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D. Mud Program and Type

Drilling mud provides hydrostatic pressure ‐ pressure exerted by a fluid
at equilibrium due to the force of gravity ‐ to prevent formation fluids from
entering the wellbore. Drilling mud also keeps the drill bit cool and clean during
drilling, carries drilled cuttings out of the well and suspends the drill cuttings
whenever drilling is paused.

Operators have multiple options for the type of mud to use during the
drilling of a well, including oil‐based mud, synthetic oil‐based mud and water‐
based mud. Depending on reservoir conditions, operators assess which mud
type is most appropriate for the specific well being drilled.

For the Macondo well, BP chose a synthetic oil‐based invert mud system,
a system in which synthetic oil in the mud remains in a continuous fluid phase in
the lower hole sections of the well. BP chose this system for several reasons: (1)
to enhance the ability to maintain consistent fluid properties independent of the
temperature and pressure conditions of the well; (2) to improve hole cleaning
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and minimize barite sag, control of pressure spikes, and gaining gel strengths;
and (3) to deal with other problems that result when balancing equivalent
circulating density (“ECD”). In selecting the synthetic oil‐based mud, BP also
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sought to reduce fluid loss in order to minimize formation damage while
maintaining a higher drilling efficiency.

Synthetic oil‐based mud, such as the type BP used in the Macondo well,
has many positive features as described above. However, this type of mud also
presents risks relating to its effect on the crew’s ability to accurately detect
natural gas influx (kicks) into the well. A recent study of drilling fluid mixtures
and well control found that drillers might have a harder time detecting kicks

March 26, 2010; and BP’s Revised Application for Permit to Bypass, which included a production
casing, on April 15, 2010.
71 Society of Professional Engineers, SPE‐116013‐PA ‐ Study of the PVT Properties of Gas—Synthetic‐
Drilling‐Fluid Mixtures Applied to Well Control (2009). “Barite sag” is the settling of barite particles
(or other weighting materials), which can result in fluctuations in drilling fluid density. When
this occurs in drilling mud, the mud loses its integrity and can only be a temporary barrier in the
well. Gaining, or progressive, gel strengths typically require higher pump pressures to break
circulation.
72 ECD is the total effective pressure that a column of drilling mud exerts on a formation as the
mud is circulated through the drill string and back up the wellbore, accounting for frictional
forces throughout the circulatory system.

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