Patent ID: 8408294
Filing Date: 2013-04-02
Classification: E21B

Abstract:
1. A method of heating at least a part of a subsurface hydro carbonaceous earth formation, comprising: forming a borehole into or adjacent to the formation; inserting an RF electric heater into the formation, the RF electric heater including two concentric tubular conductors, at least a portion of at least one of the two concentric tubular conductors being ferromagnetic, each one of the two concentric tubular conductors including a top portion and a lower portion, the two concentric tubular conductors being electrically connected to each other proximate their bottom portions, each one of the two concentric tubular conductors being connected at the top portion to an AC power supply, the AC power supply having an AC output having a selectable output frequency and current; and selecting an output frequency greater than 1500 Hz to cause the current from the AC power supply to flow through a skin layer of at least one of the two concentric tubular conductors whose depth is independent of the thickness of at least one of the conductor walls, thereby allowing the RF heater to be constructed of components configured to meet petroleum industry standards for wall thickness, including API specification 5CT or 5A to provide strength and reliability in an oil well.