I'm not a drillsite geologist or engineer, but I believe that in...

I'm not a drillsite geologist or engineer, but I believe that in some wells drilling through shales, the hole collapses outward leaving a huge cavity. As the mud flows in under pressure, the hole just becomes larger and larger. Drilling mud is not a cheap commodity. I think normally a well is not cased unless there is a good chance of production. This just means that if the well is dry, it is going to be a much more expensive endeavor.