Inorganic colloidal additives for improving the post-aging rheological performance of polymer-free geothermal drilling fluids

Deep geothermal drilling requires water-based drilling fluids that tolerate severe thermal exposure while maintaining useful rheological properties. Polymer-free bentonite fluids are attractive from environmental and thermal-degradation perspectives, but their rheology can deteriorate following high-temperature aging. This study investigates the effects of hydrotalcite, magnesium hydroxide, aluminium hydroxide, and magnesium oxide on the post-aging rheological behaviour of bentonite–Laponite drilling fluids. The clays and additives were characterised using scanning electron microscopy of dried powders, dynamic light scattering, zeta-potential measurements, X-ray fluorescence spectroscopy, and inductively coupled plasma atomic emission spectroscopy. Rheological properties were measured at 25 °C before and after 16 h thermal aging at 180, 240, or 300 °C in a custom stirred autoclave. Low-temperature, low-pressure filtration tests were also conducted on selected formulations. Hydrotalcite at concentrations of 0.4–1.0 wt% gave the greatest retention of yield stress following aging at 240 and 300 °C. Magnesium hydroxide also improved post-aging rheological retention, particularly following exposure to 240 °C. Magnesium oxide produced high yield stress values before aging but lost most of this effect after thermal exposure, while aluminium hydroxide provided little improvement in post-aging performance. The hydrotalcite-containing fluids remained strongly shear-thinning after cooling and rehomogenisation. The superior post-aging response of hydrotalcite is consistent with its platelet morphology and surface-charge characteristics promoting interactions with smectite particles. A mixed LDH–clay network is proposed as one possible explanation, although the present measurements do not directly resolve the suspension microstructure. The results identify hydrotalcite and magnesium hydroxide as promising inorganic additives for polymer-free drilling fluids and provide a basis for subsequent in-situ HTHP rheology, HTHP filtration, and structural-characterisation studies.