A Delicate Balance - page 295
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The Deepwater Horizon rig, of course, floated atop the water, and
the Macondo well sat at the bottom of the gulf. Between them a mile-
long connection called a riser pipe ran from the very top of the rig's
derrick, down through the drill floor and the middle of the rig, past
the lower decks, and into the water, all the way to the seafloor. Only
then, after the riser passed through a large safety valve device called
a blowout preventer, did it reach the top of the Macondo well. By
design, the well would be drilled from that point nearly three miles
farther into the earth in search of oil. To begin drilling a deepwater
well like Macondo, the crew first dangles a mile-long string of drill pipe
into the ocean to drop what they call a jet pipe-a 36-inch-diameter,
three-quarter-inch-thick heavy section of steel pipe - like a dart into
the silty ocean floor. The pipe, under the force of its own weight, sinks
three hundred feet into the mud, creating a starter wellbore in which
the drilling can safely begin without the soft mud walls collapsing in on
themselves. Once the jet pipe has hit solid rock, the drill string coming
from the rig's derrick, with a drill bit at the end and driven by the
torque of the engines way above on the floating platform, starts to burrow into the earth.
From here, a well is drilled in stages. First, a 36-inch-wide hole is
drilled for several thousand feet, and lined with a 36-inch pipe, called
a conductor casing. To fix the casing in place, cement is pumped
down into the well through the drill string into the bottom, where it
is forced out the sides and squeezed back upward, or circulated, into
the small space left between the raw dirt wall of the wellbore and the
conductor casing, sealing it in place against the earth. Then the process
is repeated with incrementally smaller sections of pipe the deeper
A Delicate Balance - page 296
the well is drilled. In the Macondo, BP would run a 28-inch casing to
6,200 feet, then a 22-inch casing to 7,900 feet, and so on, until the last
section of the well, which would wind up calling for a narrow, 7-inch-wide
pipe. The encased hole becomes the architectural structure of
the well, allowing another narrower string of pipe, called production
string, to be run from top to bottom, on the inside, to ferry the oil. In
between those telescoping casing layers and the production string is a
long open space-the well's main annulus.
One of the toughest parts of drilling a well in the Gulf of Mexico is
balancing the awesome difference in what is called hydrostatic pressure
between the inside of the well and the outside. This is where many
wells can go wrong. Far beneath the waters of the gulf and under the
immense weight of miles of rock piled on top, the natural pressure
from the earth constantly threatens to collapse the well, or force water,
oil, or gas trapped in pores in the rock into the wellbore. If hydrocarbons
under pressure hit the well-and if they find their way up through
either the drill pipe or the long open annulus-they are likely to rush
out toward the top in a violent kick, risking a blowout. To control
that dynamic, drilling companies use a heavy mud-a dirty mixture
of viscous synthetic fluids, polymers, oil, and chemicals with a lead-like
heft-to balance that pressure inside the well. The idea is to make
sure the outward pressure exerted by the mud constantly matches
the inward pressure exerted by the geologic formation. As long as the
two remain equal, there is balance and stability and the walls hold up.
The mud can be twice as dense as water, and twice the weight. It is
constantly circulated down the drill pipe, out of the drill bit, and then
flows back up the annular space between the drill pipe and the casing,
cooling the machinery, clearing away debris, and, most importantly,
maintaining the well's delicate balance of pressure.
But the earth, too, can be fragile. If the drilling mud gets too heavy,
the force of its weight can fracture the rock inside the well, risking a
collapse, or allowing the mud to seep out into the cracks it creates.
Since every drop of fluid that is pumped down into a well is circulated
back up through the annulus toward the surface, its volume is strictly
accounted for. When fluids are lost in a weakened well, the result is a
loss of circulation: a crack exists in the wellbore structure so significant
that some of the drilling mud has drained out. If that happens,
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the well can become unstable, again risking a blowout, or the loss of
the well. For a process relying on brute force and immense machinery,
it's an extraordinarily delicate balance that ultimately decides the fate
of a deepwater well.
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