viable geothermal energy supply from exhausted fracked gas wells?
Viability of an oil or gas well for geothermal energy production is dependent upon several factors:
 - Geothermal heat profile with depth near the well 
 - Depth of the exhausted well
 - Volume of fluid acting as a heat exchange around and near the well.

I think it might be possible to use exhausted natural-gas wells will be suitable for re-use as a geothermal energy supply. Likelihood of success would require site investigation and the engineering pilot study.  This idea would be ideal for university engineering / geology research.