High injection rates in water injectors leads to mobilization of particles in unconsolidated formations and creates preferential flow paths within the porous medium. Channelization in porous medium occurs when fluid-induced stresses become locally larger than a critical threshold (rock stress); grains are then dislodged and carried away, hence porosity and permeability of the medium will be altered along the induced flow paths. Additionally, rapid shut-ins result in pressure imbalance between the wellbore and formation. Flowback of the particles results in sand accumulation, and consequently loss of injectivity, which is a common problem in unconsolidated formations like the ones in deep water Gulf of Mexico. Experimental studies have confirmed the presence of dependent and independent flow patterns; however, there is no integrated model to describe flow patterns and predict probable issues for water injection at the reservoir scale. The objective of this study is to provide a model for a channel initiation/propagation during injection and flowback in injection wells. A finite volume model is developed based on multiphase fraction volume concept that decomposes porosity into mobile and immobile phases where these phases change spatially and evolve over time that leads to development of erosional channels in radial patterns depending on injection rates, viscosity, magnitude of in situ stresses and rock properties. The model accounts for both particle releasing and suspension deposition. The developed model explains injectivity change with injection rates observed in unconsolidated reservoirs.