Filtration properties of oil-in-water emulsions containing solids

This paper describes a laboratory study of the factors controlling the filtration and fluid-leakoff properties of emulsions containing solid particles. Such fluids are sometimes used as low-leakoff completion fluids. High-pressure/high-temperature (HP/HT) filtration tests clearly demonstrate that emulsions have the ability to reduce filtrate loss to the formation. However, the emulsion droplets tend to invade the formation and form an internal filter cake. This is evident from the liftoff pressures needed to initiate flowback and deep invasion of emulsion droplets in long-core experiments. Emulsions containing solids (CaCO3, in our case) had lower filtrate volume and higher return permeabilities than solids-free fluids. The effects of percent oil, filtration pressure, core permeability, temperature, and viscosity of the continuous phase were investigated. Filtration pressure and core permeability had major influences on the filtration properties of the emulsified completion fluids. Higher injection pressures increased internal damage and lowered the return permeability. Higher-permeability cores had higher filtrate loss. Emulsion droplets were observed in the effluent, and solid particles are needed to form a stable external filter cake. Long-core experiments showed that using emulsions containing acid-soluble solid particles have 100% return permeability after an acid squeeze, showing that the emulsion-droplet invasion depth is less than 1 in. It is shown that solids-free fluids had the highest formation damage and higher liftoff pressures. Analysis of the fluid-leakoff data indicates that emulsions containing solids do not behave as classical filtration theory predicts because of the invasion of emulsion droplets into the formation. Sizing the solid particles and emulsion droplets in completion fluids and muds according to the permeability of the rock is important in forming stable external filter cakes. This increases the return permeability and results in easier cake removal (low liftoff pressures).