Research On Biochemical Treatment Of Fracturing Flowback Fluid

Fracturing technology can effectively improve the diversion capacity of oil and gas reservoirs,and it is the main measure to increase production of low permeability and ultra-low permeability in oil and gas field.The fracturing fluid is usually composed of water(98-99.2%),a variety of chemical additives(0.5-2%)and propping agent,and the fracturing fluid is returned to the surface after the fracturing process is completed,which is called the fracturing flow-back fluid.The composition of the fracturing flow-back fluid is complex and contains a large amount of impurities,which is seriously polluted environment.With the increasing tension of water resources and the strengthening of national environmental protection,the treatment technology of fracturing wastewater has been developing rapidly,and new technique and process are emerging.In this paper,using Chlorella on fracturing fluid for biological treatment,combined with Box-Behnken design to optimize the conditions of partially curing the Chlorella and the fracturing flow-back fluid treatment using response surface method,and in-depth analysis the influence on the growth of Chlorella in the process of the fracturing flow-back fluid,the effect of pollutant removal and the coupling mechanism between pollutant removal and biological capacity.In order to improve cell density and pollutant removal rate,alginate was used to embed Chlorella.The effects of three factors,alginate concentration,calcium chloride concentration and crosslinking time on the growth of Chlorella,were investigated by response surface method combined with Box-Behnken design.The results showed that in three factors,alginate concentration have the greatest influence on Chlorella Growth,followed by calcium chloride concentration and crosslinking time.The quadratic polynomial regression equation between three factors and cell density was fitted,and the regression analysis proved that the equation fitted well.The three-dimensional images reveal the interaction law of the three factors,showing that the interaction of alginate concentration and crosslinking time is not significant and the interaction of alginate and calcium chloride concentration is significant.There is a interaction between the concentration of calcium chloride and the crosslinking time.The optimum conditions for the partially curing of Chlorella were obtained by solving the equation: the concentration of alginate is 2.15%,the concentration of calcium chloride is 2.56%,and the crosslinking time was 17.29 h.Partially curing Chlorella under the optimal conditions to,It is showed that the COD could reach 34.70%,turbidity removal rate of 80.62% and the cell density of 1.96g/L under the optimal conditions of treating with the fracturing flow-back fluid.A continuous variable surface model was established by Box-Behnken design,and three factors and three levels of dilution multiple of the fracturing flow-back fluid,the dosage of ?-mannanase and activated carbon were designed to investigate the influence of three factors on the treatment effect.The results show that the influences of dilution multiple have a great influence on the effect,because the dilution can reduce its biological toxicity,promote Chlorella growth proliferation and pollutant degradation,but when the dilution ratio increases to a certain value,the content of organic compounds is too low,and carbon source is insufficient,leading to growth proliferation of Chlorella is limited.At the same time,activated carbon powder helps to reinforce chlorella balls and reduce the leaks of chlorella.Moreover,It was found that ?-mannanase degraded guanidine gum as monosaccharide,provided carbon source for Chlorella,increased capacity and removal rate of pollutants,and the dosage of ?-mannanase has a great effect on the growth and proliferation of suspended Chlorella.The optimal treatment conditions for determining the pollutant removal and the capacity of Chlorella were as follows: The dilution multiple,the dosage of ?-mannanase and activated carbon were 5mg/L,135mg/L,50mg/L,respectively.And the cell density can reach 2.09g/L,and the removal rate of COD can reach 45.27% under the optimal treatment conditions,which is higher than the COD removal rate of 9.08% of the suspended Chlorella under the same condition.Using the Design Expert statistical software the data were analyzed by regression fitting,and a regression fitting equation was established between dilution multiple of the fracturing flow-back fluid,the dosage of activated carbon and ?-mannanase,COD removal rate and cell density.Regression analysis showed that the equation fitted well.The three-dimensional images reveal the interaction law of the three factors,showing that the the interaction of dilution multiple and the dosage of activated carbon is significant and the interaction of the dosage of activated carbon and ?-mannanase is significant and there is a interaction between the dilution multiple and the dosage of ?-mannanase.