Temporal variations in geochemistry of hydraulic fracturing fluid and flowback water in a tight oil reservoir

Hydraulic fracturing facilitates the development and exploitation of unconventional reservoirs. In this study, the injected hydraulic fracturing fluid (HFF) and flowback and produced water (FPW) in tight oil reservoirs of the Lucaogou Formation in the Junggar Basin are temporally sampled from day 1 to day 64. Freshwater is used for fracturing, and HFF is obtained. The chemical and isotopic parameters (including the water type, total salinity, total dissolved solids (TDS), pH, concentrations of Na⁺, Cl⁻, Ba⁺, K⁺, Fe²⁺ + Fe³⁺, and CO₃²⁻, δD, and δ¹⁸O) are experimentally obtained, and their variations with time are systematically analyzed based on the flowback water. The results show that the water type, Na/Cl ratio, total salinity, and TDS of the FPW change periodically primarily due to the HFF mixing with formation water, thus causing δD and δ¹⁸O to deviate from the meteoric water line of Xinjiang. Because of water–rock interaction (WRI), the concentrations of Fe²⁺ + Fe³⁺ and CO₃²⁻ of the FPW increase over time, with the solution pH becoming more alkaline. Furthermore, based on the significant changes observed in the geochemistry of the FPW, three separate time intervals of flowback time are identified: Stage Ⅰ (< 10 days), where the FPW is dominated by the HFF and the changes in ions and isotopes are mainly caused by the WRI; Stage Ⅱ (10–37 days), where the FPW is dominated by the addition of formation water to the HFF and the WRI is weakened; and finally, Stage Ⅲ (> 37 days), where the FPW is dominated by the chemistry of the formation water. The methodology implemented in this study can provide critical support for the source identification of formation water.