Polyacrylamide's function on increasing salinity in shale gas wastewater production

Large volumes of water containing friction-reducing additives are injected into target rock formations during hydraulic fracturing in shale gas production. The interactions between the fracking fluids and the reservoir rocks may increase the flowback and produced (FP) salinity, rendering the fluid ineffective for reuse and recycling. To explore the function of polyacrylamide (PAM; a widely applied friction-reducing additive) on the evolution of FP water salinity, three fluid-rock interaction experiments were conducted under relevant temperature conditions. Sr concentrations and 87Sr/86Sr were measured to identify mineral exchanges. Results showed that the dominant geochemical reactions were pyrite oxidation, carbonate, and silicate mineral dissolution, and halite dissolution; the hydrolysis of polyacrylamide facilitated mineral dissolution. Another one-tenth of carbonate was mobilized and the total oxidized pyrite mass was nearly 30 times greater on the function of PAM. These findings imply that the development of brine-tolerant polyacrylamide is necessary to effectively reuse and recycle FP water. This is beneficial for the environment as this minimizes the dependency on freshwater supplies. This study provides new information on the interactions between the fracturing fluid and the reservoir rocks and further elucidates the influence of polyacrylamide on wastewater salinity. •Dominant reactions were pyrite oxidation, and carbonate and silicate dissolution.•Polyacrylamide can block pores and occupy adsorption sites in the shale.•Hydrolysis of polyacrylamide facilitates the dissolution of shale minerals.•Polyacrylamide increases the salinity of flowback and produced water.