Note: Descriptions are shown in the official language in which they were submitted.
K 9031
~55~ 9
HEATER CABLE INSTALLATION
The present invention relates to a process for Eorming and
installing an electrical heater which is capable of heating a
long interval of subterranean earth formation and, where desired,
is arranged to facilitate the temperature logging of the heated
zone through a thermal well conduit extending from a surface
location to the interval being heated.
It is known that benefits can be obtained by heating inter-
vals of subterranean earth formations to relatively high tem~er-
atures for relatively long times. Such benefits may include the
pyrolyzing of an oil shale formation, the consolidating of
unconsolidated reservoir formations, the formation of large
electrically conductive carbonized zones capable of operating as
electrodes within reservoir formations, the thermal displacement
of hydrocarbons derived from oils or tars into production
locations, etc. Prior processes for accomplishing such results
are contained in patents such as the following United States
patents. US patent 2,732,195 describes heating intervals of 20 to
30 metres within subterranean oil shales to temperatures of 500
to 1000 C with an electrical heater having iron or reusable
chromium alloy resistors. US patent 2,781,851 by G.A. Smith
describes using a m~neral-insulated and copper-sheated low
resistance heater cable containing three copper conductors at
temperatures up to 250 C for preventing hydrate formation,
during gas production, with that heater being mechanically
supported by steel bands and surrounded by an oil bath for
preventing corrosion. US patent 3,104,705 describes consolidating
reservoir sands by heating residual hydrocarbons within them
until the hydrocarbons solidify, with "any heater capable of
generating sufficient heat" and indicates that an unspecified
type of an electrical heater was operated for 25 hours at
~v
3~39
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1570 F. us patent 3,131,763 describes an electrical heater for
initiating an underground combustlon reaction within a reservoir
and describes a heater with resistance wire helixes threaded
through insulators and arranged for heating fluids, such as air,
being injected into a reservoir. US patent 4,415,034 describes a
process for forming a coked-zone electrode in an oil-containing
reservoir formation by heating fluids in an uncased borehole at a
temperature of up to 1500 F for as long as 12 months.
~bject of the present invention is to provide a convenient
method of installing an electrical heater within a well. In
accordance with the invention a spooled assembly of electrically
conductiv~ cables is provided by spooling them on at least one
spooling means drum in an arrangement such that at least one
power supply cable having an innermost end adapted for subsequent
attachment to a power supply source and an outermost end con-
nected to a metal-sheathed heat-stable power-transmitting cable
which is connected to at least one metal-sheathed resistance-
heating cable having an outermost end which is, or is adapted to
be, electrically interconnected to at least one other metal-
sheathed heat-stable heating or other circuit completing
electrical conductor. A relatively flexible strand which is heat
and tension stable and is capable of supporting the weight of the
heating and pcwer transmitting cables within a well at the
temperature provided by the heating cables is arranged on a
separate spooling means with its innermost end adapted for
subsequent suspension within a wellhead and its outermost end
adapted to be attached to a weighting means capable of pulling
the strand down~ard within the well while substantially
straightening the bending imparted by the spooling means drum.
m e dimensions and properties of said cables, strand and spooling
means drums, are correlated with those of the well, the interval
to be heated and the temperature to be used, so that the power
supply cables, metal-sheathed power transmitting c bles, heater
cables and flexible strand are adapted to extend, respectively,
frcm a surface location to the subterranean locations selected
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for each of the upper ends of the power transmitting and heating
cables and a selected distance below the bottom of the heating
cables, while the electrical resistances of the cable are
arranged for conducting the current required for generating the
temperature to be employed without significant heat being
generated by the power supply cables or heat pcwer transmitting
cables. The cables and the flexible strand are concurrently
unspooled into the well with the weight being attached to the
flexible strand and the outermDst ends of the heater cables being
interconnected and all of the cables being attached to the
flexible strand before being moved into the well.
~ n a preferred embodiment the flexible strand can be a
spoolable heat stable conduit capable of serving as a thermowell
through which a temperature logging apparatus can be operated
from a Æ face location to measure the temperature with distance
along the interval being heated.
The invention will now be explained in more detail with
reference to the acccmpanying drawings, in which:
Figure 1 is a schematic illustration of a heater which can
be installed in accordance with the present invention within a
well.
Figure 2 is a schematic illustration of a preferred
arrangement involving a pair of pcwer supply cables connected to
both power transmitting and heating cables and wound on a single
drum.
Figures 3 and 4 are illustrations of splices of copper and
steel-sheathed metal cables suitable for use as cable connections
in the present invention.
Figure 5 is a three-dimensional illustration of an arrange-
3o ment for interconnecting the bottom ends of a pair of heatingcables in a manner suitable for use in the present invention.
Figure 6 is a diagrammatic illustration of a power circuit
arrangement suitable for use on a heater installed in accordance
with the present invention.
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Applicants have discovered that an electrical heater can
advantageously be made and installed by the method according to
the invention. m e dimensions and properties of power supplying
and transmitting and heating cables as well as a flexible strand
for supporting their weight, can be correlated with the pro-
perties of the well, the interval of earth formations to be
heated and the temperature at which the heating is to be con-
ducted. The completing of the necessary arrangements and
connections of the cables can be effected while part or all of
the cables are located on the drum of a spooling means. This
provides spooled assemblies which can be transported to the field
location and operated there to install long heaters wlthin wells
substantially as rapidly as is comman in running in continuous
strands which are to be strapped or clamped together. In a
preferred embodiment in which the weight supporting strand is a
continuous stainless steel tube, the resulting heater can be used
in conjunction with logging systems to provide an automatically
monitored heating system.
Figure 1 shcws a well 1 which contains a casing 2 and
2a extends through a layer of "overburden" and zones 3, 4 and 5 of
an interval of earth formation to be heated. Casing 2 is provided
with a fluid-tight bottom closure 6, such as a welded closure,
and, for example, a grouting of cement ~not shown) such as a
heat-stable but heat-conductive cement.
Such a flow preventing well completion arrangement is
preferably used in the present process for providing a means for
ensuring that heat in the borehole of the well will be con-
ductively transmitted into the surrounding earth formations. This
is ensured by preventing any flow of fluid between the sur-
3a rounding earth formations and a heater which is surrounded by an
impermeable wall, such as a well casing. This isolates the
heating elements from contact with fluid flcwing into or out of
the adjacent earth formations and places them in an environment
substantially free of heat transfer by movement of heated fluid.
Therefore, the rate at which heat generated by the heating
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elements is removed from the borehole of the well is sub-
stantially limited to the rate of heat conduction through the
earth formations adjacent to the heated portion of the well.
As seen from the top dcwn, the heater assembly consists of a
pair of spoolable electric pcwer supply cables 7 being run into
the well from spools 8. Particularly suitable spoolable cables
consist of copper conductors insulated by highly compressed
masses of particles of magnesium oxide which insulations are
surrounded by copper sheaths, the Ml power supply cables
available from BICC Pyrotenax Ltd. exemplify such cables.
Splices 9 connect the power cables 7 to heat-stable "cold
section" C pch~er transmission cables 13. m e cables 13 provide a
cold section above the "heating section" H of the heater
assembly. (Details of the splices 9 are shown in Figure 3). The
cold section cables 13 as well as the power cables to which they
are spliced are preferably spoolable cables constructed as shcwn
in Figure 3. The cold section cables 13 each have a metallic
external sheath which has a diameter near that of the power cable
but is constructed of a steel which preferably is, or is
substantially equivalent to, stainless steel. Relative to the
po~er supply cables 7, the conductors or cores of the cold
section cables 13 have cross-sections which are smaller but are
large enough to enable the cold section cables to convey all of
the current needed within the heating section H without
generating or transmitting enough heat to damage the copper or
other sheaths on the power cables or the splices that connect
them to the cold section cables.
At splices 14 the cold section cables 13 are connected to
moderate-rate heating-element cables 15. tDetails of the splices
14 are shown in Figure 4.) In the moderate-heating-rate cables 15
the cross-sectional area of a core such as a copper core is
significantly smaller than the core of the cold section cable 13.
The relationship between the cross-sectional area of the current
carrying core in cable 15 to the resistance of that in cable 13
is preferably such that cable 15 generates a selected-temperature
3~
between about 600 to 1000 C in response to a selected EMF of not
more than about 1200 volts between the cores and sheaths. Of
course, where desired, the cables used in a given situation can
include numerous gradations of higher or lcwer ra~es of heating.
At splices 16 the moderate-rate-heating cables 15 are joined
with maximum~rate heating cables 17. The constructions of the
cables 15 and 17 and splices 16 and 18 are the same except that
the cables 17 contain electrically conductive cores having
smaller cross-sectional areas for causing heat to be generated at
a rate which is somewhat higher than the moderate rate generated
by cables 15 in response to a given EMF.
Splices 18 connect the maximum rate heating cables 17 to
moderate rate heating cables 19. Splices 18 can be the same as
splices 16 and cables 19 can be the same as cables 15.
At the end-piece splice 20 the current conducting cores of
the cables 19 are welded together within a chamber in which they
are electrically insulated. (Details of the end-piece splice 20
are shown in Figure 5.) Where desirable, a single assembly of
electrical cables can be arranged to supply a heating cable 19,
serving as a single heating leg, to an electrical conductor (such
as a ground or return line) other than another heating cable.
The end-piece splice 20 is mechanically connected to a
structural support member 21 which is weighted by a sinker bar
22. The support member 21 is arranged to provide vertical support
for all of the power and heating cable sections by means of
intermittently applied mechanical connecting brackets or bands
23. Bands, such as band 23 are attached around the cables 19 and
support member 21 and tightened so that the friction between the
cables and a weight-supporting member is sufficient to support
the weight of the cables between each of the bands. Mechanical
banding or strapping devices which pull a flexible band such as a
steel band through a collar position while applying tightening
force and crimping the collar portion to hold the bands in place
are commercially available and are suitable for use in this
invention. For example, a suitable banding system comprises the
Signode Air Binder Model PNSC34 and other suitable systems, are
available fro~ Reda or Centrilift Pump Corporatlons.
Where, as shown in Figure 1, the interval of earth for-
mations to be heated contains a relatively highly heat-conductive
zone such as zone 4, the tendency for that zone to cause a zone
of relatively low temperature along the heater can be compensated
for by, for example, splicing in a relatively high rate heating
section of cables, such as cables 17.
Figure 2 shcws an arrangement for spooling one or both of
the electrical cable assemblies shown in Figure 1 on the drum of
a spooling means 8. As shcwn, the innermost end (relative to the
spooling means) of power cable 7 is equipped with an end-piece 7a
which is, or can be connected to, a connector for attachment to a
scurce of electrical power. The cable is wound onto the drum
surface 8a of the spooling means 8. The outermost end of cable 7
is connected, by splice 9, to cable 13 which is connected, by
splice 14, to cable 15, etc. Such connections are preferably
completed before or during spooling of the cables onto the
spooling means. Where a two-legged heater is to be formed by a
pair of electrical cables and both cable assemblies are to be
spooled onto the same drum, an end splice 20a for interconnecting
the heater cables can advantageously be connected to the heater
cables before the cables are unspooled into contact with the
structure support member 21, during their installation within the
well.
Figure 3 illustrates details of the splices 9. As shcwn in
the figure, the power cable 7 has a metal sheath, such as a
copper sheath, having a diameter which exceeds that of the steel
sheathed cold section cable 13. The central conductors of the
cables are joined, preferably by welding. A relatively short
steel sleeve 30 is fitted around, and welded or braised to, the
metal sheath of cable 7. The inner diameter of sleeve 30 is
preferably large enough to form an annular space between it and
the steel sleeve of cable 13 large enough to accommodate a
shorter steel sleeve 31 fitted around the sheath of cable 13.
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sefore inserting the short sleeve 31, substantially all of the
annular space between the central members 10 and lOa and sleeve
30 is filled with powdered mineral insulating material such as
magnesium oxide. That material is preferably deposited within
both the annular space between the central members and sleeve 30
and the space between sleeve 30. The sheath of cable 13 is
preferably vibrated to coNpact the mass of particles~ Sleeve 31
is then driven into the space between sleeve 30 and the sheath of
cable 13 so that the mass of mineral particles is further com-
pacted by the driving force. The sleeves 30 and 31 and the sheathof cable 13 are then welded together.
Figure 4 illustrates details of the splices 14, which are
also typical of details of other splices in the steel sheathed
heating section cables, such as splices 16 and 18. The splice
construction is essentially the same as that of the splices 9.
However, the steel sleeve 32 is arranged, for example, by
machining or welding to have a section 32a with a reduced inner
diameter which fits around the sheath of cable 13 and a larger
inner diameter which leaves an annular space between the sleeve
2Q 32 and the sheath of cable 15. After welding the central con-
ductors together, the sleeve portion 32a is welded to the sheath
of cable 13. The annular space between the sleeve 32 and the
central conductors is filled with powdered insulating materials,
a short sleeved section 33 is driven in to compact particles and
is then welded to the sheath of cable 15.
Figure 5 illustrates details of the end splice 20. As shcwn,
cables 19 are extended through holes in a steel block 20 so that
short sections l9a extend into a cylindrical cpening in the
central portion of the block. The electrically conductive cores
of the cables are welded together at weld 34 and the cable
sheaths are welded to block 20 at welds 35. Preferably, the
central conductors of the cables are surrounded by heat stable
electrical insulations such as a mass of compacted powdered
mineral particles and/or by discs of ceramic materials (not
shown), after which the central opening is sealed for example, by
- 9 -
welding-on pieces of steel (not shown). Where the heater is
supported as shcwn in Figure 1, by attaching it to an elongated
cylindrical structural member 21, a groove 36 is preferably
formed along an exterior portion of end splice 20 to mate with
the structural member and facilitate the attaching of the end
piece to that member.
When a well heater is emplaced in a borehole and operated at
a temperature of more than about 600 C, loading (i.e., weight/-
cross sectional area of weight-supporting elements) thermal
expansion, and creep, are three factors which play an important
role in how the heater can be positioned and maintained in
position (for any significant period of time). For example, for a
heater constructed and mounted as illustrated in Figure 1, where
the central structural member 21 is a stainless steel tube having
a diameter of 1.25 cm and a wall thickness of 1.8 cm, since the
coefficient for thermal expansion for both steel and copper is
about 13 times 10 6 cm per cm, per degree Celcius, a 300 m long
heating section would expand to 304 m by the time it reached a
temperature of 800 C.
When using the arrangement illustrated in Figure 1, space is
preferably allowed for such expansion. The heater is preferably
positioned so that, after expansion, the lcwer part is carrying
its weight under compression loading (because it is resting on
the bottom of the borehole or surrounding casing) while the upper
part is still hanging and is loaded under tension, with a neutral
point heing located somewhere in the middle.
Due to the creep rate of stainless steel, with a typical
loading factor of about 490 bar on stainless steel structural
members of a heater, at 700 C the length of a 300 m heating
section would increase by 0.026 cm per hour or 266 cm per year
or 27 m in 10 years - if it was not ruptured before then.
