The Study On Multivariate Oxidative Degradation Treatment Of Fracturing Sewage In Oil Field

Hydraulic fracturing technology,a common method to increase oil or gas production in petroleum engineering,could create transportation aisles for oil or gas and improve the permeability of petroleum stratum.However,this method can produce a large amount of troublesome sewage with high chemical oxygen demand(COD).As the most important index in secondary liquid preparation,reinjection and discharge,COD requires effectsive reduction for water saving and environment protection purposes.This thesis used Changqing fracturing sewage as the research object,and studied a three-step sewage treatment method:Fenton oxidation assisted by ultrasonic pretreatment,flocculation and sedimentation retreatment,and microelectrode electrocatalysis or photocatalysis combined with ultrasonication for further oxidation.After process optimization,COD decreased from 10616 to 2351 mg/L after the first step,to 1541 mg/L after the second step,and 105 or 13mg/L after the last step.Besides,reduced viscosity,discoloration,reduced turbidity and improved water quality were also achieved.To be specific,the optimum reaction conditions for Fenton oxidation were:0.44%H2O2(30%)、1%FeSO4·7H2O,pH value of 6,room temperature(20℃),and the ultrasonic power of 120W.With the reaction time of 18 minutes,the removal of COD reached 77.85%.After the flocculation and sedimentation,the COD removal reached 34.45%with the following optimized conditions:pH value of 8,400ppm of a composite flocculant,and 15ppm of coagulant aid PAM.The flocculated liquid sample was electrooxidized with various metal oxide electrocatalysts supported on active carbon(AC).The best result was obtained with CuxOy/AC microelectrodes,showing good repeatable degradation performances for sewage retreatment.Its removal of COD was 93.19%under optimized conditions:initial pH value of 9,sewage volume of 300mL,air velocity of 2L/min,the current density of 17.5mA/cm2,temperature of 30℃,3g/L of the catalyst,reaction time of 60 minutes.SEM,XRD,XRF,BET and XPS were used to characterize CuxOy/AC catalyst.It was found that the catalyst with the Cu content of 22.67%(Cu+:Cu2+to be 2.261:1)showed the best degradation activity.After several repeated experiments by CuxOy/AC,the morphology,pore volume and pore diameter of the catalyst remained unchanged.Photocatalytic oxidation for flocculated sewage was carried out by Fe2O3/TiO2 obtained by equal volume soaking method,while TiO2 was synthesized through precipitation.Photocatalytic oxidation combined with ultrasonication had a good effect of oxidative degradation under the following optimized conditions:pH value of 9,175W of ultraviolet power,32kHz ultrasonic frequency,air velocity of 3L/min,temperature of 30℃,while the removal of COD was 99.16%with the Fe loading of 5.5%.XRD results showed that the rutile and anatase ratio of TiO2 support was 39.8:60.2,while SEM results showed TiO2 was in the form of spherical particles.The kinetics of microelectrode electrocatalysis or photocatalytic oxidation combined with ultrasonic system for flocculated sewage were studied.The results showed that activation energy of photocatalytic oxidation combined with ultrasonic system was 12.304kJ/mol,lower than the activation energy of the electrocatalysis oxidation(30.075kJ/mol),facilitating the removal of COD.