Bentonite Nanoparticle Stability and the Effect of Fulvic Acids: Experiments and Modelling

Abstract

In this study, the critical coagulation concentration (CCC) for FEBEX bentonite colloids is determined by colloid coagulation studies under variation of pH, electrolyte concentration, and fulvic acid (GoHy-573FA) content. For CaCl 2 electrolyte solution, a pH-independent Ca-CCC of 1 mmol L −1 is found. In the case of NaCl background electrolyte, a pH-dependent Na-CCC can be determined with 15 ± 5 mmol L −1 at pH 6, 20 ± 5 mmol L −1 at pH 7, 200 ± 50 mmol L −1 at pH 8, 250 ± 50 mmol L −1 at pH 9, and 350 ± 100 mmol L −1 at pH 10, respectively. The addition of 1 mg L −1 dissolved organic carbon in the form of fulvic acid (FA) increases the Ca-CCC to 2 mmol L −1 . An association of FA with FEBEX bentonite colloids as surface coating can clearly be identified by scanning transmission X-ray microscopy (STXM). The experimental bentonite stability results are described by means of an extended DLVO (Derjaguin-Landau-Verwey-Overbeek) approach summing up hydration forces, short-range Born repulsion, van der Waals attraction, and electrical double layer repulsion. The measured zeta (ζ)-potential of the bentonite colloids is applied as platelet face electrokinetic potential and the edge electrokinetic potential is estimated by the combination of silica and alumina ζ-potential data in the ratio given by the FEBEX bentonite structural formula. Adjusting the montmorillonite face electrokinetic potential by a maximum of ±15.9 mV is sufficient to successfully reproduce the measured stability ratios. Due to the uncertainty in the ζ-potential measurement, only semiquantitative calculations of the stability ratio can be given.

Effect of Bentonite Prehydration Time on the Stability of Lightweight Oil-Well Cement System

“…When the clay is exposed to water, the sodium ions of the clay mineral and the ions from the bulk medium form the double layer around the particles and generate a repulsive force between them [7]. This force is responsible for the stability of the colloidal system [30]. The addition of calcium ions will suppress the double layer and compromise the network structure, leading to flocculation, sedimentation, and destabilization of the colloidal suspension [7,29,31].…”