Page 70 - Drilling Technology in Nontechnical Language

Chapter 3 – DRILLING A LAND EXPLORATION WELL 61

A hole that is bigger than the bit (called an overgauge hole) is
undesirable for several reasons, which will be discussed later. Also, to
force mud up the annulus takes pressure, and this pressure has to be
resisted by the wellbore. Circulating faster imposes more pressure on the
well. It is possible that a weaker rock could become fractured due to the

extra pressure imposed by circulating fast, leading to loss of drilling fluid
into the rock.

Drilling the rst hole section

After spudding the well, things happen very quickly. The pumps are
turning at full speed, all engines are running, and there is a lot of noise.
Large amounts of rock cuttings will come out of the well, and often the
capacity of the solids control equipment to separate out the solids from
the returning mud is exceeded. What happens then is that along with the
solids being ejected into the waste pit, a lot of mud will also go over the
side. Some of the equipment might become plugged up with drilled solids,

requiring fast action to get it fixed and back into service.
If drilling young clays that react quickly with water, the clay might
become sticky. The cuttings will start to accumulate into larger lumps, and
large balls of sticky clay will be pushed up the annulus. It can appear at
the top of the well as a column of doughnut-shaped clay (wrapped around
the drillstring; see fig. 3–14), and these are called clay rings. Sometimes

drilling must stop to clear up the problems before continuing. With clay

rings, people must get down under the drill floor with shovels to clear it.
Luckily, modern chemical knowledge is such that clay rings are largely
avoided by proper formulation of the mud.

After each 30 ft (9.1 m) drilled, another drill collar is added by screwing
it onto the top of the drill collars in the hole. When there are enough drill
collars to give all of the required weight on bit, drill pipe is added to the
drillstring, again in 30 ft (9.1m) lengths. (Each length of drill pipe is called
a joint.) Before drill pipe can be screwed onto the drill collars, a special
short length of pipe with a drill pipe connection on the top and a drill
collar connection on the bottom is added. These special short pipes are
called subs, and if a sub is used to convert one size or type of connector to
another, it is called a crossover sub. When the sub connects the drill bit to
the lowest drill collar, it is called a bit sub.

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