Temperature limited heater utilizing non-ferromagnetic conductor

Abstract

A heater is described. The heater includes a ferromagnetic conductor and an electrical conductor electrically coupled to the ferromagnetic conductor. The ferromagnetic conductor is positioned relative to the electrical conductor such that an electromagnetic field produced by time-varying current flow in the ferromagnetic conductor confines a majority of the flow of the electrical current to the electrical conductor at temperatures below or near a selected temperature.

1 - 652 . (canceled) 
     
     
         653 . A method for controlling a heater in a subsurface formation, comprising:
 providing heat to at least a portion of the subsurface formation by applying electrical power to a heater located in a wellbore in the subsurface formation, wherein the heater comprises:
 a ferromagnetic conductor; and 
 an electrical conductor electrically coupled to the ferromagnetic conductor, wherein the ferromagnetic conductor is positioned relative to the electrical conductor such that an electromagnetic field produced by time-varying current flow in the ferromagnetic conductor confines a majority of the flow of the electrical current to the electrical conductor at temperatures below or near a selected temperature; 
   assessing an electrical characteristic of the heater in the subsurface formation;   comparing the assessed electrical characteristic to predicted behavior for the electrical characteristic; and   controlling the heater based on the comparison.   
     
     
         654 . The method of  claim 653 , wherein the electrical characteristic is the resistance of the heater. 
     
     
         655 . The method of  claim 653 , wherein the electrical characteristic is a power factor of the heater. 
     
     
         656 . The method of  claim 653 , wherein the method further comprises assessing the electrical characteristic based on electrical measurements of the heater. 
     
     
         657 . The method of  claim 653 , wherein the method further comprises assessing the predicted behavior for the electrical characteristic using experimental measurements. 
     
     
         658 . The method of  claim 653 , wherein the method further comprises assessing the predicted behavior for the electrical characteristic using analytical equations. 
     
     
         659 . The method of  claim 653 , wherein the method further comprises assessing the predicted behavior for the electrical characteristic using simulations. 
     
     
         660 . The method of  claim 653 , wherein the predicted behavior for the electrical characteristic is assessed as a function of temperature of the heater. 
     
     
         661 . The method of  claim 653 , wherein comparing the assessed electrical characteristic to the predicted behavior for the electrical characteristic assesses a temperature of the heater. 
     
     
         662 . The method of  claim 653 , wherein controlling the heater comprises controlling the power provided to the heater. 
     
     
         663 . The method of  claim 653 , wherein controlling the heater comprises controlling the current provided to the heater. 
     
     
         664 . The method of  claim 653 , wherein the assessed electrical characteristic is the percentage of the heater length operating near the selected temperature of the heater. 
     
     
         665 . The method of  claim 664 , wherein the selected temperature is the Curie temperature of the ferromagnetic conductor in the heater. 
     
     
         666 . The method of  claim 653 , further comprising producing formation fluid from the subsurface formation, wherein the formation fluid comprises hydrocarbons. 
     
     
         667 . The method of  claim 666 , further comprising producing transportation fuel from at least a portion of the hydrocarbons. 
     
     
         668 - 1065 . (canceled) 
     
     
         1066 . The method of  claim 653 , wherein the subsurface formation comprises a hydrocarbon containing formation and the wellbore is positioned in a hydrocarbon containing layer in the formation. 
     
     
         1067 . The method of  claim 1066 , further comprising heating the hydrocarbon containing layer such that at least some hydrocarbons are mobilized in the layer. 
     
     
         1068 . The method of  claim 1066 , further comprising heating the hydrocarbon containing layer such that at least some hydrocarbons are mobilized in the layer, and producing at least some of the hydrocarbons from the layer. 
     
     
         1069 . A method for controlling a heater in a subsurface formation, comprising:
 providing heat to at least a portion of the subsurface formation by applying electrical power to a heater located in a wellbore in the subsurface formation, wherein the heater comprises:
 a ferromagnetic conductor; and 
 an electrical conductor electrically coupled to the ferromagnetic conductor, wherein the electrical conductor conducts a majority of time-varying electrical current passing through the heater at about 25° C., and wherein the heater provides a first heat output below the Curie temperature of the ferromagnetic conductor, and the heater automatically provides a second heat output approximately at and above the Curie temperature of the ferromagnetic conductor, the second heat output being reduced compared to the first heat output; 
   assessing an electrical characteristic of the heater in the subsurface formation;   comparing the assessed electrical characteristic to predicted behavior for the electrical characteristic; and   controlling the heater based on the comparison.   
     
     
         1070 . A method for controlling a heater in a subsurface formation, comprising:
 providing heat to at least a portion of the subsurface formation by applying electrical power to a plurality of heaters located in one or more wellbores in the subsurface formation, wherein at least one heater comprises:
 a ferromagnetic conductor; and 
 an electrical conductor electrically coupled to the ferromagnetic conductor, wherein the ferromagnetic conductor is positioned relative to the electrical conductor such that an electromagnetic field produced by time-varying current flow in the ferromagnetic conductor confines a majority of the flow of the electrical current to the electrical conductor at temperatures below or near a selected temperature; 
   assessing an electrical characteristic of the at least one heater in the subsurface formation;   comparing the assessed electrical characteristic to predicted behavior for the electrical characteristic; and   controlling the at least one heater based on the comparison.