Elsevier

Engineering Geology

Volume 295, 20 December 2021, 106435
Engineering Geology

Rheological behavior of bentonite-water suspension at various temperatures: Effect of solution salinity

https://doi.org/10.1016/j.enggeo.2021.106435Get rights and content

Highlights

  • A modified shear loop experiment has been adopted to measure rheological behavior of Na-bentonite suspension.

  • Na-bentonite suspension behaves a complex thixotropic type in non-salt conditions.

  • The negative thixotropy of bentonite suspension is attributed to the parallel arrangement of clay particles.

  • A increase of salinity is accompanied by a significantreduction in yield stress and flow index, as well as thixotropy.

Abstract

Bentonite suspension have been widely used as drilling fluid in drilling engineerings. In this work, a modified thixotropic loop test was conducted on 6 wt% bentonite suspensions with 4 solution salinities, to investigate the influence of salinity on rheological behaviors of Na-bentonite suspensions. Most of flow curves were well fitted with Herschel-Bulkley model except first upward curves, where stress overshoot appeared in the initial stage. It was found that the yield stress and flow index decreased with increasing salinity. The suspensions present shear thinning behavior in the first upward curves but shear thickening or Bingham behavior in the second loops. Besides, at 20 and 14 °C, a notable thixotropic type transition was observed from positive to negative when the second shear loop was applied to the suspensions in case of low salinity. Both the change of flow type and thixotropy mentioned before may be attributed to the further structural evolution resulting from the long-time shearing at low salinities. However, this change was suppressed at 4 g/L due to the effect of high salinity on particle association mode. The test at 7 °C showed interesting results that the twice upward flow curves presented the stress overshoot and the twice loops performed a positive thixotropic behavior, not as same as the cases of the other temperatures. It indicates the structural evolution during shear process may be affected at low enough temperatures.

Introduction

Bentonite clay suspensions have been widely used in various industries, such as oil-well drilling, seepage control, wastewater treatment and cosmetic products (da Silva Favero et al., 2019; Shaikh et al., 2017; Yoon and El Mohtar, 2014; Zhang et al., 2020). Rheological properties, including critical stress and thixotropy, play important role on many industrial processes and product quality. A complete understanding of thixotropy of bentonite suspension and its correct estimation are of great practical value for developing flow models in engineering applications, formulation of commercial production, design and process evaluation.

As a time-dependent rheological behavior, the concept of thixotropy has been expanded with further studied. In general, “positive thixotropy” is defined as “the continuous decrease in viscosity when a constant shearing is applied to a sample and the subsequent recovery of viscosity at a state of rest” (Mewis and Wagner, 2009). On the opposite, an increase in viscosity when a constant shearing is applied to a sample and the subsequent recovery at a state of rest, is called “negative thixotropy”. “Complex thixotropy” is defined as a phenomenon that materials display positive thixotropy at first and then negative thixotropy with shearing or rest time. Although thixotropic phenomena of bentonite clay have gained much mention during last decades (Zhang et al., 2017; Bekkour et al., 2005; Choo and Bai, 2015; Larson and Wei, 2019; Rubio-Hernández et al., 2020; Tanner, 2019), this phenomena remains the most challenging research object in rheology.

In former studies, much progress has been made on the understanding for the thixotropic behavior of bentonite suspensions, especially related the thixotropy to the suspension microstructures (Jamali et al., 2020, Jamali et al., 2017; Luckham and Rossi, 1999; Mewis and Wagner, 2009). A gel formation of clay particles is controlled by two kinds of forms: electrostatic attraction between the negatively charged face and the positively charged edge, and long-range electrostatic double-layer repulsion leading to face-to-face and edge-to-edge interactions (Du et al., 2020). The edge-to-edge and edge-to-face structures are called the card-house structures or continuous network structures and the face-to-face structure is called the band-like structure (Abu-Jdayil, 2011). It has been widely accepted that the positive thixotropy of bentonite suspension results from the destruction of network structure (Choo and Bai, 2015; Larson and Wei, 2019; Mewis and Wagner, 2009). On the other hand, negative thixotropy was reported in limited literatures (Dai et al., 2007; Jeong et al., 2015; Rubio-Hernández et al., 2020; Yusof et al., 2020). Some mechanisms have been proposed to explain the negative thixotropy or complex thixotropy in terms of microstructures, such as“aggregation theory”, “crystallization theory”, “two stages recovery process of structures” (Dai et al., 2007), but they may be valid for polymer solutions not clay suspensions. In general, the understanding of negative thixotropic behavior including its physical origin remains unclear and controversial.

The effect of salinity on rheology of bentonite suspensions becomes very important now that clay particle structures are formed by electric interaction. It presents great challenges to the using of bentonite drilling fluid in the ocean drilling engineering. Di Liu et al. demonstrated that the polymer bridging effect was tailed off in saline water and high-salinity water was observed to assist bentonite to settle faster (Liu et al., 2018). Abujdayil investigated three kinds of bentonite-water dispersions with different electrolyte ratio range of 0.02–0.2 M and found that bentonite suspensions behaved like shear thickening fluid at high divalent electrolyte concentration (Abu-Jdayil, 2011). Sueng Won Jeong et al. reported that bentonite exhibited an anti-thixotropic (negative thixotropic) behavior when hydrated with fresh water rather than salt water (Jeong et al., 2015). In spite of a few different results in these studies, the effect of salinity on rheological behavior was discussed always from the point of thixotropy. In this paper, some efforts were attempt to correlate the influence of salinity on thixotropy with other rheological properties including flow type and shear resistance, because these rheological properties were also controlled by inner structures of suspension (Ganley and Van Duijneveldt, 2017). Besides, temperature is also an important factor to fluid rheology. Y. Lin er al. found that the yield stress and moduli of bentonite suspensions increase with increasing temperature and the face-to-face repulsive electrostatic energy between clay particles is responsible for the temperature dependent behavior of clay suspensions (Lin et al., 2016). However, almost thixotropic measurements were conducted at a constant temperature and the effect of temperature on thixotropy has seldom been specifically studied in clay researches. It is necessary to study the influence of temperature on rheological behavior of bentonite suspensions.

Thixotropic loop test is one of the favorite ways to measuring thixotropy. A shear rate ramp from zero to a maximum value is performed, and then the ramp is reversed for a same time. The corresponding shear stress can be recorded and be plotted versus shear rate to obtain upward and downward curve. The difference between the two curves due to shear history is considered as a representation of thixotropy. If the downward curve is below the upward curve, the loop composed of two curves represents positive thixotropy; if the upward curve is below the downward curve, the loop represents negative thixotropy. However, how to identify the complex thixotropy in the loop test? X.N.Dai and W.G.Hou investigated the type of the thixotropic loop of the complex thixotropy by thixotropic loop test carried on kaolinite dispersion, and demonstrated that complex thixotropic loop had a crossover point that divided the loop into two sections (Dai et al., 2007). It is well known that, the loop is not an intrinsic material nature, but relies on the test procedure such as the time of shearing ramp period, the form of the time function and the maximum shear rate reached during testing (Mewis and Wagner, 2009; Perret et al., 1996). In order to ensure the accuracy of observation at least qualitatively, a continuous two shear loop was applied in the experiments.

In this paper, effect of solution salinity on the thixotropic behavior of 6 wt% bentonite-water suspension was examined through thixotropic loop test at 3 different temperatures. Some other rheological properties such as yield stress and initial stress overshoot are also studied and discussed in terms of observation and mechanism.

Section snippets

Materials and suspension preparation

Na-bentonite clay purchased from HaoSheng New Material Company, was used to prepare the bentonite-water suspension. The bentonite had an offwhite powder appearance and 33 μm average equivalent sphere diameters. According to the certificate of analysis from HaoSheng, this clay had an expansion multiple of 6 mL/g, a swelling volume of 14 mL (1 g in 100 ml water) and an over 85 mmol/100 g ion-exchange capacity. The bentonite consisted of 99.5% montmorillonite and 0.5% impurities. More

Rheological behavior of bentonite-water suspension

Fig. 4 showed the two thixotropic loops of the 6 wt% Na-bentonite suspension with fresh water at 20 °C. The upward curve in the first shear loop presented two divergent behaviors before and after 200 s−1. In the range of lower shear rate, the shear stress in bentonite suspension underwent a rapid increase from 19.3 Pa to 36.8 Pa and then a sharp decrease to 18.71 Pa. This is a thixotropic behavior termed as ‘stress overshoot’, which can produce a very large shear stress even at low shear rates.

Conclusions

In summary, we have reported the effect of salinity on the rheological properties of bentonite suspensions. Particularly, a complex thixotropic behavior, previously less reported, was identified in the case of 0 g/L salinity at 20 °C and 14 °C. A reasonable mechanical understanding of internal structure under shearing has been proposed to illustrate the relationship between thixotropy and structural change. The conversion for the bentonite suspensions from the complex thixotropy to the positive

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the ‘National Natural Science Fund (No.11872173)’, ‘Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJKY19_0427)’ and ‘the Fundamental Research Funds for the Central Universities (2013/B19020655X)’. The experiments were performed at Advanced Analysis and Testing Center (AATC), Nanjing Forestry University.

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