Abstract A model-assisted analysis of formation damage in the S and S3 facies of the Eocene B-Inferior formation in Ceuta field, Lake Maracaibo, western Venezuela, is carried out. The governing mechanisms and the estimated values of the phenomenological rate constants are determined using the formation damage model by Ohen and Ciyan. A skin factor chart is constructed based on this information as a practical measure of the formation damage potential. Introduction Reduction of injectivity of wells by formation damage can be of considerable magnitude during waterflooding of oil reservoirs. The origin of formation damage can be of chemical and mechanical caused by interaction between porous media and fluid flowing through it. Chemical damage occurs due to the incompatibility between the constituents of mineral rock and the flowing fluids. Mechanical damage can he caused either by external particle invasion, as in the case of suspended particles in completion fluids, or internal particle movement mainly influenced by hydrodynamic forces. Variation of pH of the flowing fluid is an indicator of the extent of chemical activity occurring in the formation. Dissolution of cementing - authigenic materials such as carbonates and/or ion exchange between fluid and minerals can cause pH variation, High pH (alkaline media) promotes formation damage by particle deposition within the porous media and consequently particle bridging at the pore throats. Occasionally, the particle bridges brake causing temporary permeability increase. This is reflected by the pulsating character of the pressure differential and permeability reduction data. Permeability reduction and skin factor are used as quantitative measures of formation damage. A detailed description of the processes involving formation damage of porous media is given in the literature(l-14). This paper presents a model-assisted anaIy;is of formation damage in sandstone core samples belonging to the facies S and S3 of the Eocene B-Inferior formation, in Ceuta field, Lake Maracaibo, western Venezuela. The details of this study are given by Millan(l5) Due to the high content of carbonaceous and siliceous type authigenic materials the Ceuta field core samples are extremely sensitive to brine, rendering the pH toward alkaline media during waterflooding. As a result particles tend to disperse to reduce the formation permeability. Permeability alteration process is numerically reproduced by means of the Ohen and Civan model(l2) and extended beyond the experimental range. The values of the phenomenological rate constants involving the hydrodynamic and electrochemical processes, which controls the trapping and/or release of particles within the porous media, are determined and a skin factor chart is constructed using the Ohen and Civan method(13). Laboratory Study A description of the materials, experimental system, and procedure is presented in this section. Core Samples Thirteen rock samples belonging to the facies S and S3 of the Eocene-B Inferior formation, in Ceuta field, Lake Maracaibo, western Venezuela have been analyzed.