Application of a Novel Temperature-Sensitive Polymer PNA-SiO₂ as a Rheological Agent in Geothermal Drilling

In geothermal drilling, the significant rheological deterioration of biopolymer drilling fluids can lead to decreased cutting-carrying efficiency and fluctuations in annulus pressure, which can easily result in unpredictable downhole accidents. To address the challenges, this study innovatively introduces temperature-sensitive monomers (NIPAM) and nanosilica into molecular chains based on temperature response effects and adsorption stability to counteract the negative effects of temperature and salt ions, aiming to develop a rheological agent (PNA-SiO₂) that is resistant to high temperature and salt. The molecular structure and thermal stability of PNA-SiO₂ were characterized by FT-IR and TGA, and it was demonstrated to have the stability of the molecular main chain within 317 °C. According to rheological and filtration studies, PNA-SiO₂ effectively addresses the issues of high-temperature viscosity attenuation and a sharp rise in the filtration volume of bentonite mud by aggressively adsorbing, creating a stable thixotropic network, and enhancing molecular repulsion. After aging at 60-180 °C, the drilling fluid with 2-4% PNA-SiO₂ has excellent rheology (AV ≥ 57.5 mPa·s, PV ≥ 42 mPa·s, Gel_{10 min} ≥ 1.533 Pa) and filtration (FL_API ≤ 9.6 mL) properties that fully satisfy API requirements. The H-B model is the preferred model to accurately describe the rheology behavior of PNA-SiO₂ drilling fluids. Moreover, under the comprehensive influence of Na⁺ on the interlayer spacing of bentonite and the thickness of the diffusion bilayer, PNA-SiO₂ has applicability in high-temperature saturated saline water. After aging at 150 °C and 5-36% NaCl, the PNA-SiO₂ drilling fluid exhibits stable viscosity (AV ≥ 55 mPa·s, PV ≥ 44 mPa·s) and low filtration volume (≤9.6 mL). In conclusion, PNA-SiO₂ is used as a rheological and fluid loss agent, offering a cost-effective solution to the technical issues of severe leakage and inadequate cutting-carrying capacity encountered by geothermal drilling, which enables the safe and quick exploration of geothermal resources.