Drilling mud is a critical component in geothermal well planning, where environmental sustainability and economic feasibility are key considerations. Conventional additives often come with significant costs and environmental impacts. The commercial calcium carbonate (CaCO₃) is an established weighting/fluid-loss control agent added to the water-based mud (WBM), valued for its thermal stability (maintaining structure and rheological behavior up to 120 °C in HPHT tests), compatibility with barite/bentonite/xanthan systems, and its ability to form thin, low‐permeability filter cakes that reduce fluid invasion. As waste eggshells are composed of ⁓70-95% CaCO₃ (mostly calcite form) and are an abundant agro-industrial waste, this study analyzes and defends the use of waste eggshells (finely ground, less than 125 µm) as a low-cost, sustainable alternative to commercial CaCO₃ in geothermal drilling fluid systems. As part of the methodology, the WBM samples were prepared using standard additives (barite, bentonite, and xanthan gum). Eggshell powder and, for comparison, commercial CaCO₃ were added to the base mud at increasing concentrations (10 g, 20 g, 30 g, and 40 g). Laboratory tests measured mud density, plastic viscosity (PV), yield point (YP), gel strength, pH, resistivity, and filtration characteristics (filtrate volume and filter cake thickness). Results demonstrate that eggshell powder performs comparably to commercial CaCO₃, showing a near-linear increase in mud density and PV correlating with concentration. Eggshell powder effectively enhanced mud stability by promoting optimal (10 min) gel strength and reducing filtrate volume through rapid formation of thin, low-permeability mud cake, matching or exceeding the filtration control achieved by commercial CaCO₃ at equivalent quantities. At intermediate concentrations (20 g), YP and filter cake thickness were optimal, while higher concentrations showed a marginal decrease in YP and filter cake thickness. The pH and resistivity profiles confirmed chemical compatibility and stability within the drilling fluid system. Considering CaCO₃’s established thermal stability (up to 120 °C under HPHT conditions), the similar performance of eggshell powder suggests promising behavior under geothermal conditions. Given its cost advantage and environmental benefits, waste eggshell powder is validated as a viable additive for WBM in geothermal drilling. Its use delivers equivalent fluid-loss control, rheological stability, and mud density performance, while significantly reducing environmental footprint, supporting the UN Sustainable Development Goals.

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