Abstract: A heating system for a subsurface formation includes an electrical conductor and an insulation layer at least partially surrounding the electrical conductor. A jacket that includes ferromagnetic material at least partially surrounds the insulation layer. The outside surface of the jacket is configured to be at little or no electric potential while the electrical conductor is conducting electricity and while the jacket is at temperatures below the Curie temperature of the ferromagnetic material.

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.

Abstract: Certain embodiments provide a system configured to heat 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. 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 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 electrical power supply is configured to provide a relatively constant amount of time-varying electrical current that remains within about 15% of a selected constant current value when a load of the electrical conductors changes.

Abstract: The invention provides a system configured to heat at least a part of a subsurface formation. The system includes: an electrical power supply configured to provide modulated direct current (DC); and one or more electrical conductors configured to be electrically coupled to the electrical power supply and placed in an opening in the formation, wherein at least one of the electrical conductors has a heater section, the heater section comprising an electrically resistive ferromagnetic material configured to provide an electrically resistive heat output when electrical current is applied to the ferromagnetic material, and the heater section is configured to provide a reduced amount of heat near or above a selected temperature during use due to the decreasing electrical resistance of the heater section when the temperature of the ferromagnetic material is near or above the selected temperature, and the heater section has a turndown ratio of at least 1.1 to 1.

Abstract: Certain embodiments provide a method for heating a subsurface formation. The method includes applying direct electrical current (DC) to a heater section below a first selected temperature to provide an electrically resistive heat output. The 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. Time-varying current is applied to the heater section when a temperature of the heater section approaches or is above the selected temperature. Heat is allowed to transfer from the heater section to a part of the subsurface formation.

Abstract: A heater may include an electrical conductor. Applying alternating current to the electrical conductor may generate resistively heat the electrical conductor. The electrical conductor may include an electrically resistive ferromagnetic material. The ferromagnetic material may at least partially surround a non-ferromagnetic material. The heater may provide a reduced amount of heat above or near a selected temperature. A conduit may at least partially surround the electrical conductor. A centralizer may maintain a separation distance between the electrical conductor and the conduit.

Abstract: A heater may include an electrical conductor. Applying alternating current to the electrical conductor may generate resistively heat the electrical conductor. The electrical conductor may include an electrically resistive ferromagnetic material. The ferromagnetic material may at least partially surround a non-ferromagnetic material. The heater may provide a reduced amount of heat above or near a selected temperature. A conduit may at least partially surround the electrical conductor. A centralizer may maintain a separation distance between the electrical conductor and the conduit.

Abstract: Systems and methods are described for heating a subsurface formation. Alternating electrical current may be applied to one or more electrical conductors. The electrical conductors may be located in a subsurface formation. The electrical conductors may provide an electrically resistive heat output upon application of the alternating electrical current. At least one of the electrical conductors may include an electrically resistive ferromagnetic material. The electrical conductor may provide a reduced amount of heat above or near a selected temperature. Heat may be allowed to transfer from the electrical conductor to a part of the subsurface formation.

Abstract: A heater system may include an alternating current supply and an electrical conductor. An alternating current may be applied to one or more electrical conductors at a voltage above about 200 volts. The electrical conductors may be located in a formation. The electrical conductors may provide an electrically resistive heat output upon application of the alternating electrical current. At least one of the electrical conductors may include an electrically resistive ferromagnetic material. An electrical conductor may provide a reduced amount of heat above or near a selected temperature. Heat may be allowed to transfer from an electrical conductor to a part of the formation.

Abstract: A heater may include an electrical conductor. Applying alternating current to the electrical conductor may resistively heat the electrical conductor. The electrical conductor may include an electrically resistive ferromagnetic material. The ferromagnetic material may at least partially surround a non-ferromagnetic material. The heater may provide a reduced amount of heat above or near a selected temperature. An electrical insulator may at least partially surround the electrical conductor. A sheath may at least partially surround the electrical insulator.