Coupling iron-carbon micro-electrolysis with persulfate advanced oxidation for hydraulic fracturing return fluid treatment.

Improving the sustainability of the hydraulic fracturing water cycle of unconventional oil and gas development needs an advanced water treatment that can efferently treat flowback and produced water (FPW). In this study, we developed a robust two-stage process that combines flocculation, and iron-carbon micro-electrolysis plus sodium persulfate (ICEPS) advanced oxidation to treat field-based FPW from the Sulige tight gas field, China. Influencing factors and optimal conditions of the flocculation-ICEPS process were investigated. The flocculation-ICEPS system at optimal conditions sufficiently removed the total organic contents (95.71%), suspended solids (92.4%), and chroma (97.5%), but the reaction stoichiometric efficiency (RSE) value was generally less than 5%. The particles and chroma were effectively removed by flocculation, and the organic contents was mainly removed by the ICEPS system. Fourier-transform infrared spectroscopy (FTIR) analysis was performed to track the changes in FPW chemical compositions through the oxidation of the ICEPS process. Multiple analyses demonstrated that PS was involved in the activation of Fe oxides and hydroxides accreted on the surface of the ICE system for FPW treatment, which led to increasing organics removal rate of the ICEPS system compared to the conventional ICE system. Our study suggests that the flocculation-ICEPS system is a promising FPW treatment process, which provides technical and mechanistic foundations for further field application.

Treatment and recycling of acidic fracturing flowback fluid.

Acidic fracturing flowback fluid (AFFF) has the characteristics of low pH value, high chemical oxygen demand (COD), high corrosiveness and complex components. Surface discharge without treatment may contaminate the environment. However, wastewater treatment after centralized transportation has potential safety risks and requires high costs. In this study, we confirmed that calcium and magnesium could affect cross-linking property of fracturing fluid prepared by flowback fluid, and conducted a three-step process, two-stage filtration, chemical precipitation, and flocculation precipitation, on AFFF. After treatment, we made new hydraulic fracturing fluid using the treated acidic flowback fluid as base fluid and compared the quality of the new hydraulic fracturing fluid to the ones used freshwater as base fluid. The results showed when concentration of sodium carbonate, polyaluminium chloride (PAC), polyacrylamide (PAM) were 145, 1000, and 20 mg/L respectively, the treatment result was optimal. After treatment, the oil content of AFFF decreased from 7400 to 26.53 mg/L and suspended solids (SS) from 650 to 18.24 mg/L, and the removal rate of high-valence metal ions was more than 99%. The rheological properties and viscoelasticity of new fracturing fluid prepared by the treated AFFF were similar to the ones prepared by freshwater, which met the requirements of high temperature and shear resistance for ultra-deep wells.