The problem is they’re having to go into the side of the BOP with the top some what open and the bottom pushing tons of fluid. They have to seal the top section before they can pump back down hole otherwise the cement will follow the path of least resistance. What they need is a 12 inch frac ball, to bad nobody makes one.
Do frac balls expand? I thought the traveled down the pipe passing through the larger choke points until they found their right sized choke point and thereby created the high pressure point for the fracture.