• • E. coli deposition is concentrated in the top 15 cm of the sand column, with a 70–80% reduction in bacterial concentration at 30 cm depth for fine gravel (T3, k = 2.98 × 10⁻¹ cm/s), indicating that clogging risk is highest near the injection point and requires targeted pretreatment or periodic backwashing.
• • The coarse-to-fine layered sequence (T1 over T2 over T3) advances the breakthrough peak by approximately 20–25% compared to homogeneous packing, as finer layers below act as a capillary barrier that accelerates microbial transport through preferential flow paths, potentially leading to deeper formation damage.
• • Permeability decay follows a first-order exponential model with a decay constant of 0.012–0.018 min⁻¹ for the tested conditions, and predicted values match experimental data within ±5% (R² > 0.95), enabling quantitative prediction of reinjection well performance decline.
• • The injection of 8 × 10⁹ CFU/mL E. coli suspension resulted in a 40–50% reduction in permeability after 120 minutes in fine gravel layers, whereas coarse gravel layers showed only 15–20% reduction, highlighting the critical role of grain size distribution in microbial clogging severity.