Buoyancy assist tool with degradable plug

Abstract

A downhole apparatus comprises a casing string with a fluid barrier connected in the casing string. A degradable plug is positioned in the casing string above the fluid barrier. The degradable plug and fluid barrier define upper and lower ends of a buoyancy chamber in the casing string. The degradable plug may be degraded to leave an open bore through the casing string after the casing string is lowered into a wellbore.

What is claimed is: 
     
         1 . A downhole apparatus comprising:
 a casing string;   a fluid barrier connected in the casing string; and   a degradable plug positioned in the casing string above the fluid barrier, the degradable plug and fluid barrier comprising upper and lower ends of a buoyancy chamber in the casing string.   
     
     
         2 . The downhole apparatus of  claim 1  further comprising a non-permeable cap covering an upper end of the degradable plug. 
     
     
         3 . The downhole apparatus of  claim 1  further comprising an outer case connected in the casing string, the degradable plug positioned in the outer case, the outer case and the degradable plug defining an annular space configured to receive a degrading fluid. 
     
     
         4 . The downhole apparatus of  claim 3 , further comprising a plug housing, the degradable plug being fixed in the plug housing, the plug housing defining ports through a wall thereof configured to communicate fluid from the annular space to the degradable plug. 
     
     
         5 . The downhole apparatus of  claim 3 , the outer case comprising:
 an upper outer case configured to connect in the casing string;   a lower outer case configured to connect in the casing string; and   a connecting sleeve connected at one end to the upper outer case and at a second end to the lower outer case, the annular space being defined between the connecting sleeve and the degradable plug.   
     
     
         6 . The downhole apparatus of  claim 3 , the outer case having a port defined therein configured to communicate the degrading fluid to the annular space. 
     
     
         7 . The downhole apparatus of  claim 3 , the outer case defining a fluid chamber containing, the degrading fluid. 
     
     
         8 . A downhole apparatus comprising:
 a plug assembly, the plug assembly comprising a plug housing with a degradable plug fixed therein;   an upper outer case connected at a lower end to the plug assembly and configured to connect in a casing string at an upper end thereof;   a lower outer case connected at an upper end to the plug assembly; and   a connecting sleeve connecting the upper and lower outer cases, the connecting sleeve and plug assembly defining an annular space therebetween configured to receive a plug degrading fluid.   
     
     
         9 . The downhole apparatus of  claim 8 , the plug housing defining a plurality of ports therein configured to communicate a degrading fluid from the annular space with the degradable plug. 
     
     
         10 . The downhole apparatus of  claim 8 , the upper outer case defining a port in a wall thereof, the port having a rupturable plug therein, the port configured to communicate degrading fluid into the annular space when the plug ruptures. 
     
     
         11 . The downhole apparatus of  claim 8 , further comprising a non-permeable cap covering an upper end of the degradable plug. 
     
     
         12 . The downhole apparatus of  claim 11 , wherein the non-permeable cap comprises a rubber coating. 
     
     
         13 . The downhole apparatus of  claim 8  connected in a casing string and defining an upper end of a buoyancy chamber in the casing string. 
     
     
         14 . The downhole apparatus of  claim 8 , the upper outer case defining a fluid chamber containing the degrading fluid. 
     
     
         15 . A downhole apparatus comprising:
 a plurality of casing joints defining a casing string;   a degradable plug configured to block flow through the casing string until the casing string reaches a desired depth in a wellbore, and   a flow barrier positioned in the casing string below the degradable plug, the degradable plug and the flow barrier defining upper and lower ends of a buoyancy chamber in the casing.   
     
     
         16 . The downhole apparatus of  claim 15  comprising:
 a plug assembly connected in the casing string, the plug assembly comprising a plug housing case with the degradable plug fixed therein. 
 
     
     
         17 . The downhole apparatus of  claim 16 , the plug housing defining a plurality of ports configured to communicate a degrading fluid therethrough to contact the degradable plug. 
     
     
         18 . The downhole apparatus of  claim 16  further comprising an outer case connected in the casing string, the outer case and the plug assembly defining an annular space configured to receive a degrading fluid. 
     
     
         19 . The downhole apparatus of  claim 18  further comprising a closed fluid chamber positioned above the plug assembly, the closed fluid chamber containing the degrading fluid. 
     
     
         20 . The downhole apparatus of  claim 18  wherein the degrading fluid is delivered through the casing string.