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the cost of a large one-step 
dilution is less than frequent small dilutions. The cost 
of dilution increases rapidly with mud density. 
An example arrangement of the solids control 
equipment for an unweighted clay/water mud is 
shown in Fig. 2.23. 5 The various components arc 
arranged in decreasing order of clay size removal to 
prevent clogging. Dilution water is introduced up-
stream of the hydrocyclones to increase their 
separation efficiency. Each device is arranged to 
prevent newly processed mud from cycling back to 
the input of the device. Chemical treatment normally 
is made downstream of all separation equipment. 
• 
DRILLING FLUIDS 
TABLE 2.9-RATED CUT POINT OF HYDROCYCLONES 
Hydrocyclone 
Size 
Rated 
Cut Point 
(microns) 
97% 
90% 
70% 
I 50% 
I 
30% I ~~J --~ero 
(in.) 
6 
4 
2 
40 
20 
10 
IN OVERFLOW 
• 3°/o 
• 10% 
• 30% 
• 50% 
• 70% 
• 95% 
• 100% 
CUT POINT 
IN UNDERFLOW 
SEPARATION DIAGRAM 
(Not to Scale) 
Fig. 2.21-Solids distribution and removal using 
hydrocyclones. 
Example 2.13 A mud cup is placed under one cone 
of a hydrocyclone being used to process an on-
weighted mud. Thirty seconds were required to 
collect 1 qt of ejected slurry. The density of the slurry 
was determined using a mud balance to be 17.4 
Ibm/gal. Compute the mass of solids and volume of 
water being ejected by the cone per hour. 
Solution. The density of the slurry ejected from the 
desilter can be expressed in terms of the volume 
fraction of low-specific-gravity solids by 
ms+mw - PsVs+PwVw 
p= =psfs +Pwfw· 
Vs + Vw Vs+ Vw 
Using the values given in Table 2. 7, the average 
density of low-specific-gravity solids is 21.7 Ibm/ gal 
59 
TABLE 2.1 0-COMMON DEFLOCCULANTS USED 
TO LOWER YIELD POINT AND GEL STRENGTH 
Approximate 
Deflocculant 
Phosphates 
Sodium acid 
pyrophosphate 
Sodium hexa· 
metaphosphate 
Sodium tetraphosphate 
Tetra sodium 
pyrophosphate 
Tannins 
Quebracho 
Alkaline tannate 
Hemlock tannin 
Des co 
Lignins 
Processed 
l1gnite 
Alkaline lignite 
Chrome lignite 
Lignosulfonates 
Calcium lignosulfonate 
Chrome lignosulfonate 
LL. 40 ~0 
:::i~ 
Ow 20 Cflu 
a: 
w 
0.. 
pH of Maximum 
Deflocculant Optimal Effective 
in a 10-wt% Mud Temperature 
Solution pH (oF) 
9.0 175 
4.8 
6.8 
7.5 
10.0 
11.5 250 
3.8 
300 
400 
4.8 
9.5 
10.0 
10.0 350 
7.2 
7.5 
0 20 40 60 80 100 120 140 
PARTICLE SIZE DIAMETER, Microns 
Fig. 2.22-Solids distribution and removal using 
hydrocyclones. 
and the density of water is 8.33 Ibm/gal. Substituting 
these values in the above equation yields 
17.4 = 21.7/5 + 8.33(1-/ 5 ). 
Solving for the volume fraction of solids gives 
17.4-8.33 fs = = 0.6784. 
21.7-8.33 
Since the slurry is being ejected at a rate of 1 qt/30 s, 
the mass rate of solids is 
0.6784 (1 qt) X ~ X 21.7 Ibm 
30 seconds 4 qt gal 
60 
O•lullon 
Screen Shaker Water 
Fig. 2.23- Schematic arrangement of solids-control equip· 
ment for unweighted mud systems.s 
3,600 seconds Ibm 
X =441.6-, 
hr hr 
and the volume rate of water ejected is 
1 qt (1.0-0.6784) x gal 
30 seconds 4 qt 
x 3,600 s/hr = 9.65 gal/hr. 
Note that to prevent the gradual loss of water from 
the mud, 9.65 gal of water must be added each hour 
to make up for the water ejected by this single cone. 
2.3.5 Chemical Additives. Unweighted clay/water 
muds are controlled primarily by removing inert 
solids, diluting, and adding bentonite when required 
to keep the active solids at the proper concentration. 
However, after using all available methods of solids 
control, one or more of the mud properties may be at 
an undesirable value and require selective ad-
justment. Chemical additives commonly are used for 
(1) pH control, (2) viscosity control, and (3) filtrate 
control. 
Caustic (NaOH) almost always is used to alter the 
mud pH. A high mud pH is desirable to suppress (1) 
corrosion rate, (2) hydrogen embrittlement, and (3) 
the solubility of Ca2 + and Mg2 +. In addition, the 
high pH is a favorable environment for many of the 
organic viscosity control additives. The pH of most 
muds is maintained between 9.5 and 10.5. An even 
higher pH may be used if H 2S is anticipated. 
Flocculation refers to a thickening of the mud due 
to edge-to-edge and edge-to-face associations of clay 
platelets. These arrangements of platelets are shown 
in Fig. 2.24. Flocculation is caused by unbalanced 
electrical charges on the edge and surface of the clay 
platelets. When the mud is allowed to remain static 
or is sheared at a very low rate, the positive and 
negative electrical charges of different clay platelets 
begin to link up to form a "house of cards" structure. 
The hydrated clay platelets normally have an 
excess of electrons and, thus, a net negative charge. 
AGGREGATION 
(FACE TO FACE) 
DISPERSION 
APPLIED DRILLING ENGINEERING 
FLOCCULATION 
(EDGE TO FACE) (EDGE TO EDGE) 
DE FLOCCULATION 
Fig. 2.24-Association of clay particles. 3 
Since like charges repel, this tends to keep the clay 
platelets dispersed. The local positive and negative 
charges on the edge of the clay platelets do not have a 
chance to link up. Anything that tends to overcome 
the repelling forces between clay platelets will in-
crease the tendency of a mud to flocculate. The 
common causes of flocculation are (I) a high active 
solids concentration, (2) a high electrolyte con-
centration, and (3) a high temperature. 
The concentration of flocculated particles in the 
mud is detected primarily by an abnormally high 
yield point and gel strength. At high shear rates, the 
"house of cards" structure is destroyed. The plastic 
viscosity, which describes fluid behavior at high 
shear rates, usually is not affected greatly by floc-
culation. 
The normal range of plastic viscosity for an on-
weighted mud is from 5 to 12 cp measured at l20°F. 
The yield point is descriptive of the low shear rates 
present in the annulus and greatly affects the cuttings 
carrying capacity and annular frictional pressure drop. 
A yield point in the range of 3 to 30 Ibm/ 100 sq ft often 
is considered acceptable for unweighted clay/water 
muds in large-diameter holes. This yield point range 
will enhance the ability of a mud to carry the cuttings 
to the surface without •. increasing the frictional 
pressure drop in the annulus enough to cause for-
mation fracture. The gel strength is descriptive of the 
mud behavior when the pump is stopped. The gel 
strength of the mud prevents settling of the solids 
during tripping operations. However, an excessive 
gel requires a large pump pressure to be applied to 
start the fluid moving and could cause formation 
fracture. A progressive gel increases with time and is 
less desirable than ajragile gel (Fig. 2.25). 
2.3.5.1 Dejlocculants. Deflocculants (thinners) are 
materials that will reduce the tendency of a mud to 
flocculate. The deflocculants are thought to 
render ineffective the positive charges located on the 
edge of the clay platelets and, thus, destroy the 
ability of the platelets to link together. A large 
number of deflocculants are available. The common 
types are listed in Table 2.10. None of the defloc-
culants are totally effective against all causes of 
I 
DRILLING FLUIDS 
:I: 
t-
<.!) 
z 
w 
a:: 
20 
15 
~ 10 
...J 
w 
<.!) 5 
10 20 30 40 50 60 
TIME, Minutes 
Fig. 2.25-Fragile and progressive gel strength. 3 
70 
flocculation. Since many of the deflocculants are 
acidic and only slightly soluble in the acid form, they 
must be used with caustic (NaOH) to increase the 
pH. 
Any of the deflocculants can be used to lower yield 
point and gel strength when flocculation is caused by 
excessive solids.