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Investigations on the Thixotropy of Bentonite SuspensionsV. C. Kelessidis a

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Energy Sources, Part A, 30:1729–1746, 2008

Copyright © Taylor & Francis Group, LLC

ISSN: 1556-7036 print/1556-7230 online

DOI: 10.1080/15567030701456261

Investigations on the Thixotropy of

Bentonite Suspensions

V. C. KELESSIDIS1

1Department of Mineral Resources Engineering, Technical University of Crete,

Chania, Greece

Abstract The thixotropic behavior of water bentonite suspensions has been investi-gated using Wyoming and Zenith bentonites at 5.0% and 6.42% concentrations with

0.0 M, 0.01 M, and 0.1 M NaCl following API preparation protocols. Rheologicaldata was collected with a Couette viscometer, deriving first the D0 curve, from 600

to 3 rpm, followed by a typical thixotropic loop, getting the U curve from 3 to600 rpm and then the D curve from 600 to 3 rpm. The Herschel-Bulkley model

describes very well all experimental data. Yield stresses did not vary, but variationsof flow consistency and of flow behavior indices were detected for the three curves. All

suspensions with no salt and 0.01 M salt exhibited anti-thixotropy, estimated from theD0–U curves, while the 0.1-M suspensions showed no thixotropy. Thixotropy indices

vary and depend on bentonite type and concentration and on the presence and amountof salt. The mechanisms of thixotropy and implications on drilling fluid performance

are discussed.

Keywords bentonite, drilling fluids, Herschel-Bulkley, rheology, suspensions,thixotropy

Introduction

Thixotropy has been defined as the variation of shear stress (shear rate) with time when a

fluid is subjected to a constant shear rate (shear stress) (McMillen, 1932a, 1932b, 1932c;

Mewis, 1979; Barnes, 1997). It is a characteristic behavior of two-phase systems (Nguyen

and Boger, 1985; Lemke et al., 1999), like clay suspensions, which contain non-spherical

particles (Barnes, 1997; Nakaishi and Yasutomi, 1994; Luckham and Rossi, 1999). The

influence of time of mixing, time of measurements, and time of shearing on rheological

properties of these two-phase systems normally is not taken into account in fluid flow

computations, but it has been the subject of continuous experimental and theoretical

research from early years, among many by Ambrose and Loomis (1933), Moore (1959),

and Singhal and Malik (1964), to recent times by Cheng (2003), Li et al. (2003), Labanda

et al. (2004), Roussel et al. (2004), and Bekkour et al. (2005), among many others.

Constitutive equations relating shear stress to shear rate with a structural parameter using

structural kinetics approach to thixotropy have been discussed and analyzed by Moore

(1959), Cheng and Evans (1965), Sestak et al. (1982), Toorman (1997), Mujumdar et al.

(2002), Dullaert and Mewis (2006), and Galindo-Rosales and Rubio-Hernandez (2006).

Address correspondence to Vassilios C. Kelessidis, Department of Mineral Resources Engi-neering, Technical University of Crete, 73100 Chania, Greece. E-mail: [email protected]

1729

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1730 V. C. Kelessidis

Thixotropy is a reversible process and occurs because of the microstructure of the

suspensions. For many materials, when they are sheared at a constant rate, the shear stress

decreases with time because the structure is broken down. When shearing is removed,

the material slowly rebuilds its structure, but the time necessary for complete structure

restoration is much higher (McMillen, 1932a; Chavan et al., 1975). These systems are

termed thixotropic or with positive thixotropy (Barnes, 1997). There are materials though

where the opposite is true, and the stress increases with time upon application of constant

shear rate. This phenomenon is termed anti-thixotropy or negative thixotropy (Barnes,

1997), and it has been reported for clay suspensions (Heckroodt and Ryan, 1978) as well

as for certain simulated waste slurries (Chang and Smith, 1996). Negative thixotropy

may result from temporary aggregation of suspension particles because of increased

collisions due to shearing (Barnes, 1997), or when certain flocs become looser and more

open under the action of shear, as reported for ferric oxide suspensions in mineral oil

(Kanai and Amari, 1994). Combination of positive and negative thixotropy, sometimes

termed complex thixotropy (Hou et al., 1998; Li et al., 2003), has been reported for clay

water suspensions, but some investigators have attributed this to the effect of measuring

time, with negative thixotropy observed for short measuring times and positive thixotropy

observed for long measurement times (Nakaishi and Yasutomi, 1994).

Water bentonite suspensions are encountered in a variety of industries and are the

main ingredients in oil well drilling fluids. They exhibit a yield stress, and it is well

accepted and well documented that yield stress fluids are also thixotropic and shear

thinning (Barnes, 1997; Møller et al., 2006). There is little information, however, on their

thixotropic behavior viewed from the drilling fluid industry point of view, although studies

indicate an increase in the static gel strength of water bentonite-based drilling fluids with

time (Speers et al., 1987), while Kok and Alikaya (2005) have shown that the addition

of polymers also increases gelation time of drilling fluids. In general, thixotropy is not

addressed when dealing with drilling fluids (Bourgogne et al., 1991), not only because of

the complexity of the issue and the unavailability of models taken into account (Barnes,

1997), but also because it is assumed that, after the intensive shearing experienced by

the fluids in the drilling process, thixotropy is kept at least to a minimum (Bourgogne

et al., 1991).

It is for this reason that the drilling fluid industry specifies comprehensive testing

protocols (API, 1993; API, 2000) for preparation and laboratory as well as field testing

of clay suspensions and, in particular, of water bentonite suspensions. According to these

specifics, the mixture is subjected to very high shearing prior to rheological measure-

ments, thus probably destroying the structure of the suspension. Deformation prehistory

of the suspensions is very significant for studying and understanding thixotropy (Barnes,

1997), and consistent initial but also testing conditions are essential not only for testing

for thixotropy but also for comparing results of other research work and even different

tests in the same laboratory. The API procedures thus ensure, to the best of experimental

accuracy at least, consistency in measurements, so that cross-comparisons can be made

with more certainty. Further evidence has been recently provided by Møller et al. (2006),

where it is stated that only by controlling the aging history of colloidal samples by large

preshearing, reproducible results can be obtained.

In oil well drilling operations, the fluids experience high shear rates in the drill pipe,

where they flow downward, and extremely high shear rates when they go through the

nozzles of the drill bits. There, the shear rates are of the order of thousands of reciprocal

seconds before they enter the annulus, where they experience low shear rates on their way

to the surface. This article attempts to provide some evidence that, despite high shearing in


Thixotropy of Bentonite Suspensions 1731

the laboratory, thixotropy is still observed in water bentonite suspensions, and it depends

on the type of bentonite used and the concentration as well as on environmental variables

like the amount of salt present in the suspensions. Data is therefore presented for the

thixotropy of water bentonite suspensions after intensive shearing, with full rheograms

derived by going from high to low shear rates followed by the standard thixotropic loop

for two bentonites at two concentrations and three salt concentrations.

Experimental

Materials

Two commercial bentonites used by the drilling fluid industry have been utilized in this

study: a Wyoming bentonite that is a naturally occurring sodium montmorillonite, kindly

provided by Halliburton-Cebo Holland, and a Zenith bentonite, a calcium montmorillonite

that is converted to sodium montmorillonite after treatment, kindly provided by S&B

Industrial Minerals, Greece. Two concentrations have been tested, a 5.0% and a 6.42%

w/w bentonite in water, of similar order to concentrations used by the drilling industry.

Tests were performed in freshwater suspensions and in saline water, with electrolyte

(NaCl) concentrations of 0.01 M and 0.1 M. The particle size of both bentonites was

less than 70 �m, with the majority of particles with diameters around 10 �m.

Instruments and Methodology

The American Petroleum Institute standard preparation and testing protocols (API, 1993;

API, 2000) were followed. Bentonite was mixed with deionized water in a high-speed

mixer (11,000 rpm) for a total mixing time of 20 min, and it was then stored in a

container for 16 h at room temperature for complete hydration of bentonite particles.

Prior to testing, the suspension was sheared for 5 min at high shear (11,000 rpm), and it

was then poured in the viscometer cup. Rheological data was obtained with a continuously

varying rotational speed Grace M3500 rotating viscometer (Houston, TX) at temperature

of 25ıC and speeds of 600, 300, 200, 100, 6, and 3 rpm. Inner cylinder diameter was

1.7245 cm and outer rotating cylinder diameter was 1.8415 cm, thus giving a diameter

ratio of ı D 1:06785. The maximum Newtonian shear rate in the viscometer was 1,021

sec�1 , while the minimum was 5.1 sec�1. For the high speed mixer, the highest Newtonian

shear rate was estimated around 2,300 sec�1 .

The measurements began immediately after pouring the sample into the viscometer

cup, starting from the high shear rate of 600 rpm and going down to 3 rpm, thus giving

the D0 curve. The time of measurement at each rotational speed was 60 sec, for a total

of six readings, which were taken at 10-sec intervals. Each of the six readings was then

averaged and associated with the particular rotational speed. A 60-sec interval followed

at zero shear rate, and then a full hysteresis thixotropy loop began by first deriving the

up curve (curve U), going from 3 rpm up to the maximum 600 rpm and then continuing

with the down curve (curve D) from 600 rpm down to the 3 rpm. The full time evolution

of the rotational speed together with shear stress measurements for a particular sample

is shown in Figure 1. Total test duration was 19 min.

The Herschel-Bulkley rheological model, given by

� D �y C K. P /n; (1)



Figure 1. Shear rate and shear stress time evolution for a particular sample.

where P is the shear rate, �y is the yield stress, K is the flow consistency index, and n is

the flow behavior index, has been used to describe each one of the three rheograms. For

every suspension and particular salt concentration, three sets of rheological parameters are

obtained by standard non-linear regression techniques, although other approaches have

also been reported to derive the three rheological parameters for the Herschel-Bulkley

model (Kok and Alikaya, 2003, 2004, 2005; Kelessidis et al., 2006). The Herschel-

Bulkley model describes very well the experimental data derived not only from the work

of this laboratory (Kelessidis et al., 2005), but also from other work for water bentonite

suspensions (Coussot and Piau, 1994; Bekkour et al., 2005). The goodness-of-fit has been

determined using two statistical indicators, the correlation coefficient, R2c , and the sum

of square errors normalized by the yield stress of the suspensions, NSSE, defined as

NSSE D

P

.�i � O�2i /

�2y

; (2)

where �i , O�i are the measured and predicted shear stress values.

The thixotropy index is defined as the area enclosed between the up curve (U) and

the down curve (D). The area of the hysteresis loop (A) essentially represents power per

unit volume (Perret et al., 1996) because

A D � � P ; (3)

with the units of (A) in the SI system,

ŒA�ŒD�ŒP a�

�

1

s

�

ŒD�

�

N

m2

� �

1

s

�

ŒD�

�

N � m

m3

� �

1

s

�

ŒD�ŒJ �

Œm3�Œs�D

power

volume: (4)



So the area represents the power required to bring the given volume of the material to

a more fluid state. This power is expended in breaking up the gel of the fluid. Large

hysteresis loop areas represent highly thixotropic material, while a null hysteresis loop

area represents either a non-thixotropic material or material for which the structure has

been completely broken up, and there was not enough time to rebuild, at least within the

timeframe of the experiment.

The thixotropy index has been estimated with two different methods. The first one

is by integration of the Herschel-Bulkley equations, as

A D AU � AD D

Z P f

0

�U d P �

Z P f

0

�Dd P ; (5)

with the subscripts U and D referring to the up and down curves, respectively. The second

method is by computing the area of the trapezoids between two successive points and

summing them all up. The computations using either method have shown that the results

differ by less than 5%. The values given in this work are derived using the integration

method.

Results

The full rheogram and the thixotropic loop for the Zenith bentonite suspensions of 6.42%

and 5.0% concentration are shown in Figure 2 for 0.0 M salt concentration, in Figure 3 for

the 0.01 M salt, and in Figure 4 for the 0.1 M salt concentration. The data are shown with

the error bands computed as one standard deviation from the six measurements taken at

each shear rate. Many interesting features can be distinguished in all figures, with small

and large variations observed for the suspensions at different salt concentrations.

For the 6.42% bentonite concentration with no salt, curve D0 is below curve U,

exhibiting an inverse thixotropy, or anti-thixotropy, if one considers the D0–U as a

thixotropic loop. Analyzing through the standard thixotropic loop U–D, normal thixotropic

behavior is observed, and the loop is similar to the one described by Perret et al. (1996),

with the time spent at the highest speed kept to a minimum. It is worth noticing the

coincidence of the D curve with the D0 curve for shear rates less than 511 sec�1.

The 5.0% bentonite suspension exhibits similar behavior to the 6.42% for the D0–U

curves, with D0 being lower than the U curve, although to a smaller extent. Differences,

however, are observed with the standard thixotropic loop, the area between U–D curves,

being smaller for the 5% suspensions because the points at the two highest shear rates

coincide with the points for the 6.42% bentonite suspension.

The results from the suspensions with the 0.01 M salt concentration (Figure 3)

show similar characteristics to the 0.0 M concentration for both bentonite concentrations.

However, a big change is observed when one considers the results for the 0.1 M salt

concentration (Figure 4) for both bentonite concentrations. The D0 curves are now higher

or slightly higher than both the U and the D curves for all conditions. Shear stress values

at the starting shear rates for the D0 curve almost coincide with shear stresses measured at

the ending shear rates for the U curve, while ending values of the D0 curves coincide with

ending values of the D curves. This behavior is observed for both concentrations. The

U–D curves show anti-thixotropy for shear rates less than 400 sec�1 and no thixotropy

for higher shear rates for both concentrations.



Figure 2. Shear stress-shear rate thixotropic curves for 6.42% and 5% Zenith suspension with no

salt. pH of the suspensions was 9.64 and 9.55, respectively.

The results for the Wyoming bentonite (Figures 5, 6, and 7) are similar to the

Zenith bentonite results. The D0 curves are again lower than the U and D curves for

both concentrations, for the 0.0 M and for the 0.01 M salt concentrations. They almost

coincide or are slightly higher than both U and D curves at the 0.1 M salt concentration

for the 6.42% and the 5.0% bentonite concentrations, respectively.



Figure 3. Shear stress-shear rate thixotropic curves for 6.42% and 5% Zenith suspension with

0.01 M salt. pH of the suspensions was 9.41 and 9.39, respectively.

For each of the three curves, D0, U, and D, the Herschel-Bulkley rheological

parameters have been computed, and the goodness-of-fit, as determined from the values

of the correlation coefficient and the normalized sum of square errors, has been very good

for all samples. In Figure 8, the two statistical indices are shown for the 6.42% bentonite

concentration for both bentonites and at all salt concentrations tested, with similar results



Figure 4. Shear stress-shear rate thixotropic curves for 6.42% and 5% Zenith suspension with


obtained for the 5% bentonite concentration. For the 6.42% concentration, the correlation

coefficients are greater than 0.99 for all samples except the Zenith bentonite at 0.01 M

salt concentration for which it is close to 0.98. Similarly, all normalized sum-of-square

errors are very small and range from 0.05% to a maximum of 0.65% of the yield stress

of each suspension, indicating the very good description of rheological data with the

Herschel-Bulkley model.



Figure 5. Shear stress-shear rate thixotropic curves for 6.42% and 5% Wyoming suspension with

no salt. pH of the suspensions was 8.95 and 8.79, respectively.

The yield stresses computed from the different rheograms for all samples are shown

in Figure 9. No significant variations are observed among the D0, the U, and the D

curves for each condition. Similar results have been obtained for the 5% concentrations.

Relevant comparison for the flow consistency index should be made together with the

flow behavior index because of the strong correlation among the two, particularly with





respect to the shape of the rheogram. In fact, the slope of the curve is derived from

Eq. (1) and is given by

d�

d P D .K/.n/. P /n�1: (6)





The slopes of all curves have been computed for all conditions and are shown versus the

shear rate in Figure 10 for the 6.42% concentration for both bentonites. All slopes for

Wyoming bentonite are larger than the slopes for the Zenith bentonite for all conditions,

meaning more shear thinning behavior of the former with respect to the latter. The degree

of thixotropy is estimated from the variation in the slopes among the three curves (D0, U,



Figure 8. Goodness-of-fit of Herschel-Bulkley equation to rheological data of all bentonite

suspensions for 6.42% bentonite concentration. (a) Correlation coefficients, (b) normalized sum

of square errors.



Figure 9. Yield stresses computed from the different rheograms for the first down curve (D0), the

up curve (U), and the second down curve (D) for all bentonite suspensions.

and D) that is observed, and there is less variation at high shear rates than at lower shear

rates for both bentonites. Similar results have been obtained for the 5.0% concentration.

The thixotropy indices, computed as described above from the area between the U

and the D curves, are shown in Figure 11 as a function of salt concentration for all

samples tested. Most samples show positive thixotropy with the exception of the 5.0%

Zenith suspension at 0.1 M salt concentration, which exhibits negative thixotropy. The

5.0% suspensions with 0.0 M salt of both bentonites and the 6.42% Zenith suspensions

with 0.1 M salt show negligible thixotropy. The Zenith samples of both concentrations

and the 5% Wyoming samples show a maximum thixotropy index at the 0.01 M salt

concentration, while the 6.42% Wyoming suspension shows a local minimum at the

same salt concentration. When no salt is present, there is no difference in the thixotropic

indices among the two bentonites tested, with the indices having high values for the 6.42%

bentonite concentration and very low values for the 5.0% bentonite concentrations. For

the suspensions with salt, differences are observed among the two bentonites, with the

most significant at 0.01 M and for the 6.42% bentonite concentration.

Discussion

The results presented above show specific trends, with all of these systems exhibiting

complex thixotropic behavior, meaning combination of positive and negative thixotropy

(Hou et al., 1998; Li et al., 2003). For both bentonites and both concentrations with no

salt in the suspension (0.0 M), the initial rheograms obtained (curves D0) after intensive

preshearing give rheograms with the lowest shear stresses at all shear rates, compared

to the subsequent U and D curves. The D0 curve is always lower than the U curve,

indicating rebuilding of the three-dimensional structure of the suspensions, thus giving

anti-thixotropic behavior. The structure rebuilding primarily results from clay platelet

interactions (Perret et al., 1996), probably from face-to-face or edge-to-edge associations

because the pH of the suspensions is greater than the pH of the isoelectric point of

these montmorillonite suspensions (van Olphen, 1977; Tombácz and Szekeres, 2004).

Anti-thixotropy of clay suspensions, however, has also been attributed to the build-up of



Figure 10. Slope of the rheograms for the Wyoming and Zenith 6.42% suspensions.



Figure 11. Thixotropy index, the area of the hysteresis loop between the U and the D curves as

a function of salt concentration for the two bentonites and the two concentrations.

edge-to-face structures at high shear rates, which hold larger void volume in the structure

(Heckroodt and Ryan, 1978), but this is not the case in the particular work because it is

observed at low shear rates.

For the low salt concentration of 0.01 M, similar behavior to the behavior exhibited

by the 0.0 M suspensions is observed as far as anti-thixotropy is concerned, with the

departure of the U curve from the D0 curve being more significant at the highest bentonite

concentration, and more intensive for Zenith compared to Wyoming bentonite.

Further shearing to get the thixotropic loop, destroys the structure again for both

0.0 M and 0.01 M salt concentrations, as it has been also reported by Perret et al. (1996),

and the material exhibits normal thixotropic behavior after it has been subjected to high

shear rates and then to low shear rates. Thus, the significant step to observe restructuring,

particularly for clay suspensions, is that the material should be sheared at low shear rates

to allow partial rebuilding, as demonstrated also by Li et al. (2003).

The intensive preshearing before measurements, where the material is subjected to

an estimated shear rate of �2,300 sec�1 for an extended period of time (5 min), most

probably has broken down the structure completely. Restructuring, however, is observed

as the shear rates are decreased and only after 6.5 min have passed, which was the

time necessary for the completion of the D0 curve. Following then with the U curve,

destruction of the structure is observed again at shear rates of similar magnitude as

during preshearing (1,021 sec�1). The degree of destruction diminishes with decreasing

bentonite concentration, similar to the results reported by Singal and Malik (1964) and

Nakaishi and Yasutomi (1994). In addition, the degree of destruction diminishes with

increasing salt concentration (up to 0.01 M) and is more intense for Wyoming bentonite

compared to Zenith bentonite.

It has been suggested that, if the flow of bentonite suspensions is strong enough,

it will partially destroy the structure, leading to a viscosity decrease and thus enabling

faster structure destruction (avalanche effect), while the opposite holds at low shear rates,



with viscosity increasing as the shear rates become less (Møller et al., 2006). Hence,

equilibrium between rebuilding at low shear rates and destruction at high shear rates

would be expected with this cycling process. The time of shearing, however, for this to

occur depends on the materials used and can be significant, as even after 1 h of shearing,

equilibrium was not reported by Møller et al. (2006). The duration of the full test in this

work, including the time spent to preshear the material, was 24 min; hence, equilibrium

between destruction and restructuring could not be expected during testing in this work.

The suspensions with 0.1 M salt concentrations reversed the above trends for both

concentrations and both bentonites tested. At this high salt concentration, flocculation

of the bentonite particles is expected (Luckham and Rossi, 1999), and the suspensions

exhibit either very small thixotropy when going from the D0 to the U curve, especially

the Zenith bentonite samples, or no thixotropy at all for all other cases. In conditions

of high salinity, the hysteresis loop also disappeared in the study of Perret et al. (1996),

and the authors indicated that the salt-induced flocculation structure was built up very

rapidly and reversibly when shearing, in a similar fashion observed in this study, either

by preshearing or when shearing during measurements.

The presence of electrolyte at high concentration leads to the compression of the elec-

tric double layer around the clay platelets (van Olphen, 1977; Luckham and Rossi, 1999;

Tombácz and Szekeres, 2004), and since no thixotropy is observed at this concentration

for both bentonites and both concentrations, it can be concluded that it is the electric

double layer, at least partially, responsible for the thixotropic behavior of these bentonite

suspensions. Furthermore, the more bentonite particles in the suspensions, the stronger

the thixotropy, while the variation observed in thixotropy for the different bentonites may

be attributed to the fact that one is naturally occurring sodium montmorillonite while the

other one is treated.

The antithixotropy observed is not due to shearing, as it has been reported for various

flocculated systems (Barnes, 1997) but due to the cycling of shearing and the passing

through the low shear rates. This conclusion is significant for the drilling fluids because

after they are sheared through the drill bits at very high shear rates, they experience very

low shear rates in the annulus for prolonged periods, even hours, and during this time,

structure will be rebuilt, significantly increasing the viscosity. Thus, thixotropic models

will have to be implemented into the modeling of flow behavior of drilling fluids.

Conclusion

The thixotropic behavior of water-bentonite suspensions has been investigated at different

bentonite concentrations in the presence of NaCl. These suspensions have exhibited

thixotropic behavior, despite intensive preshearing, with the degree of thixotropy de-

pending on the bentonite used, the bentonite concentration, and the presence and amount

of salt.

Three rheograms have been derived for each suspension after intensive preshearing,

from high to low, from low to high, and from high to low shear rates. The Herschel-

Bulkley rheological model described very well the experimental data, and for each of the

three curves, the three model parameters have been estimated. No significant variations

have been seen for the yield stresses of the suspensions among the three obtained

curves. The rheogram slopes, however, indicative of the combined variation of the flow

consistency and flow behavior indices, differ, with fewer variations at high than at low

shear rates. Negative thixotropy is observed for all suspensions at 0.0 M and 0.01 M but

not at 0.1 M salt concentrations when creating the hysteresis loop from high to low to



high shear rates, indicating partial rebuilding of the structures because of the exposition

of the suspensions to low shear rates.

Thixotropic indices computed from the normal thixotropic loop, low to high to low

shear rates, have been computed and vary from positive to negative. The higher the

bentonite concentration, the higher the index for the suspensions with no salt, while

in the presence of salt, variations of the thixotropy index are obtained among the two

bentonites, with the degree of variation dependent on bentonite concentration.

Acknowledgment

Experimental data has been collected by Mrs. C. Tsamantaki.

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