Abstract Condensation, vaporization and volumetric changes are well-studied phenomena that create differences between the properties of oil at reservoir and surface condition. But similar changes in effluent brine properties have not received the same attention. This study attempts to analyze variations in properties of the effluent brine at reservoir and surface conditions. Various research groups have conducted water flooding experiments. Concentrations of the effluent brines have been reported. In this study speciation properties of these brines have been reanalyzed. The Extended UNIQUAC Model was used to precisely compute the brine speciation. Calculations was conducted both at surface and reservoir (pressure, temperature) conditions. And possible release of fines that remained unaccounted in previous studies was explored. The calculation at high temperature and pressure showed that during water flooding, significant amounts of fines were released from the core plug. But with a decrease in temperature and pressure (i.e. at surface condition) these produced fines got dissolved in the effluent water. Thus the produced fines were no longer observable at room temperature. A significant increase in the solubility of anhydrite with decrease in temperature is found to be the principal reason for the dissolution of produced fines. The previously unaccounted fines were observed for the following types of experiments: Rock Type: Chalk, Limestone and DolostoneCoreplug Type: Outcrop and Reservoir CoreplugFlooding Type: Spontaneous imbibition and water flooding experimentEOR Strategy: Low salinity, Smart waterfloods and Advanced Ion ManagementRock Origin: Middle East, North Sea and United States Furthermore a good correlation was observed between the amount of produced fines and the reported amount of oil recovery. Herein it was observed that production of fines increased significantly when the composition of injected brine was significantly altered. The interaction between the injected brines and the existing brine could have led to formation of mobile fines. This trend was found congruent with incrementing oil recovery. The calculations show that brine speciation at reservoir condition is significantly different from that at surface conditions. During EOR waterfloods significant amounts of fines are produced which are closely correlated with the increments in oil recovery. These produced fines have previously been unaccounted for. The Extended UNIQUAC model can be used to accurately calculate the amounts of these fines.