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

1. A system for assessing a temperature in an opening in a subsurface formation, comprising:

an electrical conductor;

electrical insulation at least partially surrounding the electrical conductor, the electrical insulation comprising magnesium oxide and a ferroelectric material comprising bismuth titanate, wherein the electrical insulation contains at most 5% by weight of the ferroelectric material; and

an electrically conductive sheath at least partially surrounding the electrical insulation.

2. The system of claim 1 , wherein the electrical insulator comprises at most about 5% by weight of bismuth titanate.

3. The system of claim 1 , wherein the bismuth titanate is doped with niobium.

4. The system of claim 1 , wherein the electrical insulation comprises 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.

5. The system of claim 1 , wherein the electrical conductor and the electrical insulation comprise a length of at least about 10 m.

6. The system of claim 1 , wherein the system is located in a wellbore in a subsurface formation.

7. The system of claim 1 , wherein the system is located in a wellbore in a subsurface formation and at least one heater for providing heat to the subsurface formation is located in the wellbore.

8. The system of claim 1 , wherein the electrical conductor and the electrically conductive sheath are used to assess the dielectric properties of the electrical insulation during use.

9. A method for assessing a temperature in an opening in a subsurface formation, comprising:

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 comprising bismuth titanate, wherein the electrical insulation comprises at most 5% by weight of the ferroelectric material;

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.

10. The method of claim 9 , wherein the electrical insulator comprises at most about 5% by weight of bismuth titanate.

11. The method of claim 9 , wherein the bismuth titanate is doped with niobium.

12. The method of claim 9 , wherein at least one of the dielectric properties comprises a dielectric constant.

13. The method of claim 9 , wherein the electrical insulation comprises 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.

14. The method of claim 9 , further comprising providing heat to the subsurface formation with a heater located in a wellbore in the formation, wherein the insulated conductor is located in the wellbore proximate the heater.

15. The method of claim 9 , further comprising providing electrical power to at least a portion of the insulated conductor, and providing at least some heat to the subsurface formation from the portion of the insulated conductor.

16. The method of claim 9 , further comprising using the electrical conductor and the electrically conductive sheath to assess the dielectric properties of the electrical insulation.

17. The method of claim 9 , further comprising monitoring a trend in the profile of at least one dielectric property to assess a difference between increase in temperature and decrease in temperature for a range of values of the dielectric property.

18. The method of claim 9 , wherein assessing the temperature profile comprises comparing temperature dependence data of the dielectric properties to the assessed profile of the one or more dielectric properties.