Pressure-Dependent Yield Stress of Organoclay-Based Gels

We provide a detailed investigation of the pressure and temperature dependence of flow curves and, in particular, of the yield stress, σ_y, of a model drilling fluid formulation and the parent colloidal dispersion. We carefully considered the limitations of the high-pressure cell and designed reliable protocols covering a pressure range up to 100 MPa and a temperature range up to 85 °C. For both systems, the viscosity at high shear rates increased with pressure, scaling well with the pressure dependence of the solvent viscosity. For clay dispersions, σ_y increases slightly with temperature at a given pressure and much more with pressure across the whole temperature range (25-85 °C). For the drilling fluid formulation, σ_y increases with pressure and decreases with temperature. The time evolution of σ_y at high pressures (aging) was found to be different for the two systems. The clay dispersions did not show any aging of σ_y, whereas the drilling fluid formulation shows a pressure-dependent aging of σ_y. The aging is well captured by phenomenological kinetic models, with the amplitude of the aging being dependent on pressure but the kinetics not being dependent on pressure. The results suggest that the pressure does not affect much the volume fraction but rather affects the interactions between clay particles and emulsion droplets in the drilling fluid formulation. These new findings are in line with our previous results on the microscopic dynamics of the drilling fluid under high pressure and provide insights into the origin of the yield stress and its evolution with pressure and temperature in these technologically relevant systems.