BAKER HUGHES - Drilling Fluids Reference Manual

MECHANICAL SOLIDS CONTROL
• Specific Gravity (S.G.) – Solids in drilling fluids can be separated essentially into two density groups,
high and low specific gravities. High specific gravity will refer to those solids with a specific gravity
of 4.2 and above. High specific gravity materials are used for drilling fluid density increases, the
most commonly used of which are barite (4.2 s.g.), and hematite (5.0 s.g.). Low specific gravity
solids may range from a low of 1.6 to 2.9 for dense lime. Drilled solids have a specific gravity in the
range of 2.1 to 2.8. Most solids analysis calculations use 2.6 as the assumed S.G. of all drilled solids.
• In a drilling fluid containing only low-gravity solids and fresh water, the concentration of solids will
be a function of fluid density. The same relationship exists if a drilling fluid is composed only of
barite and water. If a drilling fluid contains low-gravity and high-gravity solids, then the solids
content will vary between the two ranges at a particular density. Figure 10-1 illustrates the effect of
specific gravity and solids concentration on fluid density.
Figure 10 - 1 Average Solids Range of Water-Base Drilling Fluids (Freshwater)
• Particle Size – Fluid solids are measured in microns because of their small size. A micron (µ) is a unit
of measure in the metric system and is 1 / 1000 of a millimeter. To better illustrate the relative size of a
micron, there are 25,400 microns to the inch. API classification of solids by size range is listed in
Table 10-1. Commercial clays such as MILGEL ® contain predominantly particles which are smaller
than 2 microns, or colloidal in size.
Particle
Size, microns
Greater than 2000
2000 to 250
250 to 74
74 to 44
44 to 2
Less than 2
Particle
Classification
Coarse
Intermediate
Medium
Fine
Ultra-Fine
Colloidal
Table 10 - 1 API Classification by Particle Size
Sieve
Size
10
60
200
325
400
–
Even though drilled solids initially may be relatively coarse, they rapidly disintegrate into smaller
particles due to chemical dispersion and mechanical action of the bit and drillstring. The rate of
disintegration will vary with formation, type of drilling fluid, bit exposure time, annular transport
BAKER HUGHES DRILLING FLUIDS
REVISION 2006 10-2
REFERENCE MANUAL