Tetrafluoroborate Versus Chloride Methylimidazolium-Based Ionic Liquids as Advanced Flowback Additives for Hydraulic Fracturing in Unconventional Reservoirs

Abstract Hydraulic fracturing has become an indispensable technique in unlocking hydrocarbons from unconventional reservoirs such as shale gas, tight sandstones, and tight oil formations. Despite its widespread application, challenges persist, particularly with fluid retention due to the inherent high capillary pressures and low permeability that characterize these reservoirs. These issues are exacerbated under extreme reservoir conditions, such as high temperature and salinity, which hinder efficient fluid recovery. Traditional flowback additives, including surfactants and microemulsions, often fall short in these harsh environments, limiting their effectiveness in improving fluid recovery. Recently, ionic liquids (ILs) have emerged as a promising solution, offering superior thermal stability, tunable properties, and enhanced interfacial behavior. This study expands on existing research by systematically evaluating the performance of methylimidazolium-based ionic liquids, specifically ST4 (1-Decyl-3-methylimidazolium tetrafluoroborate) and SX3 (1-Decyl-3-methylimidazolium chloride), under extreme conditions to explore the impact of their chemical structure, particularly the anion, on their flowback performance. A comprehensive set of experiments was conducted, starting with surface tension and critical micelle concentration (CMC) measurements to assess the surface-active properties of the ILs. These experiments were carried out using seawater with a salinity of 58,857 ppm to simulate realistic reservoir conditions, and extended to elevated temperatures up to 90°C. Both ILs demonstrated significant surface tension reduction, with ST4 achieving a low surface tension of 27 mN/m at minimal concentrations, while SX3 reached 35 mN/m. Furthermore, interfacial tension (IFT) tests revealed that both ILs maintained exceptional stability under high temperature and salinity, with ST4 showing a stable IFT of 30 mN/m and SX3 performing similarly at 29 mN/m in seawater, confirming their robustness in challenging conditions. Capillary pressure tests further demonstrated the remarkable ability of ST4 and SX3 to lower entry pressures. ST4 showed the most impressive performance, reducing the entry pressure by 87.5% at high water saturations (Sw = 95%), while SX3 achieved a 40% reduction, indicating that both ILs effectively enhanced fluid mobilization during flowback. These findings were reinforced by water recovery tests, where SX3 achieved a 50% recovery, and ST4 followed closely with 48% recovery, both significantly surpassing the 40.75% recovery from the base fluid. Additional insights from NMR analysis validated the observed improvements in fluid displacement, revealing that ST4 and SX3 were effective in reducing residual water saturation within tight pores. Specifically, ST4 and SX3 reduced residual saturations to 30.5% and 29.5%, respectively, compared to 36% for the untreated seawater base fluid. This indicates a clear enhancement in fluid mobility and a reduction in fluid entrapment within smaller pore spaces, highlighting the superior capability of these ILs in improving recovery in tight reservoirs. This study emphasizes the impact of anion type in methylimidazolium-based ionic liquids, with tetrafluoroborate (ST4) exhibiting superior performance in reducing surface tension and capillary pressures compared to chloride (SX3). While both ILs show promising results, ST4 outperforms SX3 in key aspects, including surface tension reduction and capillary pressure mitigation, making it the more effective additive for improving flowback recovery in harsh reservoir conditions.