The reasons not to: Unconsolidated material, and a high risk of losing core, running a core barrel with a sleeve designed for unconsolidated material, which is a shorter core barrel, but has a better recovery rate in unconsolidated sediment (or highly fractured rock).
Or they figure the fractured rock will slide along the fracture planes while coring and jam the inner barrel, at which point one of two things happens--
You either continue to your full barrel depth and grind up the core from the point where you jammed the barrel...
... or You stop coring anyway, because the rock will not slide up the inner barrel and that supports the weight you are trying to put on the bit (which can no longer cut the rock below it).
In the first case you think you are coring, but you lost data instead.
In the second, you just aren't going any farther and end up tripping anyway.
I have seen both events in the years I have spent in the patch.
(And yes, I have seen a number of expensive screw-ups happen, too).
“You still get all the core, ideally.”
Oh - duh! (On my part!) I only worked a couple of summers in the oil patch - we never did any coring. The geologist would get grab samples as the mud came back up. I’m guessing we must have been drilling in a field that was pretty well figured out? (This was back in the early 80’s).
What do you mean by ‘tripping’?