a method for removing filter cake from a subterranean borehole comprising drilling the borehole with a drilling fluid that includes additives to form a filter cake having an oxidation - degradable component , preferably a polysaccharide . the filter cake is contacted with a clear brine containing a mixture of a persulfate salt in a variable density brine to degrade the polymers within the filter cake . the persulfate mixture breaks down the filter cake in well bores having temperatures ranging from about 65 ° f . to 165 ° f . concentration of the persulfate sets the pace of the reaction .