cases, a combination of one or more of such materials may be suitable materials for seals.
A cross sectional view of one end of the excitation module 100 showing an example of a feed connector seal 110 is presented in
In some embodiments, inner and outer concentric structural rings such as 0-rings 140 (two at each end), or, alternatively, tolerance rings, or combination thereof, can be used to allow for tolerance/variation in the fabrication of the seal 110 and conductive sheath 108 as well as to maintain the seal 110 in position when the dimensions of the various components such as the main pipe 102 and conductive sheath 108 change due to thermal expansion of the materials. The inner ring may be provided proximally to the conductive pipe 102 and the outer ring can be positioned proximal to the conductive sheath 108. Where O-rings 140 are used, the O-rings 140 are preferably made of materials with low electrical loss (e.g. loss tangent <0.01) and can withstand high temperatures such as 100° C., 200° C., 250° C., 300° C., or 500° C. Some examples of O-ring materials include, but not limited to, Viton™, Teflon™, nitrile, neoprene and Kalrez™. The actual shape of the seal generally does not have significant influence on the operation of the excitation module 100 if the seal thickness is smaller than 0.05 wavelengths.
One or more excitation modules 100 can be coupled to a generator and deployed into the hydrocarbon formation. Once an antenna comprising one or more excitation modules 100 has been deployed into the hydrocarbon formation, the generator can be operated to deliver EM power to the antenna. It can be assumed that the “Initial State” of each of the excitation module 100 is surrounded by an unheated, electrically conductive reservoir in what can be denoted as the “unheated wet zone” 602, as shown in
Impedance matching to a variety of hydrocarbon formations or reservoirs with different electrical parameters may be achieved using these lossy single conductor transmission lines in conjunction with a selection of coaxial transmission line section length (i.e. length of the conductive sheath 108), coaxial transmission line characteristic impedance and placement of the feed connector 122 along the coaxial transmission line. Determination of the electrical length of the coaxial transmission line and placement of the connector will be discussed below.
To explain how impedance can be matched, allow, as an example, the physical length of the coaxial transmission line defined by the conductive sheath 108 and main pipe 102 be l1+l2, where the feed connector 122 is a distance l1 from a far end of the section of coaxial transmission line and distance l2 from a near end of the section of coaxial transmission line. If the operating wavelength is large compared to the outer radii of the main pipe 102 and conductive sheath 108 (the wavelength is >20 times whichever radius is largest), then the equivalent circuit of the corresponding antenna structure may be simplified to the circuit shown in
Specifically, the circuit of
At each end of the coaxial section, an effective shunt capacitance can be considered, which can result from the fringing fields at this transmission line discontinuity 702 as shown in
The outer conductor of the coaxial section, in other words, the inner surface 130 of the conductive sheath 108, connects with the outer surface 132 of the conductive sheath, the latter forming another single conductor transmission line. This other single conductor transmission line can be referred to as the outer single conductor transmission line 132, which has characteristic impedance Zo as shown in
With reference to the circuit of
In some cases, one or more feed connectors 122 may be positioned near the midpoint position to provide EM power to the excitation module.
This virtual PEC boundary 710 can be modeled by splitting the outer single conductor transmission line shown in
In some embodiments, when the coaxial transmission line is chosen to have an electrical length of half or substantially half wavelength of the irradiation frequency with one feed connector near the midpoint of the coaxial transmission line or two feed connectors near both ends of the coaxial transmission line, a perfect magnetic conductor (PMC) boundary condition using magnetic field symmetry can be formed if the magnetic fields from either end arrive at the midpoint of the conductive sheath out of phase (in the case of two feed connectors at either end of the coaxial section, the signals applied to each end will need to be in phase). The PMC boundary condition may also be located in a transverse mid-plane of the coaxial transmission line that is transverse to the main axis of the antenna structure.
In yet other embodiments, the length of the coaxial transmission line can be chosen such that it is an odd multiple of one half of the wavelength of the oscillation frequency of the EM energy.
The physical length of the coaxial transmission line can be affected by at least one of: the target frequency; outer diameter of the main pipe 102; outer diameter of the conductive sheath 108; the material occupying the cavity 124; and the electrical characteristics of the hydrocarbon formation. Having regard to the identified factors in consideration, a selection of structural dimensions of the excitation module can be made so that the resultant antenna can be electrically matched to a broad range of external media, or to media which undergo changes in its physical or electrical properties over the course of heating.
Returning back to the half wavelength conductive sheath antenna structure as an example once more, this configuration may be regarded as the structure having physical dimensions and characteristic impedances being optimized for a certain operating frequency. The reflection coefficient seen by a generator, in the form of the scattering parameter S11, is plotted versus the external medium's electrical conductivity and relative permittivity in
Electromagnetic heating can be regarded to have reached a semi steady-state of operation when the hydrocarbon formation or reservoir volume immediately adjacent to the radiating source (e.g. excitation module 100, irradiating modules, or module chain(s)) has been heated along the length of the main pipe 102 and along the length of the conductive sheath 108 to a point where water within the pore spaces of the hydrocarbon formation or reservoir has evaporated. These evaporated zones that surround the excitation module can be called “dry-out” zones. Both the dielectric constant and electrical conductivity of the dry-out zones are generally lower than the unheated volumes of the hydrocarbon formation or reservoir.
Measurements of oil sand permittivity and conductivity as a function of water saturation can be found in [F. S. Chute, F. E. Vermeulen, M. R. Cervenan, and F. J. McVea, “Electrical Properties of Athabasca Oil Sands,” Can. J. Earth Sci., vol. 16, pp. 2009-2021, 1979.]. The exact difference in electrical properties between unheated and heated zones varies reservoir to reservoir, but, as a rough example at 1 MHz based on the cited study, the permittivity and conductivity may change from about 30 and 0.02 S/m to about 10 and 0.0001 S/m. It may be noted that semi steady-state conditions is not signaled or indicated by a specific dry-out zone size, but rather, it can be achieved when the dry-out zone is present everywhere around the antenna. Furthermore, this state of operation is regarded as a “semi steady-state” since the volume of the dry-out zone is increasing as the heating process advances.
Formation of the dry-out zone can create lossy coaxial transmission lines defined along the main pipe and along the outer surface of the conductive sheath of an excitation module as shown in
Once the semi steady-state of operation is reached, impedance matching can become easier since the apertures 1140 of the conductive sheath are surrounded by the dry-out zone 1120. The operation of the excitation module in the semi steady-state can be described again by considering an example case where the coaxial transmission line formed by the conductive sheath and the main pipe is one half of the operating wavelength. The initial state circuit model as shown in
In some embodiments where several modules are connected together underground in a modular deployment, the dry-out zones surrounding each coaxial transmission line may connect, forming a longer uniform leaky-wave antenna with distributed sources. The power and phase of the EM energy of each module source may be controlled to obtain an overall heating pattern along the hydrocarbon formation or well, as suggested in U.S. patent application Ser. No. 14/508,423. For example, multiple excitation modules 100 or module chains 400 may be irradiating EM waves having the same phase. Alternatively, one or more modules may be configured to irradiate EM waves that are out of phase (e.g. 180 degrees out of phase or some other phase relationship). The emitted EM waves may generate a desired irradiation pattern as a result of constructive and destructive interference thereof. For example, if one area of the hydrocarbon formation is being underheated or overheated, the power and/or phases of the nearest modules can be configured to correct the heating of the problem region. In general, adjusting phases and power levels of modules provides new means of establishing and controlling radiation patterns of the distributed antenna (as taught in modular patent, U.S. patent application Ser. No. 14/508,423).
Numerous specific details are set forth herein in order to provide a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that these embodiments may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the description of the embodiments. Furthermore, this description is not to be considered as limiting the scope of these embodiments in any way, but rather as merely describing the implementation of these various embodiments.
| Number | Date | Country | |
|---|---|---|---|
| 62544313 | Aug 2017 | US |
