(63c) Applicable pH Range of EDTA in Lifted-Salinity Conditions

Chelating agents are chemicals that can interact with multivalent metal ions in the solution. This property makes chelating agents applicable materials for the treatment of formation rock at lifted salinity conditions. Besides, their wettability alteration potential on carbonate rock surfaces has been recognized by many studies. In the application of chelating agents, pH is an important parameter to consider as it controls the degree of hydrolysis of chelating agents, while different hydrolysates have significantly different solubilities in water. This work focuses on the solubility of chelating agents at different pH conditions, and the stability of the chelating agent + seawater at different pH conditions. EDTA, the most used chelating agent, was adopted in this study. To dissolve EDTA in deionized water, NaOH was used to raise the pH of the solution. HCl was used later to lower the pH to check the solution's stability. Salts including NaCl, NaHCO₃, Na₂SO₄, MgCl₂, and CaCl₂, are used to prepare synthetic seawater by dissolving these salts in deionized water. All solutions were prepared to have a mass of around 16g and contained in glass vials. They were conditioned at different temperatures to check their solubility and stability. Besides, an anionic surfactant SDS was tested with EDTA to assess the application of their combination at different pH and salinity conditions. Results showed that the solubility of EDTA increased with pH. At a pH of 3, EDTA could not dissolve to make a 1 wt% solution. At a pH of 5, the concentration of EDTA can be raised to 3 wt%. Further addition of EDTA will result in precipitation. At pH 7~13, EDTA concentration can reach 5 wt% with no solubility issue. However, at a high pH of 10, metal ions in the synthetic seawater are not stable, so are the complexes formed between the chelating agent and the metal ion. Ca²⁺ has a strong impact on the precipitation of SDS at room temperature. When the temperature is lifted to 50, 70, and 90 °C, the precipitations of SDS are reduced and vanish. The impact of temperature was also observed in the cases of MgCl₂ and NaCl. However, it was not observed in the synthetic seawater case. It appears that a combination of different metal ions results in more stable precipitations than a single metal ion. This study tested the solubility and stability of solutions containing EDTA of varying concentrations at different pH and temperature conditions. Results suggest a reasonable pH range of 7~9 for the application of EDTA that will not cause precipitation either from EDTA or from metal ions in synthetic seawater.