US2074951A - Bit for drilling a hole larger than the bit - Google Patents

Description

March 23, 1937. J. A. ZUBLIN 2,074,951-

BIT FOR DRILLING HOLE LARGER THAN THE BIT Filed Deo. 14, 1935 Patented Mar. 23,1937

UNITED STATES ao'zassi PAT-E BIT Fon DRILLING A non: LARGE-n THAN 'rnn nrr John A. Zublin, Angeles, Calif.

Application December 14, 1935, Serial No. 54,442

3 Claims.

While the present invention has to do generally with drill bits for drilling wells, it is more particularly concerned with bits that drill an oversize hole, that is a hole that is greater in diameter thanthe size of the bit. I

Bits as generally constructed have all the cutting edges and surfaces so disposed that they are symmetrical to the vertical geometric axis of the bit, and also often symmetrical to one or more horizontal axes. As the bit revolves in the well, it contacts the earth formation on the sides and bottom of the well and .at these points of contact resistance is encountered. This resistance as an entirety may be considered as composed of several smaller forces acting at these various points; and these forces created by the removal of material from the well will be found to be symmetrically disposed relative to the bit axis just as are the cutting edges. The result is that for each such force acting on the bit, there is another force of substantially equal and opposite effect so that all the resistance forces on one side of the bit are fairly evenly balanced by forces on the other side of the bit and the center of moments 5 of these forces coincides substantially with the geometric axis of the bit. As long as this condition prevails, the bit will rotate about its own axis, as it naturally would do were it free of all external forces, for there is no unbalanced external force tending to create a new axis of revolution. Consequently, the bit follows the path of least resistance and drills the smallest possible hole determined by the size of the bit.

The hole drilled will, at the top, be the full original diameter of the bit, but at the bottom will be reduced in size by the amount of wear on the bit. With a tight hole of this character, drilling is slower and more difiicult than with an oversize hole, and there is always the serious danger that the bit may jam in the hole and twist on, thus necessitating a very expensive fishing or milling job. In. order to continue with a well, each section of hole must be full gauge all the way to bottom, and if the bit fails to drill it so,

then the hole must be reamed before drilling is resumed, and this is an expensive and time-consuming operation.

' It is therefore a general object of my invention to provide a bit which will drill a hole larger than the bit by at least an amount suflicient that the hole will remain as large as the original bit diameter.

' It is also a main object of the invention to provide a bit which, when rotated, is forced to revolve about an axis other than its geometric axis and so revolves in a path of greater diameter than the bit.

' I have found that these aims are attained in a bit constructed according to my invention by having the several cutting edges so arranged that at least one cutting edge is asymmetrical to the geometric axis of the bit. The resistance to the bit is then greater or lesser against that cutting edge; and the forces opposed to the drilling torque, being unbalanced with respect -to the geometric axis, force the bit to revolve about some new axis aroundwhich all the forces and their moments are in equilibrium.

How the above and other objects and advantages of my invention are attained will be more readily seen from the following description and the annexed drawing, in which:

Fig. 1 is a side elevation of a three blade drag bit;

Fig. 2 is a bottom view of the same bit;

Fig. 3 is a diagrammaticview, in plan, showing the cutting action of the bit;-

Fig. 4 is a side elevation of a fourv blade bit;

and

Fig. 5 is a view similar to Fig. 2 showing a four bladebit.

Figs. 1 and 2 show a three blade bit with a body ill on which is a screw threaded pin II for attaching the bit to the drill pipe which rotates the bit. On the lower part of the body is one long or primary blade [4, which extends from the outside diameter of the bit to the center, and two short blades l5 which extend a shorter distance inwardly from the full bit diameter, as shown in Figs. 2 and 3. Though the lower edges of the blades are not necessarily horizontal, all blades extend down from the body I0 approximately the same distance in order that being approximately the same length they all can cut on the same surfaces of the hole being drilled. The blades are preferably radial, or nearly so, and are spaced about evenly around the body. Short blades l5 are ,placed away from the bit center as shown to provide additional digging surface out where the path travelled by the blades is longest, and the most material is to be removed, while in the central portion of the hole, the inner part of blade I is adequate to remove the smaller amount of material. Because they work on the bottom of the hole, the lower edges of the blades are termed bottom cutting edges, and the vertical side edges of the blades are termed reaming edges since 7 'they cut or ream the hole sides to produce a hole of larger diameter thanproduced by the bottom cutting edges.

Cutting action will best be understood by reference to Fig. 3 which shows diagrammatically the conditions and forces at the bottom of the hole. 5 external reactions or forces on the bit, as opposed to the internal forces or the rotating torque imparted by the drill pipe, may be divided into two classes. The first of these may be termed circular forces; that is, those resisting forces exerted against the cutting edges as they rotate and which oppose the drilling torque by the moments they produce. The second are radial or lateral forces exerted on the bit by the side-walls of the hole in opposition to the reaming action and tend to confine the bit to the hole produced by the bottom cutting edges.

' If it is assumed that the bit rotates about its geometric vertical axis i 1, the hole cut will be represented by the dash-line circle l8; and if the direction of rotation is clockwise as indicated by arrow 20, the circular forces exerted against the advancing faces of blades I4 and ii will be as indicated by arrows II. All three blades extend inwardly from circle i8, which also represents the full gauge of the bit, to circle 24, so that all the cutting edges outside circle 2 are symmetrically disposed with respect to vertical axis I1 and the moments produced by these forces are balanced; but the cutting edge on blade I lying inside circle 24 is not symmetrical with respect to axis I'I so that the circular forces exerted at points inwardly of circle 24, and the moments produced thereby, are not balanced by any other corresponding circular forces. The unbalanced moment thereby created causes blades 15 to swing in a larger are about some point near the blade encountering the greatest resistance, which is primary blade ll, or, in other words, this unbalanced moment tends to rotate the bit about some 40 new theoretical axis 25 removed from axis I! in the direction of blade ll. Under this new condition, the circular forces 2| pressing against the forward face of blade i4 will be reduced, and, at the extreme inner end, will press against the back side of the blade.

If the bit were rotating in a hole of unlimited size there would be no'radial forces exerted on the bit and the resultants of the circular forces on each of the blades would combine to determine 2 the location of the new axis 25 so that these resultants would be in equilibrium as the bit rotates. However, the hole is of a definite limited size and rotation about axis 2! drives the reamingedges of blades ii into the walls of the hole and this lateral movement is opposed by increased reacting radial forces which are now added to In its initial condition, the bit is rotating in a hole the diameter of circle II, and, when so ro+ 70 tating, the unbalanced moment of the circular forces is the greatest and'the tendency of blades ii to ream is also the greatest. Asthe hole is reamed larger by blades IS, the actual axis of rotation moves from axis i'l toward axis 25 with 75 the result that the circular forces against the At the outset it may be explained that the forward face of blade l4 decrease and there is a corresponding decrease from the maximum in radial force at each of blades i5, until finally the resultant of all forces on blade ll exactly counterbalances the resultants of forces on blades l5 so that radial forces against these two blades are no longer required for equilibrium. The bit then rotatesabout axis 25. Thus the reaming tendency is greatest when the hole is smallest and, since it progressively decreases as the hole increases in diameter, there will be a limited maximum size hole which the bit will naturally drill.

The exact position of actual center 28 of rotatation will depend on several factors. Speed of rotation, weight on the bit, hardness of the formatiomsize and shape of the blades all determine the external forces acting on the bit and so influence the equilibrium position of the center of rotation. From this it will be seen that the position'of axis 28 will likely shift from time to time as the bit progresses. However, the magnitude of the forces involved is always sufficient to drive one or more of the reamers far enough into the hole wall against the radial forces that the hole is larger than the original bit diameter, even after the bit has become worn and is under gauge.

Although certain latitude in proportions is permissible, I prefer that the short blades I 5 have a width radially of between one-quarter and onehalf the width of primary blade ll so that blade I! is at least twice as wide as either of the other blades.

As typical of the performance of this bit, a 12 inch cutter under actual field conditions drilled a hole which was never less than 12% inches and, as nearly as could be determined, was mainly about 14 inches in diameter. The reaming edge on blade I I was but slightly worn while the reaming edges on blades l5 were heavily worn, especially the short blade next behind the long blade which indicated that the axis 28 was slightly in advance of blade ll.

The invention is not limited to any particular style of bit, size or shape of blades, or number I of cutting edges, and may be made in many variations of the form described. One such variation is illustrated in Fig. 4, which illustrates the application to a four blade or, by omitting two blades. to a two blade bit. The bit shown has a body Ila provided with four blades asymmetrically arranged, comprising one or more wide blades" and one or more narrow blades 3 I. This type of bit may be used-in formations which are rather hard and brittle so that they ream with dimculty. Under these circumstances it is desirable to increase the circular forces to obtain increased reaming effect, and this is done by lengthening the bottom cutting edge on blade 3|. Since the cutting edges are inclined upwardly and inwardly there will be between the blades a corelike projection of formation. In other respects the cutting action will involve the same principles already described.

Fig. '5 shows another variation in which the bit of Fig. 1 has four blades instead of three. An

extra short blade 15a has been added, the blades still being radial but now spaced only 90 apart. The blade l5a may be the same as theother. two

blades i5, but is shown here as'an extra heavy blade of increased thickness and reaming sin-face next behind 'the long blade I4 and is thus reinforced to withstand the extra heavy wear falling on that reaming edge, and is typical of the many verious types of cutting edges that may be emp oyed.

It will be understood that various shapes and arrangements of the blades and cutting edges may be made without departing from the spirit and scope of the appended claims.

I claim as my invention:

1. A rotary well drilling bit comprising a body and a plurality of individual cutting blades thereon spaced at intervals around the body and all extending below the body approximately the same distance, the outer edges of the blades being substantially equi-distant from the bit axis, and one of the blades being a primary blade that extends inwardly substantially to the bit axis and is at least twice the width of any other blade.

2. A rotary well drilling bit comprising a body and a plurality of individual cutting blades thereon spaced at intervalsaround the body and all extending below the body approximately the same distance, the blades having outer reaming edges substantially equi-distant from the bit axis,

and one of the blades being a primary blade having a bottom cutting edge that extends substantially to the bit axis while the remaining blades have bottom cutting edges between a quarter and a half the width of the primary blade.

3. A rotary well drilling bit comprising a body and a plurality of individual cutting blades thereon spaced at intervals around the body and all extending below the body approximately the same distance, the outer edges of the blades being substantially equi-distant from the bit axis, and one of the blades being a primary blade that extends inwardly substantially to the bit axis and is at least twice the width of any other blade, and the blade next behind the primary blade being a reinforced blade of materially greater thickness and reaming surface than the other blades.

JOHN A. ZUBLIN.

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