Temperature limited heaters used to heat subsurface formations

摘要

A heater comprising a ferromagnetic member and an electrical conductor electrically coupled to the ferromagnetic member is disclosed. The heater provides a reduced amount of heat approximately at or above the Curie temperature of the ferromagnetic member. The electrical conductor is dimensioned to provide heat output below the Curie temperature of the ferromagnetic member. The ferromagnetic member is electrically coupled to the electrical conductor in such a way that a magnetic field produced by the ferromagnetic member confines a majority of the flow of the electrical current to the electrical conductor at temperatures below the Curie temperature of the ferromagnetic member.