Scale formation is one of the most serious oil field problems that inflict water injection systems primarily when two incompatible waters are involved. Two waters are incompatible if they interact chemically and precipitate minerals when mixed. This study was conducted to investigate the permeability reduction caused by deposition of calcium sulphate in sandstone cores from mixing of injected sea water and formation water that contained high concentration of calcium ion at various temperatures

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... 50 -80°C) and differential pressures (100 -200 psig). The solubility of calcium sulphate scale formed and how its solubility was affected by changes in salinity and temperatures (40 -90°C) were also studied. The morphology and particle size of scaling crystals formed as shown by Scanning Electron Microscopy (SEM) were also presented. The results showed that a large extent of permeability reduction caused by calcium sulphate that deposited on the rock pore surface. The rock permeability decline indicates the influence of the concentration of calcium ions. At higher temperatures, the deposition of CaSO 4 scale increases since the solubility of CaSO 4 scale decreases with increasing temperature. The deposition of CaSO 4 scale during flow of injection waters into porous media was shown by Scanning Electron Microscopy (SEM) micrographs. The results were utilized to build a general reaction rate equation to predict CaSO 4 deposition in sandstone cores for a given temperature, brine super-saturation and differential pressures.