Living in Western Pennsylvania it's well known that coal mines, mushroom and storage caves stay at around 50-60 degrees F year round.
So there is a difference in temperature (as well as pressure I would guess) most of the year. Couldn't that difference in energy levels be used to generate power at some, admittedly low level?
I suppose that is the concept behind heat pumps that are used here with some degree of success. However I'm thinking much deeper, say 1000 - 10,000 feet or more.
IIRC the temperature goes up 1 degree for every hundred feet or so in depth.
Now with the fracking going on there are lots of deep holes being drilled. Could those holes serve double duty especially if the gas flow dwindles?
You rapidly reach a point of diminishing returns. To go that deep would be very expensive. However, it isn't necessary to go that deep to get some benefit.
As you mentioned, heat pumps can tap into geothermal heat much closer to the surface. In fact, one could envision it as annualizing the same heat, extracted during the summer, to warm his house in the winter. Heat pumps have to be powered, of course, and the usual method is to use electricity for the purpose. (I presume that it is technically feasible to power a heat pump using wind power, for instance, but I am not aware of anyone doing it on a commercial basis.)
In any case, the heat being delivered is not the heat of the energy used to operate the pump. It is the heat being extracted from the heat sink of choice. It is for this reason that geothermal storage of the heat at 50oF is preferable to attempting to wrest heat from the winter air at 20oF.
A surprisingly small "footprint" can now be used for geothermal heat extraction. Basically, one can drill several small holes, from six inches to two feet, and only a few dozen feet deep, to have all the thermal mass his heat pump could use.
Anyone who is considering upgrading his whole-house air conditioning, or replacing his furnace, should consider getting a heat pump for both air conditioning and house heating. (One can even use this type of operation to heat domestic hot water!)
Note also that the amount of natural geothermal heat is dependent somewhat on your latitude, how far from the equator you are. In southern states, just circulating a working fluid through a large geothermal mass can provide sufficient temperature conditioning summer and winter.
Oil wells are commonly drilled in areas where the source rocks have reached thermal maturity (a combination of temperature and pressure which causes the organics to generate oil in the rock). Hotter than that, you will get natural gas.
Likely they are not hot enough to generate the steam needed to run turbines.
Bottom hole temperatures in the Bakken are usually below 300 degrees Fahrenheit, for example.
At a shallower depth, it can be used to produce air temps in the high 50s to low 60s (F) via heat exchangers, circulating fluid in a ground loop. Depending on where you live, that alone can save a lot. (A local tribal center in ND cut their energy bills for heating and cooling in half, no mean feat for a large building complex in an area where winter routinely gets to -30F and summer can run in the triple digits.)