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A method comprises heating a formation adjacent a selected depth level within a wellbore extending into the formation, forming a temporary liner in the wellbore, measuring a first temperature of the formation at the selected depth level at a first time after heating the formation, measuring a second

A method comprises heating a formation adjacent a selected depth level within a wellbore extending into the formation, forming a temporary liner in the wellbore, measuring a first temperature of the formation at the selected depth level at a first time after heating the formation, measuring a second temperature of the formation at the selected depth level at a second time after heating the formation, and combining the temperature measurements to derive an indication of thermal properties of the formation. An apparatus comprises a coiled tubing drillstring extending into a wellbore, an assembly supported by the drillstring, the assembly comprising an extruder that extrudes a liner material onto a wall of the wellbore, a heat source that heats the liner material and a formation, and at least one temperature sensor that measures a temperature of the formation.

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What is claimed is: 1. A method for determining the thermal properties of a downhole formation, comprising: heating the formation adjacent a selected depth level within a wellbore extending into the formation using a heater on a drilling assembly; forming a temporary liner in the wellbore using the

What is claimed is: 1. A method for determining the thermal properties of a downhole formation, comprising: heating the formation adjacent a selected depth level within a wellbore extending into the formation using a heater on a drilling assembly; forming a temporary liner in the wellbore using the heat from the heater; measuring a first temperature of the formation at the selected depth level at a first time after heating the formation; measuring a second temperature of the formation at the selected depth level at a second time after heating the formation; analyzing the first and second temperatures to determine the time rate of temperature decay of the formation; and determining a thermal property of the formation using the time rate of temperature decay. 2. The method of claim 1 further comprising measuring the ambient temperature of the formation at the selected depth level. 3. The method of claim 1 wherein forming the temporary liner in the wellbore comprises melting the formation adjacent the wellbore with a laser beam. 4. The method of claim 1 wherein forming the temporary liner in the wellbore comprises heating a material that solidifies while cooling. 5. The method of claim 4 further comprising extruding the material onto a wall of the wellbore. 6. The method of claim 4 wherein the material is conveyed into the wellbore through a conduit. 7. The method of claim 4 wherein the material is stored in the drilling assembly disposed in the wellbore. 8. The method of claim 1 wherein heating the formation comprises disposing a heat source on a composite coiled tubing drillstring and powering the heat source via conductors disposed in the wall of the drillstring. 9. The method of claim 1 wherein heating the formation comprises disposing a heat source on a drillstring and powering the heat source via a wireline disposed through the drillstring. 10. The method of claim 1 wherein measuring the temperatures comprises spacing at least two temperature sensors axially with respect to the wellbore and moving each of the temperature sensors in sequence to the selected depth level. 11. The method of claim 1 wherein measuring the temperatures comprises positioning a temperature sensor at the selected depth level and maintaining the sensor at the selected depth level while measuring the first temperature and the second temperature. 12. The method of claim 10 wherein the at least two temperature sensors comprise a fiber optic element. 13. The method of claim 11 wherein the temperature sensor comprises a fiber optic element. 14. The method of claim 1 wherein the thermal property of the formation is selected from the group consisting of at least one of thermal conductivity, thermal diffusivity, and specific heat. 15. The method of claim 1 further comprising drilling to extend the wellbore while forming the temporary liner in the wellbore. 16. An apparatus for determining the thermal properties of a downhole formation and forming a temporary liner in a wellbore extending into the formation comprising: a coiled tubing drillstring extending into the wellbore; a drilling assembly supported by the drillstring, the drilling assembly comprising: a reservoir containing a liner material; an extruder that extrudes the liner material onto a wall of the wellbore; and a heat source that heats the liner material and the formation; at least one temperature sensor that measures a temperature of the formation associated with the heat from the heat source; and a valve for selectively delivering the liner material from the reservoir to the extruder. 17. The apparatus of claim 16 further comprising a wireline extending through a bore of the coiled tubing drillstring to supply power to the heat source. 18. The apparatus of claim 16 further comprising conductors disposed in a wall of the drillstring to supply power to the heat source. 19. The apparatus of claim 16 further comprising a processor that receives temperature signals from the at least one temperature sensor and derives an indication of thermal properties of the formation. 20. The apparatus of claim 19 further comprising a wireline extending through a bore of the coiled tubing drillstring to conduct temperature signals from the at least one temperature sensor to the processor. 21. The apparatus of claim 19 further comprising conductors disposed in a wall of the drillstring to conduct temperature signals from the at least one temperature sensor to the processor. 22. The apparatus of claim 16 further comprising a conduit extending through a bore of the coiled tubing drillstring to deliver the liner material to the extruder. 23. The apparatus of claim 16 wherein the drillstring comprises at least two flow bores. 24. The apparatus of claim 16 wherein the assembly further comprises a drill bit and a drill motor. 25. The apparatus of claim 16 wherein the heat source comprises a laser. 26. The apparatus of claim 16 wherein the heat source comprises a heat tip. 27. The apparatus of claim 16 wherein the at least one temperature sensor comprises a fiber optic element. 28. The apparatus of claim 16 wherein the at least one temperature sensor comprises at least two temperature sensors spatially separated along the assembly.