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.