Temperature limited heaters using modulated DC power

摘要

Certain embodiments provide 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). The system also includes a heater section including 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 electrical current is applied to the heater section below a selected temperature. The heater section provides a second heat output approximately at and above the selected temperature during use. The heater section has a turndown ratio of at least 1.1 to 1. The system is configured to heat at least part of a subsurface formation.