Drill bit and method of drilling

Abstract

A drill bit for abrasive jet drilling having nozzles positioned to cut a central hole surrounded by a plurality of concentric grooves. The nozzle outlet diameters vary depending on the location of the nozzles in the drill bit and are coordinated with the number of nozzles positioned to cut a particular groove so that all grooves are of substantially the same depth.

Description

This invention relates to the drilling of wells and more particularly to a drill bit for use in the abrasive jet drilling process.

The abrasive jet drilling process has recently been developed for drilling wells in hard formations. In that process, abrasive particles such as iron or steel grit or shot are suspended in a liquid which is pumped down the drill string and discharged at extremely high velocities through nozzles in a bit at the lower end of the drill string. The necessary high velocity is obtained by a pressure differential from the nozzle inlet to the nozzle outlet of at least 5,000 psi. An important part of the cost of abrasive jet drilling is for power to pump the drilling liquid at the necessary high velocity. The drilling liquid is circulated up the borehole through the annulus surrounding the drill string and treated at the surface to remove cuttings and recondition the drilling liquid for recirculation through the drill bit.

The nozzles in the drill bit are positioned to cut a central hole surrounded by a plurality of concentric grooves in the bottom of the borehole. The outer wall of the outer groove forms the borehole wall as the hole is drilled. Thin ridges separate the central hole from the innermost groove and the grooves from each other. The ridges can be eccentrically loaded to be easily broken mechanically by the drill bit to contribute to the high drilling rate made possible by abrasive jet drilling. Most effective use of the power applied to the drilling liquid is obtained by cutting narrow grooves.

The rate of penetration into the bottom of the borehole in abrasive jet drilling decreases rapidly with increasing distance from the outlet of the nozzles to the bottoms of the grooves. If sufficient drilling liquid is discharged through the nozzles that cut one of the grooves to make that groove substantially deeper than the other grooves, an inefficient drilling operation results. The power utilized in making the groove is excessive because the bottom of that groove is so far from the nozzle outlets. It is, therefore, desirable that all of the grooves be of substantially the same depth.

Because the amount of rock to be removed by the high-velocity abrasive streams increases with the radius of a groove, a successively larger number of nozzles has heretofore been used for cutting each successively larger groove. Control of the depth of the grooves cut in the bottom of the borehole by varying the number of nozzles at any particular radial distance from the center of rotation of the drill bit is described in U.S. Pat. No. 3,375,887 of Goodwin et al. The outlet diameters of the nozzles have been uniform regardless of the position of the nozzle in the drill bit. A typical drill bit used in abrasive jet drilling is described in U.S. Pat. No. 3,542,142 of Hasiba et al. Sometimes the number of nozzles theoretically required to cut a groove of the same depth as the others is a fractional number. The next larger whole number of nozzles has been used with a groove of excessive depth resulting. Although a single nozzle would theoretically cut the central hole to the desired depth, two nozzles have been provided for that hole to take care of the possibility that one of the nozzles might become plugged.

This invention resides in a drill bit for use in abrasive jet drilling having a plurality of nozzles positioned in the bottom of the bit to cut a central hole surrounded by a plurality of concentric grooves. Standoff elements on the bottom of the bit ride on and break the ridges formed in the bottom of the borehole. In the drill bit of this invention, both the diameter of the throats of the nozzles and the number of nozzles for cutting each of the grooves are varied to cause cutting of grooves of substantially equal depth during the drilling.

In the drawings:

FIG. 1 is an elevational view, with the lower part shown in vertical section, of a drill bit constructed in accordance with this invention.

FIG. 2 is a plan view of the bottom of the bit illustrated in FIG. 1 showing the location and different sizes of the nozzles.

FIG. 3 is a diagrammatic vertical sectional view across the bottom of a borehole drilled with a bit utilizing this invention.

Referring to FIG. 1 of the drawings, a drill bit indicated generally by reference numeral 10 is shown with a hollow drill bit body 12 having a shank 14 extending upwardly therefrom. Shank 14 is threaded at its upper end for connection to a drill string adapted to rotate the drill bit and deliver drilling liquid to the drill bit. A throat 16 extends downwardly through the shank to deliver drilling liquid into a central cavity 18 within the drill bit. The central cavity 18 is closed at its lower end by the bottom 20 of the drill bit. A backsplash plate 22 is secured to the lower surface of the bottom 20 to protect the drill bit from erosion by abrasive particles rebounding from the bottom of the borehole during the drilling operation. Backsplash plate 22 is constructed of an abrasion-resistant material, such as a tungsten carbide alloy, and is secured to the loewr surface of bottom 20 by any suitable means, such as silver soldering.

A plurality of holes extend through the bottom 20 and backsplash plate 22 to receive nozzles 24. In FIG. 1 of the drawings, the nozzles 24 are shown as fitting in holes tapering toward the bottom of the drill bit whereby the pressure of the drilling liquid within the bit forces the nozzles more securely in place. The method of mounting the nozzles is not critical to this invention. An advantageous method of mounting nozzles in the drill bit is disclosed in U.S. Pat. No. 3,688,852 of Gaylord et al. That method has the advantages of permitting installation of the nozzles from the bottom of the bit, thereby facilitating installations of the nozzles at any desired location and angle and replacement of the nozzles in the event the nozzles should become worn excessively. Nozzles 24 are constructed of an abrasion-resistant material, such as a tungsten carbide alloy.

The drill bit body 12 illustrated in FIG. 2 is of generally square shape in horizontal section. This invention is not limited to drill bits having that shape but can be used with round drill bits, generally triangular drill bits such as disclosed in the aforementioned U.S. Pat. No. 3,542,142 of Hasiba et al or irregularly shaped drill bits of the type disclosed in U.S. Pat. No. 3,414,070 of J. L. Pekarek. The generally square bit shown in the drawings is advantageous in being symmetrical and thereby contributing to a more smoothly operating bit and also providing space between the drill bit and the borehole wall for upward flow of drilling liquid and cuttings during the drilling operation.

Referring to FIG. 2, drill bit 10 has a nozzle 24a that is positioned and inclined to cut a central hole 26 in the bottom of the borehole. Central nozzle 24a is illustrated in FIG. 2 as having its outlet at the center of the drill bit and sloping to discharge drilling liquid in a direction toward the periphery of the drill bit. Nozzle 24a also can be located a slight distance from the center of the drill bit and slope to discharge a stream toward the center of the drill bit, or can be vertical. It is only necessary for the nozzle 24a to be positioned and oriented to cut a central hole 26 that has no part within its periphery that extends upwardly to the bottom of backsplash plate 22 and preferably not as high as ridges 28, 30, 32 and 34 that separate grooves cut in the bottom of the borehole. When nozzle 24a is positioned as shown in FIG. 2 and slopes towards the periphery of the drill bit, it will leave a central knob 36 in the center of central hole 26. Positioning and orientation of nozzle 24a must be such that the stream of drilling liquid discharged from it erodes knob 36 sufficiently that its upper end does not contact the lower surface of backsplash plate 22 and support the bit. Because contact of the bottom of backsplash plate 22 with knob 36 would not eccentrically load the knob 36, such contact could support the bit high enough to slow seriously the drilling rate. Thus, the central hole 26 may be in the shape of a groove as long as it does not have a central knob 36 high enough that the central knob is broken mechanically by contact with the bit instead of by erosion.

It is important to this invention that the diameter of the outlet of nozzle 24a be large enough to minimize the chance of plugging of the nozzle during drilling operations. Ordinarily, the outlets of nozzles used in abrasive jet drill bits have a diameter that is at least about three times the diameter of the largest abrasive particles used in the abrasive jet drilling operation. Because only a single nozzle is used in the drill bit of this invention to cut the central hole 26 and the importance of avoiding leaving a central knob that will engage backsplash plate 22, the outlet diameter of nozzle 24a should be at least 3.3 and preferably 4 or more times the diameter of the largest abrasive particles to minimize chances of nozzle 24a becoming plugged. The maximum nozzle outlet diameter should not exceed approximately five times the diameter of the largest abrasive particles used. Because the volume of drilling liquid increases with the square of the diameter of the nozzle outlet, larger nozzles could greatly increase the volume, and consequently the cost of pumping the drilling liquid. It is preferred to cut enough grooves in the bottom of the borehole to leave ridges not more than 1/2 inch wide, and such ridges can be formed more economically by cutting narrow grooves than by cutting a smaller number of wide grooves.

Spaced outwardly from the center of drill bit 10 are a pair of nozzles 24b. Nozzles 24b are positioned to cut a groove 37 surrounding the central hole 26. In the embodiment of the invention shown, nozzles 24b are vertical, but it is not essential that the nozzles be vertical. Nozzles 24b will ordinarily have the smallest outlet diameter of any of the nozzles in the bit and preferably have an outlet diameter approximately three to four times the diameter of the largest abrasive particles. Nozzles 24b are positioned at a distance from the center of drill bit body 10 such that the ridge 28 separating groove 37 and central hole 26 can be readily mechanically broken.

Spaced farther from the center of the drill bit body 12 than nozzles 24b are a pair of nozzles 24c. Nozzles 24c are positioned to discharge a stream of abrasive-laden liquid that cuts groove 38 in the bottom of the borehole. Because the radius of groove 38 is larger than the radius of groove 36, it is important that either the outlet of nozzles 24c be larger than the outlet of nozzles 24b or the number of nozzles 24c exceed the number of nozzles 24b to discharge enough drilling liquid to cut groove 38 as deep as groove 36. Regardless of the number of nozzles 24c used, the diameter of the outlets of nozzles 24c, as well as the diameter of the outlets of other nozzles that cut grooves, should be at least three times the diameter of the largest abrasive particles.

The next ring of nozzles includes nozzles 24d that are positioned to cut groove 40 at a location separated from groove 38 by a thin ridge 32 which can be readily mechanically broken. Because the number of nozzles 24d is the same as the number of nozzles 24c, the outlets of nozzles 24d are larger than the outlets of nozzles 24c to allow cutting of the larger amount of rock required to form groove 40. The outermost ring of nozzles includes four nozzles 24e which in the embodiment illustrated are located at each of the corners of the drill bit. The relatively large number of nozzles 24e allows use of nozzles having an outlet diameter smaller than in nozzles 24d. In the embodiment of the invention illustrated in the drawings, nozzles 24a, 24c, 24d, and 24e slope at an angle of approximately 20.degree.. The slope of the nozzles is a matter of design and is not of significance to this invention.

The outlet diameter of the nozzles is selected so that the abrasive streams discharged from the nozzles will cut grooves of substantially the same depth in the bottom of the borehole. To accomplish this, the total volume of drilling liquid discharged into any groove divided by the diameter of that groove should be substantially constant. Thus, the square of the diameter of the outlets of the nozzles for cutting a particular groove times the number of nozzles positioned to cut that groove divided by the diameter of the groove is substantially constant. The central hole cut in the bottom of the borehole really has no diameter corresponding to the diameter of a groove. Usually, the amount of drilling liquid from the central nozzle 24a will exceed the proportional amount for the grooves and, consequently, cut deeper than the grooves because it is particularly important when there is only one nozzle cutting a hole or groove in the bottom of the borehole that that nozzle not become plugged, and for that reason nozzle 24a may be made slightly oversize. In a specific embodiment of the drill bit shown in the drawings designed to use 20-50 mesh U.S. Sieve ferrous abrasive particles, the outlet of nozzle 24a is 0.140 inch, of nozzles 24b is 0.120 inch, of nozzles 24c is 0.130 inch, of nozzles 24d is 0.150 inch, and of nozzles 24e is 0.130 inch. The radial distance from the center of the drill bit to the outlets of nozzles 24b is 1-3/16 inches, to the outlets of nozzles 24c is 1 1/2 inches, to the outlets of nozzles 24d is 2 1/4 inches, and to the outlets of nozzles 24e is 2 3/4 inches.

Extending from the bottom of the drill bit are a plurality of standoff elements positioned to extend into the grooves and ride on the ridges between the grooves to apply sufficient load onto the ridges to mechanically break the rock in the ridges. The number and location of the standoff elements are such that each of the ridges is subjected to load from a standoff element. In the embodiment of the invention illustrated in FIG. 2, there are two inner standoff elements 44 positioned to extend into groove 37 and apply load to ridges 28 and 30. Four standoff elements 46 are positioned to ride in groove 40 and apply a load to ridges 32 and 34. Standoff elements 44 and 46, in addition to breaking the ridges, support the drill bit above the bottom of the grooves a distance such that the abrasive-laden stream will penetrate the bottom of the borehole rapidly but that erosion of the bottom of the bit by rebounding abrasive particles will not be excessive. In the embodiment of the invention illustrated in FIG. 2, at least two standoff elements engage each ridge to increase the stability of the drill bit. It is not essential that the standoff elements be of the tapered button type illustrated in FIG. 2. The standoff elements may be rollers of the type illustrated in U.S. Pat. No. 3,548,960 of Hasiba. Flat standoff elements of the type illustrated in U.S. Pat. No. 3,402,780 of Goodwin et al can also be used, in which event the standoff elements tend to ride on the ridges without extending downwardly into the grooves.

In the use of the drill bit of this invention, an abrasive-laden drilling liquid, for example, an aqueous liquid having about 4 to 10 percent 20 to 50 mesh U.S. Sieve ferrous abrasive particles suspended in it is pumped down the drill string and into the drill bit at a pressure designed to give a pressure drop of at least 5,000 psi through the nozzles. The drill bit is rotated by the drill string whereby the drilling liquid discharged from the nozzles cuts the central hole and the surrounding grooves in the bottom of the borehole. Standoff elements 44 and 46 ride on ridges separating adjacent grooves to break the ridges and allow continued lowering of the drill bit in the hole. The tapered surfaces of the standoff elements 44 and 46 eccentrically load the rock forming the ridges and place some of that rock under tension. The weakness of the rock in tension greatly facilitates breaking of the ridges. Particles of rock eroded by the streams of abrasive-laden liquid and broken by the action of the standoff element on the ridges are carried upwardly by the drilling liquid around the drill bit to the surface where the drilling liquid is treated to recondition it for recirculation in the drilling operation.

This invention is advantageous in making more effective use of the power applied to drilling liquid by reducing the variation in the depth of grooves cut in the bottom of the borehole. In the drill bits heretofore available, in which the outlets of all of the nozzles were of the same size, cutting grooves of the same depth would sometimes theoretically require a fraction of a nozzle, for example, 1 1/2 nozzles, at particular distances from the center of the drill bit. In such bits, the next larger number of nozzles over the number theoretically required were used which would result in the groove cut by those particular nozzles being deeper than the optimum.

By varying the size of the nozzles, it is possible to use a number of nozzles that will give a symmetrical bit construction and thereby improve the operating characteristics of the drill bit. In the abrasive jet drilling bits heretofore available, two nozzles might be required to cut one groove, three nozzles to cut the next larger groove, and five nozzles to cut the largest groove. Because of the problems of mounting the nozzles, a symmetrical bit was often difficult to obtain. A small increase in the size of the outlet of the central nozzle makes the chances of that nozzle becoming plugged negligible, thereby eliminating the necessity of using two nozzles. Although the volume of drilling liquid discharged from the larger central nozzle may exceed the volume theoretically required, it is substantially less than the volume discharged from two nozzles. Moreover, the chances of the single larger nozzle becoming plugged are less than of two nozzles of minimum diameter.

Claims

I claim:

1. In a drill bit for abrasive jet drilling of wells in which nozzles extend through the bottom of the drill bit to discharge from outlets of the nozzles streams of abrasive-laden drilling liquid to cut in the bottom of the borehole a central hole surrounded by a plurality of concentric grooves, said grooves being separated by ridges on which the drill bit is supported, the improvement comprising a plurality of nozzles positioned in the drill bit to discharge streams of abrasive-laden drilling liquid to cut each of the grooves, the ratio of the square of the diameter of the outlets of the nozzles times the number of nozzles for each of the grooves divided by the diameter of the grooves being substantially equal.

2. The drill bit as set forth in claim 1, in which a single central nozzle having a nozzle outlet diameter between four and five times the diameter of the largest abrasive particles to be used in the drilling liquid is positioned to cut a central hole having no part within its periphery extending upwardly as high as the ridges.

3. The drill bit of claim 2, in which the number of nozzles in the drill bit positioned to cut the innermost groove is two and such nozzles have a minimum outlet diameter at least three times the diameter of the largest abrasive particles to be used in the drilling liquid, and the diameters of the outlets of the nozzles in the drill bit for cutting the grooves other than the innermost groove are at least as large as the nozzles for cutting the innermost groove.

4. A drill bit as set forth in claim 3 in which the maximum diameter of the outlet of the nozzles is five times the diameter of the largest abrasive particles to be used in the drilling liquid.