CA2845525A1 - Method and apparatus for high temperature series/parallel heating using mineral insulated and ferromagnetic skin effect cable - Google Patents

Abstract

A heating cable (10) has a ferromagnetic steel sheath (5) with sections (2, 3, 4) having modified insulation material such as, for example, magnesium oxide blended with an electrically conductive material. Different zones having different insulation generate increased heat output via direct electrical heating of the modified insulation material permitting targeted heating along the length of the cable. The heating cable may be optionally configured as a skin effect heating system, or may be configured with an open end termination.

Description

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 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 5000Vac.

[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", which disclosure is incorporated by reference herein for all purposes.

[0009] 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 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.

[0012] 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.

[0013] 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.
BRIEF DESCRIPTION OF THE DRAWINGS

[0014] 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.

[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] FIGS. 3a through 3c depict an analysis of skin-effect tracing system ("STS")-Ml 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 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.

[0027] FIGS. 3a through 3c depict an analysis of STS-MI heater with dielectric heating option in the form of calculations and related graphical representations.

[0028] 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.

[0029] 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.

Claims (6)

1. A heating cable comprising:
a. a conductive core having a length;
b. a ferromagnetic outer sheath disposed around said conductive core;
c. a first insulant disposed between said conductive core and said outer sheath along a first portion of said cable assembly;
d. a second insulant disposed between said conductive core and said outer sheath along a second portion of said cable assembly, wherein said first and second insulants have different electrical conductivity.

2. The heating cable of claim 1, wherein said first insulant comprises magnesium oxide and said second insulant comprises a compound of magnesium oxide and an electrically conductive material.

3. The heating cable of claim 1, wherein said conductive core is a metal alloy.

4. The heating cable of claim 1, wherein said conductive core is copper.

5. The heating cable of claim 1, further comprising a sealing end cap.

6. The heating cable of claim 1, further comprising a shorting termination at said end cap.