Thermosensitive core-shell polymer microspheres for enhanced wellbore stability in deep-water water-based drilling fluids

The particular geological conditions present in deepwater wells, including specialised stratigraphic depositional processes, low mudline temperatures and high bottomhole temperatures, present a significant challenge to the construction of drilling rigs. This problem places new demands on the temperature resistance and wellbore stability performance of deepwater water-based drilling fluids. In this study, a thermosensitive core–shell polymer microsphere, NVPS, was prepared with a D50 value of 2.42 μm and a pronounced core–shell structure, which is capable of withstanding temperatures up to 180℃. At a concentration of 2 wt% of NVPS, the roll recovery at 180℃/16 h reached 98.53 % (22.95 % in clear water), and the linear expansion at 24 h was only 32.77 % (69.34 % in clear water). The material exhibited a pronounced hydration inhibition effect. The contact angle of water on the surface of shale chips treated at 180℃/16 h was 131.7° (53.9° for shale chips treated at 25℃/16 h) at a NVPS concentration of 2 wt%. It was demonstrated that NVPS produces a significant thermosensitive effect at elevated temperatures, which significantly enhances the hydrophobicity of the wellbore surface. The organic carbon adsorption of shale chips was found to be 108.11 mg/g (0.1338 mg/g in clear water) following hot rolling of the shale chips in an NVPS aqueous solution at 180℃ for 16 h. This indicates the presence of NVPS adsorption on the surface of the shale chips. The micromorphological analysis demonstrated that NVPS is capable of effectively adsorbing and residing on the surface and within the pores of a range of media. It was observed that NVPS can effectively adsorb and reside in the pore space. The thermosensitive groups in the shell layer of NVPS produce a thermosensitive effect when the temperature rises, which enhances the hydrophobicity and forms a wide and dense hydrophobic layer. This process hinders the intrusion of water molecules and inhibits the hydration and swelling of clay minerals.