I don't think you understand the geology of oil wells or geothermal wells.  They are drilled into very different formations underground.  You don't just convert old wells.  I also wonder if you realize how dirty of a process producing geothermal energy is.  The hot water that comes up from underground that is used is some pretty nasty stuff that produces alot of waste that is very similar to oil and gas waste.  Lots of salt water brine with some nasty stuff in it.

That being said it can still be a viable energy source under the right conditions.