Hydraulic Characteristics of Bentonite Cake Fabricated on Cutoff Walls

Nguyen et al. (2012) presented an interesting paper concerning the fundamental and hydraulic properties of three commercial bentonites and have provided some unusual interpretations. They are to be commended for obtaining a substantial amount of filter-cake data from a relatively simple commercial instrument – the American Petroleum Institute (API) fluid-loss apparatus. However, the present authors question whether the API fluid-loss apparatus has sufficient resolution to justify the detailed analyses and conclusions that the authors present. The present authors also have reservations about the original authors' viscosity test results and the use of the viscosity-time relationship for the interpretation of the fluid-loss results. Suitability of the vibro viscometerThe authors present viscosity–time plots for three commercial bentonites at concentrations of 4, 6, and 8% (figure 2 of Nguyen et al. , 2012). The reported slurry viscosities are from investigations with a sine-wave vibro viscometer (A&D Company Ltd, 2008). This viscometer is a relatively new instrument based on a tuning-fork vibration principle. It measures what the manufacturer refers to as the “static viscosity” of the fluid, which is defined as the product of the viscosity and the fluid density. The present authors have concerns about the suitability of this instrument for measuring the viscosity of non-Newtonian fluids such as bentonite slurries which can show significant viscoelastic behavior. For these fluids the authors' (static) viscosity will be a function of both the elastic and viscous properties of the material. Furthermore, unlike conventional viscometers, which are based on the measuring principles of concentric cylinders, cone and plate, capillary, etc. , the viscosity value from the authors' instrument cannot be associated with a shear rate. The viscosity of non-Newtonian fluids such as a bentonite slurry is very dependent on the applied shear rate and unless the shear rate is known the viscosity data are of little use for …