Synergist Effects on Stability and Friction in N2/CO2 Hybrid Foam as a Fracturing Fluid

CO2 foam fracturing is a promising technology for developing unconventional reservoirs, but its application in deep, high-pressure/high-temperature (HPHT) wells is severely hindered by the inherent instability of CO2 foam. Conversely, while highly stable, N2 foam generates excessive friction pressure. This study systematically investigates the potential of N2/CO2 mixed gas foams as a high-performance, tunable fracturing fluid. Through a dual-scale experimental approach under simulated reservoir conditions (100°C, 30 MPa), we combined macroscopic performance tests (stability, rheology, pipe friction) with micro-scale mechanistic analysis (interfacial rheology, microscopic morphology). The results reveal a profound synergistic effect. The addition of N2 does not significantly alter the static interfacial tension but dramatically enhances the interfacial dilational viscoelasticity. Specifically, adding 50% N2 increased the interfacial dilational viscoelasticity by nearly 6-fold compared to pure CO2 foam. This enhanced Marangoni effect, coupled with the "trapped gas effect" that suppresses Ostwald ripening, fundamentally improves foam stability. Macroscopically, this was evidenced by the half-life increasing from 20 min (pure CO2) to over 500 min with just 35% N2. This enhanced stability directly translates to superior rheological properties; the effective viscosity (at 10 s−1) of a 50% N2 mixed foam reached 160 mPa·s, a 3-fold increase over pure CO2 foam (55 mPa·s). However, this performance gain comes at the cost of increased friction, with the friction factor rising from 0.03 to 0.22 (at 0.6 m/s) for the same foams. Based on this comprehensive dataset, we conclude that N2/CO2 mixed foam is a highly tunable system. An optimal engineering window exists where N2 content is between 35% and 50%, which provides sufficient stability and viscosity for effective fracturing while keeping friction pressures manageable. This work provides a quantitative design basis for optimizing N2/CO2 mixed foam fracturing fluids, balancing performance and cost to advance the technology's field-scale application.