Methylimidazolium-Tetrafluoroborate Ionic Liquids Flowback Aids for Hydraulic Fracturing Applications

Abstract Hydraulic fracturing has been widely adopted to enhance hydrocarbon recovery in tight and ultra-low permeability reservoirs. However, a significant operational challenge persists due to the retention of fracturing fluids within the formation, resulting in reduced gas mobility and production efficiency. High capillary pressures, wettability alterations, and interfacial tension between fracturing fluids and formation fluids impede effective flowback, particularly under harsh reservoir conditions. The objective of this study was to evaluate the performance of tetrafluoroborate-based methylimidazolium ionic liquids (ILs) as flowback additives to reduce fluid retention, lower capillary pressure, and enhance fluid recovery under elevated temperature and salinity environments. A series of structurally varied ILs were formulated by modifying the alkyl chain length while maintaining a common imidazolium cation and a tetrafluoroborate anion. These ILs were tested across multiple screening stages, including surface tension (ST), interfacial tension (IFT), and capillary pressureexperiments. Seawater and formation brine were used as base fluids, while tight Kentucky sandstone cores with permeability around 0.1 mD were selected to simulate unconventional reservoir conditions. Surface tension and CMC determinations were performed using the Wilhelmy plate method, and thermal and salinity stability were evaluated using high-pressure, high-temperature pendant drop IFT measurements. Capillary pressure behavior was assessed using centrifuge experiments with short (3-hour) and long (20-hour) flowback cycles, complemented by NMR analysis to validate saturation trends. The IL with the longest alkyl chain, ST4, demonstrated the strongest surface activity, reducing ST to ~27 mN/m and reaching CMC at ~300 mg/L. IFT values dropped from 70.56 mN/m to 22 mN/m in formation brine and from 65.48 mN/m to 27 mN/m in seawater at 120°C. In centrifuge tests, ST4 reduced residual water saturation (Swi) from 36% to 30.5% and lowered entry capillary pressure by approximately 80%, from 40 psi to 5 psi. NMR T2 distributions further confirmed improved water removal and mobility in ST4-treated samples, highlighting its superior performance under extended flowback conditions. These results confirm the robust thermal and salinity stability of ST4 and its effectiveness in modifying interfacial properties, lowering capillary pressure, and enhancing fluid displacement. The application of ILs, especially ST4, presents a novel and efficient approach to improving flowback performance in unconventional hydraulic fracturing operations.