US3289776A

US3289776A - Underdrilling bit

Info

Publication number: US3289776A
Application number: US421159A
Authority: US
Inventors: James M Cleary
Original assignee: Atlantic Refining Co
Current assignee: Atlantic Richfield Co
Priority date: 1964-12-24
Filing date: 1964-12-24
Publication date: 1966-12-06
Anticipated expiration: 1983-12-06

Description

Dec. 6, 1966 J. M. CLEARY UNDERDRILLING BIT 2 Sheets-Sheet 2 Filed Dec. 24, 1964 -4 2 Fmm NEGATIVE ANGLE Fig. 4

Fig. 5

AiTorney United States Patent Office 3,28%,776 Patented Dec. 6, 1966 3,289,776 UNDERDRILLING Bill James M. Cleary, Dallas, Tex., assignor to The Atlantic Refining Company, Philadelphia, Pa, a corporation of Pennsylvania Filed Dec. 24, 1964, Ser. No. 421,159 20 Claims. (Cl. 175398) This application is a continuation-in-part of copending application Ser. No. 79,739, filed Dec. 30, 1960, now Patent No. 3,163,243, by the same inventor as this application.

The present invention relates to an improved underdrilling rotary percussive drill bit for drilling earth boreholes. More specifically, this invention relates to improved features of the type of underdrilling bit of said copending application, especially those features pertaining to removing drill cuttings from a pilot hole drilled by the bit.

Copending application Ser. No. 79,739 covers a rotary percussive bit having a pilot section whose central axis is offset from the central axis or centroid of the bit. At the lower end of the side of the pilot furthest from the axis of the bit is a sloping noncutting surface which slopes inwardly toward the central axis of the bit and forwardly with respect to the direction of penetration of the bit and is adapted to force the pilot toward the center axis of the bit until the pilot reaches a stable central position in the borehole.

This type of bit drills a borehole appreciably larger than the maximum width of the bit. This bit fills only a sector of the borehole and the drill bit tends to drill an undergauge or undersized hole unless the pilot remains stable. The invention sets forth certain design features which provide improved operation and stability of the pilot.

In rotary percussive drilling, this bit is coupled to or is an integral part of a shank or rod which is impacted at a high frequency with periodic axial shocks applied by a percussive engine while being rotated so that between shocks the cutters on the bit are indexed to a new position. These axial shocks are transmitted to the cutting edges which chip or cut the formation. The fragments thus produced are removed by a drilling fluid, such as compressed gas or a liquid, which is circulated into and out of the borehole. This drilling fluid is pumped at the earths surface down a hollow drill string through the percussive engine and thence to passages through the drill bit. One of them passages leads to the lower part of the pilot hole so that the fluid will remove cuttings generated by the pilot. It has been found that in this type of underdrilling bit efficient removal of the pilot cuttings is essential to maintaining the desired hole size. It has also been found that the way that drilling fluid is circulated into and out of the pilot is critical.

Accordingly, it is a primary purpose of this invention to provide an improved system for removing pilot cuttings from the pilot hole drilled by an underdrilling bit of the type described. This system improves tooth or cutter life, prevents enlargement of the pilot hole and improves the ability of the bit to regain hole gauge should the gauge of the hole he lost.

It is a further purpose of this invention to set forth features of the pilot that provide improved pilot stability and thus improved hole size and bit operation.

Other and further objects of the present invention will be apparent from the following detailed description when read in conjunction with the drawings.

In the drawings,

FIGURE 1 is an end view of a rotary percussive bit in accordance with the present invention.

FIGURE 2 is a fragmented elevational View of the pilot drilling section of the bit of FIGURE 1..

FIGURE 3 is an elevational view in section of the rotary percussive bit of FIGURE 1 taken at 3-3.

FIGURES 4 and 5 are schematic views to aid in defining and understanding some of the preferred features of the bit shown in FIGURE 1.

Briefly, in accordance with the present invention, the improved underdrilling bit for earth borehole drilling is a rotary percussion drill bit having a central longitudinal axis and an upper bit section having a first drill fluid passage means therethrough. Extending downward from this upper section is a rotary pilot percussive drilling means having first percussive cutting means located on the lower part of the pilot. The first percussive cutting means are adapted to cut a cylindrical pilot hole having substantially circular vertical sides whose circumference will just accommodate said pilot drilling means when the axis of the pilot coincides with the central longitudinal axis of the borehole. The pilot has a maximum width which is smaller than the maximum width of the bit and, preferably, the maximum width of the bit will be between 1.3 and 3.0 times as great as the maximum width of the pilot. The pilot has a central longitudinal axis which is offset from the central axis of the bit. On the lower end at the side of the pilot furthest from the axis of the bit is a negatively sloping noncutting surface which slopes inwardly toward the axis of the bit and forwardly with respect to the direction of penetration of the bit. This sloping noncutting surface is adapted during drilling to force the pilot inwardly toward the axis of the bit whenever the borehole becomes undersized due to a shifting of the axis of the pilot from the axis of the borehole. Located on the upper bit section above the first percussive cutting means is second rotary percussive cutting means which cut an annular section of earth surrounding the pilot hole cut by the pilot section and trailing the pilot hole with respect to the direction of penetration of the bit. The cutting edges of the first percussive cutting means on the pilot lead the cutting edges of the second percussive cutting means by a distance of at least two inches and, preferably, by a distance of between three inches and two times the maximum width of the bit. The pilot has a hard smooth curved or cylindrical outer bearing surface having substantially the same circular shape or curvature as the wall of the pilot hole drilled by the first percussive cutting means. This smooth cylindrical outer surface extends above and from the sloping noncutting surface for a distance of at least two inches above the cutting edges of the first percussive cutting means, and, preferably, by a distance of between three inches and two times the maximum width of the bit. This smooth cylindrical surface is located on the side of the pilot furthest from the central longitudinal axis of the bit and forms an angle with vertical of between zero and two degrees negative. The cylindrical surface is an are equal to at least 30 degrees of the pilot hole and, preferably, between 30 degrees and degrees. Extending through the pilot in communication with the bottom of the pilot hole is a second drilling fluid passage means which communicates with the first drilling fluid passage means in the upper bit section. In the pilot and at least into part of the upper bit section is a third drilling fluid passage means which extends from near the bottom of the lower end of the pilot hole to a point above the top of the pilot hole. This third passage is adapted to conduct fluid from the lower part of the pilot hole to the borehole being drilled. Preferably, this third passage will be a vertical groove in the sides of the pilot adjacent the cylindrical outer surface and be large enough to pass particles of one inch and larger. It should be noted that this groove must not lie in the arc of the smooth outer curved bearing surface.

In other embodiments the sloping noncutting surface extends inward from the smooth outer curved surface by a horizontal distance of between 1.5 and 15.0 percent of the maximum width of the pilot drilling means, and forms a negative angle with vertical of between 2 and 35 degrees.

The subject bit tends to drill a central pilot hole and the gage or second percussive cutting means of the bit then sweep about the central axis of the pilot to cut a borehole whose diameter is equal to twice the distance from the central axis of the pilot to the outermost tip of the cutters of the gage cutters.

The improved rotary percussion bit of the present invention is illustrated in detail in FIGURES 1, 2, and 3 of the drawings. In accordance with these figures, there is illustrated a bit having central longitudinal axis 11 and upper bit section 13 with first drilling fluid passage means 15 extending therethrough. In the usual manner this first passage will receive drilling fluid from either a drill string or a fluid driven percussive unit. Extending downward from this upper section is rotary pilot percussive drilling means 17 having first percussive cutting means located on the lower part of the pilot. As shown, the first percussive cutting means are made up of annular cutters 19 and radial cutter 21. The first percussive cutting means are adapted to cut a cylindrical pilot hole having a substantially circular vertical wall whose circumference will just accommodate the pilot section when central axis 23 of the pilot coincides with the central axis of the borehole being drilled by the bit. As shown, the pilot section has a maximum width which is smaller than the maximum width of the bit and, preferably, the maximum width of the bit will be between 1.3 and 3.0 times the maximum width of the pilot. As shown, the central longitudinal axis 23 of the pilot is offset from central longitudinal axis 11 of the bit.

On the lower end at the side of the pilot drilling means furthest from central longitudinal axis 11 of the bit is sloping noncutting surface 25 which slopes inwardly toward the axis of the bit and forwardly with respect to the direction of penetration of the bit into the borehole. This sloping noncutting surface forms a negative angle with vertical (as hereinafter defined) and is adapted to force the pilot inwardly toward the axis of the bit whenever axis 23 of the pilot does not coincide with the axis of the borehole.

Extending above this noncutting sloping surface for a distance of at least two inches above the cutting edges of radial cutter 21 and annular cutters 19, and located on the side of the pilot section furthest from central longitudinal axis ll of the bit is smooth curved or cylindrical outer surface 27. Smooth curved outer sruface 27 has substantially the same circular shape or curvature as the wall of the pilot hole drilled by the first percussive cutting means of the pilot section so as to conform to the side'of the pilot hole and act as a bearing surface to stabilize the position of the pilot when the pilot axis 23 and borehole axis coincide, thereby maintaining the desired size or gauge of the borehole. This bearing surface will be made of a hard wear-resistant metal, e.g., Rockwell A hardness of 87 or harder. The smooth curved outer surface must not be relieved, that is, form a positive angle with vertical (as hereinafter defined). This cylindrical outer surface must form an angle with vertical of between zero and two degrees negative so as to provide a bearing surface to resist unbalanced side thrusts of the gage cutters of the bit which are hereafter described. Smooth outer bearing surface 27 also forms an are equal to at least degrees of the pilot hole or to a circle circumscribed by this surface as the pilot rotates, and will usually form an angle of between 30 and 120 degrees with 60 to 90 degrees being preferred.

It should be noted that this cylindrical outer surface forms a smooth unbroken arc of metal. It has been found that this is essential to stable operation of the bit since if outer surface 27 were smaller than 30 degrees or contained ridges or grooves, the pilot hole would be reamed out by this surface, thereby enlarging the pilot hole and causing the bit to drill an undersized or undergauge hole.

As stated previously, smooth outer curved surface 27 extends at least two inches above the cutting edges of the first percussive drilling means of the pilot section. The vertical length of this surface will depend upon the use to which the bit is to be put; however, in deep well drilling, the length of this surface will be between three inches and two times the maximum width of the bit.

Located on the upper bit section above the first percussive cutting means are second rotary percussive cutting means. As shown, the second rotary percussive cutting means are made up of stairstepped rows of button cutters 29 which will usually be made of tungsten carbide button insert teeth. The second rotary percussive cutting means is located and adapted to cut an annular section of earth surrounding the pilot hole cut by the pilot drilling means and trailing the pilot hole with respect to the direction of penetration of the bit, that is, around the top of the pilot hole. The first percussive cutting means on the pilot lead the second percussive means (gage cutting portion of the bit) by a distance of at least two inches, and depending on how the bit is used, will lead the second percussive cutting means by a distance of between three inches and two times the maximum width of the hit.

As shown in FIGURES l and 3, there is second drilling fluid passage means 31 extending through the pilot section. For simplicity, only one passage is shown although this passage means may consist of more than one passage. This second drilling fluid passage is adapted to conduct drilling fluid from first drilling fluid passage means 15 in the upper bit section to the pilot hole where the fluid washes the first percussive drilling means and removes cuttings from the pilot hole.

In the pilot drilling means and at least part of the upper bit section is third drilling fluid passage means 33 which extends from the lower end of the pilot section to a point above the top of the pilot hole drilled by the pilot drilling means. This passage is adapted to conduct the drilling fluid carrying cuttings from the pilot hole to the borehole above the pilot hole. As shown, the third drilling fluid passage means is a semicircular-shaped groove cut in the vertical side of the pilot section. The side of the pilot thus grooved must not be that part of the side surface of the pilot forming smooth outer cylindrical surface 27.

Preferably, third drilling fluid passage means 33 will be a single passage large enough to pass particles of onehalf inch and larger. This passage could be located anywhere on or in the pilot as long as it is not in the smooth outer cylindrical surface; however, it is preferred that the passage be located as shown in FIGURES 1 and 2.

The above description uses the term negative angle or negative slope when describing the smooth outer cylindrical surface and the noncutting sloping surface. These terms and other features of the bit herein described are illutsrated in FIGURES 4 and 5, wherein similar numbers are used for representations of similar elements, and are defined and illustrated as follows:

Refer first to FIGURE 4 wherein there is shown edge 35 which has a standard slope and is relieved in the standard fashion. This edge forms a positive angle wit-h vertical, and, as used herein, a surface that has a positive slope or forms a positive angle slopes inwardly toward the center of the bit and rearwardly with relation to the direction of penetration of the bit.

In contrast, smooth outer curved surface 27' and sloping noncutting surface 25 form a negative angle with vertical, and, as used herein, a surface that has a negative slope or forms a negative angle slopes inwardly toward the center of the bit and forwardly with relation to the direction of penetration of the bit.

As shown in FIGURE 4, sloping noncutting surface 25 extends inward from smooth outer cylindrical surface 27 by a horizontal distance B. It is preferred that this distance be between 1.5 and 15.0 percent of the maximum width of the pilot drilling means. In FIGURE 4, it is further shown that sloping noncutting surface 25 forms a negative angle C with vertical. The preferred slope of this noncutting surface is more difficult to define since the slope of this surface need not be perfectly uniform; however, in general, the average negative angle formed with vertical will be between 2 and 35 degrees.

FIGURE 4 also shows negative angle D formed by smooth outer curved surface 27'. As stated previously, this angle will be between zero and two degrees negative.

FIGURE 4 also depicts schematically the distance A that smooth outer cylindrical surface 27 extends above all the cutting edges of the first percussive cutting means depicted by lines 19' and 21, and the distance F that all of the cutting edges of first percussive cutting means lead all of the cutting edges of the second percussive cutting means represented by line 29. As stated previously, distances A and F will be at least two inches, and, preferably, for deep well drilling between three inches and two times the maximum width of the bit.

FIGURE 5 shows angle E which is included by smooth outer cylindrical surface 27. As stated previously, E is at least 30 degrees, and generally between 30 and 120 degrees, with 60 to 90 degrees preferred.

Consider now the operation of the bit and how it operates to drill a bore hole much larger than the width of the bit by considering what happens when the bit is disposed in a borehole substantially equal to the width of the bit. The borehole in this case will have a central longitudinal axis which coincides with central longituidnal axis 11 of the bit. At this stage, central longitudinal axis 23 of the pilot section will be offset from the central longitudinal axis of the borehole. As the bit is vibrated and rotated negatively sloping noncutting surface 25 forces the central axis of the pilot toward the axis of the borehole. Due to the inward thrust applied to the pilot by this sloping noncutting surface, the axis of the pilot section will eventually align with the axis of the borehole. At this point, the first percussive drilling means drill a pilot hole with a vertical wall whose axis is the same as the axis of the borehole. This pilot hole is just large enough to accommodate the pilot section with negatively slopin noncutting surface 25 so that this surface no longer creates sufficient lateral or side thrust to shift the axis of the pilot. At the same time, smooth outer cylindrical surface 27 bears against the vertical curved wall of the pilot hole with a surface that is equal to at least 30 degrees of the pilot hole. This bearing surface conforms to the pilot hole wall and is on the side of the pilot remote from the gage cutting section of the bit so that this surface resists side thrust created by the gage cutters. This smooth bearing surface is sufficiently large and smooth that it does not ream out an oversize pilot hole which would cause an undersize borehole. At the same time, drilling fluid is pumped through second drilling fluid passage 31 into the pilot hole where the fluid washes and cleanses the cuttings from the pilot hole. This fluid containing cuttings passes up and out of the pilot hole by way of third drilling fluid passage means 33. This prevents recutting of the fragments by the pilot cutters, prevents clogging of the pilot cutters, improves operation of the fluid driven hammer which impacts the bit in rotary percussive drilling and helps prevent rough rotation of the pilot and bit. The above-described drill bit thereby substantially improves the operation of the type of bit described in copending application Ser. No. 79,739, now Patent No. 3,163,243, since it was found that the largest difiiculty with this type of bit is eflicient maintenance of the hole size and this problem is aggravated by clogging of pilot cutters, insufficient bearing area on the side of the pilot furthest from the gage cutters, and rough rotation of the pilot.

It will be obvious to ones skilled in this art that variations and modification other than those specifically set forth above may be practiced without departing from this invention. For example, it should be recognized that the bit is limited to rotary percussive cutting means, but not to a specific type of percussive cutter. These cutters could be one or more short V-shaped teeth, long radial V-shaped cutters, button-type insert teeth of rounded or other cross section, and the like or in combination, and all of which could be an integral part of the bit or of an insert nature, and made of any material suitable to earth drilling.

I claim:

1. In earth borehole drilling, an improved rotary percussive drill bit having a central longitudinal axis comprising an upper bit section having a first drilling fluid passage means therethrough, rotary pilot percussive drilling means extending downward from said upper bit section, said pilot drilling means having a maximum width which is smaller than the maximum width of said bit and having a central longitudinal axis which is offset from said central longitudinal axis of said bit, a sloping noncutting surface formed on the lower end at the side of said rotary pilot drilling means furthest from said axis of said bit, said noncutting surface sloping inwardly toward the axis of said bit and forwardly with respect to the direction of penetration of said bit and adapted during drilling to force said rotary pilot drilling means inwardly toward said axis of said bit, first percussive cutting means having cut-ting edges located on the lower part of said pilot drilling means and adapted to cut a cylindrical pilot hole having a substantially circular vertical wall whose circumference will accommodate said piot drilling means when the axis of said pilot drilling means coincides with the central axis of said borehole, second rotary percussive cutting means located on said upper bit section above said first rotary percussive cutting means and adapted to cut an annular section of earth surrounding said pilot hole and trailing said pilot hole with respect to the direction of penetration of said bit, said first percussive cutting means leading said second percussive means by a distance of at least two inches, said pilot drilling means having a smooth cylindrical outer surface having substantially the same curvature as the wall of the pilot hole drilled by said first percussive cutting means and extending above said sloping noncutting surface for a distance of at least two inches above the cutting edges of said first percussive cutting means and located on the side of said pilot drilling means furthest from said central axis of said bit, said smooth cylindrical outer surface forming an angle with vertical of between zero and two degrees negative and forming an are equal to at least 30 degrees of said pilot hole, second drilling fluid passage means extending through said pilot drilling means and adapted to conduct drilling fluid from said first drilling fluid passage means in said upper bit section to said pilot hole, and third drilling fluid passage mean in said pilot drilling means and said upper bit section extending from the lower end of said pilot drilling means to a point above the top of said pilot hole drilled by said pilot drilling means and adapted to conduct drilling fluid from said pilot hole to said borehole.

2. A bit in accordance with claim 1 wherein the maximum width of the bit is between 1.3 and 3.0 times as great as the maximum width of the rotary pilot percussive drilling means.

3. A bit in accordance with claim 1 wherein the first percussive cutting means lead the second percussive means and the smooth outer cylindrical surface extends above the cutting edges of said first percussive cutting means by a distance of between three inches and two times the maximum width of the bit.

4. A bit in accordance with claim 1 wherein the smooth outer cylindrical surface forms an arc equal to between 30 and 120 degrees of the pilot hole drilled by said first percussive cutting means.

5. A bit in accordance with claim 1 wherein the sloping noncutting surface extends inward from the smooth outer cylindrical surface by a horizontal distance of between 1.5 and 15.0 percent of the maximum width of the pilot drilling means.

6. A bit in accordance with claim wherein the negative sloping noncutting surface forms a negative angle with vertical of between 2 and 35 degrees.

7. A bit in accordance with claim 5 wherein the first percussive cutting means lead the second percussive means and the smooth outer cylindrical surface extends above the cutting edges of said first percussive cutting means by a distance of between three inches and two times the maximum width of the bit.

8. A bit in accordance with claim 5 wherein the smooth outer cylindrical surface forms an are equal to between 30 and 120 degrees of the pilot hole drilled by said first percussive cutting means.

9. A bit in accordance with claim 1 wherein the third drilling fluid passage is a groove cut in the side of the pilot drilling means.

10. A bit in accordance with claim 9 wherein the negative sloping noncutting surface extends inward from the smooth outer cylindrical surface by a horizontal distance of between 1.5 and 15.0 percent of the maximum width of the pilot drilling means.

11. A bit in accordance with claim 10 wherein the negative sloping noncutting surface forms a negative angle with vertical of between 2 and 35 degrees.

12. A bit in accordance with claim 10 wherein the first percussive cutting means lead the second percussive cutting means and the smooth outer cylindrical surface extends above the cutting edges of said first percussive cutting means by a distance of between three inches and two times the maximum width of the bit.

13. A bit in accordance with claim 9 wherein the first percussive cutting means lead the second percussive cutting means and the smooth outer cylindrical surface extends above the cutting edges of said first percussive cutting means by a distance of between three inches and two times the maximum width of the bit.

14. A bit in accordance with claim 9 wherein the smooth outer cylindrical surface forms an are equal to between 30 and degrees of the pilot hole drilled by said first percussive cutting means.

15. A bit in accordance with claim 1 wherein the third drilling fluid passage is large enough to pass onehalf inch particles.

16. A bit in accordance with claim 15 wherein the negative slopping noncutting surface extends inward from the smooth outer cylindrical surface by a horizontal distance of between 1.5 and 15.0 percent of the maximum width of the pilot drilling means.

17. A bit in accordance with claim 16 wherein the negative sloping noncutting surface forms a negative angle with vertical of between 2 and 35 degrees.

18. A bit in accordance with claim 16 wherein the first percussive cutting means lead the second percussive cutting means and the smooth outer cylindrical surface extends above the cutting edges of said first percussive cutting means by a distance of between three inches and two times the maximum width of the bit.

19. A bit in accordance with claim 15 wherein the first percussive cutting means lead the second percussive cutting means and the smooth outer cylindrical surface extends above the cutting edges of said first percussive cutting means by a distance of between three inches and two times the maximum width of the bit.

20. A bit in accordance with claim 15 wherein the smooth outer cylindrical surface forms an arc equal to between 30 and 120 degrees of the pilot hole drilled by said first percussive cutting means.

References Cited by the Examiner UNITED STATES PATENTS 335,373 2/1886 Marcy -389 773,962 11/1904 McCray et a1. 17539O 1,096,134 5/1914 Sims 175-389 1,463,566 7/1923 Akins 175-389 FOREIGN PATENTS 63,262 1/1914 Austria. 472,053 2/ 1929 Germany.

CHARLES E. OCONNELL, Primary Examiner.

N. C. BYERS, Assistant Examiner.

Claims

1. IN EARTH BOREHOLE DRILLING, AN IMPROVED ROTARY PERCUSSIVE DRILL BIT HAVING A CENTRAL LONGITUDINAL AXIS COMPRISING AN UPPER BIT SECTION HAVING A FIRST DRILLING FLUID PASSAGE MEANS THERETHROUGH, ROTARY PILOT PERCUSSIVE DRILLING MEANS EXTENDING DOWNWARD FRROM SAID UPPER BIT SECTION, SAID PILOT DRILLING MEANS HAVING A MAXIMUM WIDTH WHICH IS SMALLER THAN THE MAXIMUM WIDTH OF SAID BIT AND HAVING A CENTRAL LONGITUDINAL AXIS WHICH OFFSET FROM SAID CENTRAL LONGITUDINAL AXIS OF SAID BIT, A SLOPING NONCUTTING SURFACE FORMED ON THE LOWER END AT THE SIDE OF SAID ROTARY PILOT DRILLING MEANS FURTHEST FROM SAID AXIS OF SAID BIT, SAID NONCUTTING SURFACE SLOPING INWARDLY TOWARD THE AXIS OF SAID BIT AND FORWARDLY WITH RESPECT TO THE DIRECTION OF PENETRATION OF SAID BIT AND ADAPTED DURING DRILLING TO FORCE SAID ROTARY PILOT DRILLING MEANS INWARDLY TOWARD SAID AXIS OF SAID BIT, FIRST PERCUSSIVE CUTTING MEANS HAVING CUTTING EDGES LOCATED ON THE LOWER PART OF SAID PILOT DRILLING MEANS AND ADAPTED TO CUT A CYLINDRICAL PILOT HOLE HAVING A SUBSTANTIALLY CIRCULAR VERTICAL WALL WHOSE CIRCUMFENCE WILL ACCOMMODATE SAID PIOT DRILLING MEANS WHEN THE AXIS OF SAID PILOT DRILLING MEANS COINCIDES WITH THE CENTRAL AXIS OF SAID BOREHOLE, SECOND ROTARY PERCUSSIVE CUTTING MEANS LOCATED ON SAID UPPER BIT SECTION ABOVE SAID FIRST ROTARY PERCUSSIVE CUTTING MEANS AND ADAPTED TO CUT AN ANNULAR SECTION OF EARTH SURROUNDING SAID PILOT HOLE AND TRAILING SAID PILOT HOLE WITH RESPECT TO THE DIRECTION OF PENETRATION OF SAID BIT, SAID FIRST PERCUSSIVE CUTTING MEANS LEADING SAID SECOND PERCUSSIVE MEANS BY A DISTANCE OF AT LEAST TWO INCHES, SAID PILOT DRILLING MEANS HAVING A SMOOTH CYLINDRICAL OUTER SURFACE HAVING SUBSTANTIALLY THE SAME CURVATURE AS THE WALL OF THE PILOT HOLE DRILLED BY SAID FIRST PERCUSSIVE CUTTING MEANS AND EXTENDING ABOVE SAID SLOPING NONCUTTING SURFACE FOR A DISTANCE OF AT LEAST TWO INCHES ABOVE THE CUTTING EDGES OF SAID FIRST PERCUSSIVE CUTTING MEANS AND LOCATED ON THE SIDE OF SAID PILOT DRILLING MEANS FURTHEST FROM SAID CENTRAL AXIS OF SAID BIT, SAID SMOOTH CYLINDRICAL OUTER SURFACE FORMING AN ANGLE WITH VERTICAL OF BETWEEN ZERO AND TWO DEGREES NEGATIVE AND FOR MING AN ARC EQUAL TO AT LEAST 30 DEGREES OF SAID PILOT HOLE, SECOND DRILLING FLUID PASSAGE MEANS EXTENDING THROUGH SAID PILOT DRILLING MEANS AND ADAPTED TO CONDUCT DRILLING FLUID FROM SAID FIRST DRILLING FLUID PASSAGE MEANS IN SAID UPPER BIT SECTION EXTENDING PILOT HOLE, AND THIRD DRILLING FLUID PASSAGE MEANS IN SAID PILOT DRILLING MEANS AND SAID UPPER BIT SECTION EXTENDING FROM THE LOWER END OF SAID PILOT DRILLING MEANS TO A POINT ABOVE THE TOP OF SAID PILOT HOLE DRILLED BY SAID PILOT DRILLING MEANS AND ADAPTED TO CONDUCT DRILLING FLUID FROM SAID PILOT HOLE TO SAID BOREHOLE.