Page 123 - Petroleum Geology

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Fluid inflow into the borehole, or loss of mud to the formations, is detect-
ed by observing the level in the suction tank. Modern rigs record this level
automatically from a float, and an alarm bell is rung on the rig floor if the
level reaches pre-set limits. The physical protection against a blowout (which
may be defined as an uncontrolled flow of fluid through the borehole) is
provided by the casing and the surface equipment, consisting of blowout
preventers (BOPS). The BOP stack includes one that closes the open hole
(BOP with blind rams), one that closes around the drill pipe (pipe rams), and
one that combines these needs with an expandable rubber compound (Hydril).
The casing in a borehole drilled by rotary has a dual purpose: the first is
protection against caving (as with cable tool holes) and the second is for the
isolation of fluids by sealing the casing to the wall of the borehole.
The first string of casing is the conductor (Fig. 5-4), which is a large dia-
meter pipe that may be driven into the surface materials. The purpose of this
casing is to conduct the bit into the hole from below the rotary table, to
prevent the unconsolidated surface materials from collapsing into the hole,
and to return the mud through the shale-shaker to the tanks.
The surface casing is cemented into drilled hole, and also serves several func-
tions. It is chiefly a protection against caving of relatively unconsolidated
sediments at shallow depth, but also serves the important function of prevent-
ing contamination of fresh-water aquifers by the drilling mud. The depth at
which this casing is set, or landed, varies with local conditions but is usually
at least 100 my and may be set at about 10% of the planned total depth. Its
diameter depends on the depths scheduled for the well, and the size of the
anticipated production string (also called the oil string).
The protective casing is the casing on which the safety of the drilling oper-
ations will depend. It is therefore set in competent rocks, but not so deep
that any risk is run before it is set. It may be necessary to run and cement
two. protective strings before the oil or production string is run. In areas
where abnormal pressures occur at depth, it will be common to set the first
at a relatively shallow depth, followed by the second at the first kick or drill-
ing break (when the penetration rate increases significantly). Protective casing
is usually cemented to the surface. This may involve dual, or multiple stage
cementing in which the lower part is cemented as usual by displacing cement
from the casing to the annulus, and the upper part is cemented through a
sleeve in the casing that is opened by a plug pumped down inside the casing.
The geologist should note that while this procedure reduces the pressure
necessary to displace the cement, it does not reduce the pressure on bottom
due to the total column of liquid cement in the annulus. This may approach
the overburden pressure.
The production string serves the dual purpose of isolating the reservoir or
reservoirs to be produced from other reservoirs (petroleum or water), and of
providing easy access for the tubing, packers, etc., that will be necessary dur-
ing its producing life. The sealing function is of paramount importance. AS