Study on the Key Issue in the Application of Nanoemulsions in Preflush Spacer: Contamination of Cement Slurry by Nanoemulsions

Summary Nanoemulsions have the potential to efficiently remove oil-based drilling fluids (OBDFs) due to their particular structure and properties. However, the oil phase in nanoemulsions is not compatible with cement slurries. Whether this will cause contamination of cement slurry is an issue worth discussing. In this paper, the potential contamination of cement slurry by nanoemulsions and the corresponding mechanism are discussed. The results show that the addition of nanoemulsion will not cause the thickening of cement slurry but will slightly affect the mechanical properties of hardened cement. When the cement slurry is mixed with 6.375 wt% nanoemulsion, the compressive strength and elastic modulus of hardened cement decrease by 8.9 and 14.9%, while the permeability increases by 11.8%. The mixing of nanoemulsions does not affect the hydration of cement slurry, but the oil droplets in the emulsions will lead to the formation of tiny pores in the hardened cement, which would adversely affect the mechanical properties of hardened cement. The oil droplets in nanoemulsion are coated by surfactants, so it does not affect cement hydration. The oil droplets are small and evenly distributed, which can reduce the influence of nanoemulsion on the microstructure of hardened cement. The stability of emulsion droplet in cement slurry is important but easy to be ignored. The possibility of incompatibility between nanoemulsion and cement slurry can be reduced by reducing the adsorption of surfactants. Nanoemulsions would maintain good stability under the function of surfactant interfacial film and yield stress of cement slurry.