Cutters for downhole cutting devices

Improved cutter design as well as an improved design for downhole cutters, such as mandrel cutters and rotary cutter mills. A cutter is described with a rectangular, rounded “lozenge” shape. The cutter presents a cross-sectional cutting area having a pair of curvilinear end sections and an elongated central section. Preferably, the overall length of the cutter is 1.5 times the width. The cutter may also include a raised cutter edge for chip breaking during cutting.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the design and use of cutters for the cutting arms and blades of underreamers, mills and other downhole tools.

2. Description of the Related Art

Rotary cutting mills and mandrel cutters are devices that are incorporated into a drill string and used to cut laterally through metallic tubular members, such as casing on the sides of a wellbore, liners, tubing, pipe or mandrels. Mandrel cutters are used to create a separation in metallic tubular members. Cutting mills are tools that are used in a sidetracking operation to cut a window through surrounding casing and allow drilling of a deviated drill hole. On conventional tools of this type, numerous small individual cutters are attached to multiple arms or blades that are rotated about a hub. Most conventional cutters present a circular cutting face. Other conventional cutter shapes include square, star-shaped, and trapezoidal, although these are less common. However, the use of circular cutters has some inherent drawbacks when used to cut through metallic tubular members. First, there is a small amount of bond area between the cutter and the arm or blade upon which the cutter is mounted. The bond area is essentially the area of the circle. During cutting, the cutters may become loose and break off of the cutting arm. Additionally, the geometry of circular cutters results in a significant amount of interstitial space between cutters. This is detrimental, particularly, when the cutter is cutting through metal that is ductile, such as casing containing high amounts of chrome and/or nickel. These materials will enter the interstitial spaces and erode away the cutting arm during cutting.

In the instance of a rotary cutting mill, the presence of large interstitial spaces also presents a significant problem because of the cutting pattern provided by the mill. As the mill is rotated, the cutters are caused to cut repeatedly along particular paths in the material being cut. This repeated pattern of cutting will result in grooves in the cut material and undesirably force the uncut portions of the material lying between the grooves into the interstitial spaces. To prevent this from happening, half-circular cutters have been used on alternate blades to provide an offset. However, these half-cutters have little bonding area and are prone to breaking off.

Mandrel cutters have at least one cutting knife that is rotated to cut circumferentially through a surrounding metallic tubular member. Mandrel cutters are problematic because they require the use of cutting portions that are very small and narrow in order to effectively cut through the mandrel. The limitation on the size of the cutting portion exacerbates the bonding area problem described above.

The present invention addresses the problems of the prior art.

SUMMARY OF THE INVENTION

The invention provides an improved cutter design as well as an improved design for downhole cutters, for use with cutting devices such as mandrel cutters, and rotary cutter mills. In one aspect, the invention describes an improved cutter having a rectangular, rounded “lozenge” shape. The cutter may be formed of carbide or be a polycrystalline diamond compact (“PDC”) cutter. The cutter presents a cross-sectional cutting area having a pair of curvilinear, and preferably arcuate, end sections and an elongated central section having substantially straight or flat sides. Preferably, the overall length of the cutter is 1.5 times the width. In a preferred embodiment, the cutter includes a raised cutter edge for chip breaking during cutting.

The cutters of the present invention provide advantages for attachment to a cutter arm or blade. Bond area is increased. Therefore, the cutters remain in place more securely. Also, placement of the rounded, rectangular cutters on a cutting arm results in less interstitial space between cutters. In return, this results in less extrusion of ductile metals into the interstitial spaces and less resultant damage to the arm or blade carrying the cutters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2depict an exemplary cutter10that is constructed in accordance with the present invention. The cutter10has a body12that is preferably formed of hardened carbide. However, the cutter10might also be formed of PDC, as is known in the art, or another substance suitable for use in downhole cutting. The body12features a cutting face14and a sidewall16. Preferably, the cutter10features a raised chip-breaking edge18that is located proximate the outer circumference of the cutting face14. When considered from the plan view offered byFIG. 2, the body12of the cutter10is generally made up of three sections: two end sections20,22with end walls23that are semi-circular in shape, and a generally rectangular central section24that interconnects the two end sections20,22to result in a rounded, rectangular “lozenge” shape for the cutter10.

FIG. 2also illustrates the currently preferred dimensions for the cutter10. The cutter10has an overall length26, as measured from the tip of one semi-circular section20to the tip of the other semi-circular section22. The cutter10also has a width28that extends from one lateral side of the central section24to the other. The width28is also equal to the diameter of the semi-circular end sections20,22. In a currently preferred embodiment, the length26of the cutter10is approximately 1.5 times the width28of the cutter10. A currently preferred width29for the cutter10is approximately ⅜41.

FIG. 3depicts an alternative embodiment for a cutter10′ which is constructed in accordance with the present invention. The cutter10′ is similar to the cutter10described previously. However, the end sections20′ and22′ are arcuate, but not semi-circular. The end sections20′ and22′ instead, have an end wall23′ with a larger radius of curvature and, therefore, represents an arc segment that is less than a semi-circle. In this embodiment, the length of the cutter10′ still exceeds the width of the cutter10′, and the preferred length-to-width ratios described above apply to this embodiment as well. It is noted that the end walls23′ of the end sections20′,22′ do not require any particular radius of curvature and, therefore, may present a relatively flattened curvature, as inFIG. 3, or a more pronounced curvature. Additionally, the radius of curvature for the end walls23,23′ need not be a constant radius, but may otherwise be curvilinear. It is noted that the lateral sides31of the central section24are substantially straight and flat.

FIG. 4illustrates an exemplary cutting arm, or cutting member,30having a raised cutting portion32. The cutting arm30is of a type that is incorporated into a downhole cutter and used for rotary cutting into portions of the sidewall of a wellbore, as is known in the art. A plurality of prior art cutters34are affixed thereto having round-shaped cutting faces. It is noted that there is a significant amount of interstitial space36between the cutters34on the raised cutting portion32. During downhole cutting or milling, the interstitial space36between the cutters34is highly susceptible to erosion damage. Particularly where the materials being milled or cut are highly ductile, such as those having high chrome and/or nickel content, the milled material tends to flow into the interstitial space36and erode away the arm30. Also depicted inFIG. 4is a half cutter34awhich is used to help accommodate proper spacing with the other cutters34upon the raised cutting portion32. The use of half cutters34ais problematic because there is minimal bonding area and, therefore, half cutters are very likely to break off of the cutting arm30.

FIG. 5depicts an exemplary cutting arm30having a plurality of cutters10of the type described previously with respect toFIGS. 1 and 2affixed thereupon, in accordance with the present invention. The use of the rounded, rectangular cutters10results in less interstitial space36available on the raised portion32and as a result, less erosion of the arm30. Additionally, the increased length26of the cutter10as compared to a cutter34means there is increased bond area between each cutter10and the arm30as compared to the prior art cutters34. Cutters are typically affixed to a cutting arm by brazing and welding. The increased bond area results in cutters that are more securely affixed to the cutting arm30. Additionally, the width28of the cutter10is the same as the width (diameter) of the conventional circular cutters34, which allows the cutters10to be seated upon a cutting surface having a narrow width while providing improved bonding area and strength.

FIG. 6depicts an exemplary arm50for a mandrel cutting tool. The arm50includes a proximal portion52having a pin opening54into which the arm50is pivotally attached to a cutting tool mandrel (not shown) and a distal cutting portion56. The distal cutting portion56, which is more clearly depicted in the close up view ofFIG. 6A, includes a cutter retaining area58that is bounded by side surface60and shelf62. Cutters10are accommodated inside the cutter retaining area58and leave very little interstitial space.

FIG. 7illustrates an exemplary rotary cutting mill70of the type used in sidetracking operations to mill a lateral opening in wellbore casing. Cutting mills of this type are generally known in the art, and include the SILVERBACK™ window mill available commercially from Baker Oil Tools of Houston, Tex. The cutting mill70has five cutting blades, or arms,72that are rotated about hub74during operation. Each of these blades72has cutters10mounted upon them. It is pointed out that the blades72may include some rounded, conventional cutters34as well. It is noted that the cutters10,34are mounted upon the cutting blades72in a manner such that the cutters are offset from one another in adjacent blades72. For example, the distal tip of the edge of blade72A has four cutters10that are arranged in an end-to-end manner. However, the neighboring blade72B has the lead cutter10A turned at a 90 degree angle to the other cutters10, thereby causing the interstitial space36between the cutters10,10A,34to be staggered on adjacent blades72. As a result of this staggering, the blades72will become less worn in the interstitial spaces36.

Testing has shown that the use of cutters constructed in accordance with the present invention provide a number of advantages over conventional circular cutters. The rounded, rectangular shape of the cutters10allows them to be mounted upon narrow cutting surfaces, such as raised cutting portion32. Such cutters are useful on cutting arms having narrow cutting surfaces as they provide for reduced cutting load while having sufficient bond area to remain secured during cutting. The chip breaker edge18serves to break up sections of earth material that may be formed during cutting.

Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.