This paper is an attempt to study the mechanisms controlling the filtration and fluid leakoff properties of emulsions containing solid particles.

HPHT 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 lift-off pressures needed to initiate flow-back 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, injection pressure, core permeability, temperature and viscosity of the continuous phase were investigated. Injection 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 one inch. It is shown that solids free fluids had the highest formation damage and higher lift-off pressures.

Analysis of the fluid leakoff data indicates that emulsions containing solids do not behave as classical filtration theory predicts. Due to the invasion of emulsion droplets into the formation the data did not fit the calculated filtration profiles.

Sizing the solid particles and the emulsion droplets in muds according to the permeability of the rock and the external cake is important in forming stable external filter-cakes. This increases the return permeability and results in easier cake removal (low lift-off pressures).