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Drilling ahead

Once the drill stem is back in the hole, the bit drills out the cement remaining in
the lower part of the casing. It also drills out the guide shoe and begins drilling into the
formation below the casing. From this point, the rig and crew may drill the hole to a
formation that contains oil and gas. Or they may drill to another predetermined depth
and temporarily stop. Whether they drill to final depth below the surface casing or to
an intermediate depth depends on the nature of the formations the bit drills.
Some wells, especially deep ones, usually encounter formations that are easily
controlled by using a suitable drilling fluid. Later, however, as the borehole drills into a
deeper oil and gas formation, the drilling fluid used to control the upper zones is not
suitable for the productive formation. The drilling fluid could damage the producing
zone so badly that the operator could not withdraw the hydrocarbons from the zone. To
avoid such a pitfall, the operator plans the well to be drilled to an intermediate depth
above the pay zone (the productive formation). The drilling crew uses drilling fluid
formulated to control the formations to the intermediate depth. Then, they stop
drilling, come out of the hole (pull the bit and drill stem from the well), and run and cement
casing. The casing and cement seal off the intermediate part of the hole so that the
formations neither affect nor are affected by subsequent drilling operations.
When drilling the part of the hole below the surface casing, crew members
probably won't make connections as frequently as they did when drilling the surface
hole. Deeper formations tend to be hard and drilling is thus slower. Usually, at some
point, the bit dulls and crew members have to replace it. To do so, they make a round
trip. They trip out the dull bit and drill string and install a new bit. Then they trip in the
new bit, the drill collars, and the drill pipe and resume drilling. Several round trips may
be necessary before the hole reaches an intermediate or final depth.

Running and cementing intermediate casing

At this point, let's assume that the crew on our example well has drilled an
intermediate hole. The operator then calls in a casing crew and cement company to run
and cement casing through the surface casing and to the bottom of the intermediate hole.
This casing string is the intermediate string.
The crew drilled the intermediate hole of our model well with a 311.2-millimetre
bit, so the intermediate casing must fit inside this hole and leave room for cement. A
couple of casing sizes are available, but let's say our operator picks one with an outside
5
diameter of /8 inches (244.5 millimetres). The crews run and cement this intermediate
string of casing with the same equipment and techniques they used to run surface casing.
The operator may, however, opt to run an intermediate liner instead of casing. A
liner is the same as casing except that it is shorter. A casing string runs from the bottom of
the hole all the way to the surface. A liner string, on the other hand, runs from the
bottom of the hole but stops a short distance inside the casing string above it. If crew
members run an intermediate liner, they hang it in the bottom of the surface casing. The
cementing crew then cements it back into the surface casing. Because they are shorter,

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