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.