Preliminary Assessment of CO2 Compression Behavior Modified by [BMIM][AlCl4] for Engineered Geothermal Working Fluids

Closed-loop geothermal systems (CLGS) benefit from engineered working fluids, and supercritical CO 2 is a leading candidate due to its low viscosity and buoyancy-driven circulation. However, system performance depends sensitively on CO 2 compressibility and behavior near the critical region. Ionic fluids (IFs) offer a potential route to tuning these properties, yet their interaction with CO 2 under geothermal high-temperature, high-pressure (HTHP) conditions is not well understood. This study examines the mixing behavior of [BMIM][AlCl 4 ] with CO 2 using constant composition compression experiments at 30, 50 and 100 °C up to 25 MPa. The results show that [BMIM][AlCl 4 ] alters CO 2 thermodynamic response: mixtures retain substantially higher volume, near-critical compressibility peaks are strongly suppressed, and gas-like fluid-like crossover pressures shift upward. Volume-matching analysis reveals CO 2 dissolution into the IF and microstructural reorganization of the halometallate phase. These findings demonstrate that [BMIM][AlCl 4 ] acts as an active thermodynamic modifier rather than an inert additive. For geothermal applications, such IF-induced stiffening and fluctuation suppression could affect density control, circulation stability, and heat-extraction performance in CO 2 -based CLGS. The results provide foundational data for designing IF-CO 2 working fluids tailored for next-generation geothermal systems.