System for drilling a wellbore

Abstract

A system is provided for drilling a borehole into an earth formation, comprising a casing arranged in the borehole, a drill string extending through the interior of the casing to a lower end portion of the borehole, and a body of drilling fluid extending into the casing, the casing having an inner surface susceptible of wear due to frictional contact with an outer surface of the drill string during drilling of the borehole with the drill string. The system further comprises means for reducing wear of the inner surface of the casing, the means including at least one of a hardened layer at the inner surface of the casing, a friction-reducing layer at the outer surface of the drill string, and a lubricating compound contained in the body of drilling fluid.

1 . A system for drilling a borehole into an earth formation, comprising a casing arranged in the borehole, a drill string extending through the interior of the casing to a lower end portion of the borehole, and a body of drilling fluid extending into the casing, the casing having an inner surface susceptible of wear due to frictional contact with an outer surface of the drill string during drilling of the borehole with the drill string, wherein the system further comprises means for reducing wear of said inner surface of the casing, said means including at least one of a hardened layer at the inner surface of the casing, a friction-reducing layer at the outer surface of the drill string, and a lubricating compound contained in the body of drilling fluid. 
     
     
         2 . The system of  claim 1 , wherein the hardened layer at the inner surface of the casing has a first hardness and the outer surface of the drill string has a second hardness, and wherein said first hardness is larger than said second hardness. 
     
     
         3 . The system of  claim 2 , wherein the ratio of the first hardness to the second hardness is between 1 and 5. 
     
     
         4 . The system of  claim 2  wherein the first hardness has a value of between 150 Hv and 600 Hv. 
     
     
         5 . The system of  claim 1  wherein said friction-reducing layer at the outer surface of the drill string comprises poly-tetra-fluor-ethene (PTFE). 
     
     
         6 . The system of  claim 5 , wherein said outer layer forms a coating applied to the outer surface of the drill string. 
     
     
         7 . The system of  claim 1  wherein the lubricating compound includes between 64.25-90.89 wt % base oil. 
     
     
         8 . The system of  claim 7 , wherein said base oil is selected from a natural triglyceride oil, fish oil, animal oil, vegetable triglyceride oil, sunflower seed oil, soybean oil, rapeseed oil canola oil, palm nut oil, palm oil, olive oil, rapeseed oil, canola oil, linseed oil, ground nut oil, soybean oil, cottonseed oil, sunflower seed oil, pumpkin seed oil, coconut oil, corn oil, castor oil, walnut oil, a natural or synthetic oil, and an ester. 
     
     
         9 . The system of  claim 1  wherein the lubricating compound includes between 0.02-0.05 wt % metal deactivator. 
     
     
         10 . The system of  claim 1  wherein the lubricating compound includes between 0.5-3.0 wt % antioxidant. 
     
     
         11 . The system of  claim 1  wherein the lubricating compound includes between 4-12 wt % sulfurized natural oils. 
     
     
         12 . The system of  claim 1  wherein the lubricating compound includes between 4-12 wt % phosphate ester. 
     
     
         13 . The system of  claim 1  wherein the lubricating compound includes between 0.4-1.5 wt % phosphoric acid. 
     
     
         14 . The system of  claim 1  wherein the lubricating compound includes between 0.08-1.5 wt % viscosity modifier. 
     
     
         15 . The system of  claim 1  wherein the lubricating compound includes between 0.1-0.5 wt % pour-point depressant. 
     
     
         16 . The system of  claim 1  wherein the lubricating compound includes between 0.01-0.2 wt % defoamer. 
     
     
         17 . The system of  claim 1  wherein the lubricating compound includes between 0-5 wt % carboxylic acid soaps. 
     
     
         18 . The system of  claim 1  wherein the casing is formed of a plurality of casing sections, each casing section having at least a portion not overlapping with any other one of the casing sections. 
     
     
         19 . The system of  claim 18 , wherein the casing is radially expandable, the system further comprising means for radially expanding a first one of said plurality of casing sections, and means for lowering a second one of said plurality of casing sections through said first casing section after radial expansion thereof. 
     
     
         20 . (canceled)