Solids control is the most important
function in mud treating. Control of mud properties centers around the
treatment necessary to counteract the continual influx of drilled solids into
the active mud system. Almost all of the costs of treating a mud can be
directly attributed to solids control.
Undesirable solids increase drilling
costs in a number of ways in addition to increased mud costs. If not properly
controlled, they lead directly to such problems as lost circulation. Differential
pressure sticking, and reduced penetration rate.
There are basically three mechanisms
for reducing the solids content of a drilling fluid. These are dilution,
screening, and settling or centrifuging. Dilution is accomplished by adding
liquid to reduce the concentration of solids in a given volume. Solids can also
be reduced by passing the mud over a screen and discarding those particles that
will not pass through the screen openings. Solids can be separated from a fluid
by settling or by imposed centrifugal force. This is accomplished in settling
pit, hydrocyclone, and centrifuge.
Our basic aim in solids control is to reduce the
formation solids content of a mud to a practical limit that will allow us to
maintain the filtration and flow properties in a range to best handle all of
the drilling problems. This means that we must pay special attention to control
of the particles less than 2 microns in size, since they are dominant in
determining the mud properties.
Unfortunately, the small particles are
the most difficult to remove. To reduce the concentration of these particles,
it is necessary to either dilute and build mud volume or to centrifuge the mud.
When centrifuging, these small particles are discarded along with a portion of
the liquid phase which contains a large portion of the chemicals in the mud.
Both dilution and centrifuging are expensive. Centrifuging is normally
considered to be less costly in muds weights above 12 lb./gal.
The problems and cost of removing the small particles makes it
imperative that we remove as much of the large particles as possible before
they are reduced to the fine size range. This is accomplished primarily by
screening weighted muds and by using desanders and desilters on unweighted muds. The
cost of controlling solids goes up as the mud density increases.