Note: Descriptions are shown in the official language in which they were submitted.
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LOW RESISTANCE ELECTRICAL HEATER FOR OIL WELL
This invention relates to improved subterranean electrical
resistance heaters arranged in a well borehole to heat a formation.
Electrical resistance heaters suitable for heating long
intervals of subterranean earth formations have been under
development for many years. These heaters have been found to be
useful for carbonizing hydrocarbon-containing zones for use as
electrodes within reservoir formations, for enhanced oil recovery
and for recovery of hydrocarbons from oil shales.
U.S. Patent No. 2,732,195 discloses a process to create
electrodes utilizing a subterranean heater. The heater utilized is
capable of heating an interval of 20 to 30 meters within
subterranean oil shales to temperatures of 500°C to 1000°C. Iron
or chromium alloy resistors are utilized as the core heating
element. These heating elements have a high resistance and
relatively large voltage is required for the heater to extend over
a long interval with a reasonable heat flux. It would be
preferable to utilize lower resistance material. Further, it would
be preferable to use a material which is malleable to permit more
economical fabrication of the heater.
Subterranean heaters having copper core heating elements are
disclosed in U.S. Patent No. 4,570,715. This core has a low
resistance, which permits heating long intervals of subterranean
earth with a reasonable voltage across the elements. Further,
because copper is a malleable material, this heater is much more
economical to fabricate. These heaters can heat 1000-food
intervals of earth formations to temperatures of 600°C to 1000°C
with 100 to 200 watts per foot of heating capacity with a 1200 volt
power source. But copper also has shortcomings as a material for a
heating element. As the temperature of a copper heating element
increases, the electrical resistance increases at a rate which is
undesirably high. If a segment of the heating coil becomes
excessively hot, the increase in electrical resistance of the hot
segment causes a cascading effect which can result in failure of
the element.
A subterranean heater utilizing an electric resistant heater
element having a lower temperature coefficient of resistance would
not only improve temperature stability, but would simplify the
power supply circuitry.
It is therefore the object of the present invention to provide
an improved heater capable of heating long intervals of
subterranean earth wherein the heating element has a low
temperature coefficient of resistance, a low electrical resistance,
and utilizes a core of a malleable metal material.
To this end the subterranean electrical heater according to
the present invention comprises:
a) at least one electrical heating cable arranged in a well
borehole which comprises a core comprising about 6 percent by
weight of nickel and about 94 percent by weight of copper; and
b) a means for supplying electrical current through the
electrical heating cable.
When this copper-nickel alloy is incorporated into such a
heater cable the benefits of a low resistance heater are obtained
along With the benefit of having a low temperature coefficient of
resistance. The heater cable material is also malleable. Such a
heater can therefore be utilized to heat subterranean intervals of
earth to temperatures of 500°C to 1000°C utilizing voltages in
the
range of 400 to 1000V.
These heater coils are less likely to fail prematurely because
the resistance of the cable in hot segments is much nearer to the
resistance of the remaining coil. Hot spots therefore have less
tendency to continue to increase in temperature due to higher
electrical resistance, causing premature failure. The electrical
resistance of the element also varies less between the initial cool
state and the service temperatures which simplifies the power
supply circuitry. The benefits of the low resistance and low
temperature coefficient of resistance heater element of the present
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invention are most significant When the heater is one which applies
heat over large intervals of subterranean earth and at a
temperature level of 600°C to 1000°C. Intervals of 1000 feet or
more can be heated with these heaters.
The invention will now be described in more detail with
reference to the accompanying drawings, wherein
Figure 1 is a schematic illustration of a heater of the
present invention being installed within a well.
Figure 2 is a three-dimensional illustration of an insulated
and sheathed heating element of the present invention.
Figure 3 is a cross-sectional illustration of power cable to
heating cable splice of the present invention.
Figure 4 is a cross-sectional illustration of the heating
cable bottom terminal plug.
The heater of this invention is any heater wherein a long
element is utilized. The long element necessitates the use of a
material which has a low electrical resistance. Copper is such a
material, but copper is prone to forming hot spots due to its high
temperature coefficient of resistance. An alloy of about 6 percent
by weight nickel and 94 percent by weight copper, known as low
resistance material, has both a relatively low resistance, and a
low temperature coefficient of resistance. This results in a more
simple power supply circuitry, and less of a tendency to form hot
spots. The long element heaters of this invention can be utilized
in subterranean oil recovery or coal shale hydrocarbon recovery.
These types of heaters are often referred to as well heaters.
A preferred basic heater design for the practice of this
invention is described in U.S. Patent No. 4,570,715.
The well heaters may be of other designs
because the present invention is broadly an improved heater core
metallurgy which can be utilized in numerous long heater designs.
The reason for the decreased tendency to form "hot spots"
which result in premature heater core failures can be seen from
comparing the "normalized resistance" of different potential heater
core materials. The normalized resistance is the resistance of a
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metal at a temperature divided by the resistance of that metal at
room temperature. Because resistances of metal change almost
linearly with temperature, a metal with a lower normalized
resistance at an elevated temperature will have a much lower
relative change in heat output if the temperature of the core
increases. Normalized resistance of nickel and copper at 800°C are
about 5.8 and about 4.8, respectively. The normalized resistance
of "30 Alloy" at 800°C is about 2.2. The normalized resistance at
800°C of an alloy of 6$ nickel and the balance copper is only about
1.5. This reflects a significant advantage in expected heater core
life.
Nichrome alloy also has an excellent normalized resistance.
At 800°C the normalized resistance is only about 1.12. But, the
electrical resistance is over three times that of nickel at 800°C,
and about 27 times that of copper. Nichrome is also not a
malleable metal. In spite of the very low normalized resistance of
Nichrome, its high resistance and lack of malleability render it
undesirable as a long heater core metal.
In a preferred embodiment of the present invention the heater
is a well heater with a heater core inside a metal sheath. The
heater core and metal sheath are separated by a space, and the
space is packed with mineral insulation material. The uphole ends
of the sheathed heating element cables are connected to power
supply cables. Power supply cables are heat-stable similarly
insulated and sheathed cables containing cores having ratios of
cross-sectional area to resistance making them capable of
transmitting the current flowing through the heating elements while
generating heat at a significantly lower rate. The power supply
cables are preferably copper sheathed, mineral insulated, and
copper cored, and have cross-sectional areas large enough to
generate only an insignificant amount of heat while supplying all
of the current needed to generate the selected temperature in the
heated zone.
Splices of the cores in cables in which mineral insulations
and metal sheaths encase current-conducting cores are preferably
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surrounded by relatively short lengths of metal sleeves enclosing
the portions in which the cable cores are welded together or
otherwise electrically interconnected. Such electrical connections
should provide joint resistance at least as low as that of the
least electrically resistive cable core being joined. Also, an
insulation of particulate material having properties o~ electrical
resistivity, compressive strength, and heat conductance at least
substantially equalling those of the cable insulations, is
preferably compacted around the cores which are spliced.
Figure 1 shows a well borehole 1 which extends through a layer
of "overburden" 20 and zones 21 and 22 of an earth formation. Zone
22 is the zone which is to be heated.
As seen from the top down, the heater assembly consists of a
pair of spoolable electric power supply cables 1 and 2, an optional
thermowell 3. A thermocouple 4 is suspended by a thermocouple wire
5, and held taut by a sinker bar 6. The thermocouple may be raised
or lowered by rotating a spool 7. The preferred embodiment is to
cement the heating cables directly in place, as shown in Figure 1.
In the preferred heater, the casing does not extend to the zone
which the heater is to heat. At the interface of the zone which is
to be heated, zone 22, and the zone which is not to be heated, zone
21, power supply cables 1 and 2 are spliced to heater cables 9 and
10 through splices 11 and 12. The heating cables extend downward
to the bottom of the zone to be heated. At the bottom of the
heating cables the heater cores are grounded to the cable sheaths
with termination plugs 13. The termination plugs may be
electrically connected by a means such as the coupler 12.
The thermowell, power supply cable and heating cables may be
suspended within a casing. If they are suspended within a casing,
the bottom of the casing should be sealed to prevent liquids from
entering. Liquids present within the casing in the zone to be
heated would limit the temperatures which could be achieved due to
the liquids vaporizing, rising up the casing, and condensing in the
casing above the heating cables. The condensed liquids would then
fall down to the heating cables, thus preventing high temperatures
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from being achieved. The preferred embodiment, as illustrated in
Figure 1, does not include a casing in the zone 22 to be heated.
The heating cables and thermowell are cemented in the well borehole
1, by cement 23. When the heating cable is cemented in the well
S borehole, the heating cable sheath must be a material that will
protect the heating cable from corrosion due to the exposure of the
heating cable to subterranean elements.
Cementing the thermowell and heating cable into the well
borehole, and eliminating the portion of the casing in the
formation to be heated, reduces the expense of the installation
considerably. If a casing is used, it must be fabricated from
expensive materials due to the high temperature and corrosive
environment. Heat transfer is also improved when the casing is
eliminated due to the absence of the vapor space around the heating
cable. 'A smaller diameter well borehole can also be utilized. The
smaller diameter borehole may result in less cement being required
to cement the heating cables than what would be required to cement
a casing into a borehole along with reducing drilling costs. The
problems involved with hermetically sealing the casing to exclude
24 liquids from entering are also avoided by elimination of the
casing. Cementing the heating cables directly into the borehole
also eliminates thermal expansion and creep by securing the heating
cables into their initial positions.
Figure 2 shows a preferred structural arrangement of the
heating and power supply cables. Referring to Figure 2, an
electrically conductive core 100 is surrounded by an annular mass
of compressed mineral insulating material 101 which is surrounded
by a metal sheath 102. The metal sheath may optionally be
fabricated in two layers (not shown). A relatively thin inner
34 layer may be fabricated initially, and a thicker outer layer of a
material resistant to corrosion could then be added in a separate
step.
Figure 3 displays details of the splice 11, of Figure 1. The
power supply cable consisting of the electrical conductive core 100
is surrounded by compressed mineral insulation 101 covered by a
20~~~~~
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sheath 102. The electrical conductive core of the power supply
cable is preferably copper and is of a sufficiently large
cross-sectional area to prevent a significant amount of heat from
being generated under operating conditions. The sheath of the power
supply cable is preferably copper. A transition sheath 103 extends
up from the coupled end of the power supply cable in order to
protect the sheath from corrosion due to the elevated temperature
near the heating cable. This protective sheath is preferably the
same material as the sheathing material of the heating cable. The
protective sheathing could extend for a distance of between a few
feet to over 40 feet. A distance of about 40 feet is preferred.
This distance ensures that the power supply cable is not damaged as
a result of exposure to high temperatures in the vicinity of the
heating cables.
In Figure 3, the heating cable sheath is shown as the
preferred two-layer sheath of an inner sheath 108 and an outer
sheath 107. The core of the heating cable 104 is welded to the
power supply cable core 100. The heating cable 104 is of a cross
section area and resistance such as to create from 50 to 250 watts
per foot of heat at operating currents. The coupling sleeve 105
and compression sleeve 106 are slid onto either the power supply
cable or heating cable prior to the cores of the cables being
welded. After the cores 104 and 100 are welded together, the
coupling sleeve 105 is welded into place onto the power supply
cable. The space around the power supply cable core 100 to heating
cable core 104 is then filled with a mineral insulating material
101. The mineral insulating material 101 is then compressed by
sliding the compression sleeve 106 into the space between the
sleeve coupling and the heating cable. After the compression
sleeve is forced into this space, it is sealed by welded
connections to the heating cable outer sheath 107 and the coupling
sleeve 105. Splice 12 of Figure 1 is of similar design.
For use in the present invention, the diameter and thickness
of the sheath is preferably small enough to provide a cable which
is "spoolable", i.e., can be readily coiled and uncoiled from
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spools without crimping the sheath or redistributing the insulating
material. The diameter of the electrically conductive core within
the cable can be varied to allow different amounts of current to be
carried while generating significant or insignificant amounts of
heat, depending upon whether the conductive core is a heating cable
or a power supply cable.
When the heating cable is utilized in a well With a casing,
the sheath of the heating cable is preferably a single layer sheath
of 316 stainless steel or the equivalent. When the heating cable
is cemented directly into the borehole without a casing, a double
layer sheath is preferred. The inner layer and the outer layer are
both preferably INCOLOY 800 (INCOLOY is a trade mark). A total
sheath thickness of about one-quarter inch is preferred although a
thickness of from one-eighth inch to one-half inch can be
acceptable depending upon the service time desired, operating
temperatures, and the corrosiveness of the operating environment.
Figure 3 displays a one core element, but it is most preferred
that the cable be fabricated with two or three cores. The multiple .
cores can each carry electricity, and eliminate the need for
parallel heating and power supply cables. A single-phase
alternating current power supply requires two cores per cable in
the most preferred embodiment of this invention, and a three-phase
alternating power supply requires three cores per cable.
The heating cable cores are preferably grounded at the
extremity of the heating cable opposite the end of the heating
cable which is coupled to the power supply cables. Figure 1
includes the preferred termination plugs 13 connected by an
electrically conductive end coupler 12. Figure 4 displays the
preferred termination plug. The plug 13 is forced into a
termination sleeve 19 which had been previously welded onto the
sheath of the power supply cable 107. The termination plug is
forced into the sleeve to compress the mineral insulating material
101. The termination plug is then brazed onto the heating cable
core 104 and welded to the termination sleeve. The termination
plugs on each heating cable may be clamped together,~as shown in
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Figure 1. When a heating cable with multiple cores is utilized,
the termination plug has a hole for each, and the plug serves to
electrically connect the cables.
The use of low resistance material as the heater cable core
material significantly simplifies power circuitry by permitting
zero crossover rather than phase angle control of electrical
current to the heater. The prior art copper cored heater cables
have a large difference between hot and cold resistances, and
therefore large differences between hot and cold electrical current
requirements for similar amounts of heat output.
Zero crossover electrical heater firing control is achieved by
allowing full supply voltage to pass through the heating cable for
a specific number of cycles, starting at the "crossover", where
instantaneous voltage is zero, and continuing for a specific number
of complete cycles, discontinuing when the instantaneous voltage
again crosses zero. A specific number of cycles are then blocked,
allowing control of the heat output by the heating cable. The
system may be arranged to "block" 15 or 20 cycles out of each 60.
This control is not practical when the circuitry must be sized for
a resistance that varies significantly because this varying
resistance would cause the current required to vary excessively.
Zero crossover heater firing is therefore not practical with prior
art copper core heaters, but is generally acceptable with a low
resistance material core heater. The alternative firing control
which is required by prior art copper core heaters is phase angle
firing. Phase angle firing passes a portion of each power cycle to
the heater core. The power is applied with a non-zero voltage and
continues until the voltage passes to zero. Because voltage is
applied to the system starting with a voltage differential, a
considerable spike of amperage occurs, which the system must be
designed to handle. The zero crossover power control is therefore
generally preferred, and systems which may incorporate zero
crossover power control are advantageous.
A thermowell may be incorporated into a well borehole which
incorporates the heater of the present invention. The thermowell
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may be incorporated into a well either with or without a casing.
When the well does not include a casing, the thermowell must be of
a metallurgy and thickness to withstand corrosion by the
subterranean environment. A thermowell and temperature logging
process such as that disclosed in U.S. Patent No. 4,616,705 is
preferred. Due to the expense of providing a thermowell and
temperature sensing facilities, it is envisioned that only a small
number of thermowells would be provided in heating wells within a
formation to be heated.
Subterranean earth formations which contain varying thermal
conductivities may require segmented heating cables, with heat
outputs per foot adjusted to provide a more nearly constant well
heater temperature profile. Such a segmented heater is described
in U.S. Patent No. 4,570,715. The greatly reduced tendency of low
resistance material core well heaters to develop hot spots greatly
reduces the need for the well heater core to have a heat output
which is correlated with local variations in subterranean thermal
conductivities, but the technique of segmenting the heater coil may
be beneficial, and required to reach maximum heat inputs into
specific formations.
