Ultrasound-induced viscosity evolution of water-based fracturing fluid and its mechanism on altering coal pore structure

Abstract

Current hydraulic fracturing technologies rely on chemical gel-breaking agents to enhance the flowback of fracturing fluid. However, these chemicals may cause groundwater pollution. To explore the potential application of ultrasound in flowback operations, slickwater, guar gel, and viscoelastic surfactant (VES) fracturing fluid were prepared in this study. The fluid samples were subjected to ultrasonic gel-breaking at a frequency of 20 kHz for varying durations, and the changes in viscosity were analyzed using a rheometer. Subsequently, the micellar structures of the fracturing fluids were observed using scanning electron microscopy. Experimental findings confirm that ultrasound effectively disrupts the micellar structure, achieving a viscosity reduction in guar gel from 135 to 3 mPa s within 10 min. This represents a substantial efficiency improvement over conventional chemical breaker. However, the effect of ultrasound on the VES fracturing fluid was limited. Under more intense ultrasonic conditions, it even led to an increase in viscosity. Additionally, ultrasonic treatment progressively loosened the structures and enhanced porosity in both guar gel and slickwater micelles. In contrast, VES fracturing fluid micelles only experienced localized tearing while retaining their overall network integrity. After ultrasonic treatment, the total pore volume of coal samples immersed in guar gel increased from 1.6 × 10−3 to 2.2 × 10−3 cm3/g, while that of coal samples immersed in slickwater rose from 0.6 × 10−3 to 1.1 × 10−3 cm3/g. However, the pore volume of coal samples immersed in VES fracturing fluid did not show significant improvement. These results provide guidance for the development of cleaner fracturing technologies in the future.