JRI Fireside chat with Scott Macmillan Tues. Nov 28 @ 9:00 Perth/12:00 Sydney, page-96

"Although a vertical well will probably help minimise further damage to coal ledges during wiper trips?"

If you have coals prone to washing out then a wiper trip can aggravate the situation mainly because you're usually circulating while trying to ream through the ledges associated with the coals. the reaming is an attempt to "smooth off" the ledges at bottom and top of the coals. Unfortunately while you're circulating youre blasting the coal with hydraulic horsepower from the bit and you'll often wash out the coal even more.