Experimental Study On Shale Gas Pressure Cracking And Liquid Discharge Treatment In Eastern Sichuan

In this paper, shale gas fracturing flowback fluid in the eastern Sichuan Basin was taken as study subject and some treatment experiments were investigated to protect soil, surface water, groundwater and even human health from being contaminated by it. Based on the water quality characteristics of the shale gas fracturing flowback fluid in this area and its disposal targets (both underground injection and reusing the flowback fluid for a future fracturing job after treatment were targeted in this paper) and combined with the characteristics and applications of coagulation and Fenton oxidation process, the coagulation-Fenton oxidation processes were chosen to treat the wastewater.Orthogonal and single-factor experiments were carried out in the coagulation process and Fenton oxidation process, respectively, to determine the appropriate values of various factors affecting these processes. The orthogonal experiment was designed to address the favorable ranges of various affecting factors and their primary and secondary sequences. After the orthogonal experiment, based on the results of the orthogonal experiment (the primary and secondary sequences of the various factors and their preliminary optimal values), the single-factor experiment was conducted in sequence under certain conditions to determine the appropriate value of each affecting factor sequentially according to their primary and secondary sequences.In the coagulation experiment, type of coagulant, concentration of coagulant, concentration of P AM flocculants, pH value, and time of standing were chosen to be affecting factors affecting this process and orthogonal experiment of five factors and five levels was carried out in the coagulation experiment. The results of orthogonal experiment indicated that the primary and secondary sequences of the various factors that influenced the removal effect of pollutants in the coagulation experiment was the concentration of the coagulant, type of coagulant, pH value, time of standing and concentration of PAM flocculants and the preliminary optimal values of them were as follows:the concentration of the PAC coagulant was 100 mg/L, the concentration of the PAM flocculant was 20 mg/L, the pH value was 8 and the time of standing was 90 min. After the orthogonal experiment, the single-factor experiment was conducted in sequence based on the results obtained in orthogonal experiment and the appropriate value of each affecting factor was determined, and the results were:the PAC coagulant was 80 mg/L, the concentration of the PAM flocculant was 15 mg/L, the pH value was 8 and the time of standing was 60 min.In the Fenton oxidation experiment, pH value, amount of 30% H2O2, concentration of Fe2+, reaction temperature and time of stirring were chosen to be affecting factors affecting this process and orthogonal experiment of five factors and five levels was carried out in the Fenton oxidation experiment. The results of orthogonal experiment indicated that the primary and secondary sequences of the various factors that influenced the removal effect of pollutants in the Fenton oxidation experiment was pH value, the amount of 30% H2O2, the concentration of Fe2+, the time of stirring and the reaction temperature and the preliminary optimal values of them were as follows:the pH value was 3, the amount of 30% H2O2 was 6 ml/L, the concentration of Fe2+ was 15 mmol/L, the time of stirring was 150 min and the reaction temperature was 40℃. After the orthogonal experiment, the single-factor experiment was conducted in sequence based on the results obtained in orthogonal experiment and the appropriate value of each affecting factor was determined, and the results were:the pH value was 3.5, the amount of 30% H2O2 was 6 ml/L, the concentration of Fe2+ was 15 mmol/L, the time of stirring was 90 min and the reaction temperature was 40℃. Furthermore, the number of times of 30% H2O2 addition also impacted the removal effect of pollutants in the Fenton oxidation experiment, the optimal number of times of 30% H2O2 addition was determined to be 4 according to comprehensively considering the operability of the addition and COD removal.The rejection adaptability of the effluent after the coagulation-Fenton oxidation processes was evaluated by comaparing the water quality of the effluent and the water quality indexes of re-injected wastewater in the petroleum and natural gas industry standard of Recommended Practice for Produced-water Reinjection in Gas Field (SY/T 6596-2004). The result showed that the water quality of the effluent met the water quality requirements for underground injection. Besides, the effluent has well compatibility with formation water. Thus, the effluent was qualified to be injected into disposal wells according to the requirements of disposal wells and targeted formation.The reusing adaptability of slick water prepared by the effluent was evaluated, and the result of the evaluation and the performance requirements of slick water in the local petroleum and natural gas standards of Technical Specifications of Sliding Water for Fracturing (DB 61/T 575-2013) were compared. By the comparsion of them we found that the performances of slick water prepared by the effluent treated by the coagulation-Fenton oxidation processes met the performance requirements of slick water. Therefore, the effluent was qualified to be reused to prepare slick water for a future fracturing job.