Research On The Treatment Technology Of Produced Wastewater From Polymer Flooding In Shengli Oil Field

With polymer flooding applied to enhance oil recovery and increase oil production in many oilfields, large amount of wastewater has been produced. The produced water from polymer flooding contains a quantity of residual hydrolyzed polyacrylamide (HPAM), which seriously impedes the oil-water separation. The old wastewater treatment process in the oilfield cannot meet the standard in treating produced wastewater from polymer flooding. The combined method "coagulation-hydrolysis acidification-dynamic membrane bioreactor" and "hydrolysis acidification-dynamic membrane bioreactor-coagulation" processes were developed.The applicable coagulants were chosen and compounded based on the comparison of the single coagulation effect of each coagulant at first. The coagulation mechanism of produced wastewater from polymer flooding by aluminum salt coagulants and the coagulation dynamics of different coagulates were studied. In the combined processes, the optimal dosage of the coagulant, the optimal hydraulic retention time in hydrolysis acidification reactor and the rule in dynamic membrane bioreactor operating were determined. As the combined processes running for 30 days, the feasibility of discharging the produced wastewater from polymer flooding was examined.The main contents and results are as follows:When the produced wastewater from polymer flooding was treated by coagulation method, the oil or turbidity removal efficiencies of traditional aluminum salt or iron salt coagulants were much better than those of the inorganic polymer coagulants or organic polymer flocculants. When the aluminum chloride (AC) and iron chloride (FC) were compounded with the dosage mass ratio of 1:3, it could not only improve the settling performance and decrease the volume of sludge, but also obtained high oil and turbidity removal efficiencies. When the compounded aluminum-iron coagulant was compounded with anionic polyacrylamide (APAM), the oil and turbidity removal efficiencies were dominated by the dosage of the inorganic compounded aluminum-iron coagulant. And at the dosage of 0.5 mg/L, APAM could bridge the flcos and improve the settling performance and decrease the volume of sludge.When the actual wastewater was treated by aluminum salt coagulants, the mainly removal effect was the crosslinking action between Al3+ and residual HPAM in the produced wastewater. But the crosslinking action could be hindered by the crude oil and salt. Aluminum sulfate had better efficiencies of removing oil and HPAM because of the better crosslinking action with HPAM. The dosages were increased and the treatment effects were reduced as crude oil, HPAM and salt existing in the wastewater.The flcos size was not decreased when compounded aluminum-iron coagulant reacted with the produced wastewater in the coagulation dynamics research. Inorganic coagulant bonded with HPAM in produced wastewater from polymer flooding through chemical bond, so the flcos were steady and not easy to be broken.The combined process of "coagulation-hydrolysis acidification-dynamic membrane bioreactor" was operated for 30 days. The optimal operation parameters were as follow:the dosage of compounded aluminum-iron coagulant was 140 mg/L, the HRT in hydrolysis acidification reactor was 8 h and the head drop of dynamic membrane bioreactor was 5 cm with 140 h running cycle. The results showed that the combined process was stable after optimizing the operation parameters. After coagulation, COD could decrease from 612.4 mg/L to 168.9 mg/L. After hydrolysis acidification, the COD fell to 147.7 mg/L. After aerobic biological process, the COD reduced to 79.7 mg/L. The COD of effluent could steadily reach the class I National Wastewater Discharge Standard before 1998, and the class II National Wastewater Discharge Standard after 1998. The average concentration of NH3-N in influent was 27.3 mg/L, after the combined coagulation, hydrolysis acidification and dynamic membrane bioreactor processes, the concentration decreased to 1.5 mg/L, and could steadily reach the class I National Wastewater Discharge Standard after 1998.The combined process of "hydrolysis acidification-dynamic membrane bioreactor-coagulation" was operated for 30 days. The optimal operation parameters were as follow:the HRT in hydrolysis acidification reactor was 12 h, the head drop of dynamic membrane bioreactor was 5 cm with 140 h running cycle and the dosage of compounded aluminum-iron coagulant was 120 mg/L. The results showed that the combined process was stable after optimizing the operation parameters. After hydrolysis acidification, COD could decrease from 627.7 mg/L to 570.9 mg/L. After aerobic biological process, the COD fell to 476.6 mg/L. After coagulation, the COD reduced to 75.9 mg/L. The COD of effluent could steadily reach the class I National Wastewater Discharge Standard before 1998, and the classâ…¡National Wastewater Discharge Standard after 1998. The average concentration of NH3-N in influent was 29.8 mg/L, after the combined coagulation, hydrolysis acidification and dynamic membrane bioreactor processes, the concentration decreased to 1.6 mg/L, and could steadily reach the class I National Waste water Discharge Standard after 1998. In despite of the shortcoming of "hydrolysis acidification-dynamic membrane bioreactor-coagulation" process, it provided a new biological method to treat produced wastewater from polymer flooding with the development of produced wastewater, the HPAM degrading bacteria and the new biological technology.