Page 365 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 341

3.2.1
Drilling Fluid [ ]

open cylinder. This pipe is called the vortex finder. As the mud moves
around and reapproaches the feed inlet, it is forced downwards (as the
top is closed) by more mud entering the chamber. The mud then fol-
lows a spiral path down along the cone, with its circular velocity
increasing due to the decreasing diameter of the cone. This imparts
high centrifugal forces on the mud, forcing solid particles to the out-
side of the mud stream and against the cone wall.
The volume of mud flowing down through the cone cannot exit at
the relatively small hole at the bottom. As mud is forced towards the
bottom of the cone, the pressure in the mud stream increases, forcing
the mud to turn back on itself, spiraling upwards inside the downward
stream (forced against the wall by centrifugal force). This mud exits at
the overflow pipe at the top.
In the balanced design the fluid changes direction just above the
opening. However, the solids, being heavier, cannot change direction
so readily and are forced to exit from the cone at the bottom. The cone
is adjusted by changing the size of the bottom opening.
Dissolved gas will reduce the efficiency of the hydrocyclones. The
degasser should be lined up to suck from the tank immediately
downstream of the sand trap and may discharge into the desander
suction tank.
Unlike shale shakers, the operational part of a hydrocyclone is hid-
den from view. When problems occur, they may pass unnoticed or even
be deliberately ignored by the crew assigned to the tanks and solids
control equipment during drilling. It is imperative that the drilling
supervisor is familiar with the operation of cyclones and knows how to
recognize and correct problems. Ideally, the rig contractor should take
steps to train drillers, ADs, and derrickmen to properly maintain this
equipment, since malfunctions can be as costly and unacceptable as
shale shaker problems. Figure 3-5 illustrates the process.
Hydrocyclones: types available. There are two types of hydrocy-
clone design: balanced and flood bottom. The flood bottom design has
a constant discharge at the bottom, the same size as the adjustable bot-
tom opening. Adjustment is a compromise between insufficient solids
removal and excessive mud loss. No modern rig should have flood bot-
tom hydrocyclones as primary solids removal equipment. The only
place they should be found is upstream of a decanting centrifuge,
where they are used for increasing the solids content of the mud sent

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