I'm glad everyone got out OK.
Do you know what caused this?
Yes, I know what caused it, but it's too simplistic to say, "human error".
When we cement casing in the well, we usually pick up the Blow Out Preventors to set the slips on the casing to hold it in place.
When we do this the well is wide open to the world.
We count on the cement to hold any gas downhole, but if the cement is lighter than the bottom hole pressure, the well will come in.
The way to avoid this is to wait longer on the cement to set and monitor any gas migration to surface.
Obviously, this wasn't done.
Could have been a FNG.