Patent ID: 8875788

Claim:
A method for heating a hydrocarbon containing formation, comprising: providing time-varying electrical current at a first frequency to a substantially u-shaped elongated electrical conductor located in the formation, wherein the electrical conductor extends between at least a first electrical contact at a first location on the surface of the formation and a second electrical contact at a second location on the surface of the formation; inducing electrical current flow in a ferromagnetic conductor with the time-varying electrical current at the first frequency, wherein the ferromagnetic conductor at least partially surrounds and at least partially extends lengthwise around the electrical conductor in a hydrocarbon containing layer in the subsurface formation, and wherein the ferromagnetic conductor has no direct electrical connection to the electrical conductor; resistively heating the ferromagnetic conductor with the induced electrical current flow such that the ferromagnetic conductor resistively heats up to a first temperature, wherein the first temperature is at most about 300° C.; allowing heat to transfer from the ferromagnetic conductor at the first temperature to at least a part of the formation; vaporizing at least some water in the formation with the ferromagnetic conductor at the first temperature; providing time-varying electrical current at a second frequency to the elongated electrical conductor; inducing electrical current flow in the ferromagnetic conductor with the time-varying electrical current at the second frequency; resistively heating the ferromagnetic conductor with the induced electrical current flow such that the ferromagnetic conductor resistively heats up to a second temperature, wherein the second temperature is above about 300 ° C.; allowing heat to transfer from the ferromagnetic conductor at the second temperature to at least a part of the formation; and mobilizing at least some hydrocarbons in the part of the formation with the ferromagnetic conductor at the second temperature.