Abstract: A method and fluid for selectively reducing the permeability of an oil-bearing subterranean formation is disclosed, wherein a cross-linkable polymer, a cross-linking agent and a spacer fluid are alternately injected. The spacer, such as sodium citrate, permits repeated injection cycles of the polymer and cross-linking agents without excessive polymer retention at or near the formation face. Such retention filters out polymers and inhibits polymer propagation throughout the formation. The present invention also permits controlled deep penetration of the cross-linked polymer into the formation to increase oil recovery.

Abstract: A viscoelastic polymer such as polyacrylamide is first prehydrated in an aqueous fluid having a low degree of hardness and the prehydrated viscoelastic polymer solution is mixed with hard brine to provide a polymer bank with improved mobility control. This improved mobility control is obtained by mixing the hard brine with the prehydrated viscoelastic polymer at high volume ratios of hard brine to the polymer.