Drilling fluids require effective shale inhibition while maintaining rheology and fluid-loss control, especially in polymer-based water-based drilling fluids (WBMs). Alum (KAl(SO₄)₂·12H₂O) is an eco-oriented chemical widely used in water treatment, yet its concentration-dependent performance in active WBMs remains poorly quantified. This study systematically evaluated alum (0–10 lb/bbl) using a component-based approach and a field-representative polymer WBM, focusing on pH evolution, rheology, filtration, and volumetric swelling in both bentonite and grey marl (calcareous shale).

Alum caused strong acidification in all systems, driving the initial pH of the field WBM down from 10.1 to 4.6 at a concentration of 6 lb/bbl. In the field-representative mud, alum progressively improved rheological profiles up to a critical concentration of 6 lb/bbl, where plastic viscosity and yield point reached their peak performance zone due to temporary polymer–Al³⁺ interactions. Beyond these 6 lb/bbl thresholds, severe pH reduction below 4.0 promoted acid-catalyzed polymer degradation, causing rheological collapse and sharply increasing API fluid loss from 1.1 ml to 2.4 ml.

Swelling tests showed a monotonic reduction in sodium bentonite swelling, stabilizing at 22.77% expansion at 6 lb/bbl. Conversely, grey marl exhibited a non-linear response; swelling was initially minimized to 17.56% at 7 lb/bbl but experienced a distinct swelling reversal at higher dosages, rising back to 23.78% at 10 lb/bbl due to acid–carbonate reactions that undermined mechanical matrix integrity.

These quantitative results define a narrow operational window for alum in polymer-rich WBMs (≤ 6 lb/bbl) and demonstrate that carbonate-bearing shales suffer severe chemo-mechanical instability at elevated dosages, bounding alum's practical applicability in active drilling environments.