Variable frequency temperature limited heaters

• US 7,490,665 B2
• Filed: 04/22/2005
• Issued: 02/17/2009
• Est. Priority Date: 04/23/2004
• Status: Expired due to Fees

• Current Assignee: Shell Oil Company (Shell Plc)
• Original Assignee: Shell Oil Company (Shell Plc)
• Inventors: Vinegar, Harold J., Harris, Christopher Kelvin, Sandberg, Chester Ledlie
• Primary Examiner(s): KRECK, JOHN J
• Application Number:
  US11/112,863
• Publication Number:
  US 20050269093A1
• Time in Patent Office: 1,397 Days
• Field of Search: 166/302, 166/60, 392/301
• US Class Current: 166/60
• CPC Class Codes: E21B 36/04 using electrical heaters
• CPC Class Codes: E21B 43/12 Methods or apparatus for controlling the flow of the obtained fluid to or in wells E21B43/25 takes precedence; valve arrangements E21B34/00
• CPC Class Codes: E21B 43/122 Gas lift
• CPC Class Codes: E21B 43/24 using heat, e.g. steam injection
• CPC Class Codes: E21B 43/2401 by means of electricity
• CPC Class Codes: E21B 43/2405 in association with fracturing or crevice forming processes E21B43/247 takes precedence
• CPC Class Codes: E21B 43/38 in the well
• CPC Class Codes: H05B 3/141 Conductive ceramics, e.g. metal oxides, metal carbides, barium titanate, ferrites, zirconia, vitrous compounds

Abstract

Certain embodiments provide a heating system configured to heat at least a part of a subsurface formation. The system includes an electrical power supply. A heater section includes one or more electrical conductors electrically coupled to the electrical power supply. The heater section is configured to be placed in an opening in the formation. At least one of the electrical conductors includes ferromagnetic material. The heater section provides a first heat output when time-varying electrical current is applied to the heater section below a selected temperature, and provides a second heat output approximately at and above the selected temperature during use. The second heat output is reduced compared to the first heat output. The system is configured to allow heat to transfer from the heater section to a part of the formation. The system is configured to allow a frequency of the applied time-varying electrical current to be varied.

Claims

1. A heating system configured to heat at least a part of a subsurface formation, comprising:

an electrical power supply, wherein the electrical power supply is configured to provide modulated DC;

a heater section comprising one or more electrical conductors electrically coupled to the electrical power supply and configured to be placed in an opening in the formation, at least one of the electrical conductors comprising ferromagnetic material;

wherein the heater section (a) provides a first heat output when modulated DC is applied to the heater section below a selected temperature, and (b) provides a second heat output approximately at and above the selected temperature during use, the second heat output being reduced compared to the first heat output;

the system configured to allow heat to transfer from the heater section to a part of the formation; and

the system configured to allow a frequency of the modulated DC to be varied.

2. The system of claim 1, wherein the frequency of the modulated DC is configured to be varied based on one or more subsurface conditions at or near the electrical conductors.

3. The system of claim 1, wherein the frequency of the modulated DC is configured to be varied as a function of a temperature at or near the electrical conductors in the subsurface formation.

4. The system of claim 1, wherein the frequency of the modulated DC is configured to be varied to adjust a turndown ratio of the electrical conductors.

5. The system of claim 1, wherein the electrical power supply comprises a variable frequency DC electrical power supply.

6. The system of claim 1, wherein the electrical power supply is configured to provide square wave modulated DC.

7. The system of claim 1, wherein the electrical power supply is configured to provide modulated DC in a pre-shaped waveform, and wherein the pre-shaped waveform is shaped to at least partially compensate for phase shift and/or harmonic distortions in the electrical conductors.

8. The system of claim 1, wherein the heater section automatically provides the second heat output.

9. The system of claim 1, wherein at least a portion of the heater section is positionable adjacent to hydrocarbon material in the formation to raise a temperature of at least some of the hydrocarbon material to or above a pyrolysis temperature.

10. The system of claim 1, wherein the second heat output is at most 90% of the first heat output, the first heat output being at about 50°

C. below the selected temperature.

11. The system of claim 1, wherein the electrical resistance of the heater section decreases above the selected temperature to provide the second heat output.

12. The system of claim 1, further comprising a non-ferromagnetic material coupled to the ferromagnetic material, wherein the non-ferromagnetic material has a higher electrical conductivity than the ferromagnetic material.

13. The system of claim 1, wherein the selected temperature is approximately the Curie temperature of the ferromagnetic material.

14. The system of claim 1, wherein the heater section has a turndown ratio of at least about 1.1 to 1.

15. The system of claim 1, wherein at least a portion of the heater section is longer than about 10 m.

16. A method for heating a subsurface formation, comprising:

applying modulated DC to a heater section to provide an electrically resistive heat output, the heater section comprising one or more electrical conductors electrically coupled to the electrical power supply and configured to be placed in an opening in the formation, and at least one of the electrical conductors comprising ferromagnetic material;

providing a first heat output when modulated DC is applied to the heater section below a selected temperature;

providing a second heat output approximately at and above the selected temperature during use, the second heat output being reduced compared to the first heat output;

allowing the heat to transfer from the heater section to a part of the subsurface formation; and

varying a frequency of the applied modulated DC.

17. The method of claim 16, wherein the method further comprises assessing one or more subsurface conditions at or near the electrical conductors in the subsurface formation and varying the frequency of the applied modulated DC based on at least one assessed condition.

18. The method of claim 16, wherein the method further comprises assessing a temperature at or near the electrical conductors in the subsurface formation and varying the frequency of the applied modulated DC based on the assessed temperature.

19. The method of claim 16, wherein the method further comprises varying the frequency of the applied modulated DC to adjust a turndown ratio of the electrical conductors.

20. The method of claim 16, wherein the method further comprises applying modulated DC electrical current in a pre-shaped waveform, wherein the pre-shaped waveform is shaped to at least partially compensate for phase shift and/or harmonic distortions in the electrical conductors.

21. The method of claim 16, wherein the heater section automatically provides the second heat output.

22. The method of claim 16, wherein the method further comprises placing one or more of the electrical conductors in a wellbore in the formation.

23. The method of claim 16, wherein an electrical resistance of the heater section decreases above the selected temperature to provide the second heat output.

24. The method of claim 16, wherein the selected temperature is approximately the Curie temperature of the ferromagnetic material.

25. The method of claim 16, wherein the subsurface formation comprises a hydrocarbon containing formation.

26. The method of claim 16, wherein the subsurface formation comprises a hydrocarbon containing formation, and the method further comprises heating at least some hydrocarbons in the formation such that at least some of the hydrocarbons are pyrolyzed.