A downhole tool system includes a downhole tool string configured to couple to a downhole conveyance that extends in a wellbore from a terranean surface through at least a portion of a subterranean zone, the subterranean zone including a geologic formation; and a heating device coupled with the downhole tool string, the heating device configured to transfer heat to the geologic formation in the wellbore at a specified temperature sufficient to adjust a quality of the geologic formation associated with a rock strength of the geologic formation.