There is an oil field outside my hometown. I wonder if it is feasible to use the same oil wells for geothermal production after oil production rate drops below profitable point.
In most cases, probably not.
Oil could be considered a metamorphic mineral, formed by "gentle" heating. That is gently on a geological scale - still enough to burn your hands!
Geothermal systems work most efficiently with a large temperature difference (Third Law of Thermodynamics). As soon as the temperatures increase enough to be interesting from a geothermal perspective, you are quickly losing ("overcooking") the oil on a geological timescale.
Therefore most oil fields have had the heat applied in the past and it has since been removed. However this does not discount the situation where an oil field is still technically "being made" and the hydrocarbons are still being heated, and we just happened to drill it at just the right time. Hence the "in most cases": You might find the odd field where it is practical.
An old field, especially gas ones, could be used for carbon sequestration though. The field has stored methane for 10s if not 100s million years, why not the heavier CO2 instead?
Geothermal reservoirs are very different from hydrocarbon reservoirs. A geothermal reservoir is made of highly fractured igneous/metamorphic rocks which have low intrinsic permeability. Fluids get heated as they flow through fractures to temperatures well above the boiling point of water. The reason they flow is because of thermal gradients which causes convection.
Oil/gas reservoirs are contained in sedimentary rocks which have much higher permeabilities and relatively lower temperatures. Also hydrocardon reservoirs in most cases are confined. In theory you can utilize some of the heat in the depleted reservoir by injecting cold water and utilizing the hot water as done in an enhanced geothermal system or EGS. But it wont be efficient as the reservoir will loose heat quickly. And since the fluids extracted will still contain some hydrocarbons you have the additional headache of disposing them properly.
Note: I am talking about geothermal reservoirs of the kind you see in Western US (e.g., The Geysers), Iceland, Philippines, etc.
Do you know the (TVD) true vertical depth of these wells. Are you certain the wells are tapped into an oil formation. Oil wells deplete in the production sense but the mess still remains as well over time oil will seep back into the fractured zone. The clean up and maintenance of equipment would be great. There may also be the presence of other toxic gases such as h2s. Think of a oil spill on land or ocean and the coast of cleaning it up. In my opinion I be very impractical and coastly. In most cases impossible because of the differences in formations