Note: Descriptions are shown in the official language in which they were submitted.
METHOD AND APPARATUS FOR HIGH TEMPERATURE
SERIES/PARALLEL HEATING USING MINERAL INSULATED AND
FERROMAGNETIC SKIN EFFECT CABLE
TECHNICAL FIELD
[0001] The present invention pertains to a method and apparatus for
heating
crude oil and other fluids. More particularly, the present invention pertains
to a
method and apparatus for high temperature series/parallel heating of crude oil
and
other fluids using mineral insulated and ferromagnetic skin effect cable.
BACKGROUND ART
[0002] Heating technology is generally well known in the art and is
currently
utilized in a broad range of fluid-flow applications. By way of illustration,
but not
limitation, heating technology can be used for freeze protection, temperature
maintenance and pipeline flow optimization. Heating technology can also be
utilized
to improve the flow characteristics of crude oil, thereby improving ultimate
recovery
of such crude oil.
[0003] Crude oil often comprises a mixture of liquids, solids and/or
solution gases
that flows freely at room temperature. Free-flowing crude oil typically has
some
combination of relatively low density, low viscosity, low specific gravity
(high API
gravity) and/or low wax content. In many cases, these characteristics are also
due to
the presence of a relatively high proportion of light hydrocarbon fractions,
as well
as a lack of hydrates, solids, wax or other impurities.
[0004] However, not all crude oil exhibits such characteristics or will
flow freely at
room temperature. In some cases, certain crude oil (including, but not
necessarily
limited to, so-called "heavy oil") is too viscous to flow in its naturally
occurring state.
In such instances, the ability of such crude oil to flow can often be improved
by
applying heat to such crude oil using some suitable technique. In certain
cases, heat
can be applied directly to a subterranean reservoir to permit oil to flow to
one or
more well(s) completed within such reservoir. In other cases, heat can be
applied
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within a wellbore in order to facilitate oil flow within said wellbore.
[0005] Both steam and electrical heaters have been used as sources of
heat to
promote enhanced crude oil recovery. One technique, referred to as heat
tracing,
involves the use of mechanical and/or electrical components placed on a piping
system to maintain the system at a predetermined temperature. Steam may be
circulated through tubes, or electrical components may be placed on the pipes
to
supply the desired heat.
[0006] Mineral Insulated (MI) cables can be used for electro-thermal
heating.
Such MI cables are typically constructed of one or more conductors embedded
within a mineral powder electrical insulant, normally magnesium oxide (MgO),
which
is surrounded by a continuous (typically metal) sheath. Depending on
temperature
and chemical exposure requirements, such sheath material can be copper,
stainless
steel or high nickel alloys. MI cables having a nickel alloy sheath can
typically
maintain temperatures up to 550 C (1022 F), can be exposed to temperatures up
to
670 C (1238 F), and can frequently operate up to 600 volts in alternating
current
(Vac) in a single- or three-phase configuration. MI cables are typically
series-
resistance type heaters that are usually factory-terminated and supplied in
fixed
lengths.
[0007] Ferromagnetic skin-effect heating systems (SEHS) represent a form
of
impedance heating in which a single electrically insulated copper conductor is
installed inside a ferromagnetic envelope (carbon steel heat tube). The
conductor is
connected to the heat tube at the distal end and an ac power source is
connected
between the conductor and the heat tube at the supply end. This method of
heating
is called skin-effect heating because the return path of the circuit current
is pulled to
the inner surface of the heat tube by both the skin effect and the proximity
effect
between the heat tube and the conductor. SEHS heaters can typically maintain
temperatures up to 200 C (392 F) and be exposed to 250 C (482 F), and can
operate at voltages as high as 5000 Vac.
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[0008] A subterranean heating system employing electro-thermal heating
technology is disclosed in United States Patent No. 7,322,415 entitled
"Subterranean
Electro-Thermal Heating System and Method".
[00091 Unfortunately, existing heating techniques have certain drawbacks.
Steam
injection systems may be encumbered by inefficient energy use, maintenance
problems, environmental constraints, and an inability to provide accurate and
repeatable temperature control. Although electrical heating is generally
considered
advantageous over steam injection heating systems, electrical heating systems
frequently cause unnecessary heating in regions that do not require heating in
order
to facilitate oil flow. Such unnecessary heating is associated with
inefficient power
usage and may also cause environmental issues such as undesirable thawing of
permafrost in arctic locations.
[0010] Accordingly, there is a need for an electro-thermal heating system
that is
capable of efficiently and reliably delivering thermal input to localized
areas,
particularly in a subterranean environment. The heating system should be
capable
of providing different zones of increased heating in specific regions. In the
case of
oil wells, the heating system should be capable of beneficially providing
increased
thermal characteristics at different locations along the length of such wells.
The
desired range of heat output for a single application should be up to an order
of
magnitude difference between low output zone(s) and the highest output
zone(s),
and the system should be durable and easy to install using conventional
installation
methods.
DISCLOSURE OF INVENTION
[0011] The preferred embodiment of the present invention comprises a
heating
cable having a ferromagnetic steel sheath with sections having modified
insulation
material comprising magnesium oxide blended with an electrically conductive
material to achieve zones of increased heat output via direct electrical
heating of the
modified insulant within the cable in a parallel circuit configuration. The
heater of the
present invention may be optionally configured as a typical skin effect
heating
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. .
system, or may be designed with an open end termination. Any number of
separate
parallel heating and non-heating zones can be included, each having different
heating characteristics, resulting in targeted heating zones.
100121 In the preferred embodiment of the present invention,
some or all of the
MgO electrical insulation in a ferromagnetic sheathed MI cable is replaced
with a
compound of MgO (that is, a modified insulant) having substantially reduced
insulation characteristics. The modified MgO insulant can be obtained by
adding
iron or other conductive fillers to the standard insulating MgO.
100131 An existing MI cable manufacturing method using preformed
blocks of
MgO can be directly utilized to produce the cable of the present invention,
simply by
utilizing the blocks of modified MgO insulant during the manufacturing process
where
areas of reduced insulation (and increased heating effects) are desired.
[0013A] The invention in one broad aspect provides a heating cable which
comprises a conductive core having a length. A ferromagnetic outer sheath is
disposed
around the conductive core, and a first insulant is disposed between the
conductive
core and the outer sheath along a first portion of the cable assembly. The
first insulant
extends from the conductive core to the outer sheath, and a second insulant is
disposed
between the conductive core and the outer sheath along a second portion of the
cable
assembly. The first and second insulants have different electrical
conductivity, and the
second insulant extends from the conductive core to the outer sheath.
BRIEF DESCRIPTION OF THE DRAWINGS
100141 The foregoing summary, as well as the following detailed
description of the
best mode for carrying out the invention, is better understood when read in
conjunction with the appended drawings. For the purpose of illustrating the
invention,
the drawings show certain preferred embodiments. It is understood, however,
that
the invention is not limited to the specific methods and devices disclosed.
Further,
dimensions, materials and part names are provided for illustration purposes
only and
not limitation.
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[0015] FIG. 1 depicts a side sectional view of a representative
configuration of the
cable assembly present invention.
[0016] FIGS. 2a through 2c depict an analysis of several possible power
output
distributions of the present invention in the form graphical representations.
[0017] FIG. 3 depicts an analysis of skin-effect tracing system ("STS")-
MI heater
with dielectric heating option in the form of calculations and related
graphical
representations.
BEST MODE FOR CARRYING OUT THE INVENTION
[0018] Referring to the attached FIG. 1 , the present invention comprises
cable
assembly 10. Cable assembly 10 of the present invention further comprises a
plurality of insulation zones, 2, 3 and 4, each comprising insulant having
different
electrical conductivity. Typical applications of the present invention would
utilize 2 to
4 different zones, but additional zones are not excluded.
[0019] A ferromagnetic outer sheath 5 is provided. Said outer sheath 5
has a wall
thickness determined based on in-service corrosion rates and allowing a
minimum
thickness as required to substantially contain the heater's electrical current
via
ferromagnetic skin effect. For typical power line frequency applications,
outer sheath
has a minimum wall thickness of approximately 0.1 inches. It is to be observed
that the thickness of outer sheath 5 can be varied as needed for particular
applications and conditions to be encountered.
[0020] A conductor core 6 is disposed within insulation zones 1 through
4, and
outer sheath 5. Although said conductor core 6 is typically constructed of
copper,
other conductive materials and metals are not excluded. In some cases, the use
of
metal alloys for said conductor core 6 may be desirable to increase the series
heating portion of the total heat output of cable assembly 10.
[0021] An optional shorting termination 7 within end sealing cap 8 can be
provided. When shorting termination 7 is installed, cable assembly 10
functions
electrically more like a typical skin effect heater as is commonly used in
industrial
pipeline heating applications; however, the cable assembly of the present
invention
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includes zones of increased heat output through the use of multiple different
insulation zones.
[0022] End sealing cap 8 can be welded or otherwise attached to
ferromagnetic
sheath 5 and packed with MgO insulation in order to provide an environmental
seal
(suitable for withstanding elevated pressures and temperatures frequently
encountered in oil wells and reservoirs).
[0023] Due to the inclusion of zones of dielectric insulation having
reduced
resistivity (increased conductivity), substantial electric heating of the
insulation
occurs when voltage is applied between core 6 and sheath 5. As a result, cable
assembly 10 of the present invention provides the ability to selectively
increase the
heat output of a cable by using insulations of different resistivity, in
combination with
series skin effect heating of core 6 and sheath 5. Also, the relatively high
temperature-withstanding capability of MI cable, and the possibility for very
high
heat output rates (high power density), yield improved results over existing
heating
systems.
[0024] Providing different zones of heat output along the length of a
single heater
cable that can be easily installed in wellbores using conventional coiled
tubing
installation methods represents a significant improvement over prior art
heating
systems. The present invention offers the ability to provide different zones
of heat
output in a single readily deployed and extremely rugged coiled tube
configuration.
[0025] It is to be observed that insulation materials other than MgO can
be used
in connection with the present invention; a wide variety of conductive
materials can
be utilized to alter or adjust the resistivity of insulant used in cable
assembly 10.
Further, manufacturing methods other than pre-formed insulation blocks in a
tube
reduction mill can also be used to manufacture heater cable assembly 10 of the
present invention. Similarly, the number of different insulation zones can be
varied,
and an optional shorting end termination can be provided.
[0026] FIGS. 2a through 2c depict an analysis of several possible power
output
distributions of the present invention in the form graphical representations.
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[0026A] A basic analysis of STS-MI heater with dielectric heating option is
set out
below.
pow 1..10841cm electilcat resistivity of standard MgO
4,10611.ent electricat resistivity of doped M90
4- 101`11, cal
0.01)4.4¨ ... combined core+sheath skin effect impedance
0.9io ID of the sheath
Li := 025in .,. OD of the core
txt t200V applied voltage
27273 A
7..$020081
Define the heater configuration:
'20' Prato
110 Pt*. zones defines the =measured depth
to
1100
70110 .4ZZ MiSii YWCA which the matthing resistivity goes
Phi
030D,
Pk)
Mtn segment length (allows means to reduce the vector
size)
Rom :==, 10120 terminating reWstance
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Build the IR list:
p,Ist ¨
d
calculation ot Insulation resistance value kir length sand hailer"
resistivity p
no tt--z 0 001(Yc 1"11) n1:--t= 74"1"1 al= 2
sl
si2 11
(zone s2
Ctil-) (zoctnes
110
l
j )
n4 tt= 130
:',== no,. ni IRAM.rcsistivaleso) IRõ 2.039 x
ni +1.. n1 1Rn IRAslottSitniviticti) IR =41.54711
142-1
`t I IR, IR(s! tresistividesa) 1Rnr., :z X15.46841
n :al n3+ 1-n4 IRõ stgistivities3) IR,81.5470
kits4:= !Item
Build the impedance matft and voltage vector
tows(tR) 13i
diagonals:
IR14 1;41 4' slags 7. IR04 + slam + Rum
4Ra At-1 ,1R 4Rft diagonals
sowa(Z) aZ 131
Vao :P.- V, Va, OV Va 0
rOWS1VE0*; 131
Solve the system;
Z.- 1-vs
Itõopo Itz 31&239A 11; .289.44.11A
e 2
(hoop* ¨ kop 4)2 ^Mg (1.4goopr .Zioe apparent power tor sash loop
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[00271 FIG. 3 depicts the results of the above analysis of STS-MI heater
with
dielectric heating option.
[00281 In addition to the applications described above, the present
invention can
be used for many different applications including, without limitation: down
hole
heating for well-stream flow assurance, bottom hole heating for reservoir
stimulation,
a combination of both down hole and bottom hole heating, pipeline heating
(onshore
or subsea and particularly but not limited to cases requiring zones of
increased heat
output) and conventional pipeline heating applications (even where only a
single
heat output zone is required). Such examples are illustrative only, and should
not be
construed as being limiting in any way, or as representing a comprehensive
list of all
applications for the technology disclosed herein.
[00291 The above-described invention has a number of particular features
that
should preferably be employed in combination, although each is useful
separately
without departure from the scope of the invention. While the preferred
embodiment
of the present invention is shown and described herein, it will be understood
that the
invention may be embodied otherwise than herein specifically illustrated or
described, and that certain changes in form and arrangement of parts and the
specific manner of practicing the invention may be made within the underlying
idea
or principles of the invention.
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