The relief wells stop the pressure at the source, at the point the well enters the oil field. They are not dependent upon a leaking casing and riser to hold a column of mud.
How a relief well works
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OK, I see it better now. The idea is to reduce the flow from the damaged well, not stop it. Once the flow is reduced then they will have a better shot at capping it.
Still, seems like an awful lot of pressure to be relieved by just two other outlets. But hey, it’s hard to argue with success.