Patent Publication Number: US-10316644-B2

Title: Temperature assessment using dielectric properties of an insulated conductor heater with selected electrical insulation

Description:
This non-provisional application claims priority from U.S. Provisional Application Ser. No. 61/808,494 filed Apr. 4, 2013, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to methods and systems for production of hydrocarbons, hydrogen, and/or other products from various subsurface formations such as hydrocarbon containing formations. 
     2. Description of Related Art 
     Hydrocarbons obtained from subterranean formations are often used as energy resources, as feedstocks, and as consumer products. Concerns over depletion of available hydrocarbon resources and concerns over declining overall quality of produced hydrocarbons have led to development of processes for more efficient recovery, processing and/or use of available hydrocarbon resources. In situ processes may be used to remove hydrocarbon materials from subterranean formations that were previously inaccessible and/or too expensive to extract using available methods. Chemical and/or physical properties of hydrocarbon material in a subterranean formation may need to be changed to allow hydrocarbon material to be more easily removed from the subterranean formation and/or increase the value of the hydrocarbon material. The chemical and physical changes may include in situ reactions that produce removable fluids, composition changes, solubility changes, density changes, phase changes, and/or viscosity changes of the hydrocarbon material in the formation. 
     Large deposits of heavy hydrocarbons (heavy oil and/or tar) contained in relatively permeable formations (for example in tar sands) are found in North America, South America, Africa, and Asia. Tar can be surface-mined and upgraded to lighter hydrocarbons such as crude oil, naphtha, kerosene, and/or gas oil. Surface milling processes may further separate the bitumen from sand. The separated bitumen may be converted to light hydrocarbons using conventional refinery methods. Mining and upgrading tar sand is usually substantially more expensive than producing lighter hydrocarbons from conventional oil reservoirs. 
     Typical temperature measurement methods may be difficult and/or expensive to implement for use in assessing a temperature profile of a heater located in a subsurface formation for heating in an in situ heat treatment process. The desire is for a temperature profile that includes multiple temperatures along the length or a portion of the heater in the subsurface formation. Thermocouples are one possible solution; however, thermocouples provide only one temperature at one location and two wires are generally needed for each thermocouple. Thus, to obtain a temperature profile along a length of the heater, multiple pairs of wires are needed. In some cases, the wellbore for the heater may not be large enough to accommodate the number of wires needed for adequate temperature monitoring. The risk of failure of one or more of the thermocouples (or their associated wires) is increased with the use of multiple wires in the subsurface wellbore. In addition, thermocouples installed in high temperature applications (&gt;300° C.) may suffer from a phenomenon known as temperature measurement drift. Temperature measurement drift may be a significant source of error. 
     Another possible solution is the use of a fiber optic cable temperature sensor system. The fiber optic cable system provides a temperature profile along a length of the heater. Commercially available fiber optic cable systems, however, typically only have operating temperature ranges up to about 300° C. and are susceptible to mechanical damage because of the softening of the fiber and/or fiber coatings resulting in the various materials adhering to each other. Thus, these systems are not suitable for measurement of higher temperatures encountered while heating the subsurface formation during the in situ heat treatment process. Some experimental fiber optic cable systems are suitable for use at these higher temperatures but these systems may be too expensive for implementation in a commercial process (for example, a large field of heaters). Thus, there is a need for a simple, inexpensive system that allows temperature assessment at one or more locations along a length of the subsurface heater used in the in situ heat treatment process. 
     SUMMARY 
     Embodiments described herein generally relate to systems, methods, and heaters for treating a subsurface formation. Embodiments described herein also generally relate to heaters that have novel components therein. Such heaters can be obtained by using the systems and methods described herein. 
     In certain embodiments, the invention provides one or more systems, methods, and/or heaters. In some embodiments, the systems, methods, and/or heaters are used for treating a subsurface formation. 
     In certain embodiments, a system for assessing a temperature in an opening in a subsurface formation includes: an electrical conductor; electrical insulation at least partially surrounding the electrical conductor, the electrical insulation comprising magnesium oxide and a ferroelectric material; and an electrically conductive sheath at least partially surrounding the electrical insulation; wherein a profile of one or more dielectric properties of the electrical insulation along a length of the electrical insulation are assessed during use to assess a temperature profile with spatial resolution along the length of the electrical insulation. 
     In certain embodiments, a method for assessing a temperature in an opening in a subsurface formation, includes: assessing a profile of one or more dielectric properties with spatial resolution along a length of an insulated conductor located in the opening, wherein the insulated conductor comprises: an electrical conductor; electrical insulation at least partially surrounding the electrical conductor, the electrical insulation comprising magnesium oxide and a ferroelectric material; and an electrically conductive sheath at least partially surrounding the electrical insulation; and assessing a temperature profile with spatial resolution along the length of the insulated conductor based on the assessed profile of the one or more dielectric properties. 
     In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments. 
     In further embodiments, treating a subsurface formation is performed using any of the methods, systems, or heaters described herein. 
     In further embodiments, additional features may be added to the specific embodiments described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings in which: 
         FIG. 1  shows a schematic view of an embodiment of a portion of an in situ heat treatment system for treating a hydrocarbon containing formation. 
         FIG. 2  depicts an embodiment of an insulated conductor heat source. 
         FIG. 3  depicts another embodiment of an insulated conductor heat source. 
         FIG. 4  depicts another embodiment of an insulated conductor heat source. 
         FIG. 5  depicts an example of a plot of dielectric constant versus temperature for magnesium oxide insulation in one embodiment of an insulated conductor heater. 
         FIG. 6  depicts an example of a plot of loss tangent (tan δ) versus temperature for magnesium oxide insulation in one embodiment of an insulated conductor heater. 
         FIG. 7  depicts dielectric constant versus temperature for undoped bismuth titanate at several frequencies. 
         FIG. 8  depicts dielectric constant versus temperature for bismuth titanate doped with 0.74% by weight niobium at several frequencies. 
         FIG. 9  depicts an embodiment of a length of an insulated conductor. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The following description generally relates to systems and methods for treating hydrocarbons in the formations. Such formations may be treated to yield hydrocarbon products, hydrogen, and other products. 
     “Alternating current (AC)” refers to a time-varying current that reverses direction substantially sinusoidally. AC produces skin effect electricity flow in a ferromagnetic conductor. 
     “Annular region” is the region between an outer conduit and an inner conduit positioned in the outer conduit. 
     “API gravity” refers to API gravity at 15.5° C. (60° F.). API gravity is as determined by ASTM Method D6822 or ASTM Method D1298. 
     “ASTM” refers to American Standard Testing and Materials. 
     In the context of reduced heat output heating systems, apparatus, and methods, the term “automatically” means such systems, apparatus, and methods function in a certain way without the use of external control (for example, external controllers such as a controller with a temperature sensor and a feedback loop, PID controller, or predictive controller). 
     “Asphalt/bitumen” refers to a semi-solid, viscous material soluble in carbon disulfide. Asphalt/bitumen may be obtained from refining operations or produced from subsurface formations. 
     “Condensable hydrocarbons” are hydrocarbons that condense at 25° C. and one atmosphere absolute pressure. Condensable hydrocarbons may include a mixture of hydrocarbons having carbon numbers greater than 4. “Non-condensable hydrocarbons” are hydrocarbons that do not condense at 25° C. and one atmosphere absolute pressure. Non-condensable hydrocarbons may include hydrocarbons having carbon numbers less than 5. 
     “Coupled” means either a direct connection or an indirect connection (for example, one or more intervening connections) between one or more objects or components. The phrase “directly connected” means a direct connection between objects or components such that the objects or components are connected directly to each other so that the objects or components operate in a “point of use” manner. 
     “Curie temperature” is the temperature above which a ferromagnetic material loses all of its ferromagnetic properties. In addition to losing all of its ferromagnetic properties above the Curie temperature, the ferromagnetic material begins to lose its ferromagnetic properties when an increasing electrical current is passed through the ferromagnetic material. 
     A “fluid” may be, but is not limited to, a gas, a liquid, an emulsion, a slurry, and/or a stream of solid particles that has flow characteristics similar to liquid flow. 
     A “formation” includes one or more hydrocarbon containing layers, one or more non-hydrocarbon layers, an overburden, and/or an underburden. “Hydrocarbon layers” refer to layers in the formation that contain hydrocarbons. The hydrocarbon layers may contain non-hydrocarbon material and hydrocarbon material. The “overburden” and/or the “underburden” include one or more different types of impermeable materials. For example, the overburden and/or underburden may include rock, shale, mudstone, or wet/tight carbonate. In some embodiments of in situ heat treatment processes, the overburden and/or the underburden may include a hydrocarbon containing layer or hydrocarbon containing layers that are relatively impermeable and are not subjected to temperatures during in situ heat treatment processing that result in significant characteristic changes of the hydrocarbon containing layers of the overburden and/or the underburden. For example, the underburden may contain shale or mudstone, but the underburden is not allowed to heat to pyrolysis temperatures during the in situ heat treatment process. In some cases, the overburden and/or the underburden may be somewhat permeable. 
     “Formation fluids” refer to fluids present in a formation and may include pyrolyzation fluid, synthesis gas, mobilized hydrocarbons, and water (steam). Formation fluids may include hydrocarbon fluids as well as non-hydrocarbon fluids. The term “mobilized fluid” refers to fluids in a hydrocarbon containing formation that are able to flow as a result of thermal treatment of the formation. “Produced fluids” refer to fluids removed from the formation. 
     A “heat source” is any system for providing heat to at least a portion of a formation substantially by conductive and/or radiative heat transfer. For example, a heat source may include electrically conducting materials and/or electric heaters such as an insulated conductor, an elongated member, and/or a conductor disposed in a conduit. A heat source may also include systems that generate heat by burning a fuel external to or in a formation. The systems may be surface burners, downhole gas burners, flameless distributed combustors, and natural distributed combustors. In some embodiments, heat provided to or generated in one or more heat sources may be supplied by other sources of energy. The other sources of energy may directly heat a formation, or the energy may be applied to a transfer medium that directly or indirectly heats the formation. It is to be understood that one or more heat sources that are applying heat to a formation may use different sources of energy. Thus, for example, for a given formation some heat sources may supply heat from electrically conducting materials or electric resistance heaters. Some heat sources may provide heat from combustion and some heat sources may provide heat from one or more other energy sources (for example, chemical reactions, solar energy, wind energy, biomass, or other sources of renewable energy). A chemical reaction may include an exothermic reaction (for example, an oxidation reaction). A heat source may also include an electrically conducting material and/or a heater that provides heat to a zone proximate and/or surrounding a heating location such as a heater well. 
     A “heater” is any system or heat source for generating heat in a well or a near wellbore region. Heaters may be, but are not limited to, electric heaters, burners, combustors that react with material in or produced from a formation, and/or combinations thereof. 
     “Heavy hydrocarbons” are viscous hydrocarbon fluids. Heavy hydrocarbons may include highly viscous hydrocarbon fluids such as heavy oil, tar, and/or asphalt. Heavy hydrocarbons may include carbon and hydrogen, as well as smaller concentrations of sulfur, oxygen, and nitrogen. Additional elements may also be present in heavy hydrocarbons in trace amounts. Heavy hydrocarbons may be classified by API gravity. Heavy hydrocarbons generally have an API gravity below about 20°. Heavy oil, for example, generally has an API gravity of about 10-20°, whereas tar generally has an API gravity below about 10°. The viscosity of heavy hydrocarbons is generally greater than about 100 centipoise at 15° C. Heavy hydrocarbons may include aromatics or other complex ring hydrocarbons. 
     Heavy hydrocarbons may be found in a relatively permeable formation. The relatively permeable formation may include heavy hydrocarbons entrained in, for example, sand or carbonate. “Relatively permeable” is defined, with respect to formations or portions thereof, as an average permeability of 10 millidarcy or more (for example, 10 or 100 millidarcy). “Relatively low permeability” is defined, with respect to formations or portions thereof, as an average permeability of less than about 10 millidarcy. One darcy is equal to about 0.99 square micrometers. An impermeable layer generally has a permeability of less than about 0.1 millidarcy. 
     Certain types of formations that include heavy hydrocarbons may also include, but are not limited to, natural mineral waxes or natural asphaltites. “Natural mineral waxes” typically occur in substantially tubular veins that may be several meters wide, several kilometers long, and hundreds of meters deep. “Natural asphaltites” include solid hydrocarbons of an aromatic composition and typically occur in large veins. In situ recovery of hydrocarbons from formations such as natural mineral waxes and natural asphaltites may include melting to form liquid hydrocarbons and/or solution mining of hydrocarbons from the formations. 
     “Hydrocarbons” are generally defined as molecules formed primarily by carbon and hydrogen atoms. Hydrocarbons may also include other elements such as, but not limited to, halogens, metallic elements, nitrogen, oxygen, and/or sulfur. Hydrocarbons may be, but are not limited to, kerogen, bitumen, pyrobitumen, oils, natural mineral waxes, and asphaltites. Hydrocarbons may be located in or adjacent to mineral matrices in the earth. Matrices may include, but are not limited to, sedimentary rock, sands, silicilytes, carbonates, diatomites, and other porous media. “Hydrocarbon fluids” are fluids that include hydrocarbons. Hydrocarbon fluids may include, entrain, or be entrained in non-hydrocarbon fluids such as hydrogen, nitrogen, carbon monoxide, carbon dioxide, hydrogen sulfide, water, and ammonia. 
     An “in situ conversion process” refers to a process of heating a hydrocarbon containing formation from heat sources to raise the temperature of at least a portion of the formation above a pyrolysis temperature so that pyrolyzation fluid is produced in the formation. 
     An “in situ heat treatment process” refers to a process of heating a hydrocarbon containing formation with heat sources to raise the temperature of at least a portion of the formation above a temperature that results in mobilized fluid, visbreaking, and/or pyrolysis of hydrocarbon containing material so that mobilized fluids, visbroken fluids, and/or pyrolyzation fluids are produced in the formation. 
     “Insulated conductor” refers to any elongated material that is able to conduct electricity and that is covered, in whole or in part, by an electrically insulating material. 
     “Kerogen” is a solid, insoluble hydrocarbon that has been converted by natural degradation and that principally contains carbon, hydrogen, nitrogen, oxygen, and sulfur. Coal and oil shale are typical examples of materials that contain kerogen. “Bitumen” is a non-crystalline solid or viscous hydrocarbon material that is substantially soluble in carbon disulfide. “Oil” is a fluid containing a mixture of condensable hydrocarbons. 
     “Pyrolysis” is the breaking of chemical bonds due to the application of heat. For example, pyrolysis may include transforming a compound into one or more other substances by heat alone. Heat may be transferred to a section of the formation to cause pyrolysis. 
     “Pyrolyzation fluids” or “pyrolysis products” refers to fluid produced substantially during pyrolysis of hydrocarbons. Fluid produced by pyrolysis reactions may mix with other fluids in a formation. The mixture would be considered pyrolyzation fluid or pyrolyzation product. As used herein, “pyrolysis zone” refers to a volume of a formation (for example, a relatively permeable formation such as a tar sands formation) that is reacted or reacting to form a pyrolyzation fluid. 
     “Superposition of heat” refers to providing heat from two or more heat sources to a selected section of a formation such that the temperature of the formation at least at one location between the heat sources is influenced by the heat sources. 
     “Tar” is a viscous hydrocarbon that generally has a viscosity greater than about 10,000 centipoise at 15° C. The specific gravity of tar generally is greater than 1.000. Tar may have an API gravity less than 10°. 
     A “tar sands formation” is a formation in which hydrocarbons are predominantly present in the form of heavy hydrocarbons and/or tar entrained in a mineral grain framework or other host lithology (for example, sand or carbonate). Examples of tar sands formations include formations such as the Athabasca formation, the Grosmont formation, and the Peace River formation, all three in Alberta, Canada; and the Faja formation in the Orinoco belt in Venezuela. 
     “Temperature limited heater” generally refers to a heater that regulates heat output (for example, reduces heat output) above a specified temperature without the use of external controls such as temperature controllers, power regulators, rectifiers, or other devices. Temperature limited heaters may be AC (alternating current) or modulated (for example, “chopped”) DC (direct current) powered electrical resistance heaters. 
     “Thickness” of a layer refers to the thickness of a cross section of the layer, wherein the cross section is normal to a face of the layer. 
     “Time-varying current” refers to electrical current that produces skin effect electricity flow in a ferromagnetic conductor and has a magnitude that varies with time. Time-varying current includes both alternating current (AC) and modulated direct current (DC). 
     A “u-shaped wellbore” refers to a wellbore that extends from a first opening in the formation, through at least a portion of the formation, and out through a second opening in the formation. In this context, the wellbore may be only roughly in the shape of a “v” or “u”, with the understanding that the “legs” of the “u” do not need to be parallel to each other, or perpendicular to the “bottom” of the “u” for the wellbore to be considered “u-shaped”. 
     “Upgrade” refers to increasing the quality of hydrocarbons. For example, upgrading heavy hydrocarbons may result in an increase in the API gravity of the heavy hydrocarbons. 
     “Viscosity” refers to kinematic viscosity at 40° C. unless otherwise specified. Viscosity is as determined by ASTM Method D445. 
     The term “wellbore” refers to a hole in a formation made by drilling or insertion of a conduit into the formation. A wellbore may have a substantially circular cross section, or another cross-sectional shape. As used herein, the terms “well” and “opening,” when referring to an opening in the formation may be used interchangeably with the term “wellbore.” 
     A formation may be treated in various ways to produce many different products. Different stages or processes may be used to treat the formation during an in situ heat treatment process. In some embodiments, one or more sections of the formation are solution mined to remove soluble minerals from the sections. Solution mining minerals may be performed before, during, and/or after the in situ heat treatment process. In some embodiments, the average temperature of one or more sections being solution mined may be maintained below about 120° C. 
     In some embodiments, one or more sections of the formation are heated to remove water from the sections and/or to remove methane and other volatile hydrocarbons from the sections. In some embodiments, the average temperature may be raised from ambient temperature to temperatures below about 220° C. during removal of water and volatile hydrocarbons. 
     In some embodiments, one or more sections of the formation are heated to temperatures that allow for movement and/or visbreaking of hydrocarbons in the formation. In some embodiments, the average temperature of one or more sections of the formation are raised to mobilization temperatures of hydrocarbons in the sections (for example, to temperatures ranging from 100° C. to 250° C., from 120° C. to 240° C., or from 150° C. to 230° C.). 
     In some embodiments, one or more sections are heated to temperatures that allow for pyrolysis reactions in the formation. In some embodiments, the average temperature of one or more sections of the formation may be raised to pyrolysis temperatures of hydrocarbons in the sections (for example, temperatures ranging from 230° C. to 900° C., from 240° C. to 400° C. or from 250° C. to 350° C.). 
     Heating the hydrocarbon containing formation with a plurality of heat sources may establish thermal gradients around the heat sources that raise the temperature of hydrocarbons in the formation to desired temperatures at desired heating rates. The rate of temperature increase through the mobilization temperature range and/or the pyrolysis temperature range for desired products may affect the quality and quantity of the formation fluids produced from the hydrocarbon containing formation. Slowly raising the temperature of the formation through the mobilization temperature range and/or pyrolysis temperature range may allow for the production of high quality, high API gravity hydrocarbons from the formation. Slowly raising the temperature of the formation through the mobilization temperature range and/or pyrolysis temperature range may allow for the removal of a large amount of the hydrocarbons present in the formation as hydrocarbon product. 
     In some in situ heat treatment embodiments, a portion of the formation is heated to a desired temperature instead of slowly heating the temperature through a temperature range. In some embodiments, the desired temperature is 300° C., 325° C., or 350° C. Other temperatures may be selected as the desired temperature. 
     Superposition of heat from heat sources allows the desired temperature to be relatively quickly and efficiently established in the formation. Energy input into the formation from the heat sources may be adjusted to maintain the temperature in the formation substantially at a desired temperature. 
     Mobilization and/or pyrolysis products may be produced from the formation through production wells. In some embodiments, the average temperature of one or more sections is raised to mobilization temperatures and hydrocarbons are produced from the production wells. The average temperature of one or more of the sections may be raised to pyrolysis temperatures after production due to mobilization decreases below a selected value. In some embodiments, the average temperature of one or more sections may be raised to pyrolysis temperatures without significant production before reaching pyrolysis temperatures. Formation fluids including pyrolysis products may be produced through the production wells. 
     In some embodiments, the average temperature of one or more sections may be raised to temperatures sufficient to allow synthesis gas production after mobilization and/or pyrolysis. In some embodiments, hydrocarbons may be raised to temperatures sufficient to allow synthesis gas production without significant production before reaching the temperatures sufficient to allow synthesis gas production. For example, synthesis gas may be produced in a temperature range from about 400° C. to about 1200° C., about 500° C. to about 1100° C., or about 550° C. to about 1000° C. A synthesis gas generating fluid (for example, steam and/or water) may be introduced into the sections to generate synthesis gas. Synthesis gas may be produced from production wells. 
     Solution mining, removal of volatile hydrocarbons and water, mobilizing hydrocarbons, pyrolyzing hydrocarbons, generating synthesis gas, and/or other processes may be performed during the in situ heat treatment process. In some embodiments, some processes may be performed after the in situ heat treatment process. Such processes may include, but are not limited to, recovering heat from treated sections, storing fluids (for example, water and/or hydrocarbons) in previously treated sections, and/or sequestering carbon dioxide in previously treated sections. 
       FIG. 1  depicts a schematic view of an embodiment of a portion of the in situ heat treatment system for treating the hydrocarbon containing formation. The in situ heat treatment system may include barrier wells  100 . Barrier wells are used to form a barrier around a treatment area. The barrier inhibits fluid flow into and/or out of the treatment area. Barrier wells include, but are not limited to, dewatering wells, vacuum wells, capture wells, injection wells, grout wells, freeze wells, or combinations thereof. In some embodiments, barrier wells  100  are dewatering wells. Dewatering wells may remove liquid water and/or inhibit liquid water from entering a portion of the formation to be heated, or to the formation being heated. As shown, barrier wells  100  extend only along one side of heat sources  102 , but the barrier wells typically encircle all heat sources  102  used, or to be used, to heat a treatment area of the formation. 
     Heat sources  102  are placed in at least a portion of the formation. Heat sources  102  may include heaters such as insulated conductors, conductor-in-conduit heaters, surface burners, flameless distributed combustors, and/or natural distributed combustors. Heat sources  102  may also include other types of heaters. Heat sources  102  provide heat to at least a portion of the formation to heat hydrocarbons in the formation. Energy may be supplied to heat sources  102  through supply lines  104 . Supply lines  104  may be structurally different depending on the type of heat source or heat sources used to heat the formation. Supply lines  104  for heat sources may transmit electricity for electric heaters, may transport fuel for combustors, or may transport heat exchange fluid that is circulated in the formation. In some embodiments, electricity for an in situ heat treatment process may be provided by a nuclear power plant or nuclear power plants. The use of nuclear power may allow for reduction or elimination of carbon dioxide emissions from the in situ heat treatment process. 
     When the formation is heated, the heat input into the formation may cause expansion of the formation and geomechanical motion. The heat sources may be turned on before, at the same time, or during a dewatering process. Computer simulations may model formation response to heating. The computer simulations may be used to develop a pattern and time sequence for activating heat sources in the formation so that geomechanical motion of the formation does not adversely affect the functionality of heat sources, production wells, and other equipment in the formation. 
     Heating the formation may cause an increase in permeability and/or porosity of the formation. Increases in permeability and/or porosity may result from a reduction of mass in the formation due to vaporization and removal of water, removal of hydrocarbons, and/or creation of fractures. Fluid may flow more easily in the heated portion of the formation because of the increased permeability and/or porosity of the formation. Fluid in the heated portion of the formation may move a considerable distance through the formation because of the increased permeability and/or porosity. The considerable distance may be over 1000 m depending on various factors, such as permeability of the formation, properties of the fluid, temperature of the formation, and pressure gradient allowing movement of the fluid. The ability of fluid to travel considerable distance in the formation allows production wells  106  to be spaced relatively far apart in the formation. 
     Production wells  106  are used to remove formation fluid from the formation. In some embodiments, production well  106  includes a heat source. The heat source in the production well may heat one or more portions of the formation at or near the production well. In some in situ heat treatment process embodiments, the amount of heat supplied to the formation from the production well per meter of the production well is less than the amount of heat applied to the formation from a heat source that heats the formation per meter of the heat source. Heat applied to the formation from the production well may increase formation permeability adjacent to the production well by vaporizing and removing liquid phase fluid adjacent to the production well and/or by increasing the permeability of the formation adjacent to the production well by formation of macro and/or micro fractures. 
     More than one heat source may be positioned in the production well. A heat source in a lower portion of the production well may be turned off when superposition of heat from adjacent heat sources heats the formation sufficiently to counteract benefits provided by heating the formation with the production well. In some embodiments, the heat source in an upper portion of the production well may remain on after the heat source in the lower portion of the production well is deactivated. The heat source in the upper portion of the well may inhibit condensation and reflux of formation fluid. 
     In some embodiments, the heat source in production well  106  allows for vapor phase removal of formation fluids from the formation. Providing heating at or through the production well may: (1) inhibit condensation and/or refluxing of production fluid when such production fluid is moving in the production well proximate the overburden, (2) increase heat input into the formation, (3) increase production rate from the production well as compared to a production well without a heat source, (4) inhibit condensation of high carbon number compounds (C 6  hydrocarbons and above) in the production well, and/or (5) increase formation permeability at or proximate the production well. 
     Subsurface pressure in the formation may correspond to the fluid pressure generated in the formation. As temperatures in the heated portion of the formation increase, the pressure in the heated portion may increase as a result of thermal expansion of in situ fluids, increased fluid generation and vaporization of water. Controlling rate of the fluid removal from the formation may allow for control of pressure in the formation. Pressure in the formation may be determined at a number of different locations, such as near or at production wells, near or at heat sources, and near or at monitor wells. 
     In some hydrocarbon containing formations, production of hydrocarbons from the formation is inhibited until at least some hydrocarbons in the formation have been mobilized and/or pyrolyzed. Formation fluid may be produced from the formation when the formation fluid is of a selected quality. In some embodiments, the selected quality includes an API gravity of at least about 20°, 30°, or 40°. Inhibiting production until at least some hydrocarbons are mobilized and/or pyrolyzed may increase conversion of heavy hydrocarbons to light hydrocarbons. Inhibiting initial production may minimize the production of heavy hydrocarbons from the formation. Production of substantial amounts of heavy hydrocarbons may require expensive equipment and/or reduce the life of production equipment. 
     In some hydrocarbon containing formations, hydrocarbons in the formation may be heated to mobilization and/or pyrolysis temperatures before substantial permeability has been generated in the heated portion of the formation. An initial lack of permeability may inhibit the transport of generated fluids to production wells  106 . During initial heating, fluid pressure in the formation may increase proximate heat sources  102 . The increased fluid pressure may be released, monitored, altered, and/or controlled through one or more heat sources  102 . For example, selected heat sources  102  or separate pressure relief wells may include pressure relief valves that allow for removal of some fluid from the formation. 
     In some embodiments, pressure generated by expansion of mobilized fluids, pyrolysis fluids or other fluids generated in the formation may be allowed to increase although an open path to production wells  106  or any other pressure sink may not yet exist in the formation. The fluid pressure may be allowed to increase towards a lithostatic pressure. Fractures in the hydrocarbon containing formation may form when the fluid approaches the lithostatic pressure. For example, fractures may form from heat sources  102  to production wells  106  in the heated portion of the formation. The generation of fractures in the heated portion may relieve some of the pressure in the portion. Pressure in the formation may have to be maintained below a selected pressure to inhibit unwanted production, fracturing of the overburden or underburden, and/or coking of hydrocarbons in the formation. 
     After mobilization and/or pyrolysis temperatures are reached and production from the formation is allowed, pressure in the formation may be varied to alter and/or control a composition of produced formation fluid, to control a percentage of condensable fluid as compared to non-condensable fluid in the formation fluid, and/or to control an API gravity of formation fluid being produced. For example, decreasing pressure may result in production of a larger condensable fluid component. The condensable fluid component may contain a larger percentage of olefins. 
     In some in situ heat treatment process embodiments, pressure in the formation may be maintained high enough to promote production of formation fluid with an API gravity of greater than 20°. Maintaining increased pressure in the formation may inhibit formation subsidence during in situ heat treatment. Maintaining increased pressure may reduce or eliminate the need to compress formation fluids at the surface to transport the fluids in collection conduits to treatment facilities. 
     Maintaining increased pressure in a heated portion of the formation may surprisingly allow for production of large quantities of hydrocarbons of increased quality and of relatively low molecular weight. Pressure may be maintained so that formation fluid produced has a minimal amount of compounds above a selected carbon number. The selected carbon number may be at most 25, at most 20, at most 12, or at most 8. Some high carbon number compounds may be entrained in vapor in the formation and may be removed from the formation with the vapor. Maintaining increased pressure in the formation may inhibit entrainment of high carbon number compounds and/or multi-ring hydrocarbon compounds in the vapor. High carbon number compounds and/or multi-ring hydrocarbon compounds may remain in a liquid phase in the formation for significant time periods. The significant time periods may provide sufficient time for the compounds to pyrolyze to form lower carbon number compounds. 
     Generation of relatively low molecular weight hydrocarbons is believed to be due, in part, to autogenous generation and reaction of hydrogen in a portion of the hydrocarbon containing formation. For example, maintaining an increased pressure may force hydrogen generated during pyrolysis into the liquid phase within the formation. Heating the portion to a temperature in a pyrolysis temperature range may pyrolyze hydrocarbons in the formation to generate liquid phase pyrolyzation fluids. The generated liquid phase pyrolyzation fluids components may include double bonds and/or radicals. Hydrogen (H 2 ) in the liquid phase may reduce double bonds of the generated pyrolyzation fluids, thereby reducing a potential for polymerization or formation of long chain compounds from the generated pyrolyzation fluids. In addition, H 2  may also neutralize radicals in the generated pyrolyzation fluids. H 2  in the liquid phase may inhibit the generated pyrolyzation fluids from reacting with each other and/or with other compounds in the formation. 
     Formation fluid produced from production wells  106  may be transported through collection piping  108  to treatment facilities  110 . Formation fluids may also be produced from heat sources  102 . For example, fluid may be produced from heat sources  102  to control pressure in the formation adjacent to the heat sources. Fluid produced from heat sources  102  may be transported through tubing or piping to collection piping  108  or the produced fluid may be transported through tubing or piping directly to treatment facilities  110 . Treatment facilities  110  may include separation units, reaction units, upgrading units, fuel cells, turbines, storage vessels, and/or other systems and units for processing produced formation fluids. The treatment facilities may form transportation fuel from at least a portion of the hydrocarbons produced from the formation. In some embodiments, the transportation fuel may be jet fuel, such as JP-8. 
     An insulated conductor may be used as an electric heater element of a heater or a heat source. The insulated conductor may include an inner electrical conductor (core) surrounded by an electrical insulator and an outer electrical conductor (jacket). The electrical insulator may include mineral insulation (for example, magnesium oxide) or other electrical insulation. 
     In certain embodiments, the insulated conductor is placed in an opening in a hydrocarbon containing formation. In some embodiments, the insulated conductor is placed in an uncased opening in the hydrocarbon containing formation. Placing the insulated conductor in an uncased opening in the hydrocarbon containing formation may allow heat transfer from the insulated conductor to the formation by radiation as well as conduction. Using an uncased opening may facilitate retrieval of the insulated conductor from the well, if necessary. 
     In some embodiments, an insulated conductor is placed within a casing in the formation; may be cemented within the formation; or may be packed in an opening with sand, gravel, or other fill material. The insulated conductor may be supported on a support member positioned within the opening. The support member may be a cable, rod, or a conduit (for example, a pipe). The support member may be made of a metal, ceramic, inorganic material, or combinations thereof. Because portions of a support member may be exposed to formation fluids and heat during use, the support member may be chemically resistant and/or thermally resistant. 
     Ties, spot welds, and/or other types of connectors may be used to couple the insulated conductor to the support member at various locations along a length of the insulated conductor. The support member may be attached to a wellhead at an upper surface of the formation. In some embodiments, the insulated conductor has sufficient structural strength such that a support member is not needed. The insulated conductor may, in many instances, have at least some flexibility to inhibit thermal expansion damage when undergoing temperature changes. 
     In certain embodiments, insulated conductors are placed in wellbores without support members and/or centralizers. An insulated conductor without support members and/or centralizers may have a suitable combination of temperature and corrosion resistance, creep strength, length, thickness (diameter), and metallurgy that will inhibit failure of the insulated conductor during use. 
       FIG. 2  depicts a perspective view of an end portion of an embodiment of insulated conductor  112 . Insulated conductor  112  may have any desired cross-sectional shape such as, but not limited to, round (depicted in  FIG. 2 ), triangular, ellipsoidal, rectangular, hexagonal, or irregular. In certain embodiments, insulated conductor  112  includes core  114 , electrical insulator  116 , and jacket  118 . Core  114  may resistively heat when an electrical current passes through the core. Alternating or time-varying current and/or direct current may be used to provide power to core  114  such that the core resistively heats. 
     In some embodiments, electrical insulator  116  inhibits current leakage and arcing to jacket  118 . Electrical insulator  116  may thermally conduct heat generated in core  114  to jacket  118 . Jacket  118  may radiate or conduct heat to the formation. In certain embodiments, insulated conductor  112  is 1000 m or more in length. Longer or shorter insulated conductors may also be used to meet specific application needs. The dimensions of core  114 , electrical insulator  116 , and jacket  118  of insulated conductor  112  may be selected such that the insulated conductor has enough strength to be self supporting even at upper working temperature limits. Such insulated conductors may be suspended from wellheads or supports positioned near an interface between an overburden and a hydrocarbon containing formation without the need for support members extending into the hydrocarbon containing formation along with the insulated conductors. 
     Insulated conductor  112  may be designed to operate at power levels of up to about 1650 watts/meter or higher. In certain embodiments, insulated conductor  112  operates at a power level between about 500 watts/meter and about 1150 watts/meter when heating a formation. Insulated conductor  112  may be designed so that a maximum voltage level at a typical operating temperature does not cause substantial thermal and/or electrical breakdown of electrical insulator  116 . Insulated conductor  112  may be designed such that jacket  118  does not exceed a temperature that will result in a significant reduction in corrosion resistance properties of the jacket material. In certain embodiments, insulated conductor  112  may be designed to reach temperatures within a range between about 650° C. and about 900° C. Insulated conductors having other operating ranges may be formed to meet specific operational requirements. 
       FIG. 2  depicts insulated conductor  112  having a single core  114 . In some embodiments, insulated conductor  112  has two or more cores  114 . For example, a single insulated conductor may have three cores. Core  114  may be made of metal or another electrically conductive material. The material used to form core  114  may include, but not be limited to, nichrome, copper, nickel, carbon steel, stainless steel, and combinations thereof. In certain embodiments, core  114  is chosen to have a diameter and a resistivity at operating temperatures such that its resistance, as derived from Ohm&#39;s law, makes it electrically and structurally stable for the chosen power dissipation per meter, the length of the heater, and/or the maximum voltage allowed for the core material. 
     In some embodiments, core  114  is made of different materials along a length of insulated conductor  112 . For example, a first section of core  114  may be made of a material that has a significantly lower resistance than a second section of the core. The first section may be placed adjacent to a formation layer that does not need to be heated to as high a temperature as a second formation layer that is adjacent to the second section. The resistivity of various sections of core  114  may be adjusted by having a variable diameter and/or by having core sections made of different materials. 
     Electrical insulator  116  may be made of a variety of materials. Commonly used powders may include, but are not limited to, MgO, Al 2 O 3 , Zirconia, BeO, different chemical variations of Spinels, and combinations thereof. MgO may provide good thermal conductivity and electrical insulation properties. The desired electrical insulation properties include low leakage current and high dielectric strength. A low leakage current decreases the possibility of thermal breakdown and the high dielectric strength decreases the possibility of arcing across the insulator. Thermal breakdown can occur if the leakage current causes a progressive rise in the temperature of the insulator leading also to arcing across the insulator. 
     Jacket  118  may be an outer metallic layer or electrically conductive layer. Jacket  118  may be in contact with hot formation fluids. Jacket  118  may be made of material having a high resistance to corrosion at elevated temperatures. Alloys that may be used in a desired operating temperature range of jacket  118  include, but are not limited to, 304 stainless steel, 310 stainless steel, Incoloy® 800, and Inconel® 600 (Inco Alloys International, Huntington, W. Va., U.S.A.). The thickness of jacket  118  may have to be sufficient to last for three to ten years in a hot and corrosive environment. A thickness of jacket  118  may generally vary between about 1 mm and about 2.5 mm. For example, a 1.3 mm thick, 310 stainless steel outer layer may be used as jacket  118  to provide good chemical resistance to sulfidation corrosion in a heated zone of a formation for a period of over 3 years. Larger or smaller jacket thicknesses may be used to meet specific application requirements. 
     One or more insulated conductors may be placed within an opening in a formation to form a heat source or heat sources. Electrical current may be passed through each insulated conductor in the opening to heat the formation. Alternately, electrical current may be passed through selected insulated conductors in an opening. The unused conductors may be used as backup heaters. Insulated conductors may be electrically coupled to a power source in any convenient manner. Each end of an insulated conductor may be coupled to lead-in cables that pass through a wellhead. Such a configuration typically has a 180° bend (a “hairpin” bend) or turn located near a bottom of the heat source. An insulated conductor that includes a 180° bend or turn may not require a bottom termination, but the 180° bend or turn may be an electrical and/or structural weakness in the heater. Insulated conductors may be electrically coupled together in series, in parallel, or in series and parallel combinations. In some embodiments of heat sources, electrical current may pass into the conductor of an insulated conductor and may be returned through the jacket of the insulated conductor by connecting core  114  to jacket  118  (shown in  FIG. 2 ) at the bottom of the heat source. 
     In some embodiments, three insulated conductors  112  are electrically coupled in a 3-phase wye configuration to a power supply.  FIG. 3  depicts an embodiment of three insulated conductors in an opening in a subsurface formation coupled in a wye configuration.  FIG. 4  depicts an embodiment of three insulated conductors  112  that are removable from opening  120  in the formation. No bottom connection may be required for three insulated conductors in a wye configuration. Alternately, all three insulated conductors of the wye configuration may be connected together near the bottom of the opening. The connection may be made directly at ends of heating sections of the insulated conductors or at ends of cold pins (less resistive sections) coupled to the heating sections at the bottom of the insulated conductors. The bottom connections may be made with insulator filled and sealed canisters or with epoxy filled canisters. The insulator may be the same composition as the insulator used as the electrical insulation. 
     Three insulated conductors  112  depicted in  FIGS. 3 and 4  may be coupled to support member  122  using centralizers  124 . Alternatively, insulated conductors  112  may be strapped directly to support member  122  using metal straps. Centralizers  124  may maintain a location and/or inhibit movement of insulated conductors  112  on support member  122 . Centralizers  124  may be made of metal, ceramic, or combinations thereof. The metal may be stainless steel or any other type of metal able to withstand a corrosive and high temperature environment. In some embodiments, centralizers  124  are bowed metal strips welded to the support member at distances less than about 6 m. A ceramic used in centralizer  124  may be, but is not limited to, Al 2 O 3 , MgO, or another electrical insulator. Centralizers  124  may maintain a location of insulated conductors  112  on support member  122  such that movement of insulated conductors is inhibited at operating temperatures of the insulated conductors. Insulated conductors  112  may also be somewhat flexible to withstand expansion of support member  122  during heating. 
     Support member  122 , insulated conductor  112 , and centralizers  124  may be placed in opening  120  in hydrocarbon layer  126 . Insulated conductors  112  may be coupled to bottom conductor junction  128  using cold pin  130 . Bottom conductor junction  128  may electrically couple each insulated conductor  112  to each other. Bottom conductor junction  128  may include materials that are electrically conducting and do not melt at temperatures found in opening  120 . Cold pin  130  may be an insulated conductor having lower electrical resistance than insulated conductor  112 . 
     Lead-in conductor  132  may be coupled to wellhead  134  to provide electrical power to insulated conductor  112 . Lead-in conductor  132  may be made of a relatively low electrical resistance conductor such that relatively little heat is generated from electrical current passing through the lead-in conductor. In some embodiments, the lead-in conductor is a rubber or polymer insulated stranded copper wire. In some embodiments, the lead-in conductor is a mineral insulated conductor with a copper core. Lead-in conductor  132  may couple to wellhead  134  at surface  136  through a sealing flange located between overburden  138  and surface  136 . The sealing flange may inhibit fluid from escaping from opening  120  to surface  136 . 
     In certain embodiments, lead-in conductor  132  is coupled to insulated conductor  112  using transition conductor  140 . Transition conductor  140  may be a less resistive portion of insulated conductor  112 . Transition conductor  140  may be referred to as “cold pin” of insulated conductor  112 . Transition conductor  140  may be designed to dissipate about one-tenth to about one-fifth of the power per unit length as is dissipated in a unit length of the primary heating section of insulated conductor  112 . Transition conductor  140  may typically be between about 1.5 m and about 15 m, although shorter or longer lengths may be used to accommodate specific application needs. In an embodiment, the conductor of transition conductor  140  is copper. The electrical insulator of transition conductor  140  may be the same type of electrical insulator used in the primary heating section. A jacket of transition conductor  140  may be made of corrosion resistant material. 
     In certain embodiments, transition conductor  140  is coupled to lead-in conductor  132  by a splice or other coupling joint. Splices may also be used to couple transition conductor  140  to insulated conductor  112 . Splices may have to withstand a temperature equal to half of a target zone operating temperature or, in some cases, up to about 85% of the target zone operating temperature. Density of electrical insulation in the splice should in many instances be high enough to withstand the required temperature and the operating voltage. 
     In some embodiments, as shown in  FIG. 3 , packing material  142  is placed between overburden casing  144  and opening  120 . In some embodiments, reinforcing material  146  may secure overburden casing  144  to overburden  138 . Packing material  142  may inhibit fluid from flowing from opening  120  to surface  136 . Reinforcing material  146  may include, for example, Class G or Class H Portland cement mixed with silica flour for improved high temperature performance, slag or silica flour, and/or a mixture thereof. In some embodiments, reinforcing material  146  extends radially a width of from about 5 cm to about 25 cm. 
     As shown in  FIGS. 3 and 4 , support member  122  and lead-in conductor  132  may be coupled to wellhead  134  at surface  136  of the formation. Surface conductor  148  may enclose reinforcing material  146  and couple to wellhead  134 . Embodiments of surface conductors may extend to depths of approximately 3 m to approximately 515 m into an opening in the formation. Alternatively, the surface conductor may extend to a depth of approximately 9 m into the formation. Electrical current may be supplied from a power source to insulated conductor  112  to generate heat due to the electrical resistance of the insulated conductor. Heat generated from three insulated conductors  112  may transfer within opening  120  to heat at least a portion of hydrocarbon layer  126 . 
     Heat generated by insulated conductors  112  may heat at least a portion of a hydrocarbon containing formation. In some embodiments, heat is transferred to the formation substantially by radiation of the generated heat to the formation. Some heat may be transferred by conduction or convection of heat due to gases present in the opening. The opening may be an uncased opening, as shown in  FIGS. 3 and 4 . An uncased opening eliminates cost associated with thermally cementing the heater to the formation, costs associated with a casing, and/or costs of packing a heater within an opening. In addition, heat transfer by radiation is typically more efficient than by conduction, so the heaters may be operated at lower temperatures in an open wellbore. Conductive heat transfer during initial operation of a heat source may be enhanced by the addition of a gas in the opening. The gas may be maintained at a pressure up to about 27 bars absolute. The gas may include, but is not limited to, carbon dioxide and/or helium. An insulated conductor heater in an open wellbore may advantageously be free to expand or contract to accommodate thermal expansion and contraction. An insulated conductor heater may advantageously be removable or redeployable from an open wellbore. 
     In certain embodiments, an insulated conductor heater assembly is installed or removed using a spooling assembly. More than one spooling assembly may be used to install both the insulated conductor and a support member simultaneously. Alternatively, the support member may be installed using a coiled tubing unit. The heaters may be un-spooled and connected to the support as the support is inserted into the well. The electric heater and the support member may be un-spooled from the spooling assemblies. Spacers may be coupled to the support member and the heater along a length of the support member. Additional spooling assemblies may be used for additional electric heater elements. 
     Temperature limited heaters may be in configurations and/or may include materials that provide automatic temperature limiting properties for the heater at certain temperatures. Examples of temperature limited heaters may be found in U.S. Pat. No. 6,688,387 to Wellington et al.; U.S. Pat. No. 6,991,036 to Sumnu-Dindoruk et al.; U.S. Pat. No. 6,698,515 to Karanikas et al.; U.S. Pat. No. 6,880,633 to Wellington et al.; U.S. Pat. No. 6,782,947 to de Rouffignac et al.; U.S. Pat. No. 6,991,045 to Vinegar et al.; U.S. Pat. No. 7,073,578 to Vinegar et al.; U.S. Pat. No. 7,121,342 to Vinegar et al.; U.S. Pat. No. 7,320,364 to Fairbanks; U.S. Pat. No. 7,527,094 to McKinzie et al.; U.S. Pat. No. 7,584,789 to Mo et al.; U.S. Pat. No. 7,533,719 to Hinson et al.; and U.S. Pat. No. 7,562,707 to Miller; U.S. Patent Application Publication Nos. 2009-0071652 to Vinegar et al.; 2009-0189617 to Burns et al.; 2010-0071903 to Prince-Wright et al.; and 2010-0096137 to Nguyen et al., each of which is incorporated by reference as if fully set forth herein. Temperature limited heaters are dimensioned to operate with AC frequencies (for example, 60 Hz AC) or with modulated DC current. 
     In certain embodiments, ferromagnetic materials are used in temperature limited heaters. Ferromagnetic material may self-limit temperature at or near the Curie temperature of the material and/or the phase transformation temperature range to provide a reduced amount of heat when a time-varying current is applied to the material. In certain embodiments, the ferromagnetic material self-limits temperature of the temperature limited heater at a selected temperature that is approximately the Curie temperature and/or in the phase transformation temperature range. In certain embodiments, the selected temperature is within about 35° C., within about 25° C., within about 20° C., or within about 10° C. of the Curie temperature and/or the phase transformation temperature range. In certain embodiments, ferromagnetic materials are coupled with other materials (for example, highly conductive materials, high strength materials, corrosion resistant materials, or combinations thereof) to provide various electrical and/or mechanical properties. Some parts of the temperature limited heater may have a lower resistance (caused by different geometries and/or by using different ferromagnetic and/or non-ferromagnetic materials) than other parts of the temperature limited heater. Having parts of the temperature limited heater with various materials and/or dimensions allows for tailoring the desired heat output from each part of the heater. 
     Temperature limited heaters may be more reliable than other heaters. Temperature limited heaters may be less apt to break down or fail due to hot spots in the formation. In some embodiments, temperature limited heaters allow for substantially uniform heating of the formation. In some embodiments, temperature limited heaters are able to heat the formation more efficiently by operating at a higher average heat output along the entire length of the heater. The temperature limited heater operates at the higher average heat output along the entire length of the heater because power to the heater does not have to be reduced to the entire heater, as is the case with typical constant wattage heaters, if a temperature along any point of the heater exceeds, or is about to exceed, a maximum operating temperature of the heater. Heat output from portions of a temperature limited heater approaching a Curie temperature and/or the phase transformation temperature range of the heater automatically reduces without controlled adjustment of the time-varying current applied to the heater. The heat output automatically reduces due to changes in electrical properties (for example, electrical resistance) of portions of the temperature limited heater. Thus, more power is supplied by the temperature limited heater during a greater portion of a heating process. 
     In certain embodiments, the system including temperature limited heaters initially provides a first heat output and then provides a reduced (second heat output) heat output, near, at, or above the Curie temperature and/or the phase transformation temperature range of an electrically resistive portion of the heater when the temperature limited heater is energized by a time-varying current. The first heat output is the heat output at temperatures below which the temperature limited heater begins to self-limit. In some embodiments, the first heat output is the heat output at a temperature about 50° C., about 75° C., about 100° C., or about 125° C. below the Curie temperature and/or the phase transformation temperature range of the ferromagnetic material in the temperature limited heater. 
     The temperature limited heater may be energized by time-varying current (alternating current or modulated direct current) supplied at the wellhead. The wellhead may include a power source and other components (for example, modulation components, transformers, and/or capacitors) used in supplying power to the temperature limited heater. The temperature limited heater may be one of many heaters used to heat a portion of the formation. 
     In certain embodiments, the temperature limited heater includes a conductor that operates as a skin effect or proximity effect heater when time-varying current is applied to the conductor. The skin effect limits the depth of current penetration into the interior of the conductor. For ferromagnetic materials, the skin effect is dominated by the magnetic permeability of the conductor. The relative magnetic permeability of ferromagnetic materials is typically between 10 and 1000 (for example, the relative magnetic permeability of ferromagnetic materials is typically at least 10 and may be at least 50, 100, 500, 1000 or greater). As the temperature of the ferromagnetic material is raised above the Curie temperature, or the phase transformation temperature range, and/or as the applied electrical current is increased, the magnetic permeability of the ferromagnetic material decreases substantially and the skin depth expands rapidly (for example, the skin depth expands as the inverse square root of the magnetic permeability). The reduction in magnetic permeability results in a decrease in the AC or modulated DC resistance of the conductor near, at, or above the Curie temperature, the phase transformation temperature range, and/or as the applied electrical current is increased. When the temperature limited heater is powered by a substantially constant current source, portions of the heater that approach, reach, or are above the Curie temperature and/or the phase transformation temperature range may have reduced heat dissipation. Sections of the temperature limited heater that are not at or near the Curie temperature and/or the phase transformation temperature range may be dominated by skin effect heating that allows the heater to have high heat dissipation due to a higher resistive load. 
     An advantage of using the temperature limited heater to heat hydrocarbons in the formation is that the conductor is chosen to have a Curie temperature and/or a phase transformation temperature range in a desired range of temperature operation. Operation within the desired operating temperature range allows substantial heat injection into the formation while maintaining the temperature of the temperature limited heater, and other equipment, below design limit temperatures. Design limit temperatures are temperatures at which properties such as corrosion, creep, and/or deformation are adversely affected. The temperature limiting properties of the temperature limited heater inhibit overheating or burnout of the heater adjacent to low thermal conductivity “hot spots” in the formation. In some embodiments, the temperature limited heater is able to lower or control heat output and/or withstand heat at temperatures above 25° C., 37° C., 100° C., 250° C., 500° C., 700° C., 800° C., 900° C., or higher up to 1131° C., depending on the materials used in the heater. 
     The temperature limited heater allows for more heat injection into the formation than constant wattage heaters because the energy input into the temperature limited heater does not have to be limited to accommodate low thermal conductivity regions adjacent to the heater. For example, in Green River oil shale there is a difference of at least a factor of 3 in the thermal conductivity of the lowest richness oil shale layers and the highest richness oil shale layers. When heating such a formation, substantially more heat is transferred to the formation with the temperature limited heater than with the conventional heater that is limited by the temperature at low thermal conductivity layers. The heat output along the entire length of the conventional heater needs to accommodate the low thermal conductivity layers so that the heater does not overheat at the low thermal conductivity layers and burn out. The heat output adjacent to the low thermal conductivity layers that are at high temperature will reduce for the temperature limited heater, but the remaining portions of the temperature limited heater that are not at high temperature will still provide high heat output. Because heaters for heating hydrocarbon formations typically have long lengths (for example, at least 10 m, 100 m, 300 m, 500 m, 1 km or more up to about 10 km), the majority of the length of the temperature limited heater may be operating below the Curie temperature and/or the phase transformation temperature range while only a few portions are at or near the Curie temperature and/or the phase transformation temperature range of the temperature limited heater. 
     The use of temperature limited heaters allows for efficient transfer of heat to the formation. Efficient transfer of heat allows for reduction in time needed to heat the formation to a desired temperature. For example, in Green River oil shale, pyrolysis typically requires 9.5 years to 10 years of heating when using a 12 m heater well spacing with conventional constant wattage heaters. For the same heater spacing, temperature limited heaters may allow a larger average heat output while maintaining heater equipment temperatures below equipment design limit temperatures. Pyrolysis in the formation may occur at an earlier time with the larger average heat output provided by temperature limited heaters than the lower average heat output provided by constant wattage heaters. For example, in Green River oil shale, pyrolysis may occur in 5 years using temperature limited heaters with a 12 m heater well spacing. Temperature limited heaters counteract hot spots due to inaccurate well spacing or drilling where heater wells come too close together. In certain embodiments, temperature limited heaters allow for increased power output over time for heater wells that have been spaced too far apart, or limit power output for heater wells that are spaced too close together. Temperature limited heaters also supply more power in regions adjacent to the overburden and underburden to compensate for temperature losses in these regions. 
     Temperature limited heaters may be advantageously used in many types of formations. For example, in tar sands formations or relatively permeable formations containing heavy hydrocarbons, temperature limited heaters may be used to provide a controllable low temperature output for reducing the viscosity of fluids, mobilizing fluids, and/or enhancing the radial flow of fluids at or near the wellbore or in the formation. Temperature limited heaters may be used to inhibit excess coke formation due to overheating of the near wellbore region of the formation. 
     In some embodiments, the use of temperature limited heaters eliminates or reduces the need for expensive temperature control circuitry. For example, the use of temperature limited heaters eliminates or reduces the need to perform temperature logging and/or the need to use fixed thermocouples on the heaters to monitor potential overheating at hot spots. 
     The temperature limited heaters may be used in conductor-in-conduit heaters. In some embodiments of conductor-in-conduit heaters, the majority of the resistive heat is generated in the conductor, and the heat radiatively, conductively and/or convectively transfers to the conduit. In some embodiments of conductor-in-conduit heaters, the majority of the resistive heat is generated in the conduit. 
     In some embodiments, a relatively thin conductive layer is used to provide the majority of the electrically resistive heat output of the temperature limited heater at temperatures up to a temperature at or near the Curie temperature and/or the phase transformation temperature range of the ferromagnetic conductor. Such a temperature limited heater may be used as the heating member in an insulated conductor heater. The heating member of the insulated conductor heater may be located inside a sheath with an insulation layer between the sheath and the heating member. 
     Current techniques allow for the measurement of dielectric properties of insulation along a length of the insulation (measurement of dielectric properties distributed along the length of the insulation). These techniques provide a profile of the dielectric properties with a spatial resolution (space between measurements) based on the type of insulation and the abilities of the measurement system. These techniques are currently used to assess dielectric properties and detect insulation flaws and/or insulation damage. Examples of current techniques are axial tomography and line resonance analysis. A version of axial tomography (Mashikian Axial Tomography) is provided by Instrument Manufacturing Company (IMCORP) (Storrs, Conn., U.S.A.). Mashikian Axial Tomography is disclosed in U.S. Pat. Application Pub. No. 2008-0048668 to Mashikian. A version of line resonance analysis (LIRA) is provided by Wirescan AS (Halden, Norway). Wirescan AS LIRA is disclosed in International Pat. Pub. No. WO 2007/040406 to Fantoni. 
     The assessment of dielectric properties (using either the current techniques or modified versions of these techniques) may be used in combination with information about the temperature dependence of dielectric properties to assess a temperature profile of one or more energized heaters (heaters that are powered and providing heat). The temperature dependence data of the dielectric properties may be found from simulation and/or experimentation. Examples of dielectric properties of the insulation that may be assessed over time include, but are not limited to, dielectric constant and loss tangent.  FIG. 5  depicts an example of a plot of dielectric constant versus temperature for magnesium oxide insulation in one embodiment of an insulated conductor heater.  FIG. 6  depicts an example of a plot of loss tangent (tan δ), measured at 60 Hz, versus temperature for magnesium oxide insulation in one embodiment of an insulated conductor heater. 
     It should be noted that the temperature dependent behavior of a dielectric property may vary based on certain factors. Factors that may affect the temperature dependent behavior of the dielectric property include, but are not limited to, the type of insulation, the dimensions of the insulation, the time the insulation is exposed to environment (for example, heat from the heater), the composition (chemistry) of the insulation, moisture content, and the compaction of the insulation. Thus, it is typically necessary to measure (either by simulation and/or experimentation) the temperature dependent behavior of the dielectric property for the embodiment of insulation that is to be used in a selected heater. 
     In certain embodiments, one or more dielectric properties of the insulation in a heater having electrical insulation are assessed (measured) and compared to temperature dependence data of the dielectric properties to assess (determine) a temperature profile along a length of the heater (for example, the entire length of the heater or a portion of the heater). For example, the temperature of an insulated conductor heater (such as a mineral insulated (MI) cable heater) may be assessed based on dielectric properties of the insulation used in the heater. Since the temperature dependence of the dielectric property measured is known or estimated from simulation and/or experimentation, the measured dielectric property at a location along the heater may be used to assess the temperature of the heater at that location. Using techniques that measure the dielectric properties at multiple locations along a length of the heater (as is possible with current techniques), a temperature profile along that heater length may be provided. 
     In some embodiments, as shown by the plots in  FIGS. 5 and 6 , the dielectric properties are more sensitive to temperature at higher temperatures (for example, above about 900° F. (482° C.), as shown in  FIGS. 5 and 6 ). Thus, in some embodiments, the temperature of a portion of the insulated conductor heater is assessed by measurement of the dielectric properties at temperatures above about 400° C. (about 760° F.). For example, the temperature of the portion may be assessed by measurement of the dielectric properties at temperatures ranging from about 400° C., about 450° C., or about 500° C. to about 800° C., about 850° C., or about 900° C. These ranges of temperatures are above temperatures that can be measured using commercially available fiber optic cable systems. A fiber optic cable system suitable for use in the higher temperature ranges may, however, provide measurements with higher spatial resolution than temperature assessment by measurement of the dielectric properties. Thus, in some embodiments, the fiber optic cable system operable in the higher temperature ranges may be used to calibrate temperature assessment by measurement of dielectric properties. 
     At temperatures below these temperature ranges (for example, below about 400° C.), temperature assessment by measurement of the dielectric properties may be less accurate. Temperature assessment by measurement of the dielectric properties may, however, provide a reasonable estimate or “average” temperature of portions of the heater. The average temperature assessment may be used to assess whether the heater is operating at temperatures below about 500° C., below about 450° C., or below about 400° C. 
     Temperature assessment by measurement of dielectric properties may provide a temperature profile along a length or portion of the insulated conductor heater (temperature measurements distributed along the length or portion of the heater). Measuring the temperature profile is more useful for monitoring and controlling the heater as compared to taking temperature measurements at only selected locations (such as temperature measurement with thermocouples). Multiple thermocouples may be used to provide a temperature profile. Multiple wires (two per thermocouple), however, would be needed. Temperature assessment by measurement of dielectric properties uses only one wire for measurement of the temperature profile, which is simpler and less expensive than using multiple thermocouples. In some embodiments, one or more thermocouples placed at selected locations are used to calibrate temperature assessment by measurement of dielectric properties. 
     In certain embodiments, the dielectric properties of the insulation in an insulated conductor heater are assessed (measured) over a period of time to assess the temperature and operating characteristics of the heater over the period of time. For example, the dielectric properties may be assessed continuously (or substantially continuously) to provide real-time monitoring of the dielectric properties and the temperature. Monitoring of the dielectric properties and the temperature may be used to assess the condition of the heater during operation of the heater. For example, comparison of the assessed properties at specific locations versus the average properties over the length of the heater may provide information on the location of hot spots or defects in the heater. 
     In certain embodiments, an assessment (scan) of the insulated conductor heater before the heater is used to provide heat (for example, a baseline assessment (scan)) provides information on locations of defects and/or other discontinuities (for example, splices) that cause abnormalities in the dielectric properties of the heater. The baseline assessment may be used to later detect the presence of hot spots or defects that occur after the heater is energized. For example, assessment of the heater after energizing the heater may show new abnormalities in the dielectric properties of the heater that were not present in the baseline assessment. In some embodiments, the baseline assessment is used to assess the quality of insulation in the heater. For example, the baseline assessment may be used to compare the quality of sections of insulation (for example, blocks of insulation) along the length of the heater. 
     In some embodiments, the dielectric properties of the insulation change over time. For example, the dielectric properties may change over time because of changes in the oxygen concentration in the insulation over time and/or changes in the water content in the insulation over time. Oxygen in the insulation may be consumed by chromium or other metals used in the insulated conductor heater. Thus, the oxygen concentration decreases with time in the insulation and affects the dielectric properties of the insulation. 
     Variations in water (moisture) content in the insulation may also affect the dielectric properties of the insulation. Experimentation and/or simulation may be used to provide correlation data between dielectric properties of the insulation and water content in the insulation. Thus, in certain embodiments, assessment of the dielectric properties is used to detect the water content in the heater during heating of the formation. For example, a decrease in the dielectric value of the insulation relative to an initial dielectric value of the insulation having a known water content indicates water has permeated the insulation. 
     In some embodiments, assessment of water content along the heater is used to inhibit overheating of the heater at locations assessed to have no water content. If the heater is treated as though there is water along the entire length of the heater, portions of the heater (especially for relatively long heaters) that do not have water may overheat because of the lack of water and high energy input into the heater. Thus, assessment of water content using the dielectric properties may provide locations (or at least approximate locations) of little or no water (non-detectable) content along the length of the heater and the power provided to the heater can be adjusted to inhibit overheating such locations on the heater. 
     In certain embodiments, water content along the heater is related to water content in the formation (for example, if the heater is in an open wellbore, water in the formation may contact the heater in the open wellbore). Thus, assessment of water content along the heater using assessment of the dielectric properties may be used to monitor the presence of water in the formation surrounding the heater. For example, assessment of water content using dielectric properties may provide information on water inflow into the heater wellbore. Inflow of water may cause the temperature of the heater to decrease until water begins to boil off (for example, evaporate), at which time, the heater temperature will begin increasing. 
     In certain embodiments, assessment of the dielectric properties allows assessment of when water boils off in the wellbore surrounding the heater. Thus, assessment of the dielectric properties may be used in commissioning (startup) of the heater to detect when water surrounding the heater is evaporated. During startup of the heater in a water environment, the heater may be at high power to boil off the water. If the power is not turned down when the surrounding environment changes from predominantly liquid water to predominantly steam, the heater may overheat due to reduced thermal conductivity. Monitoring the dielectric properties to assess when the water is boiled off allows the heater power to be turned down in time to inhibit overheating of the heater. Water boiling off may be detected as a hot spot or defect (for example, a rapid change in temperature) using the assessment of the dielectric properties. Thus, the heater may be turned down when the hot spot or defect is detected using the assessment of the dielectric properties to avoid overheating of the heater during startup of the heater. 
     In some embodiments, assessment of dielectric properties to assess water content is used in flow assurance operations (for example, flow assurance operations for offshore heating). The assessment of water content using the dielectric properties in the heater allows for detection of leaks or other failures that may allow water into the heater or pipeline with the heater while the heater is still in position offshore. Assessing water inflow while the heater is in position offshore reduces the need to remove the heater for assurance on operation of the heater. 
     Changes in dielectric properties of the heater over time may be measured and compensated for through experimental and/or simulated data. For example, the insulated conductor heater to be used for temperature assessment may be heated in an oven or other apparatus and the changes in dielectric properties can be measured over time at various temperatures and/or at constant temperatures. In addition, thermocouples may be used to calibrate the assessment of dielectric properties changes over time by comparison of thermocouple data to temperature assessed by the dielectric properties. 
     In certain embodiments, temperature assessment by measurement of dielectric properties is performed using a computational system such as a workstation or computer. The computational system may receive measurements (assessments) of the dielectric properties along the heater and correlate these measured dielectric properties to assess temperatures at one or more locations on the heater. For example, the computational system may store data about the relationship of the dielectric properties to temperature (such as the data depicted in  FIGS. 5 and 6 ) and/or time, and use this stored data to calculate the temperatures on the heater based on the measured dielectric properties. In some embodiments, the computational system is used to subtract out feedback from power (remove the affect of power on the measurement) provided to the heater when the heater is energized. 
     In certain embodiments, the computational system is used to monitor the dielectric properties of the heater over a period of time and report and/or control the heater while monitoring the dielectric properties. For example, the computational system may be used to provide an alarm or other report to a user when the dielectric properties (or temperature) vary by more than a selected percentage. In some embodiments, the computational system (automatically) turns down the power to the heater when the dielectric properties (or temperature) vary by more than a desired amount. For example, the power may be ramped down by about 33%, about 50%, or more when the temperature of the heater varies above the desired temperature by more than a desired amount. In some embodiments, the power is turned down while notifying the user with an alarm or other report. 
     In certain embodiments, temperature assessment by dielectric properties measurement is performed on an energized heater providing heat to the subsurface formation (for example, at least a portion of an insulated conductor heater provided with electric power to resistively heat and providing at least some heat to the subsurface formation from the portion of the insulated conductor). Assessing temperature on the energized heater allows for detection of defects in the insulation on the device actually providing heat to the formation. Assessing temperature on the energized heater, however, may be more difficult due to attenuation of signal along the heater because the heater is resistively heating. This attenuation may inhibit seeing further along the length of the heater (deeper into the formation along the heater). In some embodiments, temperatures in the upper sections of heaters (sections of the heater closer to the overburden, for example, the upper half or upper third of the heater) may be more important for assessment because these sections have higher voltages applied to the heater, are at higher temperatures, and are at higher risk for failure or generation of hot spots. The signal attenuation in the temperature assessment by dielectric properties measurement may not be as significant a factor in these upper sections because of the proximity of these sections to the surface. 
     In some embodiments, power to the insulated conductor heater is turned off before performing the temperature assessment. Power is then returned to the insulated conductor heater after the temperature assessment. Thus, the insulated conductor heater is subjected to a heating on/off cycle to assess temperature. This on/off cycle may, however, reduce the lifetime of the heater due to the thermal cycling. In addition, the heater may cool off during the non-energized time period and provide less accurate temperature information (less accurate information on the actual working temperature of the heater). 
     In certain embodiments, temperature assessment by dielectric properties measurement is performed on an insulated conductor that is not to be used for heating or not configured for heating. Such an insulated conductor may be a separate insulated conductor temperature probe. In some embodiments, the insulated conductor temperature probe is a non-energized heater (for example, an insulated conductor heater not powered). The insulated conductor temperature probe may be a stand-alone device that can be located in an opening in the subsurface formation to measure temperature in the opening. In some embodiments, the insulated conductor temperature probe is a looped probe that goes out and back into the opening with signals transmitted in one direction on the probe. In some embodiments, the insulated conductor temperature probe is a single, hanging probe with the signal transmitted along the core and returned along the sheath of the insulated conductor. 
     In certain embodiments, the insulated conductor temperature probe includes a copper core (to provide better conductance to the end of the cable and better spatial resolution) surrounded by magnesium oxide insulation and an outer metal sheath. The outer metal sheath may be made of any material suitable for use in the subsurface opening. For example, the outer metal sheath may be a stainless steel sheath or an inner sheath of copper wrapped with an outer sheath of stainless steel. Typically, the insulated conductor temperature probe operates up to temperatures and pressures that can be withstood by the outer metal sheath. 
     In some embodiments, the insulated conductor temperature probe is located adjacent to or near an energized heater in the opening to measure temperatures along the energized heater. There may be a temperature difference between the insulated conductor temperature probe and the energized heater (for example, between about 50° C. and 100° C. temperature differences). This temperature difference may be assessed through experimentation and/or simulation and accounted for in the temperature measurements. The temperature difference may also be calibrated using one or more thermocouples attached to the energized heater. 
     In some embodiments, one or more thermocouples are attached to the insulated conductor used for temperature assessment (either an energized insulated conductor heater or a non-energized insulated conductor temperature probe). The attached thermocouples may be used for calibration and/or backup measurement of the temperature assessed on the insulated conductor by dielectric property measurement. In some embodiments, calibration and/or backup temperature indication is achieved by assessment of the resistance variation of the core of the insulated conductor at a given applied voltage. Temperature may be assessed by knowing the resistance versus temperature profile of the core material at the given voltage. In some embodiments, the insulated conductor is a loop and current induced in the loop from energized heaters in the subsurface opening provides input for the resistance measurement. 
     In certain embodiments, insulation material properties in the insulated conductor are varied to provide different sensitivities to temperature for the insulated conductor. Examples of insulation material properties that may be varied include, but are not limited to, the chemical and phase composition, the microstructure, and/or the mixture of insulating materials. Varying the insulation material properties in the insulated conductor allows the insulated conductor to be tuned to a selected temperature range and/or to increase the sensitivity in assessment of temperature using the insulated conductor. The selected temperature range may be selected, for example, for a desired application of the insulated conductor. 
     Insulated conductors (for example, mineral insulated conductors) used for in situ heat treatment processes in hydrocarbon containing formations typically use magnesium oxide as the electrical insulator. Although assessing temperatures using dielectric properties of insulated conductors with magnesium oxide insulation may provide some desired information, the sensitivity to temperature for the dielectric constant of magnesium oxide may not be sufficient for reliable detection of temperature at desired heater temperatures used during the in situ heat treatment process. For example, as shown in FIG.  5 , the dielectric constant of magnesium oxide increases from about 8 at about 800° F. (about 427° C.) to about 16 at about 1600° F. (about 871° C.), and thus the dielectric constant increases only about 0.018 per every degree Celsius increase in temperature. These small changes in the dielectric constant in the desired temperature ranges for magnesium oxide may make it difficult to detect temperature differences along a length of the insulated conductor. 
     In certain embodiments, ferroelectric materials are added to the electrical insulator in the insulated conductor to increase the sensitivity for temperature assessment using dielectric properties of the electrical insulator. Adding the ferroelectric material may cause the dielectric constant of the electrical insulator to change more rapidly with temperature and increase the sensitivity in measuring the dielectric constant versus temperature. Examples of ferroelectric materials that may be added to the electrical insulator include, but are not limited to, bismuth titanate and doped bismuth titanate (for example, niobium doped bismuth titanate). In some embodiments, it may be possible to use other materials that exhibit similar properties as ferroelectric materials such as any material having a high dielectric constant with the dielectric constant varying strongly with temperature in a desired temperature range. 
     In certain embodiments, a selected amount of the ferroelectric material (for example, bismuth titanate) is added to magnesium oxide to form the electrical insulator for the insulated conductor. For example, the selected amount of the ferroelectric material may be added to electrical insulator  116 , shown in  FIG. 2 . In certain embodiments, the amount of ferroelectric material in the electrical insulator is small relative to the amount of magnesium oxide in the electrical insulator. In certain embodiments, the electrical insulator includes at most about 5% by weight ferroelectric material (with the remainder being substantially magnesium oxide). In other embodiments, the electrical insulator includes at most about 10% by weight ferroelectric material, at most about 7.5% by weight ferroelectric material, or at most 2.5% by weight ferroelectric material with the remainder being substantially magnesium oxide. 
     Magnesium oxide is typically used as the electrical insulator in insulated conductors for subsurface applications as magnesium oxide has good breakdown voltage values and low electrical conductivity to avoid electrical shorting between the core and the sheath in the subsurface. The amount of ferroelectric material added to magnesium oxide may be kept small to maintain desired electrical properties in the electrical insulator while increasing the dielectric constant and increasing the sensitivity to temperature of the dielectric constant for the insulated conductor. The amount of ferroelectric material may be kept at or below the selected amount to, for example, keep the electrical conductivity low enough to maintain the attenuation of the signal used to determine the distribution of dielectric constant at an acceptable level. As noted above, keeping the breakdown voltage above the desired value is important in subsurface heating applications to avoid electrical shorts at the high voltages used for long insulated conductors (for example, lengths greater than 10 m or greater than 100 m). 
       FIG. 7  depicts dielectric constant versus temperature for undoped bismuth titanate at several frequencies. Bismuth titanate is a ferroelectric ceramic material that has a higher dielectric constant (which ranges between about 200 and about 500000, as shown in  FIG. 7 ) than magnesium oxide (which ranges between about 7 and 17, as shown in  FIG. 5 ). In addition, the dielectric constant of bismuth titanate is more sensitive to temperature than the dielectric constant of magnesium oxide. For example, as shown in  FIG. 7 , at the frequency of 100 kHz, the dielectric constant increases from about 500 at about 400° C. to about 1000 at about 700° C., and thus the dielectric constant increases about 1.67 per every degree Celsius increase in the temperature. 
     The sensitivity to temperature for bismuth titanate at 100 kHz is about 92 times the sensitivity of the dielectric constant of magnesium oxide in a similar temperature range (shown in  FIG. 5 ). Thus, adding ferroelectric material (such as bismuth titanate) to magnesium oxide in the electrical insulator (for example, electrical insulator  116 , shown in  FIG. 2 ) may increase the sensitivity to temperature of the dielectric constant in the electrical insulator. Increasing the sensitivity to temperature of the dielectric constant increases the ability to assess temperature in the insulated conductor using dielectric properties of the insulated conductor. Adding even only a small amount of ferroelectric material (for example, at most about 5% by weight as described above) may improve the sensitivity to temperature of the dielectric constant in the electrical insulator to levels that allow reliable assessment of the temperature in the insulated conductor using the dielectric properties of the electrical insulator. 
     In certain embodiments, the electrical insulator with ferroelectric material added to magnesium oxide has a dielectric constant that increases by a factor of at least about 1.5 for every degree Celsius increase in temperature in a desired temperature range. The desired temperature range may be, for example, at or near an operating temperature range of a heater used to heat a subsurface formation such as a hydrocarbon containing formation. In some embodiments, the desired temperature range is above about 300° C., about 400° C., or about 500° C. up to about 800° C., about 900° C., or about 1000° C. In some embodiments, the electrical insulator with ferroelectric material added to magnesium oxide has a dielectric constant that increases by a factor of about at least about 2, at least about 5, or at least about 10 for every degree Celsius increase in temperature in a desired temperature range. 
     As shown in  FIG. 7 , lowering the frequency applied to bismuth titanate may further increase the sensitivity to temperature of the dielectric constant. For example, the dielectric constant of bismuth titanate is higher and has more sensitivity to temperature at 1 kHz versus 100 kHz and is higher and more sensitive at 20 Hz versus 1 kHz. Thus, in some embodiments, higher sensitivities are achieved by adjusting the frequency applied to the electrical insulator containing ferroelectric material and magnesium oxide. 
     In some embodiments, the ferroelectric material is doped with an additional material to adjust the temperature dependency of the dielectric constant. For example, bismuth titanate may be doped with niobium (Nb) or a similar material.  FIG. 8  depicts dielectric constant versus temperature for bismuth titanate doped with 0.74% by weight niobium at several frequencies. As shown in  FIG. 8 , doping bismuth titanate with niobium moves the onset of increased temperature sensitivity to higher temperatures. For example, at 20 Hz, the dielectric constant of undoped bismuth titanate shows high temperature sensitivity starting at about 100° C. (as shown in  FIG. 7 ) while the dielectric constant of niobium doped bismuth titanate shows high temperature sensitivity starting at about 400° C. (as shown in  FIG. 8 ). 
     Shifting the onset of high temperature sensitivity to higher temperatures also increases the sensitivity to temperature of the dielectric constant in the high temperature sensitivity range. Thus, in certain embodiments, doping the ferroelectric material with the additional material may be used to increase the sensitivity to temperature of the dielectric constant in the desired temperature range. In some embodiments, doping the ferroelectric material with the additional material may decrease electrical conductivity in the electrical insulator and/or increase dielectric breakdown voltage in the electrical insulator. 
     As shown in  FIGS. 7 and 8 , in some embodiments, the dielectric constant of bismuth titanate (and thus the electrical insulator) may increase to a selected temperature and then decrease slightly above the selected temperature (for example, about 700° C. in  FIGS. 7 and 8 ). In certain embodiments, a computational system such as a workstation or computer is used to perform temperature assessment by measurement of dielectric properties. As described above, the computational system may receive measurements (assessments) of the dielectric properties along the heater and correlate these measured dielectric properties to assess temperatures at one or more locations on the heater. Such a computational system may be used to assess the trend in the profile of the dielectric constant with temperature (for example, whether the dielectric constant is increasing or decreasing). The computation system may be used to assess whether the dielectric constant being measured is at either the lower temperature (below the selected temperature) or the higher temperature (above the selected temperature) based on the assessed trend in the dielectric constant profile. 
     In certain embodiments, the electrical insulator with ferroelectric material added to magnesium oxide is used in the separate insulated conductor temperature probe described above. The insulated conductor temperature probe may include a copper core and a copper sheath. The insulated conductor temperature probe may operate at lower voltage than an energized insulated conductor. Operating at lower voltage may provide lower attenuation in the signal in the insulated conductor temperature probe and thus provide higher sensitivity for measurement and more reliable measurements of the dielectric properties of the electrical insulator. Using an insulated conductor temperature probe with ferroelectric material added to magnesium oxide in the electrical insulator may provide reliable temperature measurement along a length of a wellbore in which the probe is placed at desired operating temperatures for in situ heat treatment processes. For example, the probe may be placed in a heater wellbore with an energized heater also in the wellbore to assess temperatures along the length of the heater. 
     In certain embodiments, insulation material properties are adjusted to change the capacitance of the insulation in the heater.  FIG. 6  depicts an example of loss tangent versus temperature for magnesium oxide insulation in an insulated conductor heater. Loss tangent (tan δ) is the real capacitance divided by the imaginary capacitance. Thus, the curve in  FIG. 6  is a capacitance curve for the insulation in the insulated conductor heater. 
     The takeoff temperature of the curve (the temperature at which the curve begins to go vertical or increases dramatically—for example, about 1400° F. (about 760° C.) in the example of  FIG. 6 ) is dependent on the capacitance of the insulation. Thus, in certain embodiments, the capacitance of the insulation of the heater is adjusted to change the takeoff temperature of the capacitance curve. For example, the capacitance curve, as depicted in  FIG. 6 , may be shifted left or right to change the takeoff temperature. If the change in takeoff temperature of a portion insulation of a heater is known, monitoring of the loss tangent allows assessment of when a takeoff temperature of the heater is being reached. 
     In some embodiments, insulation material properties are varied along the length of the insulated conductor (the insulation material properties are different at selected points within the insulated conductor). Varying properties of the insulation material at known locations along the length of the insulated conductor allows the measurement of the dielectric properties to give location information and/or provide for self-calibration of the insulated conductor in addition to providing temperature assessment. 
     In certain embodiments, the heater has insulation with different known takeoff temperatures (different capacitances) at different known locations along the length of the heater.  FIG. 9  depicts an embodiment of a length of insulated conductor  112 . Insulated conductor  112  may include, for example, a plurality of insulated conductor sections  112 A- 112 G. Sections  112 A- 112 G may each have different capacitances (takeoff temperatures) with the locations of the transitions (for example, the locations of the changes in capacitance or takeoff temperature) between the sections being known. In some embodiments, the capacitance of the insulation is varied by providing insulating blocks with different capacitance along the length of the heater. For example, sections  112 A- 112 G may each have blocks with different capacitances. Knowing the locations of the capacitance changes (for example, the junctions or borders between sections  112 A- 112 G) and their associated takeoff temperatures allows monitoring of the loss tangent of individual sections to provide information on when certain portions of the heater reach certain temperatures. In some embodiments, the capacitance of the insulation is tailored to provide temperature assessment of sections of the heater using the different takeoff temperatures. For example, a length of a heater may have several sections (for example, sections  112 A- 112 D) with takeoff temperatures that incrementally step between two temperatures, and the loss tangent may be monitored to assess the temperature of the length based on which section&#39;s loss tangent is at its respective takeoff temperature. 
     In some embodiments, capacitance is measured as a function of length along the heater. The capacitance versus length data may be used to provide temperature information for the heater, when the heater is energized, based on the takeoff of the loss tangent. 
     In some embodiments, the insulated conductor includes a portion with insulation material properties that allow the portion to act as a reflector. The reflector portion may be used to limit temperature assessment to specific portions of the insulated conductor (for example, a specific length of insulated conductor). One or more reflector portions may be used to provide spatial markers along the length of the insulated conductor. For example, reflector portions may be provided at the transition (junctions) between sections  112 A- 112 G. 
     Varying the insulation material properties adjusts the activation energy of the insulation material. Typically, increasing the activation energy of the insulation material reduces attenuation in the insulation material and provides better spatial resolution. Lowering the activation energy typically provides better temperature sensitivity. The activation energy may be raised or lowered, for example, by adding different components to the insulation material. For example, adding certain components to magnesium oxide insulation will lower the activation energy. Examples of components that may be added to magnesium oxide to lower the activation energy include, but are not limited to, titanium oxide, nickel oxide, and iron oxide. 
     In some embodiments, temperature is assessed using two or more insulated conductors. The insulation material in the insulated conductors may have different activation energies to provide a variation in spatial resolution and temperature sensitivity to more accurately assess temperature in the subsurface opening. The higher activation energy insulated conductor may be used to provide better spatial resolution and identify the location of hot spots or other temperature variations more accurately while the lower activation energy insulated conductor may be used to provide more accurate temperature measurement at those locations. 
     It is to be understood the invention is not limited to particular systems described which may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification, the singular forms “a”, “an” and “the” include plural referents unless the content clearly indicates otherwise. Thus, for example, reference to “a layer” includes a combination of two or more layers and reference to “a fluid” includes mixtures of fluids. 
     In this patent, certain U.S. patents and U.S. patent applications have been incorporated by reference. The text of such U.S. patents and U.S. patent applications is, however, only incorporated by reference to the extent that no conflict exists between such text and the other statements and drawings set forth herein. In the event of such conflict, then any such conflicting text in such incorporated by reference U.S. patents and U.S. patent applications is specifically not incorporated by reference in this patent. 
     Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.