Subterranean cutting tool structure tailored to intended use

#The field of the invention is subterranean milling tools and more particularly those mills that employ discrete cutting structures at different locations on the mill body to address the expected type of wear unique to that location so that overall milling effectiveness is improved.

#There are occasions where downhole devices such as packers or bridge plugs or cement shoes are milled out. Other times there is a tubing string or portion of a tubing string that needs to be cut so that subsequent operations can continue. Over time the design of such mills has evolved to address the need for greater speed and cutting efficiency. In the 1980s Baker Hughes came out with a line of mills known as Metal Muncher® as illustrated in U.S. Pat. Nos. 5,038,859 or 5,086,838; 4,796,709 and 5,456,312.

#One example of this design is shown in FIGS. 1 and 2. The typical mill of this type had a body 10 with a central flow passage 12 that lead to a plurality of outlets 14 shown on the bottom face view of FIG. 2. A series of spaced apart vertical blades 16 had their leading face covered with a nested array of round inserts 18 made of a hardened cutting material such as tungsten carbide. These inserts were arranged in rows such as 20 and 22 and as one row would wear away with the blade that supported it the next row would take over the cutting task. Fluid such as drilling mud would be pumped through the outlets 14 located ahead of the inserts 18 on a given blade 16 in the direction of rotation. A matrix material 24 is deployed behind the blades 16 for structural support and for some limited cutting capability. The cuttings made by the inserts have to clear the outside edge of the mill and are carried off by the circulating fluid that also removes some of the heat generated from the milling operation.

#FIG. 1 is a side view of a Metal Muncher® prior art mill;

#FIG. 2 is a bottom view of FIG. 1;

#FIG. 3 is a bottom view of a prior art junk mill;

#FIG. 6 shows the bottom view of a mill 32 that has a central zone 34 and a plurality of leading peripheral spaced apart zones 36 as determined by the direction of rotation represented by the arrow 38. In between are leading 40 and trailing 42 cutting regions where most of the cutting takes place and the chips off the fish are formed. The trailing region is also disposed behind the peripheral zones 36 in the direction of rotation. A series of ports 44 border the trailing region 42 and are there to allow pumped fluid to drive the cuttings to the edges where they can make a turn uphole through gaps such as 46. The cutting elements 48 in zone 34 are illustrated in detail in FIG. 7. This shape is formed from a starting shape of a sphere and cut with a cylindrical drill that passes through the shape at four perpendicular orientations. This leaves end rounded shapes 50 and 52 that are spherical and are separated by four part cylindrical walls of which three are visible in FIG. 7 namely 54, 56 and 58. The idea, regardless of the fabrication technique is to create some cutting edges in an otherwise fairly rounded shape so that the impact resistance is high while the penetration into the fish from such shapes dispersed in a matrix in zone 34 is fairly low. The main objective in zone 34 being to withstand the impact loads and heat generated and to remain in position long enough to prevent coring the center zone 34 by having parts of the carbide shapes break off. In past designs the inserts that were best suited for cutting in zones 40 or 42 were also used in the central zone where the loading was different so that the performance of those inserts such as 18 in FIG. 2 when in the center of the mill did not optimally prevent coring. In the present invention the inserts 48 are fairly rounded but still have some cutting edges such as 60 and 62 in generally parallel planes and other cutting edges such as 64, 66 and 68 that are generally parallel to each other and in planes approximately perpendicular to edges 60 and 62. Insert 48 can also be used in the peripheral zones 36 where the ability to resist high impact is more significant than cutting ability. Thus with a modest amount of cutting edges and a rounded overall shape coupled with placement at the bottom center zone 34 and the peripheral locations 36 the problems associated with the prior designs and their uniform insert distribution are avoided. At the center zone 34 the rounded shape tolerates high impacts such as can occur when milling a string inside another string where the inner string being milled is laying up against the outer string so that the mill center is over a wall portion of the fish trying to mill it out. The center zone 34 can receive impact loads as the mill bounces against the pipe wall of the fish. Heat is also generated there and the material for the matrix as well as the insert is selected to tolerate this expected heat load. The insert material can be tungsten carbide, cubic boron, polycrystline diamond compacts and other hard material cutting elements and the matrix in which it is embedded can be brazing materials like nickel bronze, silver solder, copper bronze and other braze materials having a significant wetting ability and high yield strengths.

#The leading zone for creating the chips off the fish is 40. Here it is desirable to have a robust structure that can hold an edge or generate an edge as the milling progresses. The insert 70 shown in FIG. 8 is preferred for this service. Also suitable are cylindrically shaped inserts and inserts where cutting edges are defined by surfaces that meet at 90 or more degrees. In the illustrated embodiment it features opposed truncated pyramid shapes 72 and 74 on opposed sides of a rectangular block 76. Cutting edges abound in this shape and its limited protrusion distance from the block 76 makes the pyramid shape edges such as 78, 80 82 and 84 more likely to remain in position rather than be chipped off. What characterizes the optimal shape for zone 40 is a massive core structure regardless of shape with protruding and opposed extending shapes that have multiple cutting edges that are generally shorter than the edge dimensions of the core. The extending shapes tend to have a height in the order of magnitude of the smaller core dimension and long and thin shapes are avoided in favor of short bulky shapes with a center of mass closer to the core periphery than the smallest dimension of the core.

#FIG. 9 illustrates a design of lesser strength than in FIG. 8 and one that is best suited for the zone marked 42 in FIG. 6. These designs are characterized by cutting edges where surfaces meet at an angle of under 90 degrees. The bulk of the cutting is taking place in zone 40 where more robust shape with blunter angles such as shown in FIG. 8 is used. The trailing zone 42 does not require the same strength because the cuttings are already for the most part initiated in zone 40. Another advantage of putting a less aggressive shape of FIG. 9 behind the shape of FIG. 8 is when the demand for what is being milled changes. For example, when milling a packer body the need for strength initially is there as the mandrel and body are milled away. When the seal and slips are reached the cutting demand is different. The slips are typically cast iron that breaks more easily. Very aggressive cutting shapes such as in FIG. 8 work far less well in trying to cut a rubber shape. A less aggressive structure minimizes balling of the rubber or composite on the cutting structure. The less aggressive structure is also suitable for milling cast iron slips.