bol1986

Bentonite Quality and
Quality-Evaluation Methods
G.M. Bol, Koninklijke/Shell E&P Laboratorium
Summary. Problems with bentonite quality and bentonite-quality-control methods have consistently been observed in the field in recent years. These problems are investigated and proposals are made on how to mitigate
them. A number of commercial bentonites have been analyzed and their performances evaluated (1) to determine
to what extent quality variations occur between commercial bentonites from different manufacturers, (2) to
determine the cause(s) of these variations, and (3) to develop a proper quality-control method for bentonite.
The results show significant variations in quality between Oil Companies Materials Assn. (OCMA) and APIgrade bentonite. The main cause of these variations is the different type and amount of extending chemical
.
added.
Differences in rheology and fluid loss between bentonite suspensions in fresh water, seawater, and hydrogenperoxide (H 20 2) solutions give a quantitative measure of bentonite extension. Differences in performance
before and after hot-rolling do the same.
Introduction
Commercial bentonite (sodium montmorillonite) is used
worldwide as a drilling-fluid additive. Its main functions
are to viscosify the mud and to reduce fluid loss to the
formation. We have noticed a general decline in bentonite
. quality over the past years that is reflected not so much
in noncompliance with OCMA or API specifications, but
rather in an unpredictable performance in a number of
applications: (1) bentonite properties change after longterm storage; (2) addition of bentonite to potassiumchloride (KCI)/polymer muds is ineffective in building
viscosity; (3) flocculation of cement slurries occurs in
cases where bentonite is used as an extender; (4) properties of bentonites change significantly from one batch to
another; and (5) poor and/or unpredictable performance
of bentonite has been reported for drilling under harsh
conditions.
As a result of these observations, an investigation was
started to determine whether significant quality variations
occur between commercial bentonites and, if so, to determine their nature, and to develop a proper bentonitequality-control method.
Bentonite Quality and Present
Quality-Evaluation Methods
Bentonite Quality. Bentonite quality is determined mainly
by four parameters: (1) the content of materials other than
montmorillonite; (2) the type of counter-ion(s) present on
the montmorillonite platelets; (3) the presence or absence
of small amounts of extending polymers (about 0.025 to
1.0 wt%); and (4) the size and charge of the montmorillonite platelets (depending on the source).
Pure bentonite should contain only montmorillonite. In
practice, because of the declining reserves of high-quality
bentonite, other materials-such as illites, kaolinites,
chlorites (all clays), quartz, and feldspar-are usually
Copyright 1986 Society of Petroleum Engineers
288
present. Because montmorillonitic clays have the highest
swelling capacity (which is responsible for viscosity buildup and formation of a low-permeability filter cake), the
presence of the other materials will have an adverse effect on bentonite quality.
Charge deficiencies in the montmorillonite crystallattice, caused by the exchange of A1 3 + for Si 4 + and
Mg2+ for Al 3+ , result in a negative surface charge on
the clay platelets. This negative charge is compensated
for by positive counter-ions. The type of counter-ions has
a great effect on the swelling capacity of the montmorillonite. By far the best performance is obtained with sodium montmorillonite (sodium as counter-ion). If the
montmorillonite contains counter-ions other than sodium,
the swelling properties, and thus the viscosity buildup and
filter-cake permeability, will be adversely affected.
If the mineral composition of a bentonite is such that
its viscosifying power is insufficient, an extender can be
added. The extender can be either a salt or a polymer,
and it enhances viscosity buildup by slightly flocculating
the bentonite suspension. 1 Sodium carbonate is an example of a salt that can be added as an extender. In some
cases, it is already present in the bentonite because it can
also be used for pretreatment. Soaking raw bentonite with
a solution of a sodium salt results, through ion exchange,
in a higher sodium montmorillonite content.
High-molecular-weight, linear polymers are generally
far more effective extenders than inorganic salts. Examples of polymers used for this purpose-polyanionicsare copolymers of vinyl acetate and maleic anhydride,
acrylic polymers, and hydrolyzed polyacrylamides.
Variation.s in charge and size of the montmorillonite
platelets will occur, depending on their source. These variations affect the swelling characteristics of the material
and thus the qUality. The thickness of the montmorillonite
platelets varies between 1 and 8 nm and the surface diameter from 0.01 to 0.1 /Lm.
SPE Drilling Engineering, August 1986
TABLE 1-COMPARISON OF API AND OCMA SPECIFICATIONS
Property
Fann-600 reading (22.5-lbm/bbl [64.2-kg/m 3] slurry)'
YP (22.5-lbm/bbl [64.2-kg/m 3] slurry)
Yield of 15-cp (apparent viscosity) slurry
API filtrate (22.5-lbm/bbl [64.2-kg/m 3] slurry)
API filtrate (26.3-lbm/bbl [75.0-kg/m 3 ] slurry)
Wet screen analysis
Residue on U.S. Sieve No. 200
Residue on 100-mesh screen
Moisture content
API Requirement
~30 Ibf/100 ft
::;:;3 PV
OCMA Requirement
2
::;:; 15.0 mL
::;:;15.0 mL
::;:;2.5 wt%
::;:;2.0 wt%
::;:;15 wt%
::;:;4.0 wt%
::;:;1Owt%
'Fann-600 reading = Fann-35 viscometer reading at 600 rev/min.
A measure for the surface charge is given by the cationexchange capacity (CEq-the amount of counter-ions on
the platelets. Values can range from 80 to 150 meqllOO g.
Disadvantages of Extenders. In view of the above, it is
clear that only two simple methods are available for increasing bentonite quality: ion exchange and extender addition. The first method has no drawbacks if the bentonite
is properly washed after the treatment. The addition of
an extender, however, can give rise to unexpected bentonite performance, in particular when polymeric extenders (polyanionics) have been added. The long-term
performance of the extended material will certainly not
be as good as that of the pure, high-quality bentonite. Under severe drilling conditions (high temperatures, presence of bivalent cations, high bacterial activity, etc.), the
extended bentonite can show an unexpected deterioration
in quality owing to polymer breakdown or polymer incompatibility with mud/cement chemicals or formation
materials. Additional chemical treatment will be required,
resulting in extra costs and/or drilling problems.
Another type of problem that can occur with extended
bentonite is the "over-the-hump" phenomenon. Small
amounts of high-molecular-weight poly anionics can cause
a marked increase in (particularly) yield point (YP). There
is, however, an optimum polymer concentration, 2,3 and
if this concentration is exceeded, the YP will decrease
again. The Appendix gives a brief description of this phenomenon.
In a mud system, if an extended bentonite is used
together with a polymer similar to the extender, the optimum concentration (which is generally fairly low) will
be surpassed, resulting in a significant reduction of (in
particular) the YP. This phenomenon has been observed
repeatedly in our polymer mud systems (KCl/polyacrylamide mud). A third problem that may arise is unwanted
flocculation. An extender is essentially a flocculating polymer. Sodium montmorillonite is so strongly hydrated that
no distinct flocs will be formed; however, other solids
(e.g., weighting materials) might easily form unwanted
distinct aggregates. Finally, addition of extended bentonite
to cement may lead to unacceptably high slurry viscosities.
Current Quality-Evaluation Methods. General2uality
standards for bentonite are laid down in OCMA * and
API 5 specifications. Table 1 summarizes the OCMA and
API requirements. Both the OCMA and API methods include only tests at room temperature on suspensions in
distilled water. Under such mild conditions, extending
SPE Drilling Engineering, August 1986
TABLE 2-CEC OF SOME COMMON CLAY MINERALS
CEC
Clay Type
Montmorillonite
Illite
Kaolinite
Chlorite
(meq/100 g)
70
10
3
10
to
to
to
to
130
40
15
40
polymers will certainly be stable. The presence or absence
of extenders thus will not show up directly from the test
results. The API specification does contain the requirement that the ratio of YP and plastic viscosity (PV) should
be lower than 3. This does represent an indirect control
on extension of bentonite.
Test Program and Procedures
Various commercially available bentonites have been tested, including a nonoilfield product (Sample F). This product, used in wine clarification, was included because it
is a high-purity-grade bentonite, not intended to be used
for viscosity development and thus, most probably, does
not contain any extending chemicals. Neither of the specifications of the bentonites mentioned the presence of any
extending chemical.
The bentonites were subjected to the standard OCMA
tests, modified OCMA measurements with suspensions
of prehydrated bentonite in seawater, rheology and fluidloss measurements before and after hot-rolling, rheology
measurements on suspensions of bentonite in H 20 2 solutions, CEC measurements, API methylene blue tests
(MBT's), clay-content analyses, mineralogical analyses
of bulk and clay fractions, and free-water tests.
OCMA Tests. The following OCMA measurements were
carried out: (1) the amount of bentonite slurry with an
apparent viscosity of 15 cp [15 mPa' s] after 24 hours'
aging that can be obtained from 1 t [1 Mg] of material
(yield in cubic meters per tonne), (2) the API fluid loss
of 26-lbm/bbl [74-kg/m3] bentonite suspensions; and
(3) the bentonite's moisture content.
Suspensions were prepared with distilled water. No
caustic was added-i.e., the suspensions had a neutral pH.
Sieve analyses were not carried out; the clay fraction was
determined by sedimentation analysis.
'The OCMA has been replaced by the Engineering Equipment and Materials Users
Organization (EEMUA). The EEMUA now cooperates with an API task group on establishing fluid and cement materials and on reviewing material specifications for the
European market, concentrating mainly on North Sea operations.
289
AV (c P)
r40
35
30
25
20
i5
iO
5
0
6
8
9
10
H
12
13 14
-15
H 2O 2 solution is almost the same as that of water; therefore, no effect on mud viscosity is expected. The reaction products (H 20, O 2 , CO 2 , and partially oxidized
polymer), however, could have an indirect effect on viscosity by changing the pH. Hence the pH was monitored
during the oxidation experiments.
For comparison, rheologies were measured in solutions
of a pure bentonite extender (commercially available) in
water and in 3 and 10 % H 2 0 2. The chemical structure
of the commercial bentonite extender is not known.
Because it is difficult to prepare bentonite slurries
reproducibly, special precautions were taken. Test samples were always taken from a bulk stock and then diluted either with distilled water or 30% H 20 2 solutions.
In this way, differences can never originate from differences in preparation of the initial bentonite suspensions.
_pH
Fig. 1-API apparent viscosity of a bentonite suspension
as a function of pH (room temperature).
Modified OCMA Tests With Seawater. Bentonites are
often used in seawater-based muds. Thus the various products were also evaluated in seawater after prehydration
in fresh water. The seawater used in the test was treated
with a mixture of caustic (sodium hydroxide) and soda
ash (sodium carbonate) to precipitate calcium and magnesium ions, as is common practice in the field. Because
of this treatment, the pH was approximately 10 in these
tests. OCMA yield and fluid loss were determined for
these seawater suspensions.
Hot-Rolling Tests. To simulate the circulation of
bentonite-based muds in deep, hot holes, suspensions of
24.5 lbm/bbl [70 kg/m3] were hot-rolled for 16 hours at
338 to 347°F [170 to 175°C]. Rheology measurements
were carried out before and after hot-rolling. No caustic
soda was added.
H 2 0 2 Oxidation Tests. To break down extending polymers, suspensions were prepared in 3% H 20 2 solutions.
Oxidation of the polymers results in suspensions with viscosities that are attributable only to the clays. Rheology
measurements are then able to give a measure of the actual clay quality. The bentonite concentrations were 24.5
lbm/bbl [70 kg/m3].
H202 attacking the clays is not a problem because the
clays are already fully oxidized. The viscosity of the
CEC Tests. Table 2 lists the average CEC of various
clays. \,2 A 100% pure bentonite should have a CEC
value about the same as that of montmorillonite. The pres
ence of other clays or nonclay material decreases the CEC,
and hence this can give an indication of bentonite purity.
The CEC is measured by conductometrical titration after
barium exchange. The CEC does not give information on
the type of counter-ions on the clay.
API MBT's. In the field, the API MBT6 is used to estimate the CEC. In this test, the adsorption of the dye onto
the clay is measured and correlated to the CEC or to the
specific surface of the clay.
Clay-Content Analyses. Clays are generally defined as
particles smaller than 2 /-tm. On the basis of this definition, the clay content can be determined by sedimentation analysis. The result of this test again gives an
indication of any contamination of the bentonite, and especially of the amount of silt and sand present.
Mineralogical Analyses. With X-ray diffraction, semiquantitative analyses can be carried out to determine the
types of clay present in the clay fraction and the types
of material other than clay present in the bulk.
Free-Water Content of the Suspensions. A good-quality
bentonite suspension should not settle out. The presence
of free water after 24 hours' settling indicates the pres-
TABLE 3-RESULTS OF OCMA TESTS IN DISTILLED WATER AND SEAWATER
Bentonite
Sample
A
B
C
D
E
F
G
H
OCMA Standard
Moisture Content
(wt%)
12.0
12.0
12.5
7.5
7.4
6.7
9.2
8.7
< 15.0
Distilled Water
Yield
Fluid Loss
(m 3 /t)
(mL)
20.1
14.2
19.1
18.6
18.8
16.0
23.7
16.5
>16.0
10.2
10.2
9.4
15.0
8.3
8.7
9.4
7.6
<15.0
Seawater'
Yield
(m 3 /t)
Fluid Loss
(mL)
13.9
17.9
20.4
< 12.2
25.4
>25.4
24.4
>25.4
52.0
23.0
38,6
32.8
24.3
21.3
25.1
27.1
• Seawater = 63.2·lbm/bbl [180·kg/m 3 J prehydrated bentonite, diluted to required concentration with treated seawater
(pH=10).
290
SPE Drilling Engineering, August 1986
-
YIELD (m 3/TONNE)
G
Lo
~
MINIMUM REQUIRED
i6
~~,=-U_~
__________ -=--=--
o
-
E
-
rA
C
H
i2
8
4
Fig. 2-0CMA yields of various commercial bentonites (pH = 7).
ence of non-sodium-montmorillonitic material and/or an
extending polymer. The polymer will have some flocculating effect, causing increased settling.
Free-water tests were carried out in distilled water only.
Suspensions of 8.8 Ibm/bbl [25 kg/m3] were left in a
250-mL cylinder for 24 hours, and the amount of free
water was subsequently noted.
R.splts and Discussion
In general, the pH has a considerable effect on the viscosity ofbentohite suspensions. Fig. 1 gives an example
of thisfpr one particular brand of bentonite. In view of
this, the pH was always checked during the tests. It was
found to be 7 in all tests, except the one with caustictreated seawater, where it was 10.
Results of the OCMA Te~ts in Distilled Water and in
Seawater. The results of all tests are given in Table 3.
the OCMA yields and fluid losses are also shown schematically in Figs. 2 and 3 for easy comparison. All bentonites, except Samples F and B, meet the OCMA
specifications. Sample F is the nonoilfield bentonite used
in wine clarification. The relatively high fluid loss of Sam-
OCMA
pIe D indicates flocculation (extension). Because of flocculation (by salt and caustic), both the yield and the fluid
loss of the bentonites in seawater should be significantly
higher than the standard values for distilled water. Only
Samples B, E, F, and H, however, show this significant
increase in yield. This again indicates that the other four
samples were already in a partially flocculated state and
thus probably extended.
The most likely explanation for a decrease in yield
(Samples A and D) is the effect of the salt.on the extending polymers present in these bentonites. The configuration of the polymer molecules will be affected by a change
in the ionic strength of the solution, which can result in
a significant change in flocculating efficiency, depending on the type of polymer.
The high fluid loss in seawater as compared to fresh
water is caused by a difference in cake structure. The seawater/bentonite suspension creates a relatively loose,
permeable cake because it is built from flocs instead of
individual bentonite particles.
Results of the Tests at Elevated Temperature. The results of hot-rolling tests on 24.5-lbm/bbl [70-kg/m3] bentonite suspensions are given in Table 4 and Fig. 4.
FLUID LOSS(ml)
r i6 r-MAXIMUM
- - - - - - -ALLOWABLE
-- - - - - - - VALUE
- - -- 0
---- ---- --- -- - -- - - - --
i2
-F
-
8
-A
rB
'ti
r-- '
G
E
4
0
Fig. 3-0CMA fluid losses of various commercial bentonites (pH = 7).
SPE Drilling Engineering, August 1986
291
TABLE 4-RESULTS OF HOT-ROLLING TESTS ON 24.5 Ibm/bbl [70 kg/m3]
BENTONITE SUSPENSIONS
Bentonite
Sample
A
B
C
D
E
F
G
H
PV
(cp)
Hot-Rolled
No
-7
10
16
4
15
9
26
16
YP
(lbf/100 ft2)
Hot-Rolled
Yes
No
13
25
23
9
29
16
26
31
42
6
26
46
27
8
91
14
-- --
In general, the rheology of bentonite suspensions improves during the conditioning process (hot-rolling). The
application of shear and the long contact time enhance the
dispersion and hydration of the clay platelets. In addition,
some gelling occurs at high temperatures, which is reflected in the slight increases in fluid loss resulting from the
formation of more permeable, partially "card-house"structured filter cakes. Samples A, D, and G again gave
deviating results. They did not show an improved rheology upon hot-rolling, and a very pronounced decrease in
the yield points of Samples A and D was observed after
hot-rolling. Thermal degradation of the extending polymer must have taken place.
Results of the Tests in H 2 0 2 Solutions. The results of
rheology measurements on 24.5-lbm/bbl [70-kg/m3] bentonite suspensions in both fresh water and 3% H 20 2 solutions are given in Fig. 5.
Tests were also carried out on a 0.8-lbm/bbl [2-kg/m3]
solution of a commercial bentonite extender. The results
are shown in Fig. 6. Clearly, the extender is almost completely broken down by the H 20 2 , but to complete the
reaction takes a long time (at room temperature).
The results of the oiidation tests on bentonites show
that most products suffer from a significant deterioration
Fluid
Loss
Hot-Rolled
No
Yes
-- -11.2
7
11.0
14
10.0
33
15.6
4
49
8.9
11.0
17
9.7
92
7.8
38
Yes
15.1
12.9
12.8
18.0
9.8
12.4
10.7
10.0
in quality after H202 treatment. Because measurements
were taken after 70 hours of curing time, it can be assumed
that the oxidation reaction is almost completed and that
the rheological properties are representative of the actual
clay quality.
The reduction of rheological parameters ranges from
medium (Samples Hand C) to very pronounced (Samples D, E, and G). Only Samples F and B do not show
any deterioration (differences are within the error range
of ±2). Sample D showed a peculiar result. The tests both
in seawater and at high temperature strongly indicated the
presence of an extending polymer. The H 20 2 treatment,
however, did not affect its properties when used at a 3 %
concentration. Treatment with 10% H 20 2 , however, reduced the YP by more than 50%. Either the extender was
very stable, or the amount of natural organic products that
will also react with H 20 2 was very high.
The resulting YP's all fall within 5 to 11 IbfllOO ft2
[2.4 to 5.3 Pal, which is much more acceptable than the
initial values (10 to almost 100 IbfllOO ft2 [4.8 to almost
47.9 Pa]).
Some comments should also be made on the execution
of the H 20 2 tests.
1. Because the reagent is a solution of a reactive gas
(H 2 0 2) in water, which deteriorates rather rapidly, fresh
reagent should be used for each test.
YP (\b/iOOft 2 )
60
50
+
t:S:J BEFORE HOT ROLLING
92
91
I2Zl AFTER HOT ROLLI NG
40
30
20
10
E
G
Fig. 4-YP's of 70-kg/m 3 bentonite suspensions, before and after hot-rolling at 170°C for
16 hours (pH;:: 7),
292
SPE Drilling Engineering, August 1986
YP (Ib/ 100f1 2)
30
70kg/m 3 BENTONITE IN
'\
I
ES3
96
WATER
o
70kg/m 3 BENTONITE IN
3 % - H202
20
Fig. 5-YP's of bentonite suspensions in water and H 2 0
2. A suspension of bentonite in an H 20 2 solution will
always show reaction (bubbling). The presence of the bubbles alone, however, does not prove that an extending
polymer is present. Both catalytic decomposition of
H 2 0 2 on the clay surfaces and oxidation of natural organic products (i.e., humic acids) will occur.
3. The oxidation reaction can take a long time to complete. The suspension should be left for at least 24 hours
before conclusions are drawn on differences between the
normal and the H 2 0 2 -treated bentonite. If required, reaction conditions could be optimized to shorten the reaction time.
4. During catalytic decomposition of H202 and oxidation of organic material, gases (0 2 and CO 2 ) are
formed. These gases can cause excessive foaming of the
mud. A foam volume five times the original suspension
volume can readily be generated. This introduces a problem in the rheology measurement because foams have
much higher viscosities than nonaerated solutions. There-
2
solutions (pH=7).
fore, care should be taken to ensure that suspensions are
properly defoamed before measurements are carried out.
5. Reaction products can change. the pH of the suspension. the pH should always be checked and, if necessary,
adjusted.
Ratio Between YP and PV. An extra indication of extension is given by the ratio of YP and PV. Initial ratios
higher than about 2 to 3 should arouse suspicion. Ratios
this high will normally be encountered only in flocculated
systems in which the "card-house" structure is strengthened by polymers.
It should be noted, however, that a bentonite with a
YP/PV ratio in the range of 0.5 to 2.0 is not necessarily
nonextended (as shown by, e.g., the performance of Sample E in H 2 0 2 solution).
Table 5 shows the ratios of YP's and PV's under various conditions. Three out of eight samples have a YP/PV
ratio greater than 3. Six of these eight samples, however,
F-600 Cib/100ff 2)
60 "
. .
,
~
~.\~
.. --------------------~------------------~----r-.WATER
50
, ..... .
.......
\
\
40
".
\
\
\
\
30
\
,
20
....
",
"-
.....
....
"-
""
'.q.,
............
.....
" ,
···0 .............. .
· ...................... 0 3%
H202 SOL
- ... - - - - - - - - - - ---""-iO%H202 SOL
20
30
40 h TIME
Fig. 6-Rheology of bentonite extender solutions in water and H 2 0 2 -Fann-600 reading
vs. time (extender concentration: 2 kg/m 3; pH = 7).
SPE Drilling Engineering, August 1986
293
TABLE 5-RATIOS OF YP AND PV UNDER VARIOUS CONDITIONS FOR
24.5-lbm/bbl [70-kg/m 3 ] BENTONITE SUSPENSIONS
YP/PV Ratios
Before
Hot-Rolling
After
Hot-.Rblling
Before H 2 0 2
Treatment
After H 202
Treatment
A
B.O
B
C
0.6
1.6
11.5
0.5
0.6
1.4
21.7
0.8
1.0
0.4
4.0
1.1
1.1
4.0
11.3
0.8
0.6
1.6
1.0
1.0
0.3
0.5
O.~
Bentonite
Sample
D
1.8
E
F
1.7
1.0
3.5
1.2
1.0
3.5
0.9
G
H
cation of the ranking in actual viscosity-building capacity of the nonextended clays.
The results show hardly any correlation between the
ranking in CEC, MBt, and Fann-600 values. Only Samples A and H consistently have a relatively low ranking.
The results of these tests probably indicate only a small
variation in clay quality between the various bentonites.
TABLE 6-CEC's, MBT API VALUES, AND
FANN-600 READINGS IN 3% H 2 0 2 SOLUTIONS
Bentonite
Sample
CEC
E
(illeq/100 g)
67 (3)* *
83 (1)
72 (2)
80 (1)
65 (3)
F
74 (2)
G
H
73 (2)
66 (3)
A
B
C
o
MBT Results
(mL)
1.30
1.35
1.53
1.50
1.58
1.48
1.65
1.35
F-600 in H 202 *
(lbf/100 ft 2)
(3)
(3)
(2)
(2)
(1)
(2)
(1)
(3)
28
31
25
32
30
30
23
(1)
(1)
(2)
(1)
(1)
Clay Contents and :Mineralogical Compositions. The
results of the clay-content determinations and the mineralogical analyses are given iIi Table 7. Because the bentonites yield very viscous suspensions and because clay
content is determined by seqimentation analysis, some
problems were encountered in this test. Therefore, the
results are probably on the low side and the data given
~hould be interpreted as follows: clay content is equal to,
or higher than, tbe figure given.
The results of t\le mineralogical analyses agree well with
what may be expected for bentonites: "predominant to
almost exclusive presence" of montmorillonite, a generally "clearly evident" to "considerable" presence of
quartz (sand, silt, or everi finer), and "traces" of
kaolinite, illite, calcite, dolomite, pyrite, feldspar, and
siderite (as can be expected for a natural product). The
only two exceptions are Samples C and D, which contain
amounts df kaolinite and illite in the clay fraction that are
on the high side.
In general, it can agaih be concluded from these results
that there is hot much variation in mineralogical composition between the various bentonites.
(1)
(2)
• F·600 in H 2 0 2 = Fann·35 viscometer reading at 600 rev/min for a 24.5 Ibm/bbl
3
[70 kg/m l bentonite suspension in 3% H 2 0 2 solution .
•• Numbers within parentheses represent quality ranking in groups; 1 = highest
quality.
contain an extender, as shown in the H 2 0 2 tests. This
indicates that the API requirement is not strict enough.
Excessive ratios always return to "normal" after heat or
H 2 0 2 treatment. Here it c~n also clearly be seen that
Sample A was still reacting with H 2 0 2 at the time of
measurement, as was also observed in the laboratory. The
only two products with reproducible YP/PV ratiO!! (Samples F and B) are also the only two shown to be nonextended in the rheology tests. This clearly illustrates the
unpredicta.bility of the extended products and the reliability of the pure clays.
CEC and MBT Results. The results of the CEC and
MBT measurements are given in Table 6. The Fann-600
readings of bentonite suspensions in 3 % H 2 0 2 solutions
have been included because they give a reasonable indi-
Free-Water Determinations. The results of the settling
tests are given in Table 8. Two products (Samples D and
G) showed very significant top-settling, while two prod-
TABLE 7-MINERALOQ!CAL COMPOSITION OF THE VARIOUS BENTONITES
Clay Composition
Bulk Composition
Bentonite Clay Fraction
Sample
Pyrite Feldspar Siderite Kaolinite Illite Montmorillonite
(wt%)
Quartz Calcite Dolomite Clay
A
B
C
D
E
F
G
H
78
90
79
76
79
83
93
82
-----0
2
3
:3
3
2
2
2 to 3
0
0
0
0
1
2
0
1
0
0
0
0
1
0
0
1
4
4
4
4
4
4
5
to
to
4
to
to
to
to
5
5
5
5
5
5
0
o to
0
o to
o to
0
o to
o to
i
1
1
1
1
1
2
1 to 2
1
1
1
1 to 2
1 to 2
--0
o to
0
1
o to
o to
1
o to
1
--
0
0
2
2
1
1
1
0
o to
o to
0
1
1
0
0
1
2
1
o to 1
o to 1
1
5
5
5
4 to 5
5
5
5
5
The composition was approximated by the following code: 5 = almost exclusively present; 4 =predominantly present; 3 =considerably present; 2 =clearly evident;
1 =scarcely evident; and 0 =not apparent.
294
SPE Drilling Engineering, August 1986
ucts (Samples A and H) showed slight top-settling. Usually
1 or 2 % free water is considered to be within the error
range and thus insignificant.
TABLE 8-TOP SETTLING IN 8.8-lbm/bbl
[2S-kg/m 3 ) SUSPENSIONS OF THE VARIOUS
BENTONITES IN A 2S0-mL MEASURING CYLINDER
General Discussion
Table 9 gives a summary of all the relevant results obtained from the bentonite-quality tests. It can be concluded
from Table 9 that only two products (Samples B and F)
are not extended. They are the same two products that
fail to meet OCMA requirements. The rest of the products except Sample C can be divided into two groups:
(1) bentonites with strong, resistant extenders (Samples
E and H) and (2) bentonites with relatively easily degradable extenders (Samples A, D, and G). Sample C appears
to be only slightly extended.
If we compare the rnineralogies, CEC and MBT values,
and rheologies of the true clay suspensions after oxidation, we can conclude that the nonextended clays are of
similar, rather high qUality. These observations, together
with the fact that the pure, nonoilfield bentonite fails to
meet the OCMA specification, lead to the conclusion that
the OCMA and the API standards are very high.
Apparently, it is very difficult to fulfill API and OCMA
specifications with good-quality bentonites unless extenders are added. Because extender addition can lead to
erratic, unpredictable bentonite performance, however,
it would probably be better to have nonextended bentonites
of a slightly lower API or OCMA quality with regard to
yield. To attain this, the oilfield standards for bentonite
yield should be lowered somewhat (e.g., OCMA yield
from 16 to 12 m 3 It [102.3 to 76.7 bbllU .K. ton]).
The current OCMA and API bentonite-qualityevaluation methods allow manufacturers to add extenders.
On the one hand, the requirements regarding bentonite
yield are high; on the other hand, the test procedure is
such that any material may be added, provided that it is
stable at room temperature in art aqueous environment for
24 hours.
Therefore, lowering the OCMA bentonite yield
requirement-e.g., from 16 to 12 m 3 /t [102.3 to 76.7
bbI/U.K. ton]-should be accompanied by an extension
of the test procedure with a hot-rolling, H2 O 2 , or similar test.
Because dry-blending of small amounts of polymer with
bentonite powder will certainly result in inhomogeneities
in polymer concentration, differences observed between
different batches of the saine product most probably
originate from the dry-blending process.
Finally, it should be stressed that bentonite extension
is not necessarily harmful. Extension, however, could be
Bentonite
Sample
Top-Settling
(% Free Water)
5
A
o
B
C
1
o
46
E
F
o
G
23
H
4
1
carried out better at the rig site or in the mud plant for
obvious reasons.
1. A much more homogeneous mixing of the polymer
can be obtained in a suspension than by dry-blending.
2. The degree of extension can be chosen to meet specific requirements.
3. If the type and amount of extender are known, the
chance of unexpected problems with mud quality will be
much smaller.
Conclusions
1. Bentonite clay qualities of the samples tested did not
differ much. Major differences in performance resulted
from the addition of extending polymers.
2. The OCMA and API standards for bentonite quality are such that addition of an extender will often be required to meet the specifications.
3. The OCMA and API test procedures should include
a test to control the practice of extender addition.
Acknowledgments
I thank 1.1. Hartog and B.H.1. van der Linden for their
contributions to this investigation.
References
1. van Olphen, H.: An Introduction to Clay Colloid Chemistry, second edition, John Wiley and Sons, New York City (1963).
2. Gray, G.R. and Darley, H.C.H.: Compositions and Properties of
Oil Well Drilling Fluids, fourth edition, Gulf PUblishing Co.,
Houston (1980). .
3. Finch, C.A.: Chemistry and Technology of Water-Soluble Polymers,
first edition, Plenum Press, New York City (1983).
4. "Drilling Fluid Materials-Bentonites," Oil Companies Materials Organization, Specification No. DFCP-4 (Oct. 1973).
TABLE 9-0VERVIEW OF ALL BENTONITE-QUALITY-EVALUATION RESULTS
Bentonite
Sample
A
B
C
0
E
F
G
H
Mee.ting
OCMA
Specifications
Detrimental
Effect
Seawater
Detrimental
Effect of
of High
Temperature
Detrimental
Effect of
H20 2
Addition
Deviation in
Mineral
Composition
Yes
No
Yes
Yes
Yes
No
Yes
Yes
Strong
None
Slight
Strong
None
None
Medium
None
Strong
None
None
Strong
None
None
Medium
None
Medium
None
Slight
Medium
Strong
None
Very strong
Strong
No
No
Slight
Slight
No
No
No
No
SPE Drilling Engineering, August 1986
Does
TopSettling
Occur
Is YP/PV
Ratio
Too High
Slight
No
No
Strong
No
No
Strong
Slight
Yes
No
No
Yes
No
No
Yes
No
---
295
:.~
",1
""Q~:I\
............
...\-: ...... .
I!
, ••••••: . :
+
+
"
+
\
+
.' .'
/
,.:..,
~
;:-: ••'
.. ....
,
+ ...•.....•
+
~
...
....\ ..:.
-~...
~~....
o,
+
., ;.;.
;.
\
/\
"\
CLAY PLATELET WITH NO
OR POSITIVE CHARGE ON
EDGES
,••:Of
\\
INCREASED REPULSION \
BETWEEN ADSORBED
.. '
/ .'
....
ANIONIC (-)POLYMERS
....
...... .....
Fig. A-1-(a) Low polymer concentration-flocculated system; (b) high polymer concentration-dispersed system ..
5. "API Specification For Oil Well Drilling Fluid Materials," APIRP l3A, tenth edition, API, Dallas (April 1984).
6. "Standard Procedures for Testing Drilling Fluids," API-RP l3B,
eighth edition, API, Dallas (April 1980).
Appendix-The Effect of Polymer
Concentration on Polymerl
Clay Interaction
It is well-known from flocculation theory and practice that
if an anionic polymer is present in a clay suspension, the
polymer concentration has a significant effect on the stability of the suspension. This effect is explained by the
different configurations existing at high and low polymer
concentrations.
1. At a low concentration, the polymer links the clay
platelets together, which results in flocculation and subsequent increased sedimentation. The polymer destabilizes
the suspension. A schematic representation is given in Fig.
A-1a.
2. At a high concentration, more polymer molecules
are available per clay platelet. Various polymers adsorb
onto a platelet, and the repulsion between the negatively
charged tangling ends of the polymers prevents flocculation. The polymer now stabilizes the suspension. A
schematic representation is given in Fig. A-1 b.
296
In a bentonite suspension, the same phenomena will
occur after the addition of an extending polymer. The only
difference is that sedimentation will not occur, owing to
the large hydration mantle of the montmorillonite platelets. Instead, a marked effect Oil rheology will be observed. In general, the following rheological observations
are made on bentonite suspensions: (1) pure bentonite suspension: dispersed system with normal PV and YP;
(2) slightly extended bentonite suspension: flocculated
system with normal PV but strongly increased YP; and
(3) overextended bentonite suspension: dispersed system
with normal PV and strongly decreased YP, even lower
than for the pure bentonite suspension.
SI Metric Conversion Factors
bbI/U.K. ton
cp
X
X
E-Ol
E-03
(OF - 32)/1.8
OF
Ibf/100 ft2
lbm/bbl
1.564 763
1.0*
X
X
* Conversion factor is exact.
4.788026
2.853 010
E-Ol
E+OO
m 3 /t
Pa's
°C
Pa
kg/m3
SPEDE
Original manuscript received in the Society of Petroleum Engineers office March
1, 1985. Paper accepted for publication Feb. 3, 1986. Revised manuscript reo
ceived April 7.1986. Paper (SPE 13454) first presented at the 1985 IADCISPE
Drilling Conference held in New Orleans March 6-8.
SPE Drilling Engineering, August 1986