Zeta potential of limestone in a large range of salinity

Experimental investigations have been conducted to elucidate the variation of the zeta potential (ζ) of limestone in a large range of salinity. The effects of the type and concentration of inorganic salts, the pH, and the adsorption of the oil polar fractions, naphthenic acid and cationic surfactant on the ζ have been investigated in the paper. The results show that the ζ rises toward the positive with the increase of the concentration of inorganic salt, but the increase rate gets very low in the high-concentration brine. The mineral composition of the limestone has also some influence on the ζ. The high pH value results in the large ζ, and the adsorption of oil polar fractions and naphthenic acid is beneficial to the increase of the ζ. However, increasing the salinity reduces the effect of the pH, oil polar fractions and naphthenic acid on the ζ, and the ζ is nearly not affected by the above influencing factor in the formation water (total salinity: 223,802.8 mg/L). Among the three surfactants of dodecyl trimethyl ammonium chloride, tetradecyl trimethyl ammonium chloride and octadecyl trimethyl ammonium chloride, the adsorption of octadecyl trimethyl ammonium chloride onto the rock surface is more efficient to increase the ζ, and the more adsorption leads to the higher ζ in the surfactant solutions with the same salinity. The adsorption quantity of octadecyl trimethyl ammonium chloride onto the limestone is the largest when the surfactant concentration is the same, and the high salinity promotes the adsorption of the surfactant onto the limestone. Nevertheless, the ζ decreases rather than rising due to the simultaneous increase of the salinity and surfactant adsorption. The above results indicate that the salinity is the dominant influencing factor on the ζ of the limestone in various conditions, which can be used as theoretical guidance for improved oil recovery in the carbonate reservoir with different salt contents.