Note: Descriptions are shown in the official language in which they were submitted.
sackground of the Invention
Structures for supplying RF energy to subsurface formations have
been proposed such as those disclosed in Patent No. 4,140,179 wherein a coaxial
line extending through an outer casing terminates in a dipole arrangement in
a body of oil shale. Ilowever, in such structures, portions of the energy were
lost due to RF currents flowing back up the bore hole on the outside of the
coaxial line. Thus, the heating of the subsurface body occurred partly above
the region where the heating was desired. The dipole arrangement was such
that the impedance match to the coaxial line and the radiation pattern were
very sensitive to changes in the impedance of the shale due to changes in
temperature and content of organic material.
Summary of the Invention
In accordance with this invention, there is provided an RF
applicator supplied wi~h energy through a coaxial transmission line whose outer
conductor terminates in a choking structure comprising an enlarged coaxial stub
extending back along said outer conductor. ~lore specifically, the applicator
comprises an enlarged cylindrical member connected to the central conductor of
the transmission line. The outer conductor of the coaxial transmission line is
connected to a section of coaxially positioned conductive tubing having a sub-
stantially larger diameter than said outer conductor of said coaxial trans-
mission line.
~lore specifically, this invention provides for a conductive seal-
ing casing extending from the surface through loose material to consolidated
over burden formations. A coaxial transmission line has a pipe acting as an
outer conductor extending from the surface to an RF applicator which may be a
radiator or a field defining electrode with said outer conductor being elec-
trically connected to an enlarged conductor structure surrounding thc outer
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conductor adjacent its lower end with the structure forming a reen~ran~ region
extending back along the outer conductor to reduce RF currents flowing back
up the outer conductor from the RF applicator. An inner conductor of the
coaxial transmission line extending from the surface into the subsurface
formation to be heated is directly connected to an enlarged conductive elec-
trode structure to form the primary electrode structure for coupling RF energy
into the formation either as a radiator or as an electrode of a captive field
structure.
This invention further provides for supplying fluid through the
transmission line from the surface to the applicator. More specifically, the
fluid may be high pressure liquid for injection into the formation being heated
or may be a gaseous medium for improving the dielectric strength of the regions
of the RF applicator or may be either liquid or gaseous medium for the purpose
of flushing the products of pyrolysis collected below the RF applicator to the
surface.
This invention further discloses a transmission line system for
supplying power to a subsurface RF applicator through a variable impedance
matching unit from a tr~msmitter so that variations in the impedance of the oil
shale formation due to variations in its temperature or due to variations in
the frequency of the RF energy applied may be matched to the output impedance
of the transmitter.
In accordance with the present invention, there is provided a
system for radiating energy into a subsurface body comprising a coaxial trans-
mission line extending from the surface of said body to an RF applicator; said
coaxial transmission line comprising inner and outer cylindrical conductors
and said inner conductor being attached to a cylindrical radiating element at a
point below the lower end of said outer conductor; and the diameter of the lower
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end of said outer conductor being substantially larger than the average dia-
meter of said outer conductor.
In accordance with the present invention, there is further pro-
vided a radiating system comprising a coaxial transmission line having a radiat-
ing element connected to the inner conductor of said transmission line and a
cylindrical choke structure connected to the outer conductor of said trans-
mission line; and the diameter of said choke structure being substantially
greater than the diameter of said outer conductor.
In accordance with the present invention, there is further provided
a system for transferring RF energy into a subsurface body comprising: a coaxial
transmission line extending from the surface of said body to an RF applicator;
said coaxial transmission line comprising inner and outer cylindrical conductors
and said inner conductor being attached to a cylindrical radiating element at a
point below the lower end of said outer conductor; the diameter of the lower
end of said outer conductor being substantially larger than the average diameter
of said outer conductor; and means for supplying said transmission line with
said RF energy.
In accordance with the present invention, there is further provided
a subsurface radiating system comprising: a coaxial transmission line having a
~0 radiating element connected to the inner conductor of said transmission line;
a cylindrical structure connected to the outer conductor of said transmission
line; and the diameter of said cylindrical structure being substantially greater
than the diameter of said outer conductor.
In accordance with the present invention~ there is further provided
a system for radiating RF energy into a subsurface body comprising: means for
generating said RF energy; a coaxial transmission line extending from the sur-
face of said body to an RF applicator for supplying said applicator with said
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energy; said coaxial transmission line comprising inner and outer cylindr:ical
conductors and said inner conductor being attached to a cylindrical radiating
element at a point below the lower end of said outer conductor; and the diameter
of the lower end of said outer conductor being substantially larger ~han the
average diameter of said outer conductor.
In accordance with the present invention, there is further provided
a subsurface radiating system comprising a coaxial transmission line having a
radiating element connected to the inner conductor of said transmission line
and a coaxial impedance transformation structure connected to the outer con-
cluctor of said transmission line; and the diameter of an element of said
structure being substantially greater than the diameter of said outer conductor.
In accordance with the present invention, there is further provided
a subsurface radiating system comprising: a coaxial transmission line having
a radiating element connected to the inner conductor of said transmission
line; and means connected ~o the outer conductor of said transmission line for
substantially restricting propagation of energy upwardly outside said trans-
mission line.
In accordance with the present invention, there is further provided
a subsurface radiating structure comprising: a radiating element having a first
portion connected to the inner conductor of a coaxial transmission line and a
second portion connected to said first portion; and the diameter of said second
portion being greater than the diameter of said first portion.
Brief Description of the Drawings
Other and further objects and advantages of this invention will
be apparent as the description thereof progresses, reference being had to the
accompanying drawings wherein:
Figure 1 illustrates a longitudinal sectional view of a subsurface
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RF applicator incorporated in a system em~odying the invention;
Figure 2 is a transverse sectional view of the applicator trans-
mi.ssion line of Figure l taken along line 2-2 of Figure l;
Figure 3 is a transverse sectional view of the RF applicator choke
structure of Figure 1 taken along line 3-3 of Figure l;
Figure 4 is a transverse sectional view of the lower end of the
choke structure of Figure 3 taken along line 4-4 of Figure 2;
Figure 5 is a transverse sectional view of the structure of Figure
1 taken along line 5-5 of Figure 1 illustrating the lower dipole of the
radiating structure of Figure l; and
Figure 6 is a plan view illus~rating a power layout and control
system for utilizing a plurality of the systems of Figure l.
Description of the Preferred Embodiments
Referring now to Figures 1-5 there is shown an oil shale forma-
tion 10 positioned beneath an overburden 12 and on top of a substrate 14. A
bore hole 16 has been drilled from the surface through the overburden 12 and
through the oil shale 10 into the substrate 14. Overburden 12 may be sedimen-
tary material forming a substantially gas tight cap over the oil shale region
10.
In accordance with well-known practice a seal to the overburden
12 is formed by a steel casing 18 extending from above the surface downwardly
in bore hole 16 to a point beneath the loose surface material and is sealed
to the walls of the bore hole by concrete region 20 surrounding steel casing
18. ~hile any desired bore hole size can be used dependent on the size of the
RF applicator to be used, the example illustrated herein may have a steel
casing 18 whose inner diameter is a standard 18 inches. A well head assembly
comprising a flanged bushing 22 and a cap 24 is attached to the top of the steel
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casing 18, for example, by welding. Such a structure is preferably used to
enable pressure to be maintained in the bore hole 16 and to prevent contamina-
tion of the bore hole, -for example, by ground water.
A coaxial transmission line 26 extends from the cap 24 ~hrough the
overburden 12 to an RF applicator 28 positioned in the oil shale region 10.
The transmission line 26 is pre~erably formed with an outer conductor 30 of
steel pipe having, for example, an internal diameter of approximately 6 inches
and a thickness of approximately a half inch. Several lengths of pipe 30
are joined together by threaded couplings 32 and the upper end of the upper
length of pipe 30 is thr~aded into an aperture in cap 24 while the lower length
of pipe 30 is threaded into an adaptor coupling structure 34 which provides an
enlarged threaded coupling to a coaxial stub 36 extending back up the bore
hole 16 for a distance of around an electrical eighth of a wavelength of the
frequency band to be radiated into the formation 10 by radiator 28. A stub
38 of the same diameter as stub 36 also extends downwardly from adaptor 34 for
a distance equal to approximately an eiectrical quarter wavelength of said
frequency band. If desired, a ceramic sleeve 40 having perforations 41 may be
placed in ~he formation 10 to prevent caving of said formation during the heat-
ing process.
Coaxial transmission line 26 has an inner conductor 42 made, for
example, of steel pipe lengths. The upper end of the upper pipe lengths is
threaded into cap 46. A ceramic plate 44 which is attached to cap 24 spaces
the inner conductor electrically from the outer conductor 30. Cap plate 46 is
mounted on top of plate 44 and threaded to pipe 42 so that pressure may be main-
tained inside the outer conductor 30 of the coaxial transmission line 26.
Several lengths of pipe 42 connected together by metal couplings 48 and spaced
from the inner wall of outer conductor 30 by ceramic spacer 50 extend from cap
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46 downwardly through outer conductor 30 to a point beyond the lower end of
tubular stub 38. An enlarged ceramic spacer 52 surrounds the pipe 42 adjacent
its lower end and the lower end of tubular stub 38 to space pipe 42 centrally
within coaxial stub 38. Preferably, ceramic spacers 50 rest on top of couplings
48 so that they may slide easily on the pipe lengths before being screwed into
the couplings. Enlarged spacer 52 is held in axial position by metal collars
54 welded to the bottom length of pipe 42.
An enlarged section of pipe 56 is threadably attached to the lower
end of tlle bottom pipe 42 by an enlarging coupling adaptor 58 and the lower
end of enlarged tubular member 56 has a ceramic spacer 60 attached to the outer
surface thereof to space member 56 from the bore face 16. In the example
disclosed herein using approximately 6-inch size for pipe 30, the diameter of
pipe 42 is approximately 2 inches inside and 2 3/8 inches outside. This pro-
duces a characteristic impedance for the transmission from the surface to the
RF applicator 28 of approximately 50 ohms. By choosing the interior diameter
of the stubs 36 and 38 to be, for example, of 12.715 inches, the characteristic
impedance of the coaxial line sections comprising pipe 42 and stub 38, may be
approximately 100 ohms. The outer diameter of the tubular radiating member 56
may be selected to be 8 5/8 inches to produce a radiating surface which may be
~0 easily inserted into the well bore 16 through the previously installed steel
casing 18. Preferably the size of tubing 56 should be as large as practicable
to reduce the voltage gradient on the surface of the tubing 56 during the
radiation of high RF power into the formation 10.
In accordance with this invention the region from the upper end of
tubular member 36 to the lower end of tubular member 38 is made an odd number
of quarter wavelengths effective in shale in the operating frequency band of the
device and forms an impedance matching section 106. ~lore specifically, the
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distance from the adaptor 34 to the lower end of tubular member 3~ is made
approximately a q~arter wavelength effective in air at the operating frequency
of the system. The section 106 of applicator 28 comprising stub 38 together
with the portions of member 42 adjacent thereto, act as an impedance matching
transformer which improves the impedance match between coaxial line 26 and the
radiator section 108 of applicator 28. Section 106 also substantially reduces
the current from the RF power that would flow back up the outside of pipa 30
from the lower end thereof until the power had been lost by radiation into
overburden 12 or absorbed by loss in the surface of pipe 30. With the structure
of this invention, however, the power loss by current flow along the outer
surface of the pipe 30 is reduced very substantially so that it is only a few
percent of the power transmitted down the transmission line 26.
In accordance with this invention it is to be noted that the di-
electric constant and loss tangent, and hence impedance, of the formation 10
change with temperature as may be seen from Patent 4,140,179. In accordance
~ith this invention, the impedance of the radiating section 108 changes very
little over a wide range of temperatures of formation 10. To compensate for
any such temperature impedance variation, an imyedance matching device 62 is
provided at the surface which may provide an adjustable impedance to the trans-
mission line 26. The adjustment of the impedance matching circuit may be
achieved by measuring the effective power reflected from the applicator 28 back
along the transmission line 26 to determine the standing wave ratio on the
transmission line 26. Thus it may be seen that the radiating structure 108 may
be excited to produce a radiation pattern directed primarily radially outward
in the plane of the oil shale medium with the bulk of the power being confined
to the medium. While the frequency may, for example, be varied between 1 and
10 megahertz for the dimensions given herein, the tubular member 56 is preferably
a quarter wavelength long, effective in shale. The spacing between the upper
end of tubing 56 an~ the lower end of tubing 38 is preferably a quarter wave-
length along, effective in shale with a substantial air gap.
The lengths of the enlarged section 56 and the portion of the
section 42, which together form a substantially half wave monopole radiator 108
depend on the frequency of the transmitter 64 and the effective radiation wave-
length in the medium lO as well as the radiation impedance of the medium. Good
results have been achieved, for example, at 10 megahertz, if tne total length
of the radiator 108 had the enlarged radiating section 56 (represented by the
portion thereof below cutting line 5-5 of Figure 1) approximately a seventh
of a wavelengtll in air, and the section of the monopole radiator 108 represent-
ed by the extension of the inner conductor 42 beyond the lower end of the
cylinder 38 ~the portion between cutting line 4-4 and cutting line 5-5 in
Figure 1) approximately a sixth of a wavelength in air. When the medium 10 has
a substantial quantity of water therein, for example, when the medium is first
being heated, the effective wavelength 108 will be somewhat greater than a
half wavelength. ~lowever, as heating progresses and the water is either con-
verted to steam or driven off, the dielectric constant in the medium drops and
the effective wavelength increases. Operating the monopole radiator 108 with
an effective electrical wavelength greater than one-half wavelength reduces
the vertical directionality of the pattern. Therefore, radiator 28 preferably
has dimensions which in wet shale, having a dielectric constant of, for example,
16 and in spent shale having a dielectric constant as low as 3, result in the
radiating monopole 108 being approximately a half wavelength long. Thus, for
example, for a transmitter frequency of 10 megahertz in which the free space
wavelength is 3 X 10 centimeters or 30 meters which is lO0 feet, the length
of section 56 is chosen to be approximately 14 feet and the distance from the
bottom of cylinder 38 to the top of casing 58 is chosen to be 16 feet.
In operation, the bulk of the power is radiated from the section
108 and the section 106 acts as a resonant impedance transformer The stubs 36
and 3~ act as a non-resonant or inductive choking structure whose length may
be determined empirically to optimize the directive pattern in the horizontal
direction as measured in the vertical plane. By varying the -frequency, the
pattern radiated can also be varied.
Transmitter 64 supplies variable frequency RF power to the im-
pedance matching structure 62 through a coaxial line 66 and the impedance match-
ing structure 62 supplies the RF power to the coaxial line 26 through a coaxial
line 6~ whose central conductor is connected to the cap 46 and whose outer
conductor is connected to the cap 24.
As shown in Figure 6, transmitter 64 preferably is located remote-
ly from several sites 16 and transmission lines 66 extend distances up to in
excess of 1,000 feet. Thus, one large transmitter installation can be used
to feed sequentially different sites 16. It is, therefore, preferable that
the standing wave ratio on the transmission lines 66 be maintained as close to
~mity as possible so that RF losses in the transmission line are mimimized.
In addition, it is also desirable that little or no power be fed back into the
transmitter 64 to avoid damage to the transmitter equipment as well as to allow
the transmitter equipment to be tuned for maximum RF power generating efficiency.
Thus, the impedance matching circuits 62, which may use conventional inductors
and capacitors, is adjusted in accordance with well-known practice to produce
such impedance matching of the transmission lines 66.
While the radiator 56 may be sized for optimum radiation char-
acteristic and/or power at a particular frequency, for example, by making the
length of the element 56 an effective electrical quarter wavelength at that
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frequency in the bore 16, it is desirable that the frequency of transmitter
64 be variable to adjust for the different impedances or different formations
and/or the different impedances of the formation encountered during different
portions of the heating sequence. Such impedance matches may also be achieved
by variation of the output impedance of impedance matching circuit 62 so that
by means of a standing wave the proper impedance is reflected through the
relatively short transmission line stub 68 and the transmission line 26 to the
radiating structure in the formation 10.
The impedance matching structure 62 is preferably adjusted for
the desired impedance match into the radiating structure 26 with the transmitter
64 at low power, and the impedance match to produce low standing wave ratio
in transmission line 66 is then adjusted. Ilowever, it should be clearly
understood that such impedance matching functions can be controlled in accor-
dance with a preprogrammed schedule.
It has been found that good impedance match to oil shale forma-
tions can be obtained over a thirty percent frequency band without substantial
loss in the efficiency of transferring RF power to the formation 10.
The transmission line 26 is preferably pressuri~ed with an inert
gas, such as nitrogen, from a source 70 through a pipe 72 tapped into bushing
22, through a pipe 74 tapped into cap 24 as well as to the interior of pipe
42 through a pipe 76 connected by a insulating coupling 78.
The source of nitrogen 70 may be of sufficient pressure to continu-
ously bleed nitrogen into the pipes 42 and 30 as well as the casing 18 so
that nitrogen flushes down the face of the bore 16 and through the region
between the pipes 42 and 30. Preferably, the ceramic spacers have apertures
in the peripheries thereof to allow the passage of the nitrogen. The nitrogen
thenpresses against liquids 80 collected in the bottom of the bore 16 and
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forces them up through a producing tubing 82 which may be steel with a ceramic
coupling 84 approximately at the lower end of the radiating cylinder 56 Cera-
mic coupling 84 isolates the tubing 82 which is essentially at ground potential
from a tubing 86 extending upwardly through pipes 42 to the surface and
through a cap 88 attached to the top of cap 46 and thence through an insulating
coupling 90 to a collection tank 92 where the nitrogen can be recovered~ if
desired, and re-injected via the source 70 into the formation.
Such a circulation of nitrogen, in addition to aiding in pro-
duction of kerogen products from the base of the bore 16, may serve to cool
overheated portions of the transmission line and/or radiating structure so
that high powers may be transmitted from the transmi-tter 64 into the oil shale
body 10 without voltage breakdown at high voltage points in the structure.
In order to control the flow of gas from supply 70 to the various
regions of the transmission line and radiator, pipes 72 and 74 contain valves
94. Pipe 76 contains a valve 96 on the grounded side of bushing 78 and the
pipe from bushing 90 to the collection tank 92 contains a valve 98 so that by
opening and closing the valvesS gas from the well bore may be increased, held
constant or decreased during various cycles of the production process. By
maintaining an appropriate purging flow of nitrogen through the well bore 16
before and during application of RF power, danger of explosion in the region
of the RF applicator may be minimized. Such an explosion could occur, for
example, if oxygen, driven off from components of the formation or present
after installation of the well transmission line, combined with hydrocarbons in
gaseous form driven off from the formation when a corona discharge or arc at
the R~ applicator caused ignition of an explosive mixture. The length of the
transmission line 26 should be sufficient to reach any desired region of the
oil shale 10 and for thick beds of oil shale may be gradually changed by raising
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or lowering the transmission line 26. This, in turn, raises or lowers the
radiator 28 to expose a different horizontal layer of the oil shale to the maxi-
mum intensity of the radiation.
RF breakdown is minimized by the use of the ceramic spacers 50,
52, 40 and 60 which maintain the various electrical conductors substantially
concentric with each other and with the bore hole 16 so that impedance varia-
tiolls along the transmission line due to eccentricities which could otherwise
occur between the inner and outer conductors of the coaxial line 26 are mini-
mized. These eccentricities could cause standing wave ratios in excess of
those contemplated thereby causing higher voltage nodes at points on the trans-
mission line or in the RF radiator.
The edges of the insulators are preferably beveled to facilitate
relative motion between the conductors during installation and a large in-
sulating spacer 52 is positioned between the lower end of stub 38 and inner
conductor pipe 42 since in this region a voltage maximum can occur. Such a
voltage maximum is likely to increase as the standing wave ratio on the trans-
mission line 26 increases so that at large power levels, corona breakdown might
occur. Maximum power handling capability~ in addition to being limited by
voltage breakdown, is limited by the power dissipation of the transmission line
0 and for the structure shown fabricated of conventional steel with surfaces
coated with highly conductive material, such as copper, powers in excess of one
megawatt may be transmitted through the transmission line 26 and the radiator
28 into the formation lO.
In the event that the RF applicator 28 is not sufficiently deep,
that is, the overburden 12 is not sufficiently thick, some of the RF energy at
high powers radiated into the formation lO may appear at low intensity on the
surface. In accordance with this invention, wires, for example steel cables
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100, may be welded to cap 22 and stretched radially for several hundred feet
to reflect such radiation back into the overburden thereby preventing radiation
interference when frequencies of, for example, 10 megahertz or below are used.
Generally, frequencies above 10 megahertz are sufficiently absorbed in most
overburden formations and lower frequencies are absorbed in those cases where
~here is substantial moisture content in the overburden. The spacing between
the radial wires can be any desired amolmt and branch wires from the radial
wires may also be attached, if necessary. In addition, where more than one
structure is placed in a given region, the wires can extend between adjacent
structures.
As indicated previously in connection with Patent 4,140,179,
the impedance changes due to both the absorption of the microwave energy because
of changes in conductivity and because of changes in the dielectric constant due
to removal of that portion of the water which originally existed in the oil
shale body. The temperature at which such water changes to steam and is pro-
duced out of the formation depends on the pressure maintained in the well bore.
For example, if the valve 9S remains closed and the bore face having first been
flushed with nitrogen is pressurized to 500 psi, the temperature in the oil
shale 10 may be raised at the bore face to several hundred degrees fahrenheit
~0 with the water still remaining in liquid form in the pores of the oil shale
body. Water on the order of 3 to 30 percent may be encountered and will absorb
substantial amounts of the RF power.
In accordance with this invention the temperature in the bore face
may be sensed, for example, by a thermo-couple 102 of a type shown in Patent
4,140,179, and as item 102 in Figure 1, connected to the surface via a wire
104. When the temperature reaches, for example, 700 F, opening the valve 98
will cause the pressure in the bore face to produce steam from the water cooling
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the bore face to a temperature below 700 ~ and preventlng undesired hot spots
at the surface of the formation 10.
While the coaxial line 26 has surfaces providing RF current flow
which are large and hence low in current density for a given power level the
coaxial lines 66 and 68 may be, for example, conventional conductive copper
coaxial lines having, for example, an outer diameter of 3 1/8 inches. Such
lines may be run for several hundred yards from a central transmitter and pre-
ferably have the impedance matching structure 62 positioned close to the sur-
face of the well bore 16. Thus~ the impedance of the transmitter 64 may be
substantially matched to the input lmpedance of the matching structure 62 to
maintain a standing wave ratio in line 66, for example, below 1.5 whereas the
transmission line 26 may have a standing wave ratio thereon of 1.5 to 5 depend-
ing on the matching required to optimize the radiation from radiator 28.
Referring now to Figure 6, there is shown a plan view of a plurality
of well bores 16 in a well field spaced apart by distances such as several hun-
dred feet and connected via coaxial cabling through impedance matching structures
62 to a central transmitter 64 via coaxial lines 66. The RF power may be
sequentially shifted in any desired pattern to different radiators in different
well bores 16 from a single transmitter housing which may be in, for example,
a control station. Signals fed from the impedance matching structures 62 to
the control station may be used to monitor and/or adjust the frequency and im-
pedance matching of the transmitter output to each of the wells
This completes the description of the particular embodiment of the
invention illustrated herein. However, many modifications thereof will be
apparent to persons skilled in the art without departing from the spirit and
scope of this invention. For example, parallel wire lines could be used to
feed the structures in the wells, other frequencies could be used than those
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indicated and a wide variety of conductive materials could be used for the
transmission lines and radiating structures in the wells. Accordingly, it is
intended that this invention be not limited by the particular details of the
embodiments illustrated herein accept as defined by the appended claims.
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