GB2258877A

GB2258877A - Method of drilling a borehole,and a drill coupling for use in that method

Info

Publication number: GB2258877A
Application number: GB9217661A
Authority: GB
Inventors: Alan Gerard Vasey
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-08-21
Filing date: 1992-08-20
Publication date: 1993-02-24
Also published as: GB9118019D0; GB9217661D0

Abstract

A method of drilling a borehole which makes it possible to cause the direction of the drilling to deviate in a controlled manner comprises interposing a hollow flexible joint 26 in the drill string at a distance from the drill bit 20 and applying thrust to the drill string 11 at a point rearwardly of the joint. The flexible joint 26 may advantageously be a coupling having two generally tubular body parts 31, 32 which are connected to allow them to move into and out of angular alignment and to transmit a rotational drive irrespective of their relative axial alignment. <IMAGE>

Description

Method of drilling a borehole, and a drill coupling for use in that method.

The present invention is concerned with drilling boreholes which deviate from the rectilinear and it comprises a method of drilling such boreholes and also a drill coupling for use in that method.

When a conventional borehole is being drilled by means of a string of drill rods, if thrust is applied to the rods, the drill string tends to bow away from the rectilinear and thereby to direct the drill bit away from its original cutting direction. The subsequently cut section of the borehole is therefore directed at an angle to the axis of the previously cut length of borehole. The direction of diversion of the borehole is diametrically opposite to that of the bowing of the drill string.

This natural diversion of a borehole is illustrated schematically in Fig.l of the accompanying drawings, which is a longitudinal section along the axis of a borehole. Referring to that Figure, a drill bit 10 at the end of a drill string 11 is shown cutting a borehole 12 through a rock stratum 13. Thrust applied to the drill string 11 in the direction of the arrow 14 causes the drill string 11 to bow as shown.

Since in the illustrated case the original direction of the borehole was horizontal, the drill string sags downwardly under thrust and the drill bit 10 is diverted upwardly, which in turn leads to an upwardly-diverted subsequent cutting of the borehole.

Diversion of a borehole by the application of axial thrust in this way is a known technique in guided drilling programmes in order to divert a generally horizontal borehole upwards but it is very difficult to apply a similar technique to steer a borehole downwards, since the natural sag of the drill string under gravity is downwards.

It is an object of the present invention to provide a method of drilling boreholes by means of which a borehole may be diverted in other directions as well as downwardly, for example in an upward direction.

The method according to the present invention comprises interposing a hollow flexible joint in the drill string a predetermined distance behind the drill bit and applying axial thrust to the drill string rearwardly of the flexible joint.

The diameter of the flexible joint is selected to be less than that of the drill bit and therefore the joint is urged towards the side of the borehole by the bowing of the drill string. The string therefore changes its direction at the flexible joint and as a result the drill bit is directed in the same direction as the bowing of the drill string, for example downwardly.

The invention also includes a drill coupling in the form of a flexible joint for use in the foregoing method. The drill coupling according to the present invention comprises a first generally tubular body part for connection to a first drill rod and a second generally tubular body part for connection to a second drill rod or to a drill bit, said first and second body parts being so connected together as to allow their respective tubular axes to move into and out of mutual anguLar alignment and further being so connected together that a rotational drive applied to the first body part is transmitted to said second body part irrespective of their relative alignment.

It is not intended that the coupling according to the invention should be used to cause the direction of drilling of a borehole to deviate abruptly by more than a small amount and therefore the connection between the two body parts preferably limits the available angular non-alignment of these parts to a maximum of 5 degrees of arc. Preferably the maximum angle does not exceed 2 degrees of arc.

The second generally tubular body part may be connected directly to a drill bit or a length of drill rod may be interposed between the second body part and the bit. The length of such an interposed drill rod is one factor in determining the angular extent of the resulting deviation of the borehole.

Another factor influencing the deviation of the borehole is the ratio of the diameters of the coupling and the borehole respectively. If the outside diameter of the coupling is significantly smaller than the internal diameter of the borehole, then the deviation is greater than when the two diameters are of more closely comparable size. Preferably the outer diameter of the coupling is less than the outer diameter of the drill bit.

It is an important feature of the coupling according to the present invention that, when the two body parts are out of angular alignment, the second part should continue to rotate in response to rotation of the first body part. This difficult requirement may very satisfactorily be achieved by providing a crowned tooth internal gear coupling to link the two body parts. For example, an axial extension of one of the body parts may extend within an axial extension of the other body part and the two extensions may be provided on their facing surfaces with intermeshing gear teeth, preferably shaped to allow for the relative angular movement of the two parts.

The coupling may be protected from wear by means of tungsten carbide inserts. Preferably the radially outer surface of the coupling is provided with fluting, either parallel to the axis of the coupling or extending helically along the length of the coupling, to allow the passage of flushing medium and, where appropriate, cuttings along the outside of the drill string.

The diameter of the internal bore extending through the coupling preferably is equal, or approximately so, to that of the bore of the drill rods to which the coupling is connected in use.

The coupling may be used either when the drill string is used in conventional flushing mode, in which the flushing medium flows down within the drill rods, across the drill bit and back along the outside of the drill rods, or in a reverse circulation mode.

The invention will now be further described, by way of example only, with reference to the accompanying drawings. As already indicated, Figure 1 is a schematic illustration of a conventional, naturally deviating, borehole.

Fig.2 is a view, corresponding to that of Fig.l, showing the operation of the method and drill coupling according to the present invention; Fig.3 is an axial cross-sectional view, to a larger scale, of one preferred embodiment of the drill coupling accord ing to the present invention, with its two body parts aligned; and Fig.4 is a corresponding view of the coupling of Fig.3 with its two body parts out of axial alignment.

As illustrated in Fig.2, a drill bit 20 at the end of a drill string 21 is mounted upon a short length 25 of drill rod which in turn is linked to the main drill string by a flexible coupling 26, by means of which the rotational drive of the drill string is transmitted to the drill bit 20. The drill is shown cutting a borehole 22 in a rock formation 23.

When thrust is applied to the drill string 21 in the direction of the arrow 24, the drill string bows downwardly under gravity as shown, in the same way as the drill string 11 in Fig.l.

This again forces upwardly the forward end of the drill string. The flexible coupling 26 is of smaller external diameter than the internal diameter of the borehole 22 and the coupling is therefore moved upwardly into contact with the wall of the borehole. However flexing of the coupling 26 allows the drill bit 20 to adopt a downward inclination. Consequently, the subsequent cutting of the borehole 22 is also downwardly inclined.

Referring now to Figs.3 and 4, the illustrated embodiment of the flexible coupling according to the invention comprises two generally tubular body parts, 31 and 32. Body part 31 has a female socket portion 31a for connection at the end of a string of drill rods and body part 32 has a male connection portion 32a for connection to a further drill rod or direct to a drill bit. A tubular extension piece 33 is threaded upon the forward end of the body part 31 and extends within an extension piece 34 which is threaded upon the rearward end of the body part 32. The coupling assembly is completed by a cover 35, threaded within the rearward end of the extension piece 34, and by seals 36, 37.

A rack of crowned gear teeth 38 on the extension piece 33 is enmeshed with an internal rack of gear teeth 39 on the inner face of the extension piece 34. Shaped annular pivot parts 40 on the extension piece 33 ensure that axial thrust (or indeed pull) applied to the drill string is transmitted to the drill bit. There is sufficient space 41, 42 between the extension pieces 33 and 34 to allow the two body parts 31 and 32 to move between a position (Fig.3) in which they are axially aligned and a position (Fig.4) in which their axes are inclined to each other by a small angular amount g , which in the case of the illustrated embodiment is 1.5 degrees. The shapes of the teeth 38, 39 and of the pivot parts 40 accommodate the relative angular movement of the body parts and provide for the rotational drive of the drill string and any axial thrust applied to the string to continue to be transmitted to the drill bit whatever the relative angular position of the body parts.

Thus the method and coupling according to the present invention, in particular as illustrated, make it possible for horizontal boreholes, for example, to be diverted downwards.

The method and coupling may also be used to provide controlled diversion of boreholes orientated in directions other than horizontally, in particular to reverse the direction of any deviation which would otherwise occur naturally.

Claims

1. A method of drilling a borehole which enables the direction of drilling to be diverted, comprising interposing a hollow flexible joint in the drill string a predetermined distance behind the drill bit and applying axial thrust to the drill string rearwardly of the flexible joint.

2. A method as claimed in claim 1, wherein the diameter of the flexible joint is less than that of the drill bit.

3. A drill coupling for use as the flexible joint in a method as claimed in either of the preceding claims, comprising a first generally tubular body part for connection to a first drill rod and a second generally tubular body part for connection to a second drill rod or to a drill bit, said first and second body parts being so connected together as to allow their respective tubular axes to move into and out of mutual angular alignment and further being so connected together that a rotational drive applied to the first body part is transmitted to said second body part irrespective of their relative alignment.

4. A drill coupling as claimed in claim 3, wherein the extent of maximum angular non-alignment of the two body parts is not more than 5 degrees of arc.

5. A drill coupling as claimed in claim 4, wherein said angle is not more than 2 degrees of arc.

6. A drill coupling as claimed in any of claims 3 to 5, connected directly or indirectly to a drill bit.

7. A drill coupling as claimed in claim 6, wherein the outer diameter of the coupling is less than the outer diameter of the drill bit.

8. A drill coupling as claimed in any of claims 3 to 7, wherein the rotational drive is transmitted by a crowned tooth internal gear coupling linking the two body parts.

9. A drill coupling as claimed in claim 8, wherein an axial extension of one of the body parts extends within an axial extension of the other body part and the two extensions are provided on their facing surfaces with intermeshing gear teeth.

10. A drill coupling as claimed in any of claims 3 to 9, having fluting on its radially outer surface.

11. A drill coupling for use in a method as claimed in either of claims 1 and 2, said coupling being substantially as hereinbefore described with reference to, and as illustrated in, Figs. 3 and 4 of the accompanying drawings.