In this paper, we aim to investigate the wettability alteration of carbonate rocks by ion tuned waters (ITW). It has been widely accepted that the oil-wet carbonate surface can be altered into water-wet or mix-wet by waters that containing potential determining ions (PDI, Ca2+, Mg2+ and SO42-). However, the underlying reasons are still not well understood. In order to estimate the relative effects of PDI in determining the surface charge and wettability, a systematic experimental study was carried out. Firstly, zeta potential, contact angle and intermolecular forces were measured in four salt solutions (NaCl, MgCl2, CaCl2, and Na2SO4) of same ionic strength (IS=0.33M). It suggested that the surface charge of calcite particles and oil droplet was very sensitive to Ca2+ and Mg2+ ions. The calcite surface was changed to more water-wet by MgCl2 solution and Na2SO4 solution whereas to less water-wet by CaCl2 solution. In force measurement, the atomic force microscopy (AFM) in coupling with ‘soft tip’ technique was applied. Repulsions were observed in NaCl, MgCl2 and Na2SO4 solutions. However, oil droplet jumped into contact with calcite surface in CaCl2 solution. Accordingly, the adhesion between oil droplet and calcite surface was the largest in CaCl2 solution amongst the four salt solutions. It thus suggested that Ca2+ ions contributed negatively to the surface charge change and wettability alteration of calcite surface while Mg2+ and SO42- ions were able to enhance the water wetness of calcite surface.

Secondly, six waters were manipulated to potentially change the surface charge of chalk towards more negative and meanwhile enhance the water-wet of chalk surface. Zeta potential results proved that the chalk surfaces were modified to more negative by these ion tuned waters, in comparison to the original seawater. Contact angle results additionally proved that these ion tuned waters were efficient in altering the chalk surface to more water-wet, especially in the cases of seawater with one-fourth Ca2+ concentration or Mg2+ concentration-SW0.25Ca and SW0.25Mg, respectively. Therefore, SW0.25Ca or SW0.25Mg was potential injection water to enhance the oil recovery in carbonate reservoirs.

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