Abstract: Water-in-oil emulsions are very easy to form in reservoir water injection development, and the mechanism of the effect of brine on asphaltene precipitation in emulsions is not clear. To address the above problems, based on the self-made experimental oil and water-in-oil emulsions, static centrifugal experiments and dynamic displacement experiments were carried out by spectrophotometry and simultaneous oil-water injection method, respectively, to study the effects of water cut, salt type, and salt mass concentration on asphaltene precipitation in emulsions, and reveal the mechanism of asphaltene precipitation under the action of salt water. Research shows that when the water cut in emulsion increases from 10% to 50%, the asphaltene instability increases and the mass fraction of precipitation increases from 4.5% to 10.6%, but the precipitation intensity weakens. Asphaltenes with low aromaticity are more likely to precipitate at the water/oil interfaces mainly because of adsorption by heteroatoms on the water surfaces at the interface. Asphaltene precipitation tends to increase and decrease with the increase of salt mass concentration, reaching the maximum when the salt mass concentration reaches 40 000 mg/L. MgCl₂, CaCl₂, NaCl, and KCl affect the amount of asphaltene precipitation from high to low. The emulsion with a 40% volumetric concentration of distilled water shows the most significant decrease in permeability during the flooding, followed by a 20% mass concentration of MgCl₂ emulsion, while the 20% distilled water and 20% NaCl emulsions show the smallest decrease in permeability. The effect of brine on asphaltene precipitation in the dynamic displacement is smaller than that in the static experiment. Still, the increase in water cut increases asphaltene precipitation and decreases core permeability. The research results are significant in improving the development effect of water injection in asphaltene reservoirs.

Key words: emulsion, asphaltene precipitation, salt mass concentration, water flooding reservoir, permeability, absorbance