Page 136 - Drilling Technology in Nontechnical Language

Chapter 5 – RIG SELECTION AND RIG EQUIPMENT 127

The active system tanks have some means of allowing mud into the
tank and also have a route out of the tank. As discussed earlier, drilling
mud usually consists of a liquid (water or oil, or both) within which are
suspended solids such as clays (for viscosity) and barite (for density

control). (Barite is a naturally occurring mineral, BaSO , of high specific
4
gravity [4.2] that is used as an additive in drilling fluids and cements to

increase density.) Other solids materials will enter the mud from the well
as drilling progresses. If the mud is allowed to stand in the tanks with little
or no movement, solids will start to settle on the bottom of the tank. To
avoid this, mud tanks are equipped with agitators. An agitator is simply a
large paddle attached to an electric motor. As the motor turns the agitator,
the mud is continually moved, and this reduces the tendency for solids to
settle on the bottom. Usually the corners of a square or rectangular tank
will still be relatively static, even with agitators in the tank, and solids will
often settle out here.
Settling of solids in the bottom of tanks is detrimental. It reduces the
effective volume of the tank. Solids settling will also reduce the density of
the mud, and this will require treatment of the mud to restore the density.
Settled solids can also plug pipes and valves in the bottom of the tank. At
some point, the tank has to be emptied, and the sludge on the bottom dug

out. It is better if agitation is sufficiently vigorous to reduce settling as
much as possible.
It is crucial that the driller knows how much mud is in the active system
and in particular is able to identify when the total volume increases or

decreases. An increase in mud volume may indicate that fluid is entering
the wellbore from a downhole formation (a kick); a decrease may mean
that mud is being lost downhole. In either case, action must be taken. Each
tank in the active system (as opposed to reserve tanks containing mud)
must have a means of accurately measuring the volume in that tank in such
a way that the driller can be alerted to any change in volume.
There are two common systems in use:

1. A float is attached to some kind of system that measures
its position relative to the bottom of the tank. As the tank
dimensions are known, the volume can be calculated from the

height from the bottom of the tank to the float (fig. 5–14).

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