Research On Biochemical Treatment Of Fracturing Flowback Fluid

Fracturing is an important oil recovery technology to improve the oil flow environment.Fracturing fluid is discharged back to the surface after the completion of the fracturing operation.Fracturing backflow fluid has the characteristics of high viscosity,high COD value,containing a variety of chemical reagents,which is difficult to treat.It has become one of the main pollutants in oilfields.Direct outflow of untreated fracturing backflow fluid will cause serious pollution to the environment and soil.Therefore,the treatment and recycling of fracturing backflow fluid is of great significance to the environmental protection and cost reduction of oilfields.In recent years,biological treatment of fracturing backflow has become a research hotspot because of its high efficiencies and no secondary pollution in treatment of fracturing reflux.In this paper,the algae-microorganism co-culture system was used to treat the fracturing reflux fluid efficiently,optimize the conditions of microalgae culture,and analyze the removal mechanism of pollutant and the regulation mechanism of algae-lipid synthesis and metabolism when microalgae-microorganism co-culture was used to treat the fracturing reflux fluid.The algae-bacteria symbiotic system was used to treat fracturing backflow fluid.The effects of dilution factor,aerobic bacteria dosage and pH on Chlorella density were studied by response surface methodology.The experimental results show that the order of influence on the growth density of Chlorella is pH,dosage of aerobic bacteria and dilution multiple of fracturing fluid.The second-order data model was established through the selection of experimental data,and the reliability and fitness of the model are good.Finally,the regression model equation between the three factors and the response value was established,and the best mixed culture of algae and aerobic bacteria was obtained by solving the equation.The optimum conditions are as follows: the dilution multiple of fracturing backflow fluid is 4.33,the dosage of aerobic bacteria is 42 mg/L,the pH is 6.7,and the optimum density of Chlorella can reach 2.522 g/L.The interaction between the dilution multiple of fracturing backflow fluid and the dosage of aerobic bacteria is significant.The interaction between dilution multiple and pH,as well as between the dosage of aerobic bacteria and pH exists.The effect of aerobic bacteria dosage,anaerobic sludge dosage and hydraulic retention time on COD removal efficiency of fracturing backflow fluid was investigated by using algae-bacteria co-solidification and anaerobic sludge treatment.The experimental results show that the order of influence on treatment effect is anaerobic sludge dosage,hydraulic retention time and aerobic bacteria dosage.Through the selection of experimental data and the establishment of a second-order data model,the results of analying the model of fitting property is good,and finally established the regression model equation.Considering the removal effect of pollutants from fracturing fluid,the optimal treatment conditions are determined as follows: the dosage of aerobic bacteria is 20mg/L,the dosage of anaerobic sludge is 43.78%,and the hydraulic retention time is 44 h.The optimal COD removal rate can reach 42.13%.The interaction of the three factors was analyzed by response surface graph.The results showed that there was a certain interaction between the dosage of aerobic bacteria and hydraulic retention time,and between the dosage of anaerobic sludge and hydraulic retention time.The interaction was significant,but the interaction between the dosage of aerobic bacteria and the dosage of anaerobic sludge was not significant.The activities of SOD(superoxide dismutase)and ACCase(acetyl coenzyme A carboxylase)were measured during biological treatment of fracturing backflow with chlorella-aerobic bacteria.The effects of fracturing reflux fluid on the antioxidant activity and lipid synthesis of Chlorella were analyzed.The results showed that both SOD activity and ACCase activity of Chlorella spp.increased significantly when the algae-bacteria symbiotic treatment was used to fracturing reflux fluid,which indicated that the co-growth of Chlorella spp.and aerobic bacteria could promote the removal of reactive oxygen species and resist the stress of adverse environment,and the algae-bacteria symbiotic treatment could promote the synthesis of algae fatty acids and improve the Bio-productivity efficiency.