Dynamic coupling mechanism of shale drilling fluid invasion depth, fracture width, and rock strength

The dynamic coupling effects of fracture width, invasion depth, and rock strength during drilling fluid invasion into shale reservoirs constitute a key scientific issue in wellbore stability control. This study establishes a dynamic coupling theoretical model of invasion depth-fracture width-rock strength, revealing the dynamic feedback mechanisms of drilling fluid invasion. Comparative analysis between experimental and simulated data shows excellent agreement in variation patterns, where all relative deviations remain under 12%, validating the model's practical effectiveness. The study reveals that the coupled system evolves via a flow-stress positive feedback mechanism between fracture width and invasion depth, demonstrating distinctive “rapid transition-progressive stabilization” behavior; variations in original fracture width primarily determine dynamic evolution speed and balance thresholds by regulating flow cross section, while showing minimal impact on final strength equilibrium conditions; modulating drilling fluid density from 1.3 to 1.7 g/cm3 increases invasion depth by 230% and shear strength attenuation by 13.4% through pore pressure gradient adjustments that alter equilibrium thresholds; fluid viscosity governs dynamic evolution speed via flow resistance, where high-viscosity formulations (35 mPa·s) prolong stabilization time to over 20 days. The study quantitatively characterizes the tripartite dynamic coupling mechanism, establishing a theoretical foundation for drilling fluid optimization and wellbore stability management in shale gas exploitation.