Green Treatment And Mechanism Of Fracturing Flowback Fluid By Oxidation Of Highly-energetic Potassium Ferrate

Fracturing technology is the major measure to ensure high and stable production of oil and gas fields.After the completion of fracturing operation,a large amount of fracturing flowback fluid is produced,which has large yield,complex composition,high viscosity,high chemical oxygen consumption(COD),high polymer content,and high emulsification degree,therefore,it is difficult to reach the discharge standard by using conventional treatment technology alone.The chemical oxidation method has many advantages of not changing the existing flow process and high operation flexibility,which is widely used.However,the application of the chemical oxidation method is restricted by the large secondary pollution of most oxidation treatment agents and the hidden safety problems of storage.Potassium ferrate(K₂FeO₄)has high redox potential,large electrochemical theoretical capacity,and strong chemical oxidation performance,meanwhile the reduced product is environmentally friendly and pollution-free,moreover,it has the advantages of demulsification,oxidation,flocculation,adsorption,sterilization,and relatively safe storage,etc.As a new type of green and efficient water treatment agent integrating multiple functions,K₂FeO₄ has important theoretical research value and practical application prospect in the wastewater treatment of oilfield.Potassium ferrate was synthesized by the hypochlorite oxidation method,and the key factors affecting the oxidation performance of the solution were investigated.A green composite oxidation system of potassium ferrate was established,and the oxidation mechanism of guar gum was explored by thermodynamic calculations and kinetic investigations combined with multiple characterization methods,in which guar gum was taken as a model compound of characteristic contaminants in fracturing flowback fluid.A new process of pretreatment-oxidation-flocculation treatment of fracturing flowback fluid was proposed,and the mechanisms of demulsification,flocculation,and oxidation were analyzed.The synergistic effect of ultrasonic and electric field on mass transfer and oxidation treatment of fracturing flowback liquid was explored.Finally,the on-site fracturing flowback fluid oxidation treatment test was conducted to provide a new idea for the advanced treatment of fracturing flowback fluid and the use of reinjection.The main results are as follows:(1)The optimal synthesis parameters of K₂FeO₄ by the hypochlorite method are as follows:Fe(NO3)3 as the iron source,the initial concentration of 2 mol/L,the KOH concentration of 11 mol/L,the reaction temperature of 15°C,and the reaction time of 40 min.Under the condition,the orthorhombic K₂FeO₄ with a yield of 38%and a purity of 88.2%was obtained.K₂FeO₄ had better stability in the alkaline solution system,and the stability was improved with the addition of KCl or(NH4)2S2O8,where the appropriate addition was m(KCl):m(K₂FeO₄)=1:1 or m((NH4)2S2O8):m(K₂FeO₄)=1:1.(2)High polymer content is a significant feature of fracturing flowback fluid.The thermodynamic calculation for the oxidation of guar gum by K₂FeO₄ was carried out.The negative value of?H indicates that the process was an exothermic reaction and the negative value of?G indicates that the reaction process was feasible.The suitable process conditions for the oxidation of guar gum by K₂FeO₄ are as follows:the pH value of 12,the temperature of 30?,initial concentration of K₂FeO₄ of 0.5 g/L,and the addition of KCl at m(KCl):m(K₂FeO₄)=1:1.Meanwhile,the COD removal rate was 65.0%,the viscosity was reduced from 1.49 m Pa·s to 1.04 m Pa·s,and the removal rate of total organic carbon(TOC)was more than 77%.The oxidation products of guar gum by K₂FeO₄ mainly included galactose,mannose,alcohols,ketones,etc with different molecular weight,and the weight average molecular weight was reduced from 2.38×10⁶ g/mol to 1752 g/mol and 708 g/mol.The oxidation reaction mechanism of guar gum by K₂FeO₄ was studied,and it was found that the1-4 bonds and 1-6 bonds of galactose and mannose in the molecular structure of guar gum were easy to be further broken,resulting in the fragments with relatively small molecular weight.The oxidation reaction of guar gum by K₂FeO₄ basically conformed to the first-order kinetic model.(3)The fracturing flowback fluid treatment process included:(1)Quiescence and the first flocculation treatment:Under the condition of the quiescence time of 24 h,the temperature of55?,the pH value of 10,and the coagulant aid polyacrylamide(PAM)of 100 mg/L,the separation effect of large solid particles and oil droplets was better.The types of flocculants were also optimized.When the polyaluminum chloride(PAC)was 0.4 g/L,the time was 30min,the temperature was 25°C,and the pH value was 7,the viscosity of the solution decreased from 1.45 m Pa·s to 1.28 m Pa·s,the solid removal rate was 60.2%(from 93 mg/L to39 mg/L),the oil content removal rate was 88.7%(from 1177 mg/L to 133 mg/L),the COD removal rate was 53.3%(from 5190 mg/L to 2424 mg/L),the corresponding TOC removal rate was 59.0%(from 648.72 mg/L to 265.98 mg/L),the removal rate of chromaticity was96%(from 500 degree to 20 degree),and the removal rate of turbidity was 93.7%(from483.10 NTU to 30.20 NTU),the flocs settled faster.(2)Oxidative demulsification treatment:when K₂FeO₄ was 5 g/L,pH=10,reaction time was 30 min,and the temperature was 40?,the demulsification efficiency was 91.8%(oil content decreased from 133 mg/L to 10.9 mg/L),the COD removal rate was 44.8%(from 2424 mg/L to 1337 mg/L),the corresponding solution viscosity decreased from 1.28 m Pa·s to 1.10 m Pa·s,and the TOC removal rate was54.0%.(3)After the second flocculation treatment,the COD value of the fracturing flowback fluid was reduced to approximately 600 mg/L,the TOC value was 41.58 mg/L,the viscosity was 1.03 m Pa·s(close to the viscosity of water),the suspended solids was 7 mg/L,the oil content was 5.2 mg/L,and the median particle size of the suspended solids was approximately2?m.The K₂FeO₄ almost completely entered into the flocs in the form of Fe(III)through flocculation and had almost no effect on the chromaticity of the solution.The turbidity at this time was 2.26 NTU,which met the standard of sewage reinjection.(4)The synergy between the enhancement of mass transfer by ultrasonic or electrochemical field and the K₂FeO₄ oxidation process could not only reduce energy consumption and oxidation cost,but also further improved the oxidative degradation efficiency of fracturing flowback fluid,which was conducive to improving economic benefits.When the ultrasonic field was strengthened,the pH of the solution was 10,the ultrasonic frequency was 40 k Hz,and the output power was 30%,the operating temperature was reduced from 40°C to 35°C,and the reaction time was shortened from 30 min to 20 min.The addition amount of K₂FeO₄ was reduced by 40%(from 5 g/L to 3 g/L),the COD removal rate of fracturing flowback fluid still increased by 12.2%(from 44.8%to 57%),and the TOC removal rate increased by approximately 12.5%(from 54.0%to 66.5%),the viscosity of the solution decreased from 1.28 m Pa·s to 1.04 m Pa·s.With the introduction of the external electric field,when the metal oxide electrode was DSA,pH=7,the electrolysis time was 30min,and the electrolysis voltage was 8 V,the reaction temperature was reduced from 40°C to20°C,the COD removal rate of fracturing flowback fluid increased by 20.2%(from 44.8%to65.0%),the TOC removal rate could be increased by approximately 17.8%(from 54.0%to71.8%),and the viscosity of the solution decreased from 1.28 m Pa·s to 1.02 m Pa·s.(5)The field application for the oxidation of the fracturing flowback fluid was tested in an oil production plant in the oilfield.Under the process condition for the treatment capacity of fracturing flowback fluid of 16 m³/h,the treatment temperature of 25?,the flocculant polyaluminum chloride(PAC)of 0.4 g/L,the coagulant aid polyacrylamide(PAM)of 0.1 g/L,and the K₂FeO₄ of 5 g/L,the combined process of flocculation-oxidation-flocculation was performed,resulting in that the COD value of the fracturing flowback fluid was less than 950mg/L,the oil content was less than 10 mg/L,the suspended solid content value was less than7 mg/L,the TOC value was less than 60 mg/L,the chromaticity value was less than 20 degree,and the turbidity value was less than 10 NTU,which met the water reinjection standard.The economic cost analyses demonstrated that the synergism oxidant or the synergistic function between the enhancement of mass transfer by outfield and chemical oxidation was an effective way to reduce the economic cost in the future.Green and high-energy K₂FeO₄ oxidizer for treating chemically driven fracturing rejection fluid,not only guarantee the smooth operation of the production process of oilfield fracturing,but also reduce the reaction cost.At the same time,the reasonable combination with the added oilfield chemicals was conducive to guiding the use of various oxidation treated agent in the field,which was of great significance to the production of oilfield.