The tapered design of conventional wells is as old as the oil and gas industry itself. And despite its refinement over all that time, it still has some fundamental limitations.
Designing a well involves detailed engineering to make sure that the progressively narrowing diameter is still sufficiently wide when it reaches the reservoir to enable economic production rates.
Unexpected encounters with unstable formations can require an additional string of steel pipe, reducing the final hole size though the reservoir and restricting the production from the well or even preventing the well from reaching the target depth.