The effect of salt composition of fluid injected into clay-bearing Berea sandstones on the water sensitivity phenomenon has been studied. Experiments were carried out with solutions of sodium and/or calcium ions which are the type of ions commonly found in formation environment. The fresh water flooding of sandstones previously exposed to sodium salt solutions results in the release of clay particles and a drastic reduction in the permeability. However, the permeability reduction is reduced when calcium ions are also present in the salt solution. Formation damage is virtually eliminated when the solution composition is adjusted to give calcium surface coverages of greater than a critical value of 75%. The effect of the fluid composition on the damage process is explained by the double-layer interactions between the surface of the clay particles and the pore walls.

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