Electric heating – Inductive heating – Specific heating application
Reexamination Certificate
2000-04-28
2001-09-04
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
Specific heating application
C219S670000, C219S672000, C219S676000, C166S060000, C166S248000
Reexamination Certificate
active
06285014
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a heating system useful in the production of oil from a subterranean reservoir. More particularly it relates to a downhole induction-heating tool that can be placed in a wellbore and has the capability to electro-magnetically generate heat in the wellbore casing. In addition the invention relates to a method of heating a segment of wellbore casing electro-magnetically.
BACKGROUND OF THE INVENTION
It is common knowledge in the oil industry that the introduction of heat into an oil reservoir, especially a reservoir containing heavy or waxy oil, is beneficial. There are several methods used to achieve reservoir heating. They include steam injection, in situ combustion and electrical heating. The present invention is concerned with electrical heating.
Electric heating can take the form of resistance heating or induction heating. The present invention is concerned with induction heating. A significant drawback of the non-conductive heating approach is that high non-conductive heating element temperatures can cause coking, scaling and other forms of deposition, which raise the thermal resistance through which heat flows from the non-conductive element to the well bore. This elevated thermal resistance either increases the operating temperature of the non-conductive element for the same power level, or reduces the operational power level for the same element temperature.
Electric heating may also be classified by the method of conveying electric power to the downhole heater. In both non-conductive and inductive heating, electric power may be conveyed downhole via an isolated production tubing string. The present invention is concerned with inductive heating, in which electrical power is conveyed downhole via a cable running from the surface power system to the downhole tool.
Induction heating tools may be run into the wellbore of an existing well on a tubing string. The induction-heating tool may be landed opposite an interval to be heated. The tool can readily be removed for repair. There is no need for a permanent modification to the well to facilitate heating, such as the incorporation of isolators into the casing string, which is the case for some electrical heating systems which use the tubing string as an electrical conductor.
The tool itself comprises a transformer-type core-coil assembly jacketed in a tubular housing. Each core-coil assembly comprises a conductive wire coil wound on a magnetically permeable, laminated core. AC power is supplied to the coil from a power source at ground surface through a bus extending down the wellbore. Application of power to the coil induces eddy currents in the adjacent steel casing or screen liner, thereby increasing its temperature. The hot casing or liner in turn heats the near-wellbore region of the reservoir and oil within the wellbore. The term “casing” as used herein broadly means casing, sand exclusion liners and similar metal tubular goods having an interior flow path that defines the well bore.
Canadian Patent Application No. 2,208,197, filed by R. E. Isted and published Dec. 18, 1998, discloses an induction-heating tool. Although the application discloses a stainless steel, magnetically-transparent housing, the housing is not electro-magnetically transparent and a high tool winding operating temperature can be expected to reduce the tool operating life.
Canadian Patent 2,090,629, issued to J. E. Bridges on Dec. 29, 1998 discloses another induction heating tool. The '629 patent discloses a method of conveying electrical power via the production tubing. This method requires modification of the well casing for the installation of an electrically non-conductive window. This well modification is expensive and likely to be the source of serious reliability concerns.
The visco-skin effect, which reduces oil recovery, arises when heavy oil, approaching the wellbore, loses light ends due to changing pressure conditions, leaving a heavier oil clogging the reservoir immediately adjacent the wellbore. As previously stated, the hot casing heats both the near-wellbore region of the adjacent reservoir and the oil entering or within the casing. This has the benefits of ameliorating the visco-skin effect and improving the production and pumpability of the oil. The application of heat in this manner thus can stimulate and significantly improve the production rates of high viscosity heavy oil and waxy wells.
SUMMARY OF THE INVENTION
The present invention addresses many of the challenges that face one designing a downhole induction-heating tool. These challenges include:
(a) Maximizing the power dissipation within the casing while minimizing power dissipation within the tool. By minimizing tool power dissipation, tool operating temperature can be kept low, thus protecting tool components and raising limits on tool input power, without raising coil temperature;
(b) Providing a tool having desirable structural strength in longitudinal tension and compression and some flexibility, so that the tool can be pushed and pulled as it moves through the wellbore and can be worked past curves and other deviations of the wellbore;
(c) Providing a lengthy tool having a series of core-coil assemblies aligned longitudinally and arranged in contiguous or spaced apart configurations;
(d) Providing a tool adapted to facilitate the sharing of supplied power so that a single string of tubing can incorporate several tools to supply heat across a long production interval;
(e) Providing a tool having several core-coil assemblies, to smooth out the temperature profile extending along the casing;
(f) Achieving a design that is generic, so that a single tool design can be used in vertical, deviated and horizontal wells.
The work underlying the present invention has demonstrated the desirability of several features described below, which can be incorporated into a downhole induction-heating tool, either singly or in various combinations.
One feature of the tool is the use of a non-magnetic, electrically insulating, housing to enable high power transfer efficiency to the casing. Prior art technology has taught that a magnetically transparent housing, such as stainless steel, is sufficient to achieve satisfactory power transfer efficiency. A stainless steel housing, of sufficient thickness, could also provide desired structural strength, which would allow the tool to be pushed and pulled through the well bore. However, our work has shown that if a stainless steel housing is used, the heating process is limited by losses and heat builds up in the core-coil assembly to such an extent that it may become damaged.
A relatively thin fibreglass housing is non-magnetic and electrically insulating, but has low axial structural strength in compression and tension. In another feature of the invention, a longitudinally rigid reinforcing member, such as one or more steel rods, extends internally and longitudinally through each core-coil assembly. Spacers or joints providing high flexibility join the reinforcing members of individual adjacent core-coil assemblies. The tool thus has an internal “skeleton” which is strong longitudinally in tension and compression yet capable of flexing sufficiently to allow the tool to manoeuvre around bends in the wellbore. The non-magnetic, electrically insulating, housing is not relied upon for any axial structural strength.
In another feature, epoxy is used to encapsulate each core-coil assembly. Epoxy has a very high dielectric value. Thus a busway can be formed in the epoxy lengthwise of the core-coil assembly, while electric isolation of the coil relative to the bus is maintained by residual epoxy remaining between the coil and busway. As a consequence, high voltage bus wire can be used, increasing the power that can be delivered to the tool. In addition the epoxy enhances heat dissipation, resistance to mechanical shock and winding protection.
The use of spacers between adjacent core-coil assemblies significantly affects the uniformity of the temperature profile developed along the casing bei
Beck Thomas
Garnall Stephen J.
Kahn David Seemab
McGee Bruce C. W.
Sallwasser Alan J.
Browning Bushman
Leung Philip H.
Neo PPG International, Ltd.
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