Page 68 - Drilling Technology in Nontechnical Language

Chapter 3 – DRILLING A LAND EXPLORATION WELL 59

To drill rock, a drill bit requires a downward force to be applied, to
force the teeth to penetrate into the rock. This force is provided by the
weight of thick-walled pipe, called drill collars, which are screwed on top
of the drill bit. With a large drill bit used to spud the well (perhaps a 23"
[584 mm] or 26" [660 mm] diameter bit inside 30" [762 mm] conductor
pipe), these drill collars might be 9½" (241 mm) outside diameter and 3" (76
mm) inside diameter steel pipe. Each foot of this particular drill collar will
weigh around 200 lb (91 kg), and drill collars come in lengths of around
30 ft (9 m). Thus a single drill collar will weigh around 6,000 lb (2,721 kg).
As the bit drills deeper, more drill collars are added on top, giving more
available weight with which to drill.
A drill bit also must be turned, in addition to having weight applied.

With sufficient “weight on bit” and rotary speed, the bit will drill rock.
Stronger rock requires greater weight on bit to be applied, so that the
pressure exerted by each tooth is greater than the compressive strength of
the rock. Close to the surface, rock is usually fairly soft and easily drilled,
so having low weight available is not a problem.

This fluid will most often be water with various chemicals mixed in,

but it may also be an oil-based fluid, a mixture of oil and water, or in some
areas where conditions permit, it might be compressed air or a mixture
of water and air (which produces a foam). The fluid used to drill the top

section of the well is called spud mud because it is the drilling mud used in
spudding the well. Spud mud is usually water that has clays and polymers
added to make it thicker (more viscous).

The speed at which the fluid moves up the annulus is measured in
feet per minute (or meters per minute on a metric operation). This speed
measurement is called the annular velocity (AV). To lift cuttings upwards,
depending on how thick or viscous the mud is and also depending on its
density, needs a minimum AV of about 50 ft/min (15 m/min). An AV

greater than 100 ft/min (30 m/min) is preferred to efficiently clean the hole.
The volume of mud to pump each minute to give any particular AV can
be easily calculated. The larger the hole size, the more volume per minute
must be pumped (fig. 3–13).

For a hole of 26" (660 mm) diameter (D) and using 5" (127 mm)
diameter drill pipe (d) inside, the annular volume equals 0.0408
(D – d ) = 26.6 gallons per foot; or 0.7854 (D – d ) = 329 liters
2
2
2
2
per meter). To achieve an AV of 50 feet per minute (FPM) (15 m/min),

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