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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 mixt

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.

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1. A method of removing filter cake from a subterranean borehole, comprising:drilling the borehole with a drill-in fluid comprising a polymer to form a filter cake; contacting the filter cake with a mixture of a persulfate salt in a variable density brine in the absence of an activating agent; and a

1. A method of removing filter cake from a subterranean borehole, comprising:drilling the borehole with a drill-in fluid comprising a polymer to form a filter cake; contacting the filter cake with a mixture of a persulfate salt in a variable density brine in the absence of an activating agent; and allowing the mixture to remain at the downhole temperatures ranging from 65° F. to 165° F. for a period of time effective to degrade the polymer filter cake. 2. The method of claim 1 further comprising the step of flushing away the decomposed filter cake with a low concentration organic or non-organic acid.3. The method of claim 1 wherein the polymer is selected from a water soluble organic polymer, a water dispersible organic polymer, a water soluble bio-polymer, a water dispersible bio-polymer and combinations thereof.4. The method of claim 1 wherein the mixture of persulfate salt in a variable density brine further comprises a surfactant.5. The method of claim 1 wherein the mixture of persulfate salt in a variable density brine further comprises a chelating agent.6. The method of claim 1 herein the persulfate salt is ammonium persulfate.7. The method of claim 1 wherein the persulfate salt is selected from an alkali metal persulfate, an alkaline earth metal persulfate and combinations thereof.8. The method of claim 7 herein the alkali metal persulfate is selected from potassium persulfate, sodium persulfate, lithium persulfate and combinations thereof.9. The method of claim 7 herein the alkaline earth metal persulfate is selected from calcium persulfate, magnesium persulfate, and combinations thereof.10. The method of claim 1 wherein the variable density brine is selected from NH4Cl, NaCl, KCl, CaCl2, ZnCl2, and combinations thereof.11. The method of claim 1 wherein the density of the brine varies within a range of from about 8.3 lbs/gal. to about 12.8 lbs/gal.12. The method of claim 1 wherein the density of the brine varies within a range of from about 8.5 lbs/gal. to about 10.4 lbs/gal.13. The method of claim 1 wherein the temperature ranges is from about 70° F. to 160° F.14. The method of claim 1 wherein the period of time to degrade the filter cake is at least 4 hours.15. A method of removing filter cake from a subterranean borehole, comprising:drilling the borehole with a drill-in fluid comprising a polymer to form a filter cake; contacting the filter cake with a mixture of a persulfate salt in a variable density brine in the absence of an activating agent, the brine selected from NH4Cl, NH4Br, NaCl, NaBr, KCl, KBr, CaCl2, CaBr2, ZnCl2, ZnBr2, and combinations thereof; and allowing the mixture to remain at the downhole temperatures below 104° F. for a period of time effective to degrade the polymer filter cake. 16. The method of claim 15 herein the persulfate salt is ammonium persulfate.17. The method of claim 15 herein the persulfate salt is selected from an alkali metal persulfate, an alkaline earth metal persulfate and combinations thereof.18. The method of claim 17 herein the alkali metal persulfate is selected from potassium persulfate, sodium persulfate, lithium persulfate and combinations thereof.19. The method of claim 17 herein the alkaline earth metal persulfate is selected from calcium persulfate, magnesium persulfate, and combinations thereof.20. The method of claim 15 herein the density of the brine varies within a range of from about 8.3 lbs/gal. to about 18 lbs/gal.21. A method of removing filter cake from a subterranean borehole, comprising:drilling the borehole with a drill-in fluid comprising a polymer to form a filter cake; contacting the filter cake with a mixture of a persulfate salt in a variable density brine in the absence of an activating agent, the brine selected from NH4Cl, NH4Br, NaCl, NaBr, KCl, KBr, CaCl2, CaBr2, ZnCl2, ZnBr2 and combinations thereof; and allowing the mixture to remain at the downhole temperatures ranging from 65° F. to 104° F. for a period of time effective to degrade the polymer filter cake. 22. A method of removing filter cake from a subterranean borehole, comprising:drilling the borehole with a drill-in fluid comprising a polymer to form a filter cake; contacting the filter cake with a mixture of a persulfate salt in a brine in the absence of an activating agent, the density of the brine within a range of from about 8.5 lbs/gal. to about 10.4 lbs/gal; and allowing the mixture to remain at the downhole temperatures ranging from 65° F. to 165° F. for a period of time effective to degrade the polymer filter cake. 23. A method of removing filter cake from a subterranean borehole, comprising:drilling the borehole with a drill-in fluid comprising a polymer to form a filter cake; contacting the filter cake with a mixture of a persulfate salt in a variable density brine in the absence of an activating agent, the brine selected from NH4Cl, NH4Br, NaCl, NaBr, KCl, KBr, CaCl2, CaBr2, ZnCl2, ZnBr2 and combinations thereof; allowing the mixture to remain at the downhole temperatures ranging from 65° F. to 135° F. for a period of time effective to degrade the polymer filter cake. 24. A method of removing filter cake from a subterranean borehole, comprising:drilling the borehole while circulating a mud therein which comprises a polymer, the polymer is selected from a water soluble organic polymer, a water dispersible organic polymer, a water soluble bio-polymer, a water dispersible bio-polymer and combinations thereof, and optionally comprises finely divided solids dispersed therein to form a filter cake on surfaces of the borehole; thereafter installing gravel pack screens and tool assemblies into the borehole; thereafter introducing sand in a non-viscosified carrier into the borehole; and introducing a filter cake removal fluid in the well bore, in contact with a subterranean formation containing hydrocarbons to be produced, for a duration effective to substantially remove the filter cake in the vicinity of the subterranean formation, wherein the filter cake removal fluid comprises a solution of a persulfate salt in a brine having a density within a range of about 8.3 lbs/gal to about 12.8 lbs/gal, the persulfate effective for oxidation at temperatures between 65° F. to 165° F. 25. The method of claim 24 wherein the non-viscosified carrier for the sand comprises said filter cake removal fluid.26. A method of removing filter cake from an existing subterranean borehole, comprising:placing a fluid loss pill into the borehole, the fluid loss pill comprising a polymer to form a filter cake; contacting the filter cake with a mixture of a persulfate salt in a variable density brine; and allowing the mixture to remain at the downhole temperatures ranging from 65° F. to 165° F. for a period of time effective to degrade the polymer filter cake. 27. A method of removing filter cake from an existing subterranean borehole, comprising:placing a fluid loss pill into the borehole, the fluid loss pill comprising a polymer to form a filter cake, the polymer is selected from a water soluble organic polymer, a water dispersible organic polymer, a water soluble bio-polymer, a water dispersible bio-polymer and combinations thereof; contacting the filter cake with a mixture of a persulfate salt in a variable density brine, the persulfate selected from ammonium persulfate, alkali metal persulfate, alkaline earth metal persulfate and combinations thereof, the brine selected form NH4Cl, NaCl, KCl, CaCl2, ZnCl2, and combinations thereof; and allowing the mixture to remain at the downhole temperatures ranging from 65° F. to 165° F. for a period of time effective to degrade the polymer filter cake. 28. The method of claim 26 wherein the mixture of persulfate salt in a variable density brine further comprises a chelating agent.29. A method of removing filter cake from an existing subterranean borehole, comprising:placing a fluid loss pill into the borehole, the fluid loss pill comprising a polymer to form a filter cake, the polymer selected from a water soluble organic polymer, a water dispersible organic polymer, a water soluble bio-polymer; contacting the filter cake with a mixture of a persulfate salt in a variable density brine, the brine selected from NH4Cl, NH4Br, NaCl, NaBr, KCl, KBr, CaCl2, CaBr2, ZnCl2, ZnBr2 and combinations thereof; and allowing the mixture to remain at the downhole temperatures ranging from 65° F. to 104° F. for a period of time effective to degrade the polymer filter cake.