Filter cake removal fluid

Abstract

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.

1 . A composition for a filter cake removal fluid comprising: 
 a solution of a persulfate salt in a brine, the concentration of persulfate effective for oxidation at temperatures between 65° F. to 180° F., the solution substantially free of an activating agent.    
   
   
       2 . A composition for a filter cake removal fluid comprising: 
 a solution of a persulfate salt in a brine having a density within a range of about 8.3 lb./gal to about 12.8 lb./gal, the concentration of persulfate ranging from about 1 lb/bbl to about 50 lb./bbl, the solution substantially free of an activating agent.    
   
   
       3 . The composition for a filter cake removal fluid of  claim 2  wherein the persulfate salt is ammonium persulfate.  
   
   
       4 . The composition for a filter cake removal fluid of  claim 2  wherein the persulfate salt is selected from alkali metal persulfate, alkaline earth metal persulfate and combinations thereof.  
   
   
       5 . The composition for a filter cake removal fluid of  claim 4  wherein the alkali metal persulfate is selected from potassium persulfate, sodium persulfate, lithium persulfate and combinations thereof.  
   
   
       6 . The composition for a filter cake removal fluid of  claim 4  wherein the alkaline earth metal persulfate is selected from calcium persulfate, magnesium persulfate, and combinations thereof.  
   
   
       7 . The composition for a filter cake removal fluid of  claim 2  wherein the brine is selected from NH 4 Cl, NaCl, KCl, CaCl 2 , ZnCl 2 , and combinations thereof.  
   
   
       8 . A composition for a filter cake removal fluid comprising: 
 a solution of a persulfate salt in a bromide brine, the persulfate effective for oxidation at temperatures less than 104° F. the solution substantially free of an activating agent.    
   
   
       9 . A composition for a filter cake removal fluid comprising: 
 a solution of a persulfate salt in a brine selected from NH 4 Cl, NaCl, KCl, CaCl 2 , ZnCl 2 , and combinations thereof, the concentration of persulfate effective for oxidation at temperatures between 65° F. to 165° F., the solution substantially free of an activating agent.    
   
   
       10 . A composition for a filter cake removal fluid comprising: 
 a solution of a persulfate salt in a brine selected from NH 4 Cl, NaCl, KCl, CaCl 2 , ZnCl 2 , and combinations thereof, the concentration of persulfate ranging from about 1 lb/bbl to about 50 lb./bbl, the solution substantially free of an activating agent.    
   
   
       11 . A composition for a filter cake removal fluid comprising: 
 a solution of a persulfate salt in a brine selected from NH 4 Cl, NaCl, KCl, CaCl 2 , ZnCl 2 , and combinations thereof, the concentration of persulfate ranging from about 4 lb./bbl to about 48 lb./bbl, the solution substantially free of an activating agent.