BAKER HUGHES - Drilling Fluids Reference Manual

MECHANICAL SOLIDS CONTROL
• Improved filter cake quality (less coarse drilled solids) results in a less-permeable, less-porous cake.
Improved cake quality (thinner and tougher) can minimize wall sticking and reduce frictional forces
between drillstring and wellbore.
• A decreased concentration of drilled solids contributes to improved rheological properties and
reduced fluid maintenance costs.
Centrifuges
Centrifuges use centrifugal force, as do hydroclones, to shorten and/or control the settling time
required to separate solids from liquids. Centrifuges utilize an external force to rotate a separation
chamber and increase centrifugal force. This G-force can be calculated using the following
formula.
where,
G = G-force
G = 0.0000142(r 2 )(d)
r = revolutions per minute
d = inside diameter of the bowl in inches.
Practical application of the barite recovery centrifuge for processing drilling fluids considers the
following.
1. It is a common misconception that centrifuges separate low-gravity solids from barite, but this
would only be possible if all barite in a system was above 3 to 4 microns and all low-gravity
solids were below the 4 to 6 micron range. Under field operating conditions, approximately 10%
of barite is below 3 microns in size and 20% to 50% of low-gravity solids are below 6 microns
in size. The solids removed with a centrifuge are the very fine particles which have a greater
relative effect on rheology than the coarser particles.
2. Centrifuging will not eliminate the need for water. Water will still be required to replace that lost
through the centrifuge and to the wellbore. Dilution requirements will however, be reduced and
drilling fluid maintenance cost reduction can be expected. This is particularly true in areas where
reactive formation solids are prevalent.
3. Day-to-day trends in plastic viscosity can give an indication of how fast solids concentration is
increasing and can be used as a guide for operating the centrifuge and/or addition of water.
Bentonite content, as measured by Cationic Exchange Capacity (CEC) using the Methylene
Blue Test (MBT), and solids content, as determined by retort analysis, can also be of assistance
in determining the need for centrifuging and/or water additions.
There are basically three types of decanting centrifuges. The types are grouped depending on the
G-force, rpm, cut point, and feed capacity.
1. Barite Recovery Centrifuge – Used primarily for viscosity control. These centrifuges
operate in an rpm
range of 1600 to 1800 rpm and generate a G-force from 500 to 700
Gs. Cut point will be between 6 to 10 microns for low-gravity solids and 4 to 7 microns
BAKER HUGHES DRILLING FLUIDS
REVISION 2006 10-15
REFERENCE MANUAL