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approximately 15,500 bbls of drilling fluids were lost during drilling, running
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casing and cementing operations.

F. Well Ballooning

Well ballooning is a common phenomenon in which the formation
absorbs drilling mud while the rig’s pumps are activated and then releases the
mud back into the well when the pumps are not active. Well ballooning is
significant because it can mimic a kick. Rig crews can therefore miss critical kick
indicators if they mistakenly believe that ballooning is occurring in the well.

Mud logging data for the Macondo well demonstrated that the production
casing zone started ballooning between 17,530 feet and 17,761 feet. The daily
IADC reports also show that the well flowed back during flow checks following
mud loss at those depths.

G. Planned and Actual Total Depth

In designing a well, engineers calculate a planned total depth of the well.
BP’s February 2009 Exploration Plan estimated the well depth at approximately
20,200 feet true vertical depth. Due to a narrowed drilling margin, BP ultimately
decided to stop drilling the well at 18,360 feet.

BP set a casing string at 17,168 feet measured depth (sometimes referred to
as “MD”) and then continued to drill the final production section of the well. In
the section below the last two casing strings, the well lost returns, indicating
possible fracturing or formation pressure regression. In response, the crew
stopped drilling, pumped in lost circulation materials to seal the fracture, and
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restored mud circulation. In this open‐hole interval (where no casing string
had been set yet) the pore pressure decreased from 14.5 ppg to 12.6 ppg between
the sand interval at 17,233 feet measured depth and the M56 target formation at
18,083 feet through 18,206 feet. Because of the decreasing pore pressure at this
depth of the well, there was no drilling margin in the open‐hole section of the
well, which meant that the mud weight necessary to prevent the formation from
flowing at the upper portion of the open hole could result in the formation
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fracturing in the lower section. BP explained to its partners, MOEX and

74 BP‐HZN‐MBI00304121.
75 BP‐HZN‐MBI00126338.
76 Id.

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