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below the fracture gradient, and the “swab” margin, which is typically 0.2 ppg
above the pore pressure. In short, the mud must be heavy enough to control the
pore pressure and ensure that the formation fluids (including hydrocarbons) do
not enter the wellbore, while not so heavy that it fractures the formation.
61

BOEMRE regulations require operators to provide various data to the
Agency demonstrating that the operator is maintaining a safe margin. For
example, operators must show, in a single graphic plot, pore pressures, mud
weights and fracture gradients for the full extent of the well. The plotted data
62
63
reflects forecasted data based on 3D seismic and offset well data. When
operators encounter unexpected pressures that differ from their forecasts,
applicable regulations require the operator to revise its casing design and apply
to the Agency for approval of the modification.
64

The final estimated Macondo well pore pressure, mud weight, and
fracture gradient plot submitted to MMS on March 26 is shown at Figure 1. The
vertical axis of Figure 1 shows the depth of the well. The horizontal axis shows
the mud weight. The red line depicts BP’s planned mud weight, which was
designed to be between the kick margin and the swab margin identified in
Figure 1.

61 Maintaining mud weight in this range is an industry‐accepted practice but is not specifically
required by BOEMRE regulations.
62
30 CFR §§ 250.413 and 414.
63 “Off‐set well data” are data obtained from wells that are drilled in an area close to the target
well.
64
30 CFR § 250.427(b).

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