US Patent 7353872 Start-up of temperature limited heaters using direct current (DC)

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.