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drilling rate, or rate of penetration. Air or gas, because they are so light in density,
allow bit cuttings to move rapidly away from the bit. With few cuttings getting in the
way of the bit cutters, the cutters always contact fresh, uncut formation.
With such an advantage, you would think that companies would drill every
hole with air or gas. Sadly, drilling with air or gas as a circulating fluid has a major
drawback. Water in a formation can enter the hole, wet the cuttings, and cause them
to ball up. If enough water enters, the balled up cuttings clog the hole and prevent
circulation. Without circulation, drilling stops. Unfortunately, most subsurface
formations contain ample amounts of water; consequently, air and gas drilling has
limited use.
Whether gaseous or liquid, drilling fluid plays several vital roles in rotary
drilling. It raises cuttings made by the bit to the surface. It also cools and lubricates
the rotating drill stem and bit. Moreover, drilling mud keeps underground pressure in
check. A hole full of drilling mud exerts pressure, just as a swimming pool full of
water exerts pressure. Mud pressure in the borehole offsets the pressure in a
formation.
The heavier, or denser, a mud is, the more pressure it exerts. Water or oil by
itself often does not weigh enough to exert the necessary pressure, especially as the
hole gets deep. A gallon of fresh water, for instance, only weighs about 8V3 pounds.
(A cubic metre of water weighs about 1,000 kilograms.) To make water or oil exert
the correct amount of pressure – not too little and not too much – the operator has the
derrickman add weighting material. A mineral called “barite” is a popular weighting
material. It is over four times heavier than water. Barite is supplied to the rig as a fine
powder, and the derrickman gradually adds it to the mud s water or oil. The mud
suspends the powdered barite uniformly throughout the hole.
At the rig, you may hear personnel talk about mud weight,” which is another
way of saying mud density. As mentioned earlier, mud weight is important because it
indicates how much pressure the mud exerts to hold formation pressure in check. In
most of the U.S., oilfield hands measure mud weight in pounds per gallon. California
is different, of course; there, they measure it in pounds per cubic foot.
In Canada, and many other countries, they measure it in kilograms per cubic

metre. So a mud that weighs 10 pounds per gallon also weighs 74.8 pounds per
cubic foot or 1,198.2 kilograms per cubic metre.
With instructions from the operator and possibly a drilling fluids engineer, the
derrickman also adds special clay to the water or oil. This clay, when thoroughly
mixed into the mud, keeps the cuttings in suspension as they move up the hole.
When the driller stops pumping the mud for any reason, the clay makes the mud
temporarily gel. The gelled mud keeps the cuttings suspended even when the mud is
not moving. When the driller breaks circulation (starts pumping again), the mud
liquefies, or ungels, to move up the hole.
As mentioned before, particles of clay also line the wall of the hole, much as
plaster sticks to the wall of a room. The clay solids form a thin but strong lining,
orwallcake, that stabilizes the hole and keeps it from caving in, or sloughing. Wall
cake may be only a few thirty-seconds of an inch (a few millimetres) thick, but the

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