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The shoe track was designed to prevent u‐tubing in two ways: (1) the
presence of 189 feet of cement as a barrier and (2) the float collar’s flow dual
flapper valves were designed to allow only one‐way flow after conversion.

The float collar dual flapper arrangement is designed to close after the
cement is in place (and starts setting up) to prevent any flow‐back into the casing
(and up the well) caused by hydrostatic pressure differences between the dense
cement and drilling mud on the outside of the casing and the less dense
displacement fluid on the inside. The float collar also acts as the landing point
for the cementing plugs used during the job. The float collar employed a
differential fill tube that allowed mud to flow into the casing as it was run into
the well. The fill tube in this case was designed to be pumped out of the float
collar if the pump rate was higher than five barrels per minute. The position of
the top of the float collar located at 18,115 feet placed the float collar across the
productive reservoir between 18,083 feet and 18,206 feet measured depth.

As described above, the crew had difficulty converting the float collar and
may not have achieved conversion despite making nine attempts. There are
three possible reasons for the failure of the float collar: (1) the high load
conditions required to establish circulation damaged the float collar; (2) the float
collar failed to convert due to insufficient flow rate; and (3) the check valves on
the float collar failed to seat due to damage, contamination, or the presence of
debris. None of these float collar failure scenarios excludes the possibility that
the cement could have failed due to defective cement design, contamination of
the cement by mud in the wellbore, commingling of cement with nitrogen due to
nitrogen breakout from the nitrified foam cement slurry, swapping of the shoe
track tail cement with the heavier mud in the rathole, a clogged reamer shoe that
possibly altered cement flow‐out of the reamer shoe, or some combination of
these factors.

The forensic examination of the BOP stack found interior erosion of the
blind shear rams, which supports this flow path as the most likely scenario. This
erosion detected on the blind shear rams likely resulted from the high pressure
flow of hydrocarbons past the rams as a result of the blowout and indicates that
hydrocarbons flowed up the well after entering through the shoe track.

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