Study On The Harmless Treatment Of Unconventional Fractured Backflow

Wastewater produced in fracturing process in oil&gas wells is flow-back wastewater after fracturing construction. Recently, many oil fields in China belong to unconventional layered oil&gas reservoirs. Also, they have large amounts of anisotropic and permeable nature fractures. Hydraulic fracturing technology is widely used to exploit unconventional oil&gas reservoirs. Produced flow-back wastewater after hydraulic fracturing has characteristics of high CODCr, viscosity, stability, oil content, chromaticity and SS content. It will bring about serious environmental pollution without handling. Sources, components, characteristics of pollutants and handling methods of unconventional fracturing flow-back wastewater were described systematically in the paper. Handling agent K2FeO4was synthetized by a new method, and a new handling technology was proposed, including oxidation and gel breaking, advanced oxidation, coagulation and sedimentation, magnetic separation.A green and efficient gel breaker K2FeO4was synthesized by a two-step wet process with Fe(NO3)3and Ca(ClO)2as initial material. Gravimetric analysis and chromite titrimetry show the productivity of K2FeO4is67.63%and the corresponding purity is93.41%. The molecular composition and molecular structure were examined by XRD, FT-IR and SEM. Using K2FeO4as gel breaker, single factor and orthogonal experiments show the optimal dosage of K2FeO4, pH, and reaction time are3000mg/L,11.0, and40min, respectively. Besides, the mechanism of gel breaking was also studied by SEM in this work.Magnetic separation technology was used to handle unconventional fracturing flow-back wastewater after gel breaking. Single factor experiment shows that the optimal dosage of nano-sized magnetic particle, dosage of Anionic flocculent P-2and stirring intensity are4000mg/L,200mg/L, and300r/min, respectively.Electric-decomposition catalytic oxidation combined with Fenton oxidation was used as advanced oxidation technology. Single factor experiment show that, under the rapid stirring, optimal oxidized voltage, current density, pH, reaction time of electric-decomposition catalytic oxidation are10.0V,16mA/cm2,11.0, and60min, respectively. In the meanwhile, orthogonal experiment shows that the optimal dosage of H2O2, dosage of Fe2+, pH, reaction time are2500mg/L,2000mg/L,3.0, and90min, respectively. Coagulant AMN made in lab was used in subsequent coagulation, and its optimal dosage was2500mg/L.With the optimal conditions of the new designed handling technology mentioned above, viscosity of flow-back fluid reduced from12.7to1.4mPa·s, which approached viscosity of pure water (1.0mPa·s). Additionally, removal rates of CODCr, SS, oil content and chromaticity are95.6%,97.4%,97.5%, and100.0%, respectively. All indexes meet the requirements of first level discharge standard of GB8978-1996.