Objectives and Methods  Large-scale hydraulic fracturing represents a key technology for shale gas exploitation. However, engineering practices indicate that the flowback rates of fracturing fluids are low and vary significantly across different blocks. Furthermore, some components in fracturing fluids may cause groundwater contamination. To explore the adsorption characteristics and mechanisms of Ba²⁺ within fracturing fluids in shale reservoirs, this study investigated four typical types of samples, including shale and three shale samples with different compositions, from the Longmaxi Formation in the Changning Block, southern Sichuan Basin. Systematic experiments were conducted on the adsorption of shales with different mineral compositions for Ba²⁺, highlighting the impacts of time, initial Ba²⁺ concentration, NaCl ionic strength, pH, and temperature.

Results and Conclusions The Ba²⁺ adsorption process of the four types of shale samples can be divided into three stages: rapid adsorption, slow adsorption, and adsorption equilibrium. The montmorillonite-dominated sample (Sample III) exhibited a far higher adsorption capacity than the sample consisting predominantly of illites, calcites, and quartz (sample I), followed by those dominated by chlorite (sample IV) and by feldspars and quartz (sample II) sequentially. The equilibrium adsorption capacities of all the samples increased with the initial Ba²⁺ concentration. With an increase in NaCl ionic strength, all samples showed increasing Ba²⁺ adsorption capacities except for sample III, which exhibited a decreasing Ba²⁺ adsorption capacity. Regarding the impacts of pH, alkaline conditions were more favorable for Ba²⁺ adsorption onto samples II, III, and IV ²⁺, whereas acidic conditions contributed more significantly to the adsorption of sample I for Ba²⁺. Temperature exerted an insignificant impact on the Ba²⁺ adsorption capacity. Kinetic investigations reveal that the Ba²⁺ adsorption process conformed to the pseudo-second-order kinetic model, indicating the predominance of chemical adsorption. The fitting results of isothermal adsorption models suggest that the adsorption of samples II, III, and IV for Ba²⁺ was primarily governed by both the monolayer chemical process and heterogeneous surface effects. In contrast, Ba²⁺ was adsorbed in the form of bimolecular or multimolecular layers onto sample I. Regulating the ionic strength and pH of fracturing fluids can effectively enhance the adsorption capacity of shales for heavy metal ions, thereby reducing the risk of groundwater contamination by the ions. The results of this study provide a technical basis for the prevention and control of heavy metal contamination during hydraulic fracturing.