Protective sheath through a casing window

Abstract

Systems, assemblies, and methods are described which relate to drilling a lateral borehole from a primary wellbore. A primary wellbore may have a casing therein. A window may be milled in the casing using a casing bit. The casing bit may pass through the window and into the surrounding formation. A protective sheath coupled to the casing bit may also extend through the window. The casing bit and protective sheath may remain in place while a drill bit slides through the protective sheath. The drill bit may drill through the casing bit and drill the formation to extend the lateral borehole. An anchor may be used to secure the protective sheath and/or casing bit in place. The drill bit may be included in a single trip assembly with the casing bit and protective sheath.

What is claimed is: 
     
         1 . A method for drilling a lateral borehole from a cased, primary wellbore, comprising:
 positioning a drilling assembly within a primary wellbore, the drilling assembly including a casing bit coupled to a protective sheath;   milling a window in a casing of the primary wellbore with the casing bit;   extending the protective sheath through the window;   guiding a drill string and drill bit through the protective sheath and through the window; and   drilling a lateral borehole using the drill bit.   
     
     
         2 . The method recited in  claim 1 , the casing bit being a drillable casing bit, wherein drilling the lateral borehole including:
 drilling through the casing bit using the drill bit.   
     
     
         3 . The method recited in  claim 1 , further comprising:
 anchoring the protective sheath following extending the protective sheath through the window.   
     
     
         4 . The method recited in  claim 3 , wherein anchoring the protective sheath includes anchoring the protective sheath to restrict at least one of axial or rotational movement of the protective sheath. 
     
     
         5 . The method recited in  claim 1 , wherein the protective sheath is a casing while drilling joint coupled to the drill string. 
     
     
         6 . The method recited in  claim 1 , wherein milling the window in the casing further includes using the casing bit to initiate drilling of the lateral borehole. 
     
     
         7 . The method recited in  claim 1 , wherein the drill string is coupled to an interior of the protective sheath. 
     
     
         8 . The method recited in  claim 7 , wherein drilling the lateral borehole includes releasing the drill string from the protective sheath. 
     
     
         9 . The method recited in  claim 1 , further comprising:
 anchoring a deflection member within the primary wellbore; and   deflecting the casing bit into the casing using the deflection member.   
     
     
         10 . The method recited in  claim 1 , wherein extending the protective sheath includes positioning the protective sheath partially through the window. 
     
     
         11 . A drilling assembly, comprising:
 a casing bit;   a protective sheath coupled to the casing bit; and   a drill string coupled to an interior of the protective sheath, the drill string including a drill bit.   
     
     
         12 . The drilling assembly recited in  claim 11 , further comprising:
 an anchor coupled to the protective sheath.   
     
     
         13 . The drilling assembly recited in  claim 11 , the casing bit being a drillable casing bit and configured to mill through casing of a wellbore. 
     
     
         14 . The drilling assembly recited in  claim 11 , further comprising:
 a fastener selectively coupling the drill string to the protective sheath.   
     
     
         15 . The drilling assembly recited in  claim 14 , the fastener including a sacrificial element. 
     
     
         16 . The drilling assembly recited in  claim 11 , further comprising:
 a deflection member releasably coupled to the casing bit.   
     
     
         17 . The drilling assembly recited in  claim 16 , further comprising:
 an anchor coupled to the deflection member.   
     
     
         18 . A drilling system for forming a window in a casing of a primary wellbore and drilling a lateral borehole off the primary wellbore, comprising:
 a drillable casing bit configured to mill a window in casing of a primary wellbore;   a protective sheath coupled to the drillable casing bit;   an anchor configured to anchor the protective sheath and restricting at least one of axial or rotational motion of the protective sheath;   a drill string coupled to, and within the protective sheath, the drill string including a drill bit configured to drill through the drillable casing bit and through formation around the primary wellbore; and   a fastener releasably coupling the drill string to the protective sheath.   
     
     
         19 . The drilling system recited in  claim 18 , the anchor being selectively expandable to anchor the protective sheath within the formation. 
     
     
         20 . The drilling system recited in  claim 18 , further comprising:
 a deflection member releasably coupled to at least one of the drillable casing bit or the protective sheath.