Water-based drilling fluids (WBDFs) are widely used due to their economic and environmental advantages; however, shale hydration remains a major limitation. This study evaluates Fe₂O₃, CuO, ZnO, and MgO nanocrystalline metal oxides synthesized via co-precipitation as inorganic shale inhibitors for WBDFs. Comprehensive characterization confirmed phase-pure nanocrystalline oxides (17–38 nm) with high thermal stability. Performance tests revealed that MgO-based WBDF exhibited the lowest plastic viscosity (17 cP), the highest pH (≈10.0), and the strongest shale inhibition (6.1% swelling), while Fe₂O₃ provided superior filtration control (6.0 mL). CuO showed balanced rheology, whereas ZnO displayed comparatively weaker inhibition. Compared with commercial inhibitors (Amine NF and Glycol), MgO- and Fe₂O₃-based systems achieved comparable or improved performance with enhanced thermal and environmental robustness. These results demonstrate the potential of nanocrystalline metal oxides as sustainable additives for improving WBDF performance under high-pressure, high-temperature (HPHT) conditions.

Water-based drilling fluids (WBMs) are widely applied in petroleum engineering due to their lower cost and reduced environmental impact compared to oil-based muds. However, their performance is severely limited in shale formations, where hydration and swelling of clay minerals lead to wellbore instability. In this study, two novel imidazoline-type inhibitors were synthesized from fatty acids: A-Lin (derived from linoleic acid) and A-Lau (derived from lauric acid). The synthesis involved amidation followed by cyclization, and the products were characterized using FTIR and TGA. Their performance as shale hydration inhibitors was evaluated in WBM formulations and compared with commercial additives (Amine NF and Glycol). The FTIR spectra confirmed successful imidazoline ring formation, while TGA demonstrated good thermal stability up to 150 °C, with A-Lin exhibiting superior resistance due to its unsaturated structure. Rheological tests showed that the synthesized additives reduced plastic viscosity, thereby improving cuttings transport efficiency. Swelling tests revealed that A-Lin achieved the lowest final swelling (6.3%), outperforming both commercial inhibitors and the saturated A-Lau analogue. Furthermore, A-Lin provided the best lubricity coefficient (0.148), reducing torque and drag during drilling. Overall, A-Lin demonstrated strong potential as an efficient, thermally stable, and environmentally compatible shale inhibitor for advanced WBM formulations. Compared to conventional inhibitors such as KCl, glycol, and amine-based additives, A-Lin uniquely combines superior swelling inhibition, enhanced lubricity, and good thermal stability, highlighting its novelty as an imidazoline derivative derived from renewable fatty acids

Direct comparisons of structurally distinct carbon-based additives in water-based drilling fluids under identical conditions remain limited. This study evaluated grapevine-wood-derived carbon (GWC), graphitic carbon nitride (GCN), and commercial activated carbon (CAC), each incorporated at 20 g per formulation. Their structural, morphological, and textural properties were characterized, and their effects on rheology, API filtration, and 72 h linear shale swelling were assessed. GWC exhibited the highest DFT-derived specific surface area (939.17 m² g⁻¹) and, among the tested formulations, provided the best filtration performance, reducing fluid loss from 8.0 to 5.6 mL and mud-cake thickness from 1.5 to 0.5 mm. Although GCN possessed the lowest specific surface area (32.50 m² g⁻¹) and produced comparatively unfavorable filtration behavior, it produced the greatest reduction in shale swelling, decreasing final swelling from 12.2% to 7.3 ± 0.2%, corresponding to a 39.9% reduction relative to the carbon-additive-free reference formulation. GWC and CAC both limited swelling to 9.2%, while CAC produced intermediate filtration performance. These contrasting responses demonstrate that specific surface area and porosity alone do not determine additive performance. Among the tested formulations, GWC provided the best filtration control, whereas GCN produced the greatest reduction in shale swelling.