Significance of ozonation on surfactant-polymer flooding produced water coagulation: Water quality deterioration inhibition, high-standard reinjection and mechanism

Surfactants and polymers are extensively being used to enhance oil recovery resulting in the generation of surfactant-polymer (SP) flooding produced water, which is more challenging to treat for reinjection compared with conventional produced water. Aiming at high-standard SP flooding produced water reinjection, this study investigated the coagulation-based treatment processes for the treatment of SP flooding produced water. The conventional coagulation process with two typical coagulants, poly-aluminum chloride (PAC) and poly-ferric sulfate (PFS), was applied. However, the effluent did not meet the reinjection requirements for suspended solids (SS) ≤ 2 mg/L, oil ≤ 6 mg/L and median diameter d₅₀ ≤ 1.5 μm according to the water quality standard for oilfield injecting waters in clastic reservoirs (SY/T 5329–2012). The presence of adsorbed surfactant (betaine) and polymer (hydrolyzed polyacrylamide, HPAM) on oil and suspended solids increased the difficulties in agglomeration and charge-neutralization. In addition, water quality deterioration occurred within 5 h leading to increased turbidity, suspended solids and d₅₀. X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis implied that water quality deterioration was caused by souring (producing sulfide), leading to the generation of suspended solids mainly containing sulfur and calcium carbonate (CaCO₃). To address these challenges, the pre-ozonation coagulation (POC) and hybrid ozonation-coagulation (HOC) processes were compared. The effluent from both processes, using either PAC or PFS, met the reinjection requirements while inhibiting water quality deterioration. Additionally, higher removal efficiency and significantly reduced optimal ozone dosage were also achieved in the HOC process compared with the POC process. The enhanced generation of hydroxyl radicals (•OH) and polymeric hydrolysed coagulant species in the HOC process contributed to higher removal efficiency of HPAM, betaine and COD. Kinetics analysis revealed that •OH oxidation and coagulation played important roles in betaine and HPAM removal, respectively. The reaction pathways of betaine and HPAM in the ozonation and HOC processes were also analysed. Furthermore, the HOC process can effectively inhibit the accumulation of small molecular weight organic matter, facilitating the simultaneous coagulation.