Injectivity decline is an issue during Produced Water Re-Injection (PWRI) for water disposal in aquifers, water flooding, chemical EOR and geothermal energy exploitation. A novel model for injectivity decline under flow conditions reminiscent of Produced Water Re-Injection (PWRI) was developed taking into account deep-bed filtration and build-up of external filter cake. A distinct feature of the model is that it describes particle retention kinetics responsible for internal filtration by an exponential decaying function of the retained particle concentration. The corresponding non-linear governing partial differential equations were solved numerically and coupled with a known analytical model for external filtration using the concept of transition time. Core-flood experiments consisting of the injection of brine containing suspended hematite particles (volume fractions in the range 1–5 ppm) were also performed. CT scans of the core were done to obtain deposition profiles along the core at different times. Effect of various parameters (particle concentration, number of grids) on injecitivty was investigated. From CT scans, Scanning Electron Microscopic (SEM) analyses and optical microscopy photographs, it was found that surface deposition in the porous medium and face plugging at the inlet of the core were responsible for decline in injectivity. A transition time from pure internal filtration and external filtration was accurately determined from the CT scan and pressure data. Newly proposed model and experiments were found to be in excellent agreement indicating that the adopted retention function is good heuristic description of particle retention.