The cement doesnt have to withstand pressure per say. They fill the hole with “mud” and it’s weight exceeds the upward pressure so nothing comes up. Then they follow up with cement which weighs similar to the “mud”, which isn’t mud at all. It’s over twice the density of water, barium something or other. Note, that if the upper part of the casing to rock seal is compromised, of if the the upper part of the casing fails in some way, then intercepting and killing might be the only solution. But the flow hasn’t been to high so erosion hopefully won’t be to big a deal.
fyi... the “mud” is mostly bentonite, or so I understand.
East, thanks for the clarification. It’s amazing that in one hole they can drill the bit, then surround it in a cement casing that allows drilling mud to flow upwards to the surface.