Failure mechanism and performance deterioration of bentonite suspensions in drilling fluids under CO2-thermal coupling conditions

Deep acidic gas reservoirs have emerged as promising geological CO2 storage sites, driven by global carbon neutrality goals and advancements in carbon capture, utilization, and storage technologies. However, during drilling operations in these reservoirs, water-based drilling fluids are frequently contaminated by acidic gases like CO2, leading to severe rheological and filtration abnormalities that jeopardize wellbore stability. As the dispersion state of bentonite governs the performance of such fluids, this study investigates the structural and colloidal stability of bentonite suspensions under CO2-thermal coupling conditions, simulating contamination at 0.2–1 MPa and temperatures of 80–150 °C. Macroscopic fluid performance, colloidal stability, surface chemistry, and microstructural evolution were systematically analyzed. Results reveal a two-stage evolutionary mechanism: under mild CO2 intrusion, slight protonation of particle surfaces enhances the three-dimensional network, improving rheological and filtration properties. In contrast, severe contamination disrupts electrostatic equilibrium, triggering particle aggregation dominated by van der Waals forces and causing performance deterioration—a process exacerbated by elevated temperatures that accelerate particle migration. These findings elucidate the microstructural degradation pathways of bentonite under CO2-thermal coupling, offering theoretical insights for designing contamination-resistant drilling fluids in deep acidic gas reservoirs.