Hydraulic fracturing is extensively used in the petroleum industry to enhance well productivity. However, the discharge of flowback fracturing fluids after fracturing brings out significant environmental challenges. Flowback fluids usually contain poisonous additives, which may cause deaths of plants and animals if discharged directly to the ground. This paper introduced an effective processing and recycling system to deal with flowback fracturing fluids in Keshen gas field of Tarim basin, China.

Apparatuses such as AA7020 atomic absorption spectrophotometer, etc. were used to test the properties of the flowback fracturing fluids in Keshen gas reservoir so as to find out which properties are needed to be processed to meet the requirements. Then various technologies of processing harmful substances in the fluids like suspended solids and high-valence metal ions were compared and selected via repeated experiments. The procedures to compound new fracturing fluids using the processed fluids were also optimized and tested. Based on the selections and optimizations, a processing and recycling system to deal with on-site flowback fracturing fluids was founded and put into operation.

The results of laboratory tests of the flowback fluid properties showed that there was more than 60% of weighing additive residual in the flowback fluid, most of which could be recycled. The contents of suspended solids and high-valence metal ions in flowback fluids far exceeded the required water quality standard for discharge, and they had to be processed to re-compound fracturing fluids. For the processing of suspended solids, the effectiveness, costs and efficiency of three methods, Chemical coagulation-sedimentation, membrane filtration and sand filtration were compared and sand filtration method was selected as the best one. For processing of high-valence metal ions, the methods of complexing stabilization, ion exchange and chemical precipitation were compared and the chemical precipitation method was selected as the most cost-effective one. After the quality of processed fluid meet requirements, the procedures of adding the other additives were also optimized to guarantee good properties of the newly-compounded fluid. A recycling system has been founded to process the flowback fracturing fluids on-site, which has a processing capability of 300m3 per day. During on-site trials, 970m3 flowback fracturing fluids have been already processed and recycled successfully, and 560m3 of fluids have been successfully used for fracturing of well KeS2-2-16.

The laboratory experiments and on-site trials have proved the feasibility of the recycling system. It reduces the volume of fluids discharged to the environment by 70% in Keshen gas field, and is expected to reduce more if the processing system is further improved. As a result, it has the potential to be popularized to relieve the environmental stress brought by hydraulic fracturing.

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