Development and Performance Optimization of a Micro–Nano-Polyamine Drilling Fluid System: Mechanistic Study on Wellbore Stability in Shaximiao Formation

Wellbore instability is a key challenge in drilling engineering, significantly affecting the efficiency and safety of oil and gas extraction. This study addresses the wellbore instability problem in the Shaximiao Formation and proposes a polyamine drilling fluid system based on micro-nanofillers sealing technology. The system’s efficiency and stability are verified through multi-scale characterization and performance optimization. Techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and contact angle measurement are employed to systematically analyze the mineral composition, microstructure, and surface properties of the Shaximiao Formation. It is revealed that the main cause of instability stems from the hydration expansion of clay minerals and the development of micro-cracks. Based on this, the synergistic effect of the micro-nanofiller sealing agent and the polyamine inhibitor is optimized to construct a high-performance drilling fluid system. Experimental results show that this system exhibits excellent rheological properties (API fluid loss < 5 mL, HTHP fluid loss < 10 mL), significant inhibition (rock sample recovery rate 96.79%, expansion rate < 4%), and long-term temperature stability (48 h). The micro-nanocomposite sealing structure effectively fills the micro-cracks in the formation and reduces pore pressure and permeability, thereby significantly enhancing wellbore strength. Compared to traditional drilling fluids, this system has significant advantages in sealing performance, inhibition, and high-temperature stability. This study provides crucial theoretical foundations and technical support for addressing wellbore instability and inadequate plugging performance of drilling fluids in unconventional reservoir drilling operations.