Removal Of Ferrous And Sulfur Ions In Oilfield Wastewater And Their Mechanisms On Reducing Viscosity Of Partially Hydrolyzed Polyacrylamide

As the enormous output of oilfield wastewater, oilfield wastewater has been treated and injected again for reducing cost and achieving sewage recycling utilization.However, metal and reductive ions in oilfield wastewater have negative impact on viscosity of partially hydrolyzed polyacrylamide (HPAM) which has been widely applied to expel oil from the ground. According to preparing polymer solution by oilfield wastewater, effects of Na+, K+, Ca2+, Mg2+, Fe3+, Fe2+ and S2-ions on the polymer solution viscosity were investigated and compared. All ions had a negative impact on polymer solution viscosity, and meanwhile Fe2+ had the most remarkable impact. What’s more, trivalent cations and divalent cations exerted much more effects than monovalent ones.In this study, oxidation and coordination methods were chose to remove the Fe2+ ions in oilfield wastewater, and effects of potassium permanganate (KMnO4), hydrogen peroxide (H2O2), sodium hypochlorite (NaClO), carbamide peroxide (CO(NH2)2·H2O2), sodium percarbonate (2Na2CO3·H2O2), potassium persulfate (K2S2O8), potassium mono-sulfate triple salt (2KHSO5·KHSO4·K2SO4), phenanthroline and potassium tartrate were investigated and compared. It was found that all oxidants could remove Fe2+ quickly and effectively, resulting in an increase of polymer solution viscosity; conversely, phenanthroline and potassium tartrate had no effects. Compared with other oxidants, NaClO was the best and cheapest oxidant, and the optimal dosage was 0.75 times of Fe2+dosage. However, NaClO itself could decrease the viscosity of HPAM, and the viscosity loss in simulated water was much more than that in deionized water. The optimal dosage of NaClO remained unchanged, regardless of the ferrous ions dose and ionic strength changed. However, the viscosity of HPAM solution could enhance with lower ionic strength.Under the condition of anaerobic, S2- ions also had an remarkable effect on polymer viscosity. KMnO4, H2O2, NaClO, CO(NH2)2H2O2,2Na2CO3·3H2O2 and 2KHSO5KHSO4K2SO4 were chose to remove the S2- ions in oilfield wastewater and the result indicated that KMnO4, H2O2, NaCIO and CO(NH2)2H2O2 could remove S2- ions quickly and increase polymer solution viscosity effectively. Compared with other oxidants, KMnO4 was the best oxidant for higher HPAM viscosity recovery and least cost, with 1.5 times of S2- dosage. When the dose of S2-was 2 mg/L, the optimal dosage of KMnO4 increased,2 times S2- dosage. The ionic strength had no effect on the dosage of KMnO4, but could change the polymer viscosity, the viscosity of HPAM solution could increase with lower ionic strength.The mechanism of reduced HPAM viscosity was studied by IR, XRD and SEM. The results showed that shear force could decrease HPAMviacosity; Na+, K+, Ca2+ and Mg2+ decreased HPAM solution viscosity by interacting with the carboxylate anions, which may lead to the polymer curl. For Fe2+ and S2-, the viscosity degradation was attributed to a series of free-radical reactions, which could cause disintegration of the polymer chains and reduction of the viscosity. The addition of oxidant could alleviate the free radical reaction caused by Fe2+ and S2-.