<p>This study presents an integrated experimental and phenomenological modeling investigation of macroscopic flow heterogeneities in bentonite-based suspensions and their control through carboxymethylcellulose (CMC) addition for drilling fluid applications. Systematic rheological measurements performed under controlled shear-rate protocols on Maghnia bentonite suspensions (2–8 wt%) reveal a concentration-driven transition, with an apparent percolation threshold identified near 3 wt%, beyond which pronounced non-monotonic macroscopic flow responses and signatures consistent with shear banding are observed. Recognizing bentonite suspensions as yield-stress fluids, we propose a physically informed nine-parameter phenomenological model designed to reproduce apparent flow curves obtained under the imposed experimental protocol. The model combines a regularized Souza Mendes core with a logistic transition function and a Gaussian stress correction, enabling an accurate representation of the macroscopic rheological response from the yield stress through apparent banding transitions to high-shear stabilization. The model parameters provide physically interpretable links between microstructural mechanisms and macroscopic behavior. Finally, through controlled CMC addition (0.25–2.00 wt%), we demonstrate that polymer concentrations exceeding 0.75 wt% effectively suppress macroscopic signatures associated with shear banding by reinforcing the colloidal network and promoting more homogeneous flow responses.</p> Graphical Abstract <p></p>