The ability to get the wellhead or get into the wellbore is a challenge because of some physical attribute related to the environment.
Injected or produced elements such as sand, wax, hydrates, cement etc. are causing reduced or blocked production or injection.
Extended reach or highly deviated wells often cause difficulty getting the toolstring or payload to the toe of the well. Sometimes for wells with restrictions or changes in upset this can also be a challenge.
The hardware installed in the well, either originally or during a workover, is not working properly and requires modification or removal.
The well requires a mechanical solution to fish out dropped or stuck toolstrings, recover other items, cut the pipe / completion, set or remove a plug in the well.
The wells are not meeting expectations regarding safety, cost, construction, efficiency, production or recovery factor.