A heating system for a subsurface formation includes an elongated electrical conductor located in the subsurface formation. The electrical conductor extends between at least a first electrical contact and a second electrical contact. A ferromagnetic conductor at least partially surrounds and at least partially extends lengthwise around the electrical conductor. The electrical conductor, when energized with time-varying electrical current, induces sufficient electrical current flow in the ferromagnetic conductor such that the ferromagnetic conductor resistively heats to a temperature of at least about 300.degree. C.
The Government has certain rights in the invention pursuant to Agreement Nos. SD 10634 and NFE 062050824 between Sandia National Laboratories (operating under Agreement DE-AC04-94AL85000Sa for the U.S. Department of Energy) and Shell Exploration and Production Company.