Study On Flocculation-sedimentation And Advanced Oxidation Treatment Of Oilfield Fracturing And Returning Fluid

With the increase of energy demand and consumption in modern society,injection of additional water into the reservoiv to maintain water pressure is a common way to increase its oil recovery.A large amount of fracturing and returning fluid will be produced during the process.These waste liquid generally contains complex components and various pollutants.High concentration of suspended soilds,salinity and organic matters is the main characteristic of fracturing and returing fluid,which increase difficulty of its treatment.Components of the fracturing and returning fluid from the No.5 Oil Production Plant?Dingbian?Oilfield of Changqing Oilfield are analyzed.The results indicat that:The fracturing is mildly acidic,and has poor biodegradability.The concentrations of SS and organic matter are 596 mg/L and 4400 mg/L respectively.The results of ion chromatography show that the salinity of the wastewater is very high.To deal with these characteristics of the fracturing and returning liquid,the coagulation is used to pretreat the wastewater to remove suspended solids.Secondly,the advanced treatments?Fenton advanced oxidation and photocatalytic?are employed to remove high concentrations of refractory organics in wastewater.The process is optimized and the conclusions are draron as follows:?1?For the flocculation treatment process,are as follows:the optimum results can be achieved under the stirring strength of 250 r/min,when the pH of the wastewater is 7,the dosage of PAC and PAM are 1.5 g/L and 0.03 g/L respectively.The optimum results is shown as follows:COD of the wastewater decreased from 4400mg/L to 1991mg/L,and the removal rate is54.75%;The SS decreased from 589 mg/L to 18 mg/L,and the removal rate is 96.9%;The removal rates of Ca²⁺,Mg²⁺,Cl^-are 59.87%,43.83%,and 47.01%.The mechanism of flocculation is investigated through the analysis of floc morphology and strength.The results suggest that the aggregation state of flocculation is better under the optimal treatment conditions,and the branching structure is more obvious.Therefore,the floc is more likely to aggregate and adsorb more particles.?2?For Fenton advanced oxidation process,the optimum result is achieved when adding 0.4 g FeSO₄?7H₂O and 4 ml H₂O₂ into 100 ml fracturing fluid?pH=4?and react under UV irradiation for 75 min.The optimum result show that COD is removed by 73.73%and finally reduced to523 mg/L.The optimum processing conditions were simulated by response surface methodology:The results indicate that when pH is 4.01,the dosage of FeSO₄?7H₂O and H₂O₂are 0.47g and 4.35ml,respectively.The removal efficiency of COD can reach up to 79.09%.The result simulated is close to the result of experiment,indicating that the model fit the process well.Under the optimum fitting parameters,the removal efficiencies of Mg²⁺and Ca²⁺can achive 78.61%and 78.94%,respectively after Fenton treatment.?3?The results of photocatalytic advanced treatment indicate that the degradation efficiency of nano-ZnO supported on coconut shell activated carbon is better than another two photocatalysts.The optimum result can be achieved when pH is 4,UV irradiation time is 4h and the dosage of photocatalyst is 2 g/L.After treatment,COD reduced to 71.54 mg/L,which meet the petrochemical industry pollutant discharge standard?GB31571-2015?.To investigate the reusability of the catalyst,the nano-ZnO supported on the coconut shell activated carbon is reused for 4 times under the optimal treatment conditions.The results show that the removal efficiency of COD is as high as the first time,which proved that the catalyst.After three processes,COD and SS of the fracturing fluid is 72 mg/L and 10 mg/L,respectively,which meet the petrochemical industry pollutant discharge standard?GB31571-2015?.At the same time,concentrations of other pollutants in wastewater have also been greatly decreased.