The measured pressure of the producing formation is about 11,800. The measured back pressure at the BOP prior to cutting off the riser was about 8,300 at sea floor level. After cutting off the riser kink the pressure at the BOP dropped to about 4,400.
One thing you can’t and won’t know is how big is the orifice or cross sectional area thru the BOP and the pinched riser pipe. Hence the BOP back pressure reduction.
I remember way back shortly after this started that the hole size for 5,000 barrels per day at the above mentioned 8,300 psi was 1/4”. That flow path is continuously being eroded by sand and is much bigger, hence the higher flow rates of today. The bigger the hole gets cut out by sand the higher the flow rates but with a reduction in back pressure.
The nile crocodle has a bite of around 5300 psi. I’ll stay with 20,000 to 70,000 psi.