RE:Cement
Its been a while, but from what I recall. Holes are sometimes plugged with a cement plug so that the drilling rig can be moved off. The plug is later drilled out by the service rig, once the BOP stack is in place, and services such as perforating, fracking and or swabbing takes place. But like I said, this was 1984. The last time a set foot on a service rig.