Mechanism of weighting salts effect on high-density polyacrylamide hydrogels for hydraulic fracturing performance

Partially hydrolyzed polyacrylamide (HPAM) hydrogels are widely used as fracturing fluids for extracting underground hydrocarbon resources. Weighting salts are incorporated to enhance hydrogel density to address the extreme pressure encountered in ultra-deep reservoirs. These salts exert significant effects on performance; however, the underlying mechanisms remain poorly understood, and the development of high-performance fracturing fluids remains a significant challenge. This study systematically examines the influence of potassium formate (HCOOK) and sodium bromide (NaBr) as weighting agents on the viscoelasticity of high-density HPAM hydrogels. The findings indicate that HCOOK exerts a more pronounced adverse impact on the viscoelasticity of HPAM hydrogels compared to NaBr. Molecular dynamics simulations reveal that the inferior viscoelasticity of the HCOOK-based polymer hydrogels is attributed to stronger supramolecular interactions between the crosslinker and HCOOK. This study develops an enhanced-performance high-density fracturing fluid with a density of up to 1.40 g/cm 3 , a maximum drag reduction rate of 75.1 %, and temperature tolerance up to 160 °C by integrating ultra-high molecular weight HPAM, a dual crosslinking strategy, and NaBr as weighting salt. This research not only provides fundamental insights into the complex interactions among weighting agents, crosslinkers, and polymers but also presents an enhanced-performance high-density fracturing fluid for ultra-deep hydrocarbon reservoirs.