Application of Electro-Oxidation Technology for Water Treatment and Its Impacts on Rock Wettability

The unconventional oil and gas industry has been facing numerous operational, economic, and environmental challenges for treating produced water. Although much research has been conducted on improving the treatment of produced water, few studies evaluated the performance of treated produced water when it is reused for fracturing and EOR. In this paper, we treated sodium chloride (NaCl) solution using an electro-oxidation (EO) treatment process and evaluated how the treatment changes brine properties and how the wetting characteristics change with treated brine. The brine properties, including density, surface tension (ST), oil-water interfacial tension (IFT), viscosity, and pH values were measured and compared before and after the treatment. Then, we conducted systematic contact-angle measurements using treated and untreated brine to study the effects of water treatment on rock-fluid interactions. The results show that the EO process slightly reduces ST, density, and viscosity, and slightly increases the pH value. Also, we observed that the IFT between oil and treated brine is slightly lower than that between oil and untreated brine. However, the contact-angle results show that the treated brine provides stronger wetting characteristics than the untreated one. We hypothesized that the change in wetting characteristics is mainly due to the generated oxidants and hydroxide (OH^-) from the EO process. To further investigate the mechanisms behind the change in wetting characteristics after treatment, we investigated the diffusion of OH^-, H⁺, hydrogen peroxide (H₂O₂), and Sodium hypochlorite (NaOCl) into untreated brine and monitored the contact-angle variations. The results suggest that H₂O₂ and OH^- are responsible for the stronger brine wetting characteristics, and NaOCl and H⁺ are responsible for the stronger oil wetting characteristics. We also found that the EO process produces a significant amount of active chlorine. However, the stability of active chlorine is low, and it can evaporate as chlorine gas. Based on the results, we concluded that the oil-saturated rock samples in treated brine generally have a higher wetting affinity to water than those in untreated brine, and H₂O₂ and OH^- are the main substances that cause the wettability alteration to more water-wet conditions.