Patent ID: 8791396

Claim:
A heating system for a subsurface formation, comprising: a first heater configuration, comprising: a conduit located in a first opening in the subsurface formation; three insulated conductors located in the conduit, wherein each insulated conductor comprises a core at least partially surrounded by an insulation layer and an electrically conductive sheath, the sheath at least partially surrounding the insulation layer; a return conductor located inside the conduit, an end of the return conductor being electrically coupled to ends of each of the cores of the insulated conductors distal from a surface of the formation; and electrical insulation positioned around the sheaths of the insulated conductors and the return conductor and substantially filling the conduit along lengths of the insulated conductors and the return conductor, wherein the electrical insulation at least partially electrically insulates the sheaths of the three insulated conductors from each other along the lengths of the insulated conductors, at least partially electrically insulates the sheaths of the three insulated conductors from the return conductor along the lengths of the insulated conductors and the return conductor, at least partially electrically insulates the sheaths of the three insulated conductors from the conduit along the lengths of the insulated conductors, and at least partially electrically insulates the return conductor from the conduit along the length of the return conductor; wherein the insulated conductors provide heat to at least a portion of the subsurface formation when an electrical current is applied to the insulated conductors during use.