Note: Descriptions are shown in the official language in which they were submitted.
Background 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. However, 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.
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_ mmary of the_Invention
In accordance with this invention, there is provided an
RF appllcator supplied with energy through a coaxial transmission
line whcse outer conductor terminates in a choking structure com-
prisin~ an enlarged coaxial stub extending bac~ along said outer
conductorO More specifically, the applicator comprises an en-
larged cylindrical member connected to the central conductor of
the transmission line. Tne outer conductor of the coaxial trans-
mission line is connected to a section of coaxially positioned
conductive tubing having a substantially larger diameter than
said outer conductor of said coaxial transmission line.
More specifically, this invention provides for a conductive
sealing casing extending from the surface through loose material
to consolidated overburden 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 conduc-tor being electrically
connected to an enlarged conductor structure surrounding the
outer conductor adjacent its lower end with the structure forming
a reentrant 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
electrode 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 fi.eld structure.
This invention ~urther provides for supp].ying fluid through
the transmisciion line erom the surface to the applicator. More
specificallyt the fluid may be high pressure liquid for injection
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into the formation being hea-ted or may be a gaseous medium for im-
proving the dielectric strength of -the regions of the RF applica-
tor or may be either li~uid or gaseous medium for -the purpose of
flushing the products of pyrolysis collected below the RF applica--
tor to the surface.
This invention further discloses a transmission line
system for supplying power to a subsurfaee RF app]ica-tor -through
a variable impedance matching unit from a transmitter so -that
variations in the impedance of the oil shale formation due to
variations in its temperature or due to variations in the frequen-
ey of the RF energy applied may be matched -to the output impedance
of the -transmitter.
In aecordance with the present invention, there is pro-
vided the method of producing organic products from a body of oil
shale beneath an overburden comprising: generating electrical
energy in the frequency range be-tween 100 kilohertz and 100 mega-
her-tz; transmitting said energy via a Eirst transmission line hav-
int a first charaeteristie impedance through a variable impedance
matching structure to a seeond transmission line having a seeond
eharaeteristic impedanee and ex-tending through said overburden to
a radiating strueture positioned in said body of oil shale; and
varying the impedanee matehing of said structure -to compensate for
ehanges in temperature of said oil shale body.
In aceordance with the present invention, there is fur-
ther provid~d a syC;tem Eor produeing organic produets frorn a body
oE o.i.l. shaLe beneat:h an ove~rburden eomprising: rneans for genera-t-
i.n~ el~etrlea:l on~t^cJy in the Erequeney rancJe between 100 kilohert~
arld l()0 me~gahert~; Ineans l-or transmitting said energy via a first
Irnr~ rli..s.qic)rl l.ine havirlg a f:irst eharaeteristie impedanee through
3() a variclb:le impedancc.~ matehing strueture to a seeond transmission
L;ine hav:ing a seeorld eharacterist:ie impedance and extending
through said overburden to a radiating structure posi-tioned in
said body of oil shale; and means for varying the frequency of
said energy to vary -the pattern of said energy radiating into said
body of oil shale.
Brief Description of the Drawings
Other and further objects and a~vantages of this invention
will be apparent as the description thereof progresses, reference
beiny had to the accompanying drawings wherein:
FIG. 1 illustrates a longitudinal sectional view of a sub-
surface RF applicator incorporated in a system embodying the
invention;
FIG. 2 is a transverse sectional view of the applicator
transmission line of FIG. 1 taken along line 2-2 of FIG. l;
FIG. 3 is a transverse sectional view of the RF applicator
choke structure of FIG. 1 taken along line 3-3 of FIG. l;
FIG. 4 is a transverse sectional view of the lower end of
the choke structure of FIG. 3 taken along line 4-4 of FIG. 2;
FIG. 5 is a transverse sectional view of the struc.ure of
FIG. 1 taken along line 5-5 of FIG. 1 illustrating the lower
dipole of the radiating structure of FIG. l; and
FIG. 6 is a plan view illustrating a power layout and
control system for utilizing a plurality of the systems of
FIG. 1.
3~
Description of the Preferred Embodiment
Referring now to FIGS. l-S there is shown an oil shale
formation 10 positioned beneath an overburden 12 and on top
of a substrate l4. 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 sedimentary
material forming a substantially gas tight cap over the oil
shale region 10.
In accordance with ~ell-known practice a seal to the over-
burden 12 is formed by a steel casing 18 extending from abovethe surface down~lardly 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. While
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 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 contamination of the bore hole, for example,
by ground water.
A coaxial transmission line 26 extends from the cap 24
through the overburden 12 to an RF applicator 28 positioned in
the oil shale region 10. The transmission line 26 is preferably
forrned ~ith an outer conductor 30 of steel pipe having, Eor
example, an internal cliameter oE approximately 6 inches and a
thickness o~ approximately a half inch. Several lengths of pipe
30 are joined together by threaded couplings 32 and the upper end
o~ the upper ]ength of pipe 30 is threaded into an aperture in
~3~
cap 24 while the lower length o~ pipe 30 i5 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 ei~3hth of a wavelength of
the freauency band to be radiated into the formation lO by radia-
tor 28. ~ stub 38 of the same diameter as stub 36 also extends
downwardly frorn adaptor 34 for a distance equal to approximately
an electrical quarter wavelenath of said frequellcy band. If
desired, a ceramic sleeve 40 having perforations 41 may be placed
in the formation lO to prevent caving of said formation during
the heating process.
Coaxial transmission line 2~ 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 a~ 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 lenyths 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 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 oE couplings 48 so that they may
slide ea.sily 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.
~:~B~3~:g
An enlarged section of pipe 56 is threadably attached to
the lower end of the bottom pipe 42 by an enlarging coupling
adaptor 58 and the lower end of enlarged t~bular member 56 has
a ceramic spacer 60 attached to the outer s~rface 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 out-
side. This produces a characteristic impedance for tihe trans-
mission from the surface to the ~ applicator 28 of approximately
50 ohrns. 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 mernber 56 may be seleçted to be ~ 5/8
inches to produce a radiating surface which may be easily inserted
into the well bore 16 through the previollsly installed steel
casing 18~ Preferably the size of tubing 56 should be as large
as practicable to reduce the vol.age 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 3~ 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 im-
pedance matching section 106. L~ore specifically, the distance
from the adaptor 34 to the lower end of tubular member 38 is
made approximately a quarter wavelength effective in air at the
operating ~requency o~ the system. The section 106 of applicator
28 comprisiny stub 38 toyether with the portions oE member 42
adjacent thereto, act as an impedance matching transformer which
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3~3~
improves the impedance match between coaxial line 26 and the
radiator section 108 of applicator 28. Section 106 also substan-
tially reduces the current from the RF power that would flow
back up the outside of pipe 30 from ~he lower end thereof until
the power had been lost by radiation into overburden 12 or ab-
sorbed by loss in the surface of pipe 30. With the structure or
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 dielectric constartt and loss tangent, and hence impedance,
of the formation 10 change with temperature as may be seen from
Patent 4,140,179. In accordance with tnis invention, the im-
pedance 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 impedance matching
device 62 is provided at the surface which may provide an adjust-
able impedance to the transmission line 26. The adjustrnent of
the impedance matching circuit may be achieved by measuring the
effectiv.e power reflected from the applicator 28 back along the
transmission line 26 to determine the standing wave ratio 011 the
transmission line 26. Thus it may be seen that the radiating
structure 10~ may be excited to produce a radiation pattern
~irected primarily radially outward in the plane of the oil
sha~le medium with the bulk of the power being confined to the
medium. While the frequency may, Eor example, be varied between
1 and 10 megahertz Eor the dimensions given herein, the tubular
member 56 is preferably a quarter wavelength long, effec-tive in
shale. The spacing between the upper end of tubing 56 and the
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lower end of tubing 38 is preferably a quarter wavelength long,
effective in shale with a s~bstantial 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 fre~uency of the
transmitter 64 and the effective radiation wavelength in the
medium 10 as well as the radiation impedance of the medium.
Good results have been achieved, for example, at 10 megahertz,
if the total length of the radiator 108 had the enlarged
radiating section 56 (represented by the portion thereof below
cutting line 5 5 of FIG. 1) approximately a seventh of a wave-
length in air, and the section of the monopole radiator 108
represented by the extension of the inner conductor 42 beyond
the lower end of the cylinder 38 (the portion between cutting
line 4-~ and cutting line 5-5 in FIG. 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. However, as heating progresses
and the water is either converted to steam or driven off, the
dielectric constant in the medium drops and the effective wave-
length 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 preferal)ly 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, ~or e~ample, for a transmitter frequency of 10 mega-
hert~ in which the free space wavelength is 3 X 103 centimeters
or 30 meters which is 100 feet, the length of section 56 ischosen to be approximately 14 feet and the distance from the
bottom of cylinder 38 to the top of casin~ ~8 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 38 act as a non-resonant or induc-
tive choking structure whose length may be deter~ined 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 impedance matching structure 62 through a coaxial line 66
and the impedance matching structure 62 supplies the RF power
to the coaxial line 26 through a coaxial line 68 whose central
conductor is connected to the cap 46 and whose outer conductor
is connected to the cap 24.
As shown in FIG. 6, transmitter 64 preferably is located
remotely from several sites 16 and transmission lines 66 extend
distanaes up to in excess of 1,000 feet. Thus, one large trans-
mitter 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
unity as possible so that RF losses in the transmission line are
minirnized. 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 equip-
ment to be tuned ~or maximLIm RF power generating efficiency.
Thus, the impedance matching circuits 62, which may use conven-
tional inductors and capacitors, is adjusted in accordance with
-10~
well-known practice to produce such impedance matching of the
transmission lines 66.
While the radiator 56 may be sized for optimum radiation
characteri.stic and/or power at a particular frequency, for
example, by making the length of the element 56 an e~fective
electrical quarter wavelength at that 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 for-
mation 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 i5
then ad3usted. However, it should be clearly understood that
such impedance matching functions can be controlled in accordance
with a preprogrammed schedule.
It has been found that good impedance match to oil shale
formations can be obtained over a thirty percent ~requency band
without subst:antial loss in the efficiency of transferring RF
power to the Eormation 10.
The transmiss.ion line 26 is preferably pressurized with
an inert gas" such as nitrogen, from a source 70 through a pipe
72 tapped int:o bushing 22, through a pipe 74 tapped into cap 24
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as well as to the interior of pipe ~2 through a pipe 76 connected
by a insulating coupling 78.
The source of nitrogen 70 may be of s~fficent pressure to
continuously 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 bet~een the pipes 42 and 30. Preferably~
the ceramic spacers have apertures in the peripheries thereof to
allow the passage of the nitrogen. The nitrogen then presses
against liquids 80 collected in the bottom of the bore 16 and
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. Ceramic coupling 84 isolates the
tubing 82 which is essentially at ground potential rrom a tubing
86 extending upwardly through pipes 42 to the s~rface 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
production 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 trans-
mitted from the transmitter 64 into the oil shale body 10 with-
out voltage breakdown at high voltage points in the structure.
In order to control the flow of gas from supply 70 to the
vi~rious regions o the transmission line and radiator, pipes
72 and 74 contain valves 9~. Pipe 76 contains a valve 96 on the
grounded side of bushing 78 and the pipe from bushing 90 ~o the
collection tank 92 contains a valve 98 so that by opening and
closing the valves, gas from the well bore may be increased,
'~8~.3~
held constant or decreased during various cycles of the production
p~ocess. 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 R~ applicator
may be minimized. Such an explosion could occur, for example,
if oxygen, driven o~E 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 RF 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 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 maximum intensity of
the radiation.
RF breakdown is minimized by the use of the ceramic spacers
50, 52, ~0 and 60 which maintain the various electrical conductors
substantially concentric with each other and with the bore hole 16
so that impedance variations along the transmission line due to
eccentricities which could otherwise occur between the inner and
outer conductors of the coaxial line 26 are minimi~ed. These
eccentricities could cause standing wave ratios in excess oE those
contemplated thereby causing higher voltage nodes at points on the
transmission line or in the RF radiator,
The edges of the insulators are preferably beveled to facili-
tate relative motion between the conductors during installation and
a large insu].ating spacer 52 is posltioned between the lower end
of stub 38 and inner conductor pipe 42 since in this region a
volta~e maximum can occur. Such a voltage maximum is likely to
increase as the standing wave ratio on the transmission 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 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 tne transmission line 26 and the radiator
28 into the formation 10.
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
10 may appear at low intensity on the surface. In accordance
with this invention, wires, for example steel cables 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 there is substantial moisture content in the overburden.
The spacing between the radial wires can be any desired amount
and branch wires from the radial wires may also be attached,
if necessary" In addition, where more than one structure is
placed in a qiven region, the wires can extend between adjacent
structures.
~ s 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 porti.on of the
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~3~3
water which originally existed in the oil shale body. The tem-
perature at which such water changes to steam and is produced
out of the formation depends on the pressure maintained in the
well bore. For example, if the valve 98 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 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 en-
countered and will absorb substantial amounts of the RF power.
In accordance with this invention the temperature in thebore 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 FIG. 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 the bore face to a temperature below 700 F and pre-
venting undesired hot spots at the surface of the formation lOo
While the coaxial line 26 has surfaces providing R~ c~rrent
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, Eor example,
an outer diameter of 3 1/8 inches. Such lines rnay be run for
several hundred yards from a central transmitter and preferably
have the impedance rnatching structure 62 positioned close to the
surface of the well bore 16. Thus, the impedance of the trans-
mitter 6~ may be substantially matched to the input impedance of
the matchin~ 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 oE 1.5 to 5 depending on
the matching required to optimize the radiation f~om radiator 28.
Referring now to FIG. 6, there is shown a plan view of a
plurality of well bores 16 in a well field spaced apart by dis-
tances such as several hundred feet and connected via coax
cabling through impedance matching structures 6~ to a central
transmitter 64 via coaxial lines 66. The ~F power may be sequen-
tially 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 impedance matching
of the transmitter output to each of the wells.
This completes the description of the particular embodiment
of the invention illustrated herein. ~owever, 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 indicated
and a wide variety of conductive materials could be used for the
transmission lines and radiating structures in the wells. Accor-
dinglyt 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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