Louise Bailey

Rheology modification in mixed shape colloidal dispersions. Part I: pure components

The flow behaviour and rheology of colloidal dispersions are of considerable interest in many applications, for example colloidal clay particles find applications in oilfield and construction-drilling fluids. The rheological properties of such fluids can be enhanced significantly by adding colloidal particles of different size and shape. To gain insight into the mechanism of this phenomenon, we have studied model mineral-colloid systems whose shape changes systematically from a plate-like aluminasol (gibbsite), through a lath-like smectite clay (hectorite), to a rod-like aluminasol (boehmite).The paper presents the results of a systematic and comprehensive multi-technique study (oscillatory, transient and steady shear) of the rheology of dispersions of these model systems. This gives a detailed account of the yield space that characterises the complex transition of these soft materials from elastoviscous gels to viscoelastic liquids, and of the effect of particle size and shape on this behaviour. The observed phenomena are underpinned by two competing flow-mediated microstructural rearrangements that have significantly different timescales. A physical model invoking flow-mediated building and disruption of fluid structure is described to rationalise the observed behaviour. The study also forms the baseline to a companion study (part II), which investigates the rheological behaviour of mixed anisometric colloid systems based on these pure components.

Rheology modification in mixed shape colloidal dispersions. Part II: mixtures

We report the results of a comprehensive study of the rheological properties of a series of mixed colloid systems where the shape of one of the components has been varied systematically. Specifically we have measured the oscillatory, transient (creep) and continuous steady shear flow behaviour of a 2.5 wt% dispersion in water of a well-characterised hectorite clay modified by the addition of a series of aluminasol colloidal particles whose shape varies systematically from rod (boehmite) to platelet (gibbsite) to sphere (alumina-coated silica), all having essentially the same smallest dimension, which is similar to that of the hectorite. The particle characterisation and rheological properties of the pure components have recently been reported in Part I of this series (Soft Matter, 2007, 3, 1145). The mixtures show the same general behaviour as the pure systems, displaying a complex yield space transition from an elastoviscous gel at low applied stresses to a viscous, weakly elastic, shear-thinning liquid at high stresses. The unifying theme of this work is that the addition of 0.25 wt% of the minor component in all cases results in dramatic enhancements to the dispersion rheological properties. At the same time the magnitude of this effect depends on the shape of the particles. Shear moduli, low stress viscosities and effective yield stresses all increase in the additive order rods < platelets < spheres, with enhancements for the latter being up to a factor of 500 and typically 20. At the same time the critical failure strains for the gels decreased in the same order - the strongest gels are also the most fragile in this sense. The physicochemical factors underlying this behaviour are discussed and a simple qualitative model described. While no complete explanation or model can be proposed at this stage, the study provides a quantitative model-system baseline for mixed colloidal dispersions already used for industrial applications (e.g. oilwell-drilling fluids) and suggests ways in which such fluids may be optimised and controlled.

Particulate invasion from drilling fluids

Particulate invasion is one of the primary mechanisms of formation damage caused by drilling fluids. During the initial stages of filtercake growth particles are forced into the formation, building an internal filtercake which plugs the near-surface pores. Where bridging is inadequate, the depth of invasion can be considerable. Removal of this internal cake can be difficult, leading to reduced productivity. Better understanding of its properties will lead to more effective cleanup and enhanced productivity. As part of an EC-Joule funded study into formation damage mechanisms, we have investigated both the invasion of solids and their mobility during clean up. We have systematically investigated the effect of mud particle size on rock substrates of varying permeabilities. The results indicate that formation damage increases with increasing substrate permeability. Substrate permeability was also found to have a marked effect on the initiation of production; flow initiation pressures were greatest on low permeability cores. Quantitative profiles of solids invasion into sandstone cores for a range of water-based drilling fluids were obtained using scanning electron microscopy/ X-ray mapping (SEM-EDS) and synchrotron energy-dispersive X-ray diffraction tomography (EDD-T). This latter technique, using a synchrotron source, samples the intact core rather than a fractured surface, and gives analytical powder diffraction data, whereas a conventional CAT scanner provides attenuation data only. The effect of back flooding on the invasion profile was then examined, and permeability damage associated with the invasion was also determined. The invasion was compared to damage profiles obtained by core sectioning. Invasion profiles were found to drop quite steeply but fines could be observed deep with in the core. The presence of external cake was found to influence the ease of clean up of the invaded zone. A deep bed filtration model has been developed for the effect of solids invasion on permeability reduction. This model is compared to the profiles obtained in the invasion experiments.

Rheology modification of montmorillonite dispersions by colloidal silica

We have studied the effect of additions of both anionic and cationic spherical silica colloids of different sizes on the rheology of dispersions of a well-characterised montmorillonite clay, SWy-2. The systems have been studied above and below the critical hydrodynamic overlap concentration, c*, of the clay. For dispersions at c < c* on replacement of ∼10 % w/w of the clay content by silica, it was found that whereas a cationic silica additive transformed a liquid-like, non-gelling montmorillonite dispersion into a substantial gel, anionic silica destroyed any nascent structure in the fluids, reducing the effective viscosity and virtually eliminating the rheological hysteresis characteristic of structured fluids. On the other hand, in the regime of c > c*, replacement of ∼10 % w/w of the clay content by silica leads to enhancements of all the rheological parameters characteristic of a gelling system, for the addition of both anionic and cationic silica. A simple tentative microstructural model for this complex behaviour is presented. This work, alongside our previous studies, confirms significant rheological modification by the addition of small quantities of nanoparticles as a general phenomenon of clay-colloid systems. It further suggests that viscosity enhancement and control of the rates of sol-gel transitions for product applications can be achieved using relatively low-cost, commercially available materials, such as silica nanoparticles and natural clays of different mineralogy.