Downhole Mills and Improved Cutting Structures

Abstract

A drill bit for cutting through a downhole metal structure includes a bit body having a central axis and a bit face. The bit body is configured to rotate about the central axis in a cutting direction. In addition, the bit includes a cutting structure disposed on the bit face. The cutting structure includes a plurality of circumferentially spaced blades and a plurality of primary cutter elements mounted to each blade. Each primary cutter element has a forward-facing primary cutting face. Each primary cutter element is made of a whisker ceramic composite.

1 . A drill bit for cutting through a downhole metal structure, the bit comprising:
 a bit body having a central axis and a bit face, wherein the bit body is configured to rotate about the central axis in a cutting direction;   a cutting structure disposed on the bit face, wherein the cutting structure includes a plurality of circumferentially spaced blades, a plurality of primary cutter elements mounted to each blade, and a plurality of secondary cutter elements mounted to each blade, wherein the primary cutter elements on each blade lead the secondary cutter elements on the same blade relative to the cutting direction,   wherein each primary cutter element has a forward-facing primary cutting face and each secondary cutter element has a forward-facing secondary cutting face;   wherein each primary cutter element has an extension height that is greater than an extension height of each secondary cutter element;   wherein each secondary cutter element comprises a substrate and an ultrahard table mounted to the substrate;   wherein each primary cutter element is made of a whisker ceramic composite;   wherein the whisker ceramic composite is configured to cut the downhole metal structure and the ultrahard table is configured to cut a subterranean formation adjacent the downhole metal structure.   
     
     
         2 . (canceled) 
     
     
         3 . The drill bit of  claim 1 , wherein the primary cutter elements on each blade are arranged in a row extending along the blade; and
 wherein the secondary cutter elements on each blade are arranged in a row extending along the blade.   
     
     
         4 . (canceled) 
     
     
         5 . (canceled) 
     
     
         6 . The drill bit of  claim 1 , wherein each primary cutter element is made of a ceramic matrix and a plurality of silicon-carbide whiskers distributed throughout the ceramic matrix. 
     
     
         7 . The drill bit of  claim 6 , wherein the ceramic matrix is an aluminum-oxide or zirconium oxide. 
     
     
         8 . The drill bit of  claim 1 , wherein each primary cutter element is brazed to the corresponding blade or pre-cast with the bit body. 
     
     
         9 . The drill bit of  claim 1 , wherein each primary cutter element is secured within a sleeve by an interference fit, and wherein each sleeve is brazed to the corresponding blade. 
     
     
         10 . A cutting device for milling a downhole metal structure, the cutting device comprising:
 a body having a central axis, a first end coupled to a pin, and a second end defining an annular cutting face;   a plurality of circumferentially-spaced cutter elements mounted to the cutting face, wherein each cutter element comprises a whisker ceramic composite.   
     
     
         11 . The cutting device of  claim 10 , wherein each cutter elements has a cylindrical or rectangular geometry. 
     
     
         12 . The cutting device of  claim 10 , wherein the body includes a throughbore extending axially therethrough from the first end to the second end. 
     
     
         13 . The cutting device of  claim 10 , wherein each cutter element is made of a ceramic matrix and a plurality of silicon-carbide whiskers distributed throughout the ceramic matrix. 
     
     
         14 . The cutting device of  claim 13 , wherein the ceramic matrix is an aluminum-oxide or zirconium oxide. 
     
     
         15 . The cutting device of  claim 10 , wherein each cutter element has a base portion secured to the body and a cutting portion with a cutting surface, wherein the cutting surface of each cutter element includes a plurality of steps. 
     
     
         16 . The cutting device of  claim 15 , wherein each cutter element is disposed at a backrake angle between 5° and 20°. 
     
     
         17 . The cutting device of  claim 10 , wherein each cutter element has a base portion secured to the body and a cutting portion with a cutting surface, wherein the cutting surface of each cutter element includes a plurality of parallel teeth. 
     
     
         18 . The cutting device of  claim 10 , wherein each cutter element is brazed to the body or removably mounted to the body with a bolt. 
     
     
         19 . A method for sidetracking from a borehole, the method comprising:
 (a) coupling a drill bit to a lower end of a drillstring, wherein the drill bit comprises:
 a bit body having a central axis and a bit face; 
 a cutting structure disposed on the bit face, wherein the cutting structure includes a plurality of circumferentially spaced blades, a plurality of primary cutter elements mounted to each blade and a plurality of secondary cutter elements mounted to each blade, wherein the secondary cutter elements on each blade trail the primary cutter elements on the same blade; 
 wherein each cutter element has an extension height and a forward-facing cutting face, and wherein the extension height of each primary cutter element is greater than the extension height of each secondary cutter element; 
   wherein the forward-facing cutting face of each primary cutter element is made of a whisker ceramic composite configured to engage and cut casing lining a borehole;   wherein the forward-facing cutting face of each secondary cutter element is made of an ultrahard table mounted to a substrate, wherein the ultrahard table is configured to engage and cut a borehole in an earthen formation;   (b) lowering the drill bit into a borehole lined with casing;   (c) rotating the bit about the central axis in a cutting direction;   (d) engaging the casing with the cutting structure during (c);   (e) milling the casing with the primary cutter elements during (d);   (f) cutting a hole through the casing with the drill bit;   (g) advancing the drill bit through the hole in the casing; and   (h) drilling a borehole in an earthen formation with the secondary cutter elements.   
     
     
         20 . (canceled) 
     
     
         21 . The method of  claim 19 , further comprising sacrificing the primary cutter elements during (h). 
     
     
         22 . The drill bit of  claim 1 , wherein the whisker ceramic composite of each primary cutter element comprises a ceramic matrix embedded with a plurality of distributed fibers, wherein at least some of the plurality of distributed fibers are oriented perpendicular to the primary cutting face.