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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.
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.```