Formation damage due to clay migration may be observed when the injected brine replaces the connate water during operations such as waterflooding, chemical flooding including alkaline, surfactant and polymer processes. This work deals with the effects of physico-chemical changes induced by brine replacement on the permeability behavior of consolidated argillaceous sandstones. The experimental investigation is made up of:-core flow experiments in clayey sandstones. Effluent pH and ionic concentration are measured at the outlet of the porous medium and permeability variations are simultaneously monitored when the injected salinity is abruptly decreased.-batch coagulation tests giving both critical salt concentration and pH in a salinity-pH diagram. Brines are prepared with sodium and potassium salts. A physico-chemical flow model is also developped based on cationic exchange with the proton when the salinity decreases. The model allows: -the interpretation of the experimental observations (pH and concentration)-the knowledge of the transition compositions -the effect of the major parameters like the clay content through the cation exchange capacity. The model gives pH and salinity profiles. The comparison of these values with the salinity-pH diagram critical ones leads to the prediction of the effect of the fluid composition on the damage process.

Formation damage due to clay migration may be observed when the injected brine replaces the connate water during operations such as waterflooding, chemical flooding including alkaline, surfactant and polymer processes. This work deals with the effects of physico-chemical changes induced by brine replacement on the permeability behavior of consolidated argillaceous sandstones. The experimental investigation is made up of: - core flow experiments in clayey sandstones. Effluent pH and ionic concentration are measured at the outlet of the porous medium and permeability variations are simultaneously monitored when the injected salinity is abruptly decreased. - batch coagulation tests giving both critical salt concentration and pH in a salinity-pH diagram. Brines are prepared with sodium and potassium salts. A physico-chemical flow model is also developed based on cationic exchange with the proton when the salinity decreases. The model allows:–the interpretation of the experimental observations (pH and concentration)–the knowledge of the transition compositions–the effect of the major parameters like the clay content through the cation exchange capacity. The model gives pH and salinity profiles. The comparison of these values with the salinity-pH diagram critical ones leads to the prediction of the effect of the fluid composition on the damage process. Introduction It has been observed that the permeability damage of clayey sandstones occurs when a brine solution initially saturating the core is replaced by a low salinity brine. This phenomenon results from the mobility of clay particles that migrate in the porous medium. The specific mechanisms leading to permeability damage depend, besides the injection flow rate, on several physico-chemical factors that affect the nature of rock-fluid interactions. Numerous studies show that the temperature, the injection fluid composition and pH are important variables. The primary factor that determines the migration of clay particles is the salt composition. It has been previously shown that little amounts of divalent cations in the injected fresh water brine are often sufficient to reduce permeability impairment. Other studies pointed out the effect of pH. It is reported that low pH minimizes clay dispersion whereas high pH causes permeability reduction. Recently, it has been recognized that ion exchange is responsible in some extent of physico-chemical permeability damage. The "water-shock" experiment from Vaidya and Fogler shows that the permeability reduction is associated with an increase of the effluent pH. They conclude that salinity and pH must be considered simultaneously and, that the damage mechanism is not only salinity-related. In this paper, the physico-chemical processes leading to permeability reduction are analyzed in following the H ion exchange mechanism proposed first by Vaidya. Specifically, the purpose of our paper is to provide quantitative guides to handle different application cases. A modeling of H/Na or K exchange is presented and validated by experimental results in a wide range of situations. Different conditions of initial brine salinity, injected water salinity and pH, that cover the range of practical situations like the secondary recovery by water injection or the injection of alkaline additives, are considered. Particularly, a detailed analysis of physico-chemical interactions in neutral pH conditions is given, which has never been done before. Transient variations of the pH leading to permeability reduction when a low salinity brine is injected are satisfactorily modeled. On the other hand, the physico-chemical effect arising when a KCl brine replaces a NaCl solution is presented. It is shown that the model can be a useful tool, when associated with a batch description of the reaction of the sandstone material, for predicting incompatibility of brines and formation damage. P. 491^