WO2004101943A2

WO2004101943A2 - Underreamer

Info

Publication number: WO2004101943A2
Application number: PCT/CA2004/000393
Authority: WO
Inventors: Bruno H. Walter; Per G. Angman; Tommy M. Warren
Original assignee: Tesco Corporation
Priority date: 2003-03-17
Filing date: 2004-03-16
Publication date: 2004-11-25
Also published as: WO2004101943A3

Abstract

An underreamer for expanding a borehole through an earthen formation. The underreamer may include a housing including an outer surface, a top end for the connection into a drill string and an opposite end for attaching a pilot bit; an inner bore through the housing to allow for the passage of the drilling fluid and a cavity in the housing with an opening to the housing outer surface, the cavity being in fluid communication with the inner bore and a cutter arm slidably moveable in the cavity and drivable from a retracted position to an expanded position by fluid pressure applied to the cavity.

Description

UNDERREAMER

Field of the Invention

This invention is directed to an underreamer for operating behind a pilot bit.

Background of the Invention

When drilling a borehole through an earthen formation a pilot hole is drilled by a pilot bit and the pilot hole can be enlarged by an underreamer. Underreamers have arms with cutters thereon that cut into the formation to enlarge the borehole to its intended gauge,

Underreamers are useful in casing drilling, wherein the pilot bit must be of a size to pass through the bore of the casing and is therefore not sized to drill a borehole of a gauge that the casing can pass therethrough. Therefore the pilot bit drills the pilot hole into the formation and underreamers enlarge the hole behind the pilot bit to a gauge greater than the casing outer diameter to permit advancement of the casing into the borehole. Underreamers are also useful when extending a borehole below installed casing. In such embodiments, the underreamer arms are collapsible to permit the underreamer to be moved through the bore of the casing and are expandable downhole to permit drilling of a borehole to a gauge greater than the outer diameter of the casing. The standard underreamer includes a plurality of arms which are pivotally or otherwise connected to a main body and moveable between a collapsed position in or against the body and an expanded position extending out from the body. A standard underreamer will generally include the plurality of arms at the same axial position along the body, so that they balance one another in the expanded position. In very small diameter casing, it may even be difficult to pass an underreamer of standard diameter therethrough. An underreamer is required, therefore that may be formed with a significantly reduced outer diameter so that it can be used in small diameter casing.

Summary of the Invention

An underreamer has been invented, which may facilitate tripping through a casing string (or other liner string) inner bore.

In accordance with one broad aspect of the present invention there is provided an underreamer for expanding a borehole through an earthen formation, the underreamer comprising: a housing including an outer surface, a top end for the connection into the drill string and an lower end for attaching a pilot bit; an inner bore through the housing to allow for the passage of the drilling fluid; a cavity in the housing with an opening to the housing outer surface, the cavity being in fluid communication with the inner bore; a cutter arm slidably moveable in the cavity and drivable from a retracted position to an expanded position by fluid pressure applied to the cavity; and a stop to stop the cutter arm from being expelled from the cavity.

The inner bore permits fluid to pass through the underreamer and on to the pilot bit. A passage opens between the inner bore and the cavity to communicate drilling fluid pressure to the cavity. A seal is provided between the cavity and the cutter arm to prevent fluid from passing out of the cavity between the arm and the cavity wall. The seal can include, for example, an "o" ring mounted in a gland on the cutter arm.

During operation, pressure of the drilling fluid inside the fluid passage is higher than the pressure surrounding the housing resulting in "dp" (difference between the pressure in the inside passage and the pressure in the bore hole) to act on the inside projected area of the cutter arm. Such a pressure differential effectively forces the cutter arm to extend outward radially from the underreamer.

A stop,- such as interacting shoulders of a stopper block, for example, which may be attached by welding, fusing, fasteners or cap . screws prevents the cutter arm from being driven entirely out of the cutter arm cavity. In one embodiment, the stop is positioned on the outer surface extending into the cavity opening. The stop may include an inner facing surface and an outer facing surface and the cutter arm being formed to extend over the outer facing surface when in the expanded position. In one embodiment, a stopper block may be used and the geometry of the contact angles between the stopper block and the matching area on the cutter arm are designed to produce reaction force which will act on the stopper block in compression. This configuration may avoid loading of the stopper block, and in particular the fasteners securing the stopper block, in tension. Thus, the force on the cutter arm, during use to drill the formation may tend not to drive the stopper block away from its stopping position against the cutter arm, but rather tends to drive the stopper block against the underreamer housing where it is positioned.

The cutter arm can include PDC cutters or other formation cutting means, as desired. Reaction . force from the cutter arm during drilling can be accommodated by a segment/pad of hard facing on the housing opposite the cutter arm, which will be in contact with the hole drilled by the pilot bit;

In order to allow for the passage of fluid from the pilot bit upward into the bore hole housing can have recesses, junk slots, formed on its outer surface.

When it is desired to pull the underreamer out of the hole, pumping of drilling fluid may be stopped, thus causing the pressure inside the housing to be reduced. This may eliminate expanding force on the cutter arm. Thus, the cutter arm may tend to retract into its cavity by gravity or by abutment against the casing shoe, when the underreamer is pulled up out of the hole.

When pulling the underreamer out of the hole, a top edge of the cutter arm may abut the bottom end of the casing shoe such that it is pushed into the cutter arm cavity. This allows the underreamer to be pulled Up through the casing string ID. As such, the cavity may be oriented in an upwardly inclined manner such that the cutter expands out radially and upwardly toward the top end.

BRIEF DESCRIPTION OF THE DRAWINGS

A further, detailed, description of the invention, briefly described above, will follow by reference to the following drawings of specific embodiments of the invention. These drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. In the drawings:

Figure 1 is an axial view of an underreamer in accordance with the present invention, with the cutter arm in extended position.

Figure 2 is a transverse sectional view through the underreamer of Figure 1 taken along line 2-2.

Figure 3 is a partial top view of Figure 1 at the underreamer arm taken along line 3-3. The underreamer being of an alternate embodiment, than that shown in Figure 1.

Figure 4 is a transverse partial sectional view taken through the fluid passage relief of Figure 1 taken along line 4-4.

Figure 5 is a transverse sectional view through the underreamer of Figure 1 taken along line 5-5. Figure 6 is an axial sectional view taken along line 6-6 of Figure 2.

Figure 7 is a transverse sectional view through the "o" ring groove taken along line 7-7 of Figure 6.

Figure 8 is a transverse sectional view through the underreamer of Figure 6 taken along line 8-8.

Figure 9 is an axial sectional view of the underreamer of Figure 2 taken along line 6-6 with the cutter arm fully inserted into the cutter arm cavity.

Figure 10 is an axial sectional view of an underreamer according to the present invention showing the forces acting on the cutter arm.

Detailed Description of Various Embodiments

The Figures show an underreamer 2. As will be appreciated, underreamers are useful to act behind a pilot bit (shown in phantom as 3) to enlarge the borehole 1 behind a pilot hole 3a formed by the pilot bit. The underreamer is engaged at the lower end of a drill string (shown in phantom as 5). If the underreamer is used when drilling with casing, the drill string is a string of casing and the underreamer may be releasably locked to the string of casing by, for example, a drilling lock assembly. The underreamer and the pilot bit may be rotated either by rotation of the casing string from surface or by use of a mud motor that is positioned between the underreamer and the underreamer's connection to the casing string.

Underreamer 2 of the illustrated embodiment includes a housing 4, which carries an underreamer arm 6. The underreamer arm is moveable between an expanded position (Figures 1 and 6) wherein it is exposed for use to enlarge the well bore diameter and a stored position (Figure 9), wherein it is retracted into the housing. The underreamer arm may be moveable into the expanded position by the pressure of drilling fluid acting against its rear surface 8 and, in particular, a pressure differential between the fluid pressure at its rear surface and the fluid pressure in the borehole about the underreamer which would be acting against cutting end 10 of the underreamer arm.

Housing 4 includes a top end 12 for the connection into drill string 5 and an opposite end 14 to which pilot bit 3 may be attached, directly or indirectly. An inner bore 16 extends from end 12 to. end 14 through the housing to allow for the passage of the drilling fluid from surface to the pilot bit. Housing 4 further includes an outer surface 18. Recesses 20, also called junk slots, can be formed in the outer surface of the housing to facilitate the circulation of drilling fluid upwardly past the underreamer toward surface. In the illustrated embodiment, bearing surfaces 21a, 21b may be formed on housing 4 to centralize/stabilize the underreamer in pilot hole 3a and to react forces generated by operation of arm, as will be discussed in more detail herein below. Hard facing or other treatments can be applied to surfaces 21a, 21b in order to increase their durability.

A cavity 22 is formed in the housing with an opening to housing outer surface 18. Cavity 22 can extend any desired depth into the housing. In one embodiment for example, the cavity may extend into the material of the housing to a depth of more than 50%, and in the illustrated embodiment more than 75% or 80%, of the outer diameter of the housing. Thus, the underreamer need only have a diameter slightly greater than the length of a cutter arm, if desired.

A passage 24 opens between inner bore 16 and cavity 22, bringing the cavity into fluid communication with the inner bore. In the illustrated embodiment, the passage extends from the cavity downwardly to a position where it opens to the inner bore. However, other passage configurations may also be useful. Passage 24 may have a smaller diameter than that of inner bore 16 since fluid pressure need only be communicated into cavity 22, while the major flow is through inner bore 16. The inner bore may bypass cavity 22 and the inner bore may be formed generally to facilitate the flow of drilling fluid therethrough without significant pressure loss.

Cutter arm 6 is slidably moveable in the cavity and drivable from the retracted position to the expanded position by fluid pressure applied through passage 24 to the cavity behind the cutter arm. A seal, which in the illustrated embodiment includes an "o"-ring 26 in gland 28, may be provided between the cavity wall and the cutter arm to prevent fluid pressure loss by passage of fluid therebetween. In this way, cutter arm 6 may act as a piston in the cavity.

During drilling operation, the pressure P1 of drilling fluid inside inner bore 16 is higher than the pressure P2 of the drilling fluid surrounding the housing resulting in differential pressure (difference between the pressure in the underreamer and the pressure in the bore hole). This relatively higher pressure P1 when applied on the inside projected area 8 of the cutter arm, effectively forces the cutter arm to extend outward radially in its cavity 22.

Passage 24 may have a diameter less than the effective diameter of cavity 22 such that the rear of cavity may act as a rear stop to cutter arm, when moving into the retracted position.

A stopper block 30 secured by cap screws 32 to housing 4 may act as a stop, in the illustrated embodiment, which prevents cutter arm 6 from being pushed entirely out of the cutter arm cavity during normal operation but can be removed to permit removal of the cutter arm. Block 30 may act against a shoulder 34 on the cutter arm to block complete expulsion, in response to differential pressure, of the arm from the cavity. In the illustrated embodiment, and with reference to Figure 9, the geometry of the contact angles between the stopper block and shoulder 34 on the cutter arm are designed to produce reaction force F1 which will act on the stopper block in compression thus avoiding loading cap screws 32, in tension. Thus, although there is force F2, by fluid pressure, on the cutter arm pushing it against the stopper block and the cap screws, the major operational forces, tend to trap the stopper block against the housing, such that even if the cap screws work loose or fail, the cutter arm holds the stopper block in place by action of force F1 against the housing.

Reaction force, such as lateral loads, from the cutter arm generated during drilling may be accommodated, if desired, by provision of bearing surfaces 21a, 21b, which may be hardened or raised relative to the outer surface and, for example, formed to be in contact with hole 3a drilled by the pilot bit. In particular, bearing surface 21a on the housing may be positioned anywhere from generally diametrically opposite to a position about % clockwise rotated (when viewed from above) from the cutter arm cavity opening, to support and stabilize the underreamer against lateral loads. Bearing surface 21b may be positioned below the cutter arm, for example at least in part in axial alignment with and below the cutter arm cavity opening such that it stabilizes the

' underreamer against being drawn against pilot hole 3a, as arm 6 engages the formation.

The cutter arm may include PDC cutters 36 exposed on cutting end 10. Cutters 36 may be arranged to engage the formation to expand the borehole. In some embodiments, it may be desirable to position further cutters or buttons 36a to ensure borehole gauge. Cutters 36a in some applications can be replaced with hardfacing 36b (Figure 3), as desired.

Cavity 22 may be oriented to facilitate movement or positioning of cutter arm 6, as desired. In the illustrated embodiment, cavity 22 is oriented to facilitate movement of cutter arm 6 from an expanded position to a retracted position. In particular, cavity 22 can be inclined the housing such that its long axis extends radially and upwardly (relative to top end 12) relative to the tool center line. As such, cutter arm 6 can move slidably along the cavity long axis radially and generally upwardly toward top end 1 . Thus, when it is desired to pull the underreamer Out of the hole by tripping it through the casing, drilling fluid pump may be slowed or stopped to reduce or effectively eliminate the differential pressure. The reduction in differential pressure ceases application of the opening force F2 on the cutter arm such that it can retract back into the cavity. Once cutter arm 6 is retracted, the underreamer outer diameter is such that it can fit through the casing. If the cutter arm is not already retracted when the underreamer is pulled uphole, a top edge 40 of the cutter arm can abut against the bottom end of the casing shoe and, due to its upwardly and radially outward orientation, can be pushed into the cutter arm cavity by the casing shoe. Once cutter arm 6 is retracted either by gravity or by abutment against the casing shoe, the underreamer may be tripped through the casing string ID. The cutter arm can be radiused or angled at its top edge 40 to further facilitate passage into the casing string ID.

The underreamer can be formed with a one-piece housing or, alternately, to facilitate manufacture, it can be formed in parts, as shown, interconnected by threads 42, welding, seals, etc. It may be useful to use methods, such as for example EDM, to create any or all of bore 16, cavity 22, and passage 24.

Underreamer 2 can include only a single cutter arm, as shown in Figure 1. Alternately, more than one arm can be provided on each underreamer. Where an underreamer of relatively small ID is desired, it may be useful to employ only a single cutter arm or it may be useful to space each of a plurality of underreamers axially along the underreamer housing. As an example, it may be desirable to provide a plurality of cavities spaced in a spiral fashion about a housing, with each cavity formed to accept a cutter arm which may be similar to that illustrated or described in accordance with one of the various options.

The underreamer as illustrated can be used alone with a pilot bit. Alternately, a plurality of underreamers can be used in series or one housing can have mounted therein a plurality of cutter arms each in an cavity at a position axially spaced along the housing.

Although various embodiments of the present invention have been described in some detail hereinabove, those skilled in the art will recognise that various substitutions and modifications may be made to the invention without departing from the scope and spirit of the appended claims.

Claims

We claim:

1. An underreamer for expanding a. borehole through an earthen formation, the underreamer comprising: a housing including an outer surface, a top end for the connection into the drill string and an lower end for attaching a pilot bit; an inner bore through the housing to allow for the passage of the drilling fluid; a cavity in the housing with an opening to the housing outer surface, the cavity being in fluid communication with the inner bore; a cutter arm slidably moveable in the cavity and drivable from a retracted position to an expanded position by fluid pressure applied to the cavity; and a stop to stop the cutter arm from being expelled from the cavity.

2. The underreamer of claim 1 wherein the stop is positioned on the outer . surface in the cavity opening. '

3. The underreamer of claim 1 wherein the stop includes an inner facing surface and an outer facing surface and the cutter arm being formed to extend over the outer facing surface when in the expanded position.

4. The underreamer of claim 1 wherein the geometry of contact angles between the stop and the cutter arm are-selected to produce a reaction force which acts on the stop in compression.

5. The underreamer of claim 1 further comprising a first bearing surface on the outer surface of the housing positioned about diametrically, opposite the cutter arm cavity opening.

6. The underreamer of claim 5 wherein the first bearing surface extends between a position about diametrically opposite the cutter arm cavity to a position about % clockwise rotated from the cutter arm cavity opening.

7. The underreamer of claim 1 further comprising a lower bearing surface on the housing outer surface and positioned along the housing between the cavity opening and the lower end.

8. The underreamer of claim 1 further comprising a bearing surface to react lateral loads and wherein the bearing surface is raised relative to the outer surface.

9. The underreamer of claim 1 further comprising a bearing surface to react lateral loads and wherein the bearing surface includes a pad of hard facing.

10. The underreamer of claim 1 wherein the inner bore extends from the top end to the lower end to permit fluid to pass through the underreamer and on to the pilot bit.

11. The underreamer of claim 1 further comprising a passage opened between the inner bore and the cavity to communicate drilling fluid pressure to the cavity.

12. The underreamer of claim 11 wherein the cavity includes a diameter and the passage includes a diameter less than the cavity diameter.

13. The underreamer of claim 11 wherein the passage extends from the cavity downwardly to a position where it opens to the inner bore.

14. The underreamer of claim 1 wherein the cutter arm includes a rear surface and an outer surface and the rear surface acts as a piston face against which drilling fluid pressure acts to drive the expansion of the cutter arm.

15. The underreamer of claim 1 wherein the cavity and the cutter arm are formed such that the cutter arm moves radially outwardly and upwardly relative to the tool center line and the top end when moving into the expanded position.

16. The underreamer of claim 15 wherein the stop is positioned such that the cutter arm extends upwardly over the stop when in the expanded position.

17. The underreamer of claim 1 wherein the cavity extends inwardly and downwardly in the housing.

18. The underreamer of claim 1 wherein the inner bore bypasses the cavity and a passage provides fluid communication between the cavity and the inner bore.

19.The underreamer of claim 1 including a single cutter arm at any one axial position along the underreamer.

20. The underreamer of claim 1 further comprising PDC cutters on the cutter arm. '