Most of the oil in the world is produced by injecting water in some wells and recovering oil in other wells. In offshore fields sea water containing organic and mineral inclusions is injected. This practice curtails the well's injectivity because the particles suspended in the fluid are trapped while passing through the porous rock. In this work, we study the deep filtration during the injection of water containing solid particles to predict the loss of injectivity in wells. The mathematical model for the filtration process is characterized by the filtration and the permeability reduction func tions which describe properties of the porous medium where the flow occurs. We develop a recovery method for determining the permeability reduction function indirectly from the pressure drop along the core, assuming that the filtration function has been deter mined previously by a separate procedure. For this recovery of permeability, we derive an integral equation of Volterra type for the rock formation damage function k(\sigma) and we discuss conditions for wellposedness of the operator equation. Finally, we describe a numerical implementation to calculate k(\sigma) within an appropriate subset of feasible solutions. The classical Tikhonov-Phillips regularization is used to reduce the illposed Volterra equation of first kind to a wellposed problem.