Rheology of silica-based dispersions and Cross-sectional modeling of settling slurries
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The stability and flow of suspensions (Rheology) is a complex field of investigation, but is of significant importance in many industrial processes in the oil industry. For colloidal particles, flow rheology becomes particularly complex when interactions between particles are present. In this thesis, silica is the primary particle, as it is one of the most common reservoir particles and can be used as flow modifiers or as model particles for various materials such as sand. The studies are performed in polar solutions such as in water or mixtures of water and glycerol. The focus is mainly on suspension rheology as a function of particle volume fraction, solvent pH, ionic strength, glycerol/water mixing ratio, temperature and pressure. The effects of pH, salinity and particle volume fraction are investigated for critical particle concentration at the transition from Newtonian to shear thinning flow in each system. It is revealed that suspensions at acidic pH exhibit high stability even at their isoelectric point and in the presence of high NaCl concentrations, due to the stabilising effect of short range repulsive hydration forces. On the other hand, suspensions with high salt concentrations and basic pH show time dependant rheology and large yield values. These effects are attributed to destabilization mechanisms such as ion exchange and particle bridging. With increasing particle concentrations or salinity, the strength of gelled networks is also found to increase. In mixtures of water and glycerol, increasing glycerol concentrations result in a transition from a highly flocculated gel to stable dispersions containing no microstructures. At intermediate glycerol concentrations, an irreversible and strong shear thickening is observed, resulting from hydrodynamic effects over shadowing colloidal interactions. The shear rate at the onset of shear thickening is found to be depressed by decreasing particle and salt concentrations as well as by increasing glycerol concentrations. Increasing temperatures accelerate aggregation kinetics. The rheology shows increased liquid-like viscoelastic properties, and a weaker gel network. In mixtures of water/glycerol, a transition from a viscoelastic solid to shear induced gelation and highly Newtonian behaviour occurs at increasing temperature. The abnormal stabilizing effect of temperature is attributed to a smaller degree of hydrogen bonding for water and glycerol, leading to formation of fewer silica bridges. The thickening behaviour is observed at intermediate temperatures. The shear thickening behaviour is enhanced by increasing pressure, due to 2 contributing effects: reduction in electrostatic stability, and increase in glycerol and water hydrogen bonding. For settling suspensions, a rheometric method is established to delineate the effects of settling on apparent viscosity and the effects of particle concentration on viscosity. Our approach is based on an inversion of a cross section model for vertical particle concentration gradients and local rheology in the gap of Couette geometries. The particle distribution model is based on equilibrium between gravitational settling and shear induced migration, and the rheology model (Krieger-Dougherty) with adjustable parameters is applied as a function of local particle concentration and shear rate. Experimentally observed shear thinning behaviour arises from a combination of Krieger-Dougherty model and particle dispersion-settling. The small influence of particle size on apparent viscosity is attributed to different intrinsic viscosity and different viscosity parameters. Good agreement is obtained for maximum packing fraction values obtained experimentally and values determined by model inversion. Experimental and theoretical consistency is observed in both the water and oil phase, with various particle sizes and densities.