US3159223A

US3159223A - Underdrilling roller bit

Info

Publication number: US3159223A
Application number: US79738A
Authority: US
Inventors: James M Cleary
Original assignee: Atlantic Refining Co
Current assignee: Atlantic Richfield Co
Priority date: 1960-12-30
Filing date: 1960-12-30
Publication date: 1964-12-01
Anticipated expiration: 1981-12-01

Description

3 Sheets-Sheet 1 Filed Dec. 50, 1960 INVENTOR. James M. Cleory Altorney ATTEST M92? J. M. CLEARY UNDERDRILLING ROLLER BIT Dec. 1, 1964 Filed Dec. 30, 1960 3 Sheets-Sheet 2 INVENTOR.

I James M. Cleory BYZ. jg-

Attorney Dec. 1, 1964 J. M. CLEARY 3,159,223

UNDERDRILLING ROLLER BIT Filed Dec. 50, 1960 s Sheets-Sheet 5 I I b Fig. 7

AT TE S T INVEN TOR.

St I James M. Cleary Attorney United States Patent 3,159,223 UNDERDRILLING BET James M. Clea, Dallas, Tern, assignor to The Atlantic Refining Company, Philadelphia, Pa, a corporation of Pennsylvania Filed Dec. Sll, H56), Ser. No. 79,738 9 Claims. (Cl. 175-335) The present invention relates to an improved bit for drilling boreholes in the earth. In a more specific aspect, the present invention relates to an improved roller cone bit adapted to drill a borehole substantially larger than the bit itself. In one more specific form, the present invention relates to an improved roller cone bit adapted to straighten an inclined borehole.

In present-day operations for drilling boreholes in the earth, particularly for the production of oil and gas, a drill bit is suspended from a tubular drill string and is rotated or rotated and vibrated while a drilling fluid designed to remove cuttings from the hole is circulated down the drill string across the bottom of the bit and back to the surface of the earth. The vast majority of bits employed in present-day drilling operations are capable of drilling a borehole no larger than the diameter of the bit itself.

' This fact causes many problems in drilling operations.

On occasion a buildup of cuttings or a combination of mud cake and cuttings on the sides of the hole wedge and stick the bit as it is pulled from the hole. This difficulty is most common in air or gas drilling when a small amount of moisture is present in the hole.

It is also conventional in present-day operations to place casing in the hole after drilling has reached total depth. This casing is then used as a conduit for the production of fluids from the well and to control caving of incompetent formations and the production of unwanted fluids. Although the casing could be run while the hole is being drilled, this is not practical since the normal drill bit drilling a hole substantially larger than the internal diameter of the casing cannot be removed or inserted through the casing. Accordingly, if casing is run during drilling under these circumstances, the casing must be pulled each time a bit is replaced. Since casing cannot be conveniently run during the drilling operation, the walls of the formation are controlled by adding certain materials to the drilling fluid. Such additives are necessary to seal the walls of the formation and prevent loss of the liquid portion of the drilling fiuid and prevent the caving of incompetent formations. It is well known, however, that such additives are detrimental to the main function of the drilling fluid since they reduce drilling rate.

In addition, in recent years, gases, such as air have been employed as drilling fluids. However, the use of gases to drill is often quite limited because of, their failure to control and treat the walls of the borehole and the interference of formation fluids entering the borehole. The entry of fluids, such as water, into the borehole causes balling of drill cuttings and consequent interference with the drilling operation. In addition, if the hole cannot be cased as the drilling progresses, the hole may become partially filled with water or oil and gas drilling is thereby rendered impossible. Uncontrolled formation iluid production may also cause the blowout of such liquids at the surface.

Finally, due to discontinuities in the formations being penetrated, uneven wear of drilling bits, bending of the drill string and the like, boreholes often tend to drift from the vertical and become inclined. Since a very small ice deviation often becomes large, such deviation can even result in the driller missing his target. It is, therefore, necessary to straighten the borehole periodically, which straightening operations interrupt normal drilling and are in themselves expensive.

It is, therefore, an object of the present invention to provide an improved roller cone bit combination.

It is another object of the present invention to provide an improved roller cone bit adapted to drill a borehole substantially larger than the bit itself.

Itis a further object of the present invention to provide an improved roller cone bit for underdrilling beneath a casing.

Still another object of the present invention is to pro vide an improved roller cone bit which will permit casing of a borehole as drilling progresses, which bit may be removed from the hole and reinserted through the interior of the casing.

Mother and further object of the present invention is to provide an improved roller cone bit adapted to maintain a borehole vertical.

A still further object of the present invention is to provide an improved roller cone bit adapted to straighten an inclined borehole.

These and other objects of the present invention will become apparent from the following detailed description when read in conjunction with the drawings.

In the drawings: I

FIGURE 1 is an elevational View, partially in section, of an improved roller cone bit constructed in accordance with the present invention.

FIGURE 2 is a plan view of the bit of FIGURE 1.

FIGURE 3 is an elevational view, partially in section, of a further modification of the roller cone bit of the present invention. 7

FIGURE 4 is a plan View of the bit of FIGURE 3.

FIGURE 5 shows a third variation of the roller cone bit of the present invention.

FIGURE 6a is a plan view of the bit of FIGURE 5, and FIGURE 6b shows a modified pilot cutter of the bit of FIGURE 6a.

FIGURE 7 is a series of views showing the manner in which the bit of the present invention operates to enlarge a borehole.

FIGURE 8 is a series of views showing how the bit of the present invention may be utilized to straighten an inclined borehole.

The conventional roller cone bit usually includes a plurality of cones mounted on a bit body in a manner such that the cones will roll on a substantially horizontal plane on the bottom of the borehole as the bit is rotated. These cones have mounted or formed thereon a plurality of cutting teeth so that as the cone rolls these teeth chip or pulverize the earth formation beneath the surface of the cone. Because of the nature and operation of the cutting teeth of the roller cone bit, this bit is well suited for rotary percussive dnilling operations where the bit is both vertically vibrated and rotated. As previously indicated, such conventional roller cone bits drill a hole substantially the same size as the bit itself and are thus subject to the same drawbacks experienced in the use of other types of bits. However, it has been found in accordance with the present invention that a roller cone bit can be constructed so that it will drill a borehole substantially larger in di ameter than the bit itself. This enlargement of a borehole is accomplished in accordance with the present invention by mounting on one side of the bit a pilot cutter whose axis of revolution is displaced from the central axis of the main bit and which is provided with a noncutting surface on the peripheral half or outer edge of the pilot most remote from the axis of the main bit. This noncutting surface is adapted to cause the pilot to be 9 a forced in toward the center of the hole as drilling prograsses. Mounted on the opposite side of the bit and trailing the pilot by a predetermined amount is a gage cutting roller cone adapted to cut an annular section of the formation immediately surrounding the pilot. As will be pointed out in more detail hereinafter this bit reaches a stabilized position with the axis of the pilot coincident with the central axis of the borehole and the gage cutting roller sweeping around the pilot with the pilot axis as its axis of rotation. It has further been found that if the cutter of the pilot is extended across the center of the pilot in the dire-ctiorrof the noncutting surface, the subject bit may be employed to straighten an inclined well bore. 7

Referring in detail to FIGURES l and 2 of the drawings, there is shown a main bit body 1 which is threaded at its upper end 3 for attachment to a conventional drill string (not shown). If the bit is operated as a rotary percussion bit the fluid motor is preferably mounted at 3 immediately adjacent the bit. Mounted on one side of the bit body 1 is roller cone 5. The mounting of roller cone 5 is such that cone 5 will roll,'for example, on hearing 7, as the bit is rotated. Suitable teeth are formed on the surface of cone 5 which cut into the formation beneatih the cone as the bit is rotated and cone 5 rolls. As

"will be seen from FIGURE 1, center line of the main bit is midway between the left side of bit body 1 and the gage cutting perimeter of roller cone 5. Also mounted on bit body 1, generally opposite to roller cone 5, is roller cone '11. The central axis 13 of the pilot is between the left side of bit body 1 and the apex of cone l1 and is offsct from axis 9 of the main-bit. Roller cone 11 is adapted to roll as the bit is rotated, as through bearing 15. Cone 11 leads cone 5 in the direction of penetration of the bit and, therefore, drills in advance of roller cone 5. Ac-

cordingly, the bit maybe divided into gage cutting portion 17 and pilot cutting portion 19 as shown by the dashed horizontal line in FIGURE 1. This lead of roller cone 11 is extremely important to the operation of the bit as will be explained hereinafter. Formed on the peripheral half or outer edge of pilot portion 19 most remote from axis 90f the main bit is a noncutti-ng surface 21 inclined inwardly and forwardly'with relation to the downward direction of bit penetration. It will be seen that inclined surface 21 extends beyond the perimeter of cutting teeth on conell, as indicated by the small dashed circle of FIGURE 2. Noncutti-ng surface 21 causes pilot portion 19 to drift in toward the center of the borehole as drillingprogresses until axis 13 of the pilot is coincident Wit-h the central axis of the borehole, and the cutters on the apex of cone 11 are cutting a pilot hole whose circumference includes the furthermost extremity of noncurting surface 21. 'At this point it should be recognized that the cutters on the periphery of cone 11 may be utilized to cut relief for noncutting surfaceZl rather than thecutters on the apex of cone 11. In this case the angle of the cone would be larger than shown and cone 11 could have a frusto conical shape. The bit becomes stabilized in this position; and cone 5 is then sweeping about the pilot hole with axis 13 of the pilot as its axis of rotation and is, therefore, cutting the gage of a hole which is substantially larger than the bit itself. The maximum size of the hole is shown by the'large dashed circle of FiG- URI-12. -It should be noted that cones 5 and 11 may be inclined so that they do not roll on a substantially horizontal plane as shown. However, if the bit is tobe employed to maintain aborehole vertical or to straighten an inclined borehole, the inclination of cones 5 and If.

should not be upwardly and outwardly by more than a .small angle. As is conventional in the construction and design of any bit, the bit is also provided with drilling fluid channels 23 which supply drilling fluid to the base of the bit. Finally, it should be recognized that more than one cone may be mounted on gage cutting portion 17 or on pilot portion 19 so long as the rollers on the pilot and in its final position.

the gage cutting portion are designed to function as previously set forth.

Referring now to FIGURES 3 and 4, it is to be recognized that numbers corresponding to those used in FIG- URES 1 and 2 have been utilized to indicate corresponding elements which perform substantially the same Way to produce the same results. In FIGURES 3 and 4, roller cone 5 is mounted in substantially the same position and in the same way as is the roller cone on the bit of FIGURES 1 and 2. This cone will, therefore, cut the gage of the hole when the bit becomes stabilized However, in this particular bit, the pilot portion has been modified by mounting a different type of cutting element thereon. Thus, pilot portion 19 has mounted thereon a generally V-shaped per 19 most remote from central axis 9 of the bit pilot portion progresses.

19 and cutting element 25 have been inclined inwardly and forwardly with relation to the direction of penetration of the bit. Thus, pilot portion f9 has formed thereon a noncutting surface 27 which will causepilot portion 19 to drift in toward the center of the borehole as drilling progresses; ward the center axis 9 of the bit a sutficient distance so that it will cut a pilot hole whose circumference includes the furthermost extremity of noncutting surface 27. Thus, the cutter 25 will eventually cut relief for 'noncutting surface 27 when the pilot portion 19 becomes cutting surface 27 is present and one of the cutters cuts relief for noncutting surface 27. .In addition, a row or rows of spaced teeth or buttons may be substituted for the elongated cutter 25.

In still another modification of the present invention, as illustrated by FIGURES 5, 6a and 6b, the pilot portion 19 has mounted thereon a drag-type cutting blade 29; The bottom edge of cutting blade 29 is inclined upwardly and rearwardly with relation to the direction of rotation of the bit so as to present a leading cutting edge which will bite into-the formation beneath the blade as drilling In the configuration shown, it is evident that the bottom of blade" 29 will be dragging tosome extent rather than cutting until the bit becomes stabilizcd. Thereafter, when the bit becomes stabilized, the inner portion of the bottom of blade 29 will be cutting and the outer portion dragging. This is not the most efiicient design. improved by splitting blade 29, sloping the inner portion as shown at 29 and reversing the slope of the outer portion as shown at 29' on FIGURE 6b. Formed on.

the outermost vertical edge of blade 2? is a noncutting surface '31. which is inclined inwardly and forwardly with relation to the direction of rotation of the bit. Because of the noncutting or dragging action of surface 31, pilot portion 19 drifts toward the center of the borehole as drilling progresses. On the opposite side of pilot portion 19, cutting surface 33 is formed on blade 29. Cutting surface 33 is designed conventionally, sloping inwardly and rearwardly with relation to the direction of rotation of the bit and, therefore, cuts into the walls of the pilot hole as the bit rotates. It should also be recognized that cutting surface 33 is designed to de scribe a circle whose circumference includes noncutting surface 31. Therefore, when the bit has become stabilized with axis 13 of pilot portion 19 on the center line of the hole, cutting surface 33 will cut relief for noncutting surface 3d and the bit will become stabilized since noncutting surface 31 can no longer force the bit inwardly.

.In this stabilized position, the entire bit will rotate about Cutter 25 extends inwardly to Accordingly, the cutting action can be axis 13 of pilot portion 19 and roller cone 5 will cut an annular section of formation immediately surrounding the pilot hole. Therefore, this bit will also drill a borehole substantially larger than the bit itself. It is obvious that a plurality of drag-type cutting blades can be sub stituted for blade so long as noncutting surface 31 is present and one of the blades cuts relief for surface 31 when the bit becomes stabilized. As will be explained later, it is the offset of the center axis 13 of pilot portion 19 from the center axis 9 of the main bit which ultimately determines the maximum diameter of the hole which will be drilled.

As previously indicated, the distance by which pilot portion 19 leads gage portion 17 is extremely important to the functioning of the bit in its intended manner. If the lead of the pilot is too small, the bit will fail to become stabilized due to the fact that the pilot hole is so shallow that the noncutting surface 31 will continuously destroy the shoulder of the pilot hole by lateral crushing and the bit will simply wobble about the hole rather than become stabilized. It has been found that the ratio of the lead of the pilot to the diameter of the main bit should be at least'0.l5 in order to overcome this problem. The maximum lead of pilot portion 19 is an important design consideration since it should be within certain limits which depends upon the strength of the pilot, the practical design of the connection at the upper end of the bit, etc. 7

FIGURE 7 of the drawings illustrates the manner in which the bit of the present invention enlarges a borehole. For this series of illustrations, a simplified version of the bit shown in FIGURES 1 and 2 is utilized.

View a of FIGURE 7 shows the bit disposed in a well bore having a diameter substantially equal to the diameter of the bit itself. It should be noted in View a that center axis 9 of the main bit is coincident with central axis of the Well bore and that central axis 13 of pilot portion 19 is offset from borehole axis 35. View b shows the configuration of the borehole after the bit has been operated for some time. At this stage of the opera tion, noncutting surface 21 has forced pilot portion 19 in toward the center of the borehole. As a result, central axis 9 of the main bit has moved off axis 35 of the borehole and is sweeping about the center axis of the borehole. By the same token, center axis 13 of pilot portion 19 has moved in toward center axis 35 of the borehole. At this stage, pilot roller cone 11 has begun to drill a pilot hole equal to the depth of pilot portion 19 of the bit but still substantially larger in diameter than pilot portion 19. Because of the shift of

axes

9 and 13, gage roller cone 5 has begun to cut into the sides of the main borehole to thereby enlarge the hole somewhat. View 0 shows another stage in the enlargement of the borehole after further penetration of the bit. Because of the continued inward drift of pilot portion 19, axis 9 of the main bit has drifted further from axis 35 of the borehole and axis 13 of pilot portion 19 has drifted closer to axis 35 of the borehole. As a result, the pilot hole has decreased in diameter by an amount equal to twice the shift of the axes and the gage of the main borehole has been enlarged by this same amount. Finally, View d shows the bit in its final stabilized position, in which position it will contnue to drill a borehole substantially larger than the diameter of the bit itself. As shown in View d, axis 13 of pilot portion 19 has drifted in toward axis 35 of the borehole until the two axes are coincident. In addition, axis 9 of the main bit has drifted further from axis 35 of the borehole. Since the pilot hole is now substantially equal in diameter to the diameter of pilot portion 19 and pilot roller 11 is now cutting a cylindrical pilot hole whose circumference includes noncutting surface 21 of pilot portion 19, noncutting surface 21 no longer forces the bit inwardly and the bit has become stabilized in the position shown. The entire bit is now rotating about axis 13 of pilot portion 19. Consequently, gage cuttter 5 of gage t5 cutting portion 17 is cutting an annular peripheral section of the hole immediately surroundingthe pilot hole.

By observing the operation as shown by FIGURE 7, it is obvious that the amount by which axis 13 of pilot portion 19 is offset from axis 9 of the main bit determines the amount by which the bit will enlarge the hole. Thus, the final diameter of the borehole is equal to twice the distance from axis 13 of pilot portion 19 to the outermost tip of gage cutter 5. Stated differently, the diameter of the borehole is larger than the bit diameter by an amount equal to twice the distance from the center axis 9 of the main bit to the center axis 13 of pilot portion 19.

FIGURE 8 shows a series of views illustrating the manner in which the bit of the present invention may be used to straighten an inclined borehole. In View e of FIG- URE 8, the bit is shown positioned in a borehole larger than the diameter of the bit itself. This view is the equivalent of View d of FIGURE 7 except that the hole is inclined from the vertical. In order to straighten the subject hole, drilling is discontinued and the bit is lifted from the bottom of the hole. The bit is then dropped to the bottom of the hole and, due to gravity and the weight of the drill string supporting the bit, the bit will come to rest on the low side of the hole. If pilot portion 19 strikes shoulder 37 on the low side of the hole, the bit is then vibrated without rotation in order to totally or partially destroy shoulder 37 as shown in View f. Having destroyed shoulder 37, rotation of the bit is resumed and the bit begins to drill a new pilot hole closer to the original vertical axis of the hole. This is shown in View g of FIGURE 8. View g of FIGURE 8 shows the condition of the hole after drilling has progressed to the stage shown in View 0 of FIGURE 7. At this stage, the bit has begun to drill a pilot hole somewhat larger than the diameter of pilot portion 19 and gage cutting roller 5 has begun to cut into the low side of the borehole. View 71 then shows the bit after it has become stabilized in its new pilot hole and the new borehole. This condition is, of course, equivalent to that shown in View d of FIGURE 7 and View e of FIGURE 8 except that the new pilot hole and the new main borehole are now closer to vertical than they are in View e of FIG- URE 8.

In order to completely return the borehole to vertical,

the above procedure may be repeated as many times as necessary. In addition, the borehole may be straightened intermittently as drilling progresses without measuring the inclination of the hole or orienting the bit so that pilot portion 19 will drop on shoulder 37. Obviously, if the straightening procedure is repeated a suflicient number of times throughout the progress of the drilling operation, pilot portion 19 will land on shoulder 37 a sufficient number of times to maintain the borehole vertical. On the other hand, if desired, the inclination of the hole may be measured at selected intervals and the bit properly oriented so that shoulder 37 will be wholly or partially destroyed by pilot portion 19 when the bit is dropped to the bottom and vibrated. By selectively orienting the bit and following the above procedure it is also possible to .start drilling an inclined hole at a diiferent angle.

There are several significant design features which must be present in the bit in order to utilize it as a means for straightening a deviated borehole. First, the cutters on the bottom of pilot portion 19 must extend across axis 13 in the direction of noncutting surface 21. If the cutters do not extend beyond center line 13, the bit will not function to destroy shoulder 37 but will simply slide off the shoulder and back into the original pilot hole. For this same reason, the bottom of the cutters on pilot portion 19 should not be inclined upwardly and outwardly by more than a small angle. If the pilot cutters are inclined upwardly and outwardly by more than a small angle, the bit will again glance otf shoulder 37 and back into the original pilot hole rather than destroy the shoulder and begin drilling a new pilot hole closer to vertical. Lastly, the

distance by which pilot portion 19 leads gage portion 17 should be as close to the minimum as possible. In other words, the ratio of the lead of the pilot to the diameter of the bit should be greater than 0.15 but as close to that value as is practical. The ratio of the lead of the pilot to the diameter of the main bit may be expressed by the formula:

where R=ratio, x=length of pilot, and d =diameter of bit.

It is necessary that at least a part of shoulder 37 of FIG- URE 8 be destroyed all the way to the bottom of the pilot hole if the bit is to form a new borehole axis shifted toward vertical. Otherwise, on renewing rotation of the. bit, the pilot would jump back into the remaining part of the pilot hole without shifting the axis of the borehole. In order to destroy shoulder 37 of FIGURE 8 by percussive action,

one must take into consideration the compressive sheer angle of the formation being drilled. The average sheer angle for most formations encountered is about 28 degrees. Therefore, in order to destroy shoulder 37 by percussive" action, the angle w of shoulder 37 (View e of FIGURE 8) must be larger than about 28 degrees.

Observation of View 2 shows that angle w is related tothe diameter ofthe bit, the diameter of pilot and the length of the pilot. The following expression can, therefore, be written:

tan in= d1 where d diameter of the pilot.

Then: 7 I

' (1 -61, tan w and since,

i a: B it then can to For example, in the bits shown in the drawings, w is 65 degrees and R==0.233.

. modifications and variations may be made without departing from the present invention. Accordingly, the present invention is limited only in accordance with-the appended claims.

I claim:

1. An irnprovedbit for drilling a borehole in the earth comprising a main bit body, rotary pilot drilling means on said bit'body whose axis of revolution is offset from the central axis of said bit body, a sloping noncutting surface on the outer edge of said pilot drilling means most rem'otefrom said axis of said bit body and adapted to force said pilot drilling means. inwardly as drilling progresses, rotary cutting means on said pilot drilling means adapted to drill a pilot hole whose circumference includes the outermost extremity of said noncutting surface when said axis of said pilot drilling means coincides with the central axis of said borehole and at least one first roller cone cutting means mounted on said bit body above said rotary cutting means on said pilot drilling means with respect to the direction of penetration of said bit, said first roller cone cutting means adapted to drill an annular section of the earth immediately surrounding said pilot hole and trailing said pilot hole with respect to the direction of penetration of said bit and the outermost tip of the cutting edges of said pilot drilling means at the bottom of said pilot hole leading the lowermost cutting edges of said at least one first roller cone cutting means by a distance sufficient to stabilize said bit in said borehole when said axis of said pilot drilling means coincides with the central axis of said borehole.

2. A bit in accordance with claim 1 wherein the rotary cutting means on the pilot drilling means includes at least one drag-type blade.

3. A bit in accordance with claim 2 wherein the outermost tips of the cutting edges of the pilot drilling means at the bottom of thewalls of the pilot hole lead the lower most cutting edges of the at least one first roller cone cutting means by a distance between 0.15 and 1.25 times the maximum width of the bit. I

4. A bit in accordance with claim 3 wherein the noncutting surface is formed on the drag-type blade and slopes inwardly and forwardly with relation to the direction of rotation of the bit.

5. A bit in accordance with claim 1 wherein the bit is adapted to drill by rotary percussive action and the rotary cutting means on the pilot drilling means has a cutting edge extending across substantially the entire pilot in the direciton of the noncutting surface.

6. A bit in accordance with claim 1 wherein the cutting means on the pilot drilling means is at least one second roller cone cutting means set inwardly from the momentting surface toward the center of the bit body.

7. A bit in accordance with'claim 6 wherein the outermost tips of the cutting edges of the pilot drilling means at the bottom of the walls of the pilot hole lead the lowermost cutting edges of the at least one first roller cone cutting means by a distance between 0.15 and 1.25 times the maximum width of the bit.

8. A bit in accordance with claim 7 wherein the noncutting surface, formed on the pilot drilling means slopes adapted to drill by rotary percussive action and the rotary cutting means on the pilot drilling means has a cutting edge extending laterally beyond the axis of said pilot 'means in the direction of the noncutting surface.

References Cited in thefile of this patent UNITED STATES PATENTS Germany Feb. 28, 1929 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0. 3, 159,223 December 1, 19641 James M. Glear'y It i h b 'bii'i requiijinfg t rgz ign 75,3 3? or @PPQZTF lbD/ln quimmi; t; lii ixxs mtg fi M s-aawcd s.

int: \B {or he" second occurrgsnce, reaa ec fi iii 29, sex-1k out "the": aolumn 5, l1ne 26, for depend 5 read depend sama column 5, l ne 63, for

"contnue" read continue Signed and Gfild this 20th day of April 1965.

(SEAL) Auest:

ERNEST W. SWIDER' EDWARD J. BRENNER Alwfiling Office! Commissioner of Patents

Claims

1. AN IMPROVED BIT FOR DRILLING A BOREHOLE IN THE EARTH COMPRISING A MAIN BIT BODY, ROTARY PILOT DRILLING MEANS ON SAID BIT BODY WHOSE AXIS OF REVOLUTION IS OFFSET FROM THE CENTRAL AXIS OF SAID BIT BODY, A SLOPING NONCUTTING SURFACE ON THE OUTER EDGE OF SAID PILOT DRILLING MEANS MOST REMOTE FROM SAID AXIS OF SAID BIT BODY AND ADAPTED TO FORCE SAID PILOT DRILLING MEANS INWARDLY AS DRILLING PROGRESSES, ROTARY CUTTING MEANS ON SAID PILOT DRILLING MEANS ADAPTED TO DRILL A PILOT HOLE WHOSE CIRCUMFERENCE INCLUDES THE OUTERMOST EXTREMITY OF SAID NONCUTTING SURFACE WHEN SAID AXIS OF SAID PILOT DRILLING MEANS COINCIDES WITH THE CENTRAL AXIS OF SAID BOREHOLE AND AT LEAST ONE FIRST ROLLER CONE CUTTING MEANS MOUNTED ON SAID BIT BODY ABOVE SAID ROTARY CUTTING MEANS ON SAID PILOT DRILLING MEANS WITH RESPECT TO THE DIRECTION OF PENETRATION OF SAID BIT, SAID FIRST ROLLER CONE CUTTING MEANS ADAPTED TO DRILL AN ANNULAR SECTION OF THE EARTH IMMEDIATELY SURROUNDING SAID PILOT HOLE AND TRAILING SAID PILOT HOLE WITH RESPECT TO THE DIRECTION OF PENETRATION OF SAID BIT AND THE OUTERMOST TIP OF THE CUTTING EDGES OF SAID PILOT DRILLING MEANS AT THE BOTTOM OF SAID PILOT HOLE LEADING THE LOWERMOST CUTTING EDGES OF SAID AT LEAST ONE FIRST ROLLER CONE CUTTING MEANS BY A DISTANCE SUFFICIENT TO STABILIZE SAID BIT IN SAID BOREHOLE WHEN SAID AXIS OF SAID PILOT DRILLING MEANS COINCIDES WITH THE CENTRAL AXIS OF SAID BOREHOLE.