Without a circulating drilling fluid, rotary drilling would be
difficult if not impossible in some cases. A fluid is any
substance that flows, so drilling fluid may be either liquid, gas,
or a mixture of the two. If liquid, drilling fluid may be water, oil,
or a combination of water and oil. Operators often put special
substances (additives) in these liquids to give them characteristics that make it possible or easier to drill the hole. Most oilfield
workers call liquid drilling fluid drilling mud. A gaseous drilling
fluid may be (1) dry air or natural gas, (2) air or gas mixed with a
special foaming agent, which forms mist or foam or, (3) air or gas
mixed with liquid, which is an aerated drilling mud.
Drilling fluids were simple in the early days of rotary drilling;
operators usually just used whatever water was available. They
dug an open pit in the ground next to the rig and filled it with
water. If they wanted to stabilize the hole—that is, keep the hole
from caving in where it penetrated soft formations—they stirred
up the pit holding the water. Stirring the pits mixed the natural
clays in the soil with the water. The solid clay particles plastered
the sides of the hole with wall cake. This wall cake often prevented
soft formations from caving or sloughing (pronounced sluffing)
into the hole. Legend has it that the Hamil brothers, who successfully drilled the Spindletop well in 1901, ran cattle through their
water pits to stir up the clay. Whatever they did to make mud,
it worked. The solids in the natural clay formed a wall cake on a
troublesome formation that enabled the Hamils to successfully drill
it. The formation had thwarted several previous attempts when
the drillers merely used clear water as a drilling fluid.
Petroleum Extension-The University of Texas at Austin
DRILLING FLUIDS
Today, drilling fluid choices are complex. The development of
special types of fluids and additives to cure all sorts of downhole
problems brings its own difficulties. Research and field employees
of drilling fluid companies, often called mud engineers, must ask,
for example, what additive will best correct a particular drilling
problem? How will the mud react to changes in the formation? Will
a certain additive interfere with or cancel the effect of another? Will
the expense of disposing of a fluid with toxic additives outweigh
the benefits of using it?
A great deal of study, analysis, and expense go into a mud
program, which is the plan for the type and properties of drilling
fluid to use. A good mud program ensures that the physical
and chemical properties of the fluid are the best ones possible
for a particular drilling situation. Miscalculation can result in
unnecessary costs in time and money. Although the design and
maintenance of a mud program are the responsibility of the mud
engineer, all rig personnel will be better equipped to do their jobs
if they understand the basics of drilling fluids.