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Pieces of rock are scraped free by the rotating
drill bit. Drilling fluid is pumped down the drill
pipe, picks up these cuttings and brings them to the
surface.
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The harsh environment in underground drilling operations
encouraged the research and development of drilling fluids
that can fill several crucial roles in the drilling process:
suspension, pressure control, stabilization of formations,
buoyancy, lubrication and cooling.
The flow of drilling fluid down the drill pipe and up
the borehole sometimes stops, either because of a problem,
or in order to raise the drill pipe up and out of the hole
to allow the bit to be changed. When drilling stops the cuttings
suspended in the fluid can sink to the bottom of the hole
jamming the drill. Drilling fluids are designed to have a
very interesting property that takes care of this problem.
The thickness, or viscosity, of the fluid increases as movement
of the fluid slows. When the fluid stops moving it forms a
thick gel that suspends the rock cuttings and keeps them from
sinking to the bottom of the borehole. When the fluid starts
moving again it becomes thinner and reverts to its previous
thin, liquid form.
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At rest Visplex* is
a gel that supports the weight of the small rock shown
here.
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Visplex* has an interesting
and unusual property. It can change form - from thick
to thin. This allows the drilling fluid to suspend cuttings
when drilling stops.
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When stirred it becomes
more liquid and the rock sinks.
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There is a popular image of oil gushing from a rig, high
into the sky, while workers rejoice at having found oil. Actually,
such blowouts are rare and no cause for celebration since
the goal is to extract the oil in a controlled manner. Mud
is designed to prevent such accidents by counteracting the
natural pressure of fluids in rock formation. A proper balance
must be achieved in which the pressure of the drilling fluid
against the walls of the borehole is enough to counter the
pressure exerted by both rock formations and by oil or gas,
but not so much that it damages the well. If the weight of
the drilling fluid is too great it could cause the rock to
fracture and the drilling fluid would be lost into the ground.
The pressure of a liquid depends on its density. Weighting
agents may be added to the drilling fluid to increase its
density, and thus, the pressure it exerts on the walls of
the borehole. The density of the liquid may be adjusted to
meet the conditions in the well.
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There are two phases of the drilling process. At first
drilling proceeds through rock that does not contain oil.
The goal is to move as quickly as possible and get to the
oil-bearing rock - the reservoir. The priority is on keeping
the exposed rock formation in the borehole stable while avoiding
the loss of drilling fluid. By maintaining drilling fluid
pressure above rock formation pore fluid pressure, there is
a natural tendency for the drilling fluid to enter permeable
rock in the formation. With special additives in the drilling
fluids this can be prevented.
The drilling fluid may interact with the surrounding rock
in other ways. For example, if the rock is laden with salt,
water will dissolve the salt and tend to make the walls of
the borehole unstable. An oil-based fluid would be better
in this situation. Rock formations with a high clay content
also may tend to be washed away by water. Such formations
require an inhibitive fluid to maintain a stable wellbore
and prevent enlargement, or wash outs. As drilling progresses,
the wellbore is lined with a steel casing which is cemented
in place to provide both wellbore stability and a route to
the surface for oil when the reservoir is reached. After reaching
the reservoir the composition of the drilling fluid may have
to be changed to avoid clogging the pores of the rock. Keeping
the pores open will allow oil to flow more freely into the
borehole and up to the surface.
A well may be many thousands of feet or meters deep. A
steel drill pipe of such great length weighs many tons. Immersing
the drill pipe in fluid produces a buoyancy effect, reducing
its weight and putting less stress on the drilling mechanism.
When metal moves against rock there is friction and heat.
Drilling fluids provide lubrication and cooling to keep the
process moving along smoothly, and to extend the life of the
drill bit. Lubrication may be especially important on extended
reach or horizontal wells where the friction between the drill
pipe, drill bit and rock surfaces must be kept to a minimum.
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