I don't think the hole deterioration problems were related to swelling clays....it was mechanical hole breakdown...I'm thinking borehole stress related.The Cass Cabora Basin sits in The East African Rift System..and active rift system with an associated regional stress regime.
SBM will help in getting a good hole drilled but if your drilling significantly over balanced then differential sticking is still a risk. See my previous comment about the main bore. Mechanical hole deterioration prevented the RCX getting down but IMO the tool/cable would have become differentially stuck if they had managed to get a working tool in the hole.
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