Assessing Cs-137 migration from abandon radioactive logging tool in Kuwait

This study integrates COMSOL Multiphysics' multiphase flow modelling with PHREEQC geochemistry through the iCP interface to evaluate Cs-137 migration from a lost-in-hole radioactive logging tool abandonment and backfilled with cement in Kuwait's Burgan oil filed. The tool contained Cs-137, Cf-252, and H³ sources; modelling targets Cs-127 due to the availability of thermodynamic data for its worst-case leakage scenario through cement into surrounding limestone host rock. High-pH leachate (12.97) from cement hydration dissolve host minerals at the cement-host rock interface releasing Ca²⁺ for CSH precipitation, vital for retarding radionuclides in limestone. Reactive transport simulation over 20 years employs thermodynamic equilibrium (modified LLNL database) under Darcy flow with 2-years timesteps and coupling species transport with mineral dissolution-precipitation. Results show a chemically disturbed zone extending ∼100 m downward, pH plume to 12.3, early precipitation of brucite (Mg drop), prehnite at cement-host rock interface (Ca sink), and ettringite along flow paths (SO₄^2-driven). Cs-137 migrates <100 m at minimal concentration, following pressure/Darcy patterns with most retained near borehole due to cement's physical blockage (low permeability) and chemical retardation characteristics. At the observation point, cement reduces CS-137 migration speed by almost a factor of 10 compared to the No-cement scenario (20 years compared to 2 years to reach the value of minimum concentration). These finding validate cementitious abandonment safety for oilfield radioactive tools and establish a baseline for low-level waste disposal analogues in limestone hosts, supporting future implementation of dynamic porosity, permeability-porosity relation, and reactive surface evolution for inhomogeneous Kuwaiti lithologies.