Principle And Application Of Modular Treatment Process For Oilfield Wastewater Treatment With Variable Water Quality

Appropriate treatment of oilfield wastewater is an important prerequisite for sustainable oil exploitation in ecologically fragile areas.The traditional treatment processes are difficult to treat oilfield wastewater with complex and viarable water quality because of their single and fixed processes.Due to the limitation of site conditions,the wastewater cannot be well homogenized,especially in the complex terrain area.As a result,pollutants such as SS and petroleum cannot meet the requirements of reinjection or reuse steadily.In this study,the characteristics of oilfield wastewater from Changqing were investigated,and the treatability of different types of oilfield wastewaters were obtained.The treatment processes for oilfield wastewater with different treatment difficulties were optimized by comparing different modular process combinations.Thus,a modular and variable-process wastewater treatment system was developed.On this basis,the treatment performance of oilfield wastewater using modular variable process in practical application was evaluated.The research results provided theoretical and technical support for efficient treatment of oilfield wastewater.The main results was summarized as follows:(1)Changqing oilfield wastewater exhibited a characteristics of high concentration,complex compostion and hyper stability.The pollutant indexes of different types of wastewater were significantly different.Based on the characteristics of wastewater,the water quality evaluation matrix was established.According to the results of water quality evaluation,6 kinds of oilfield wastewater were divided into three categories(low contaminative wastewater(LCW),medium contaminative wastewater(MCW),and high contaminative wastewater(HCW)).The treatability index of EM wastewater and well washing wastewater were below 0.15,which belongs to low contaminative wastewater.The treatability index of biopolymer fracturing fluid,thickened wastewater and acidising wastewater was between 0.15 and 0.2,which belongs to medium contaminative wastewater.The treatability index of guanidine gum wastewater was 0.287,which belongs to HCW.Based on the above research,a modular and variable-process wastewater treatment system was developed.(2)Based on the results of water quality evaluation,combined with the comparative analysis of 8 modular combination processes,the modular combination modes suitable for different types of wastewater were determined.The results showed that for the LCW,the utilization of pretreatment module(coalescent oil removal)and the solid-liquid separation module(dissolved air floatation)can achieve efficient pollutant removal.For the MCW,because of its high organic content and high suspended solid,oxidation and solid-liquid separation should be adopted on the basis of oil removal.According to the comparison and selection of different module combination processes,the combined process of oil removal pretreatment and dissolved ozone flotation(DOF)module can be used to treat this kind of wastewater efficiently.For the high viscosity of HCW,it is necessary to add oxidation and viscosity-reducing pretreatment modules on the basis of oil removal.The addition of Fe/C and DOF module can synergically enhance the removal of pollutants according to processes comparison and selection.(3)According to the results of modular configuration,the optimal operation conditions of Fe/C and DOF modules were explored.Under the optimal treatment conditions,the COD_cr removal efficiency of Fe/C could reach 53%.The COD_cr and SS removal efficiency of DOF could reach 67.2%and 82.1%,respectively.The DOF module can remove tryptophan and humus pollutants effectively,and the contents of hydrophobic neutral substances(HON)and hydrophobic acid substances(HOA)were significantly decreased.In terms of enhanced solid-liquid separation,in order to improve the separation effect of pollutants,the floc size was inversely proportional to the flow intensity and collision probability of bubbles in DOF separation zone.The floc size could be appropriately reduced to enhance the floc-bubble collision and improve the flotation efficiency.(4)The module configuration and practical application for wastewater was investigated.For the reinjection water quality of treated water,the main limiting factors of water quality were SS(?2 mg/L)and median particle size(?1.5?m).Therefore,for different three types of wastewater,the general modular combination should be coalescence oil removal,dissolved air floatation(DAF)and microfiltration(MO)module.For the MCW,DOF module should be added on the basis of the above general modular combination.For HCW,Fe/C and DOF modules should be added simultaneously.The effluent of the three types of oilfield wastewater can meet the reinjection requirements by adopting the modular variable processs.(5)For the purpose of drilling liquid preparation,the effluent was constrained by the total hardness(?300 mg/L),and the appropriate removal of organic matter should also be considered.Therefore,for the above three types of wastewater,the general combination process was oil removal,dehardening,DAF and MO module.For the MCW,DOF module should be added on the basis of the above module combination.For the HCW,Fe/C and DOF modules should be added simultaneously;For the preparation of drilling fracturing fluid,the limiting factors were COD_cr(?300 mg/L)and salt content(?20000 mg/L).In addition to the drilling fluid preparation process,the bypass reverse osmosis module was added to enhance the removal of salt and organic matter for three types of wastewater;For urban miscellaneous water consumption,considering the treatment cost and complexity of the process,only the LCW was treated.The system was composed of oil removal,dehardening,DAF,MO and RO modules.The SS?10 mg/L,color?30 times,turbidity?5 NTU,and TDS?1000 mg/L,which meets the requirements of urban miscellaneous water consumption.