Research on the comprehensive characteristics and instability causes of shale oil produced water quality

The water quality characteristics of shale oil produced water are complex and highly unstable, which poses difficulties for its treatment, storage, and resource utilization. This study focused on shale oil produced water from 8 sites in Qingyang area of Changqing Oilfield. Through multiple indicator detection techniques, the water quality characteristics and characteristic pollution composition were analyzed. Combined with sediment morphology and composition, the internal causes of water quality instability were analyzed. The results showed that shale oil produced water had characteristics of high salinity(TDS>13 000 mg/L), high chemical oxygen demand(COD>645.4 mg/L), low biodegradability(B/C<0.1), and high instability(Zeta potential tends to 0, Polydispersity Index PDI tends to 1),mainly consisting of white calcium precipitates(CaCO3, CaSO4), brown iron oxide precipitates(Fe(OH)3, Fe2O3), and black precipitates (FeS2, FeS), which were accompanied by a large amount of NaCl leading to an increase in precipitation. The instability of produced water was affected by both inorganic and organic components. Alkylbenzene sulfonic acid surfactants reduced the surface tension of produced water, promoted the aggregation and precipitation of particulate matter and high concentrations of Ca2+, HCO3-, SO42- ions under neutral conditions, and continuously precipitated Fe2+through oxidative hydrolysis reaction. The correlation analysis results showed that Cl-, Ca2+, Mg2+, and Ba2+ mainly contributed to the conductivity of the produced water. High concentrations of SO42- and HCO3- significantly reduced surface tension and were the main factors leading to unstable water quality in shale oil produced water.