GB2343470A

GB2343470A - Eccentrically weighted drilling apparatus for deviated boreholes

Info

Publication number: GB2343470A
Application number: GB9926254A
Authority: GB
Inventors: Alan Martyn Eddison
Original assignee: Andergauge Ltd
Current assignee: Andergauge Ltd
Priority date: 1998-11-07
Filing date: 1999-11-08
Publication date: 2000-05-10
Also published as: GB9926254D0; GB9824380D0

Abstract

A directional drilling apparatus (10) has an eccentric mass (20) rotatably mounted on a mandrel (18). The mandrel is coupled to a drill string (12) and a bit (24) can be offset in the borehole by an offset portion (22). There is also a flexible string section (14) forming a flex joint for coupling the mandrel to a drill string. The offset mass can be connected between two larger diameter string sections, possibly stabilisers (16, 22). The bit can be offset by a stabiliser having a non-rotating offsetting element mounted on the mandrel, where the mandrel may be common to the offset mass and the stabilizer. A non-rotating portion of the stabilizer can be connected to the eccentric mass and the angular orientation of the stabilizer may be altered to achieve a desired deviation of the bit, fig 4. The mass can be rotated with respect to the stabilizer, the rotation being achieved by an electric motor, fig 5 (40) axially mounted in the mass and having a drive pinion (50) which engages gearing (46,48) associated with the stabilizer mandrel. The motor can be battery powered (44) and may also be associated with an electrical generator that uses the relative rotation of the parts of the apparatus to supply a trickle charge to the battery.

Description

DRILLING APPARATS This invention relates to drilling apparatus, and in particular to apparatus for use in directional drilling utilising an eccentric mass to maintain an offset in a drill string element, causing a drill bit to deviate in a desired direction.

In directional or controlled trajectory drilling, the vertical inclination and azimuth of a drilled bore may be controlled such that the bore may extend from the surface to a target area which is not vertically aligned with the point on the surface where drilling commences. This permits a wide area to be accessed from a single drilling location and is therefore particularly useful in offshore drilling operations.

Rotation of the drill bit mounted on the lower end of the drill string is achieved by rotation of the entire drill string, by a turntable on the surface, and often also by a downhole motor located on the drill string adjacent the bit. The downhole motor is usually driven by the drilling fluid which is pumped through the string.

Steerable downhole motors include a"bent"housing or elbow which introduces a small deviation (around 1 ) in the end portion of the drill string. When the entire string is rotating such an elbow has little or no effect on the bore trajectory. However, if the string is stopped and then adjusted such that the motor bend is in a desired direction, rotating the drill bit using only the downhole motor will result in the trajectory of the well deviating.

Drilling in this manner without rotation of the drill string may be very time consuming as static friction between the non-rotating parts of the string and the bore wall tends to produce a stick-slip progression of the string through the bore. This results in sudden increases in the weight (downward force) being applied to the bit and motor, causing the motor to stall. The drill string must then be picked off bottom before drilling may restart.

This problem may even result in it becoming impossible to drill any further without rotating the drill string and is particularly acute in horizontal and extended reach wells.

Attempts have been made to provide drilling apparatus which will permit bore trajectory to be varied or controlled while still rotating the drill string, primarily by providing a non-rotating eccentric mass on the drill string adjacent the drill bit, and which mass engages the "low"portion of the bore wall and supports the drill string. A radially extending blade is mounted on the mass and engages the bore to produce a lateral force on the drill string causing the drill bit to deviate from its existing path, or at least prevents further deviation in the direction of the blade. However, the success of such apparatus has been limited as the mass provides an unstable support for the heavy drill string, such that the mass is likely to topple and be moved to one side by the string, which will tend to move downwards to occupy the lower part of the bore. Examples of such arrangements are illustrated in US Patents Nos 4,638,873 and 4,220,213.

W096\31679 describes a surface controlled wellbore directional steering tool comprising a mandrel for forming part of the drill string, and two eccentric sleeves. The outer sleeve has an eccentric bore that forms a pregnant or weighted side. Two stabiliser shoes are provided on either side of the sleeve at 90 to the pregnant housing. The inner sleeve has a further eccentric longitudinal bore that contains the mandrel. The relative orientations of the sleeves may be controlled to move the mandrel to one side of the pregnant housing, thus transmitting a fulcrum force to the bit. An electric motor in the housing may be activated from surface to rotate the inner sleeve. This tool has not been commercialised, and it is believed that the tool would prove difficult to operate.

Other forms of directional drilling apparatus for controlling hole direction or inclination by providing eccentric or offset blades or members are described in US Patents Nos 3,062,303,3,092,188,3,650,338,3,825,081 and 4,305,474.

It is among the objectives of the embodiments of the present invention to provide directional drilling apparatus utilising an offset or eccentric mass which obviates or mitigates the disadvantages of the prior art arrangements.

According to the present invention there is provided a directional drilling apparatus for use in drilling a deviated bore, the apparatus comprising: means for mounting on a drill string for offsetting a drill bit in a bore and including an eccentric mass rotatably mounted on a mandrel for forming part of the string, the mass tending to maintain a desired offset; and a relatively flexible string portion located adjacent said offsetting means.

The invention also relates to a directional drilling method utilising such an apparatus.

The provision of a relatively flexible string portion, such as a flex joint, in the string adjacent the offsetting means, facilitates accommodation of the deviation that may be introduced in the bore by operation of the offsetting means. Further, the flexible string portion will accommodate deviation that would otherwise have to be accommodated by bending of other elements of the apparatus, and in particular the mandrel on which the eccentric mass is mounted; it has been found that bending of the mandrel, as a result of a change in bore inclination, increases the friction between the mandrel and the eccentric mass, and may cause the mass to rotate with the string, negating the effect of the mass and thus making it difficult if not impossible to maintain a desired offset.

Preferably, the eccentric mass is selected to describe a smaller diameter than the bore. Thus, the mass is normally maintained clear of the bore wall, obviating any tendency for the heavy drill string to rest on the mass and topple to one side of the mass. Conveniently, the mass may be mounted between two larger diameter string elements, such as stabilisers.

Preferably also, the offsetting means includes a stabiliser including an offset element, and may take the form of one of the stabilisers described in our W097\47848, the disclosure of which is incorporated herein by reference. However, it is most preferred that the stabiliser comprises a non-rotating portion defining an offset outer profile, the non-rotating portion being rotatably mounted on a mandrel; the mass and the stabiliser portion may share a common mandrel. The nonrotating portion of the stabiliser may comprise a plurality of parts, and in one embodiment comprises inner and outer parts, each part defining an offset bore: by varying the relative orientation of the offset bores it is possible vary the offset of the mandrel relative to the outer circumference of the stabiliser. With such an arrangement it is possible to configure the stabiliser such that it does not create an offset, allowing a bore to be drilled straight ahead. Most preferably, the non-rotating stabiliser portion is coupled to the eccentric mass, the relative angular orientation of the mass and stabiliser portion being selected to achieve a desired deviation of the bit.

Preferably also, a downhole adjustable coupling is provided between the mass and the stabiliser portion. The coupling may include drive means for rotating the mass relative to the stabiliser portion. The drive means may comprise a motor on one part engaging a gear on the other part. In a preferred embodiment, an electric motor is axially mounted in the mass and has a drive pinion engaging gearing associated with the stabiliser mandrel. The electric motor may be powered from an appropriate battery.

If necessary, the apparatus may incorporate an alternator or other electrical generator which takes advantage of the relative rotation of the parts of the apparatus in use; the generator may provide a trickle charge for the battery. In use, the relative orientation of the mass and stabiliser portion may be controlled from surface by transmitting appropriate control signals to the drive means. The control signal may take the form of, for example, pressure pulses, acoustic pulses or electrical signals, or variations in the speed of rotation of the drill string, which may be detected by appropriate sensors. The mass will tend to occupy the lower side of the bore. However, initially following a change in relative orientation of the mass and stabiliser portion, the mass will tend to assume a position offset from the bore lower side. As drilling continues, the mass will tend to rotate to occupy the lower side of the bore and will thus rotate the stabiliser portion towards the desired orientation, thus introducing a gradual deviation in the bore; this avoids the creation of a sudden deviation, or dog-leg, in the bore which might cause the apparatus to jam in the bore. This preferred feature of the invention also facilitates adjustment of the apparatus; to adjust the setting of the apparatus all that is required is a rotation of the mass relative to the stabiliser portion to the relative orientation that will provide the desired offset once the mass has moved to the lower side of the bore. Accordingly, only a relatively low, known torque is required to rotate the mass, and thus the drive means may be of relatively low output. This contrasts to the arrangements described in, for example, W096\31679, in which adjustment of the offset direction may only be achieved by moving a portion of the drill string, and thus also the drill bit, in the direction of the desired deviation, requiring a motor having a very large torque output.

According to another aspect of the present invention there is provided directional drilling apparatus for use in drilling a deviated bore, the apparatus comprising: means for mounting on a drill string including an offset portion for offsetting a drill bit in a bore and an eccentric mass for rotatably mounting on the string and for maintaining the offset portion at a desired orientation in the bore; and downhole drive means for rotating the eccentric mass in the bore and changing the relative orientation of the mass and the offset portion.

According to a further aspect of the present invention there is provided directional drilling apparatus for use in drilling a deviated bore, the apparatus comprising: a stabiliser for mounting on a drill string and including a non-rotating offset portion for offsetting a drill bit in a bore; an eccentric mass for rotatable mounting on the string and being operatively associated with the offset portion of the stabiliser for maintaining said portion at a desired orientation in the bore; and downhole drive means for rotating the eccentric mass relative to the offset portion.

These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an elevation of directional drilling apparatus for use in drilling a deviated bore in accordance with an embodiment of the present invention: Figure 2 is an enlarged view of the leading end of the apparatus of Figure 1; Figure 3 is a sectional view on line 3-3 of Figure 2; Figure 4 is a sectional view on line 4-4 of Figure 2; Figure 5 is an enlarged sectional view of the stabiliser and eccentric mass of the apparatus of Figure 1; Figure 6 is a sectional view of a stabiliser of directional drilling apparatus in accordance with another embodiment of the present invention; Figures 7 is a sectional view on line 7-7 of Figure 6; Figure 8 is a sectional view corresponding to Figure 7, but showing the elements of the stabilisers in different relative orientations; and Figures 9 and 10 are enlarged views corresponding to Figures 7 and 8, and showing the stabiliser located in a bore.

Reference is first made to Figure 1 of the drawings, which illustrates directional drilling apparatus 10 in accordance with an embodiment of the present invention.

The apparatus 10 is adapted for mounting on the lower end of a drill string fabricated from drill pipe, Figure 1 showing a standard drill collar 12 as attached to the end of the drill pipe portion of the drill string. Attached to the drill collar 12 is a flex joint 14, that is a drill string section which is relatively flexible and, as will be described, facilitates the drilling of deviated bores.

Coupled to the lower end of the flex joint 14 is a string stabiliser 16 which rotates with the string. A mandrel 18 is coupled to the lower end of the stabiliser 16 and extends through a non-rotating eccentric mass 20 and a nonrotating offset stabiliser 22, with a drill bit 24 being mounted on the lower end of the mandrel 18.

Reference is now also made to Figures 2,3,4 and 5 of the drawings, which illustrate the offset stabiliser 22 and drill bit 24 in greater detail. The drill string is hollow to allow the passage of drilling fluid or"mud"from the surface to the drill bit 24, and thus the mandrel 18 also defines a through bore 26. The eccentric mass 20 defines a throughbore 28 to accommodate the mandrel 18, bearings 30 being provided between the mass 20 and the mandrel 18 to allow the mandrel 18 to rotate independently of the mass 20. As the centre of gravity of the mass 20 is offset from the centre of the mandrel 18, the mass 20 will tend to lie towards the"low"side of an inclined bore.

The non-rotating offset stabiliser 22 defines a throughbore 32 and, in a similar manner to the mass 20, is mounted on the mandrel 18 via bearings 34, which allow the mandrel 18 to rotate independently of the stabiliser 22.

As illustrated in Figure 4 of the drawings, the stabiliser bore 32 is offset from the circumference described by the stabiliser blades such that the stabiliser 22 offsets the mandrel 18 from the axis of the bore. As will be apparent to one of skill in the art, this offset is carried to the drill bit 24, such that the drill bit 24 will tend to drill in the direction of the offset.

The orientation of the offset may be varied by rotating the stabiliser in the bore, and this is achieved by operation of a downhole adjustable connector 38 between the mass 20 and the stabiliser 22. The connector 38 comprises an electric motor 40 located in an axial bore 42 in the mass 20, the bore 42 also containing a battery pack 44. The motor 40 drives a pinion 46 engaging gear teeth 48 provided on an inner circumference of an upper portion of the stabiliser 22. The electric motor drive shaft 50 extends beyond the pinion 46 to engage a support surface 52 defined by an inner face of an annular slot 54 formed in the stabiliser 22. The motor 40 and gearing 46,48 are isolated from well fluid by pairs of seals 56,58 between the stabiliser 22 and the mass 20 and mandrel 18, respectively.

The motor 40 may be pre-programmed on surface to rotate the mass 20 and change the relative orientation of the mass 20 and offset stabiliser 22 at predetermined intervals, but is most conveniently activated from surface by means of acoustic signals. If desired, the motor 40 may be in the form of a stepper motor.

In use, the drill string will be rotated from surface, this rotation being transferred to the drill bit 24 via the flex joint 14, string stabiliser 16 and mandrel 18. With the offset stabiliser 22 oriented as illustrated in Figure 4, the mandrel 18 will be offset in the drill bore, creating a similar offset in the drill bit 24, and causing the drilled bore to deviate in the direction of the offset.

The orientation of the stabiliser 22 is maintained by the mass 20 which will tend towards a position occupying the lower portion of the bore.

To change the drilling direction, appropriate acoustic signals are transmitted from the surface to the motor 40, actuating the motor to rotate the mass 20 on the mandrel 18 relative to the stabiliser 22. In practice, the stabiliser 22 will remain stationary in the bore, while the mass 20 rotates, driven by the motor 40. As drilling continues, the mass 20 will apply a torque to the stabiliser 22, tending to rotate the stabiliser 22 in the bore and return the mass 20 to the lower side of the bore; the stabiliser 22 will gradually rotate under the influence of the applied torque, until the mass 20 lies to the lower side of the bore. The stabiliser 22 and mass 20 will remain in this orientation in the bore until the motor 40 is actuated once more.

In order to drill"straight-ahead", the mass 20 be rotated continually at low speed, introducing a corresponding low speed rotation of the stabiliser 22, and thus negating the stabiliser offset.

As the mass 20 is"free"to rotate in the bore, due to the mass 20 describing a smaller diameter than the bore, the torque which must be applied by the motor 40 may be predetermined, and the motor 40 rated accordingly. Also, the ability to freely rotate the mass 20 to a desired orientation relative to the stabiliser 22, and the mass 20 then gradually rotating the stabiliser 22 in the bore to the desired offset, allows the introduction of gradual deviations in the bore inclination, avoiding the creation of sharp turns or dog-legs in the bore, which might otherwise cause the apparatus 10 to jam in the bore.

Further, the provision of a flex joint 14 in the apparatus 10 facilitates the introduction of a bore deviation utilising the apparatus 10. The flex joint 14 is selected to be more flexible than the mandrel 18, such that bore deviations will tend to be accommodated by bending of the flex joint 14, rather than bending of the mandrel 18, which would otherwise increase the friction between the mandrel 18 and the mass and stabiliser 20,22, potentially rendering the apparatus 10 ineffective.

Reference is now made to Figures 6 to 10 of the drawings, which illustrate an alternative configuration stabiliser 70 for forming part of apparatus in accordance with another embodiment of the present invention. The other parts of the apparatus correspond to the firstdescribed embodiment and will not be described again.

The stabiliser is mounted on a common mandrel 72 with the non-rotating mass 74 and comprises an outer sleeve 76 carrying the stabiliser blades 78 and body 80 which includes an inner sleeve 82. The sleeves 76,82 define respective offset bores 84,86, and may be rotated relative to one another by operation of an electric motor 88 mounted in a bore 90 in the inner sleeve 82 to vary the offset provided by the stabiliser 70, as described below.

With the sleeve offsets"aligned"as illustrated in Figures 7 and 9, the axis of the outer circumference described by the stabiliser 70 is offset by a maximum distance"O"from the mandrel axis; this offset provides for the introduction of a maximum deviation in the drilling direction. However, if the motor 88 is actuated, a drive pinion 92 on the motor drive shaft engages a circumferential gear 94 on the outer sleeve 76 and rotates the sleeves relative to one another. If the sleeves 76,82 are rotated through 180 , the sleeves assume the relative positions illustrated in Figures 8 and 10, in which the axis of the outer circumference described by the stabiliser 70 corresponds with the mandrel axis; accordingly, in this position, the apparatus will allow holes to be drilled straight ahead.

If desired, the sleeves 76,82 may be located at intermediate positions to provide a less aggressive offset.

Those of skill in the art will realise that the abovedescribed embodiments are merely exemplary of the present invention, and that various modifications and improvements may be made thereto, without departing from the scope of the present invention. For example, a different form of offset stabiliser may be utilised, such as the stabiliser forms described in our W097\47848, the disclosure of which is incorporated herein by reference. Further, the form of the downhole adjustable connector 38 may be varied, and in other embodiments may incorporate alternative drive means.

Claims

CLAIMS 1. A directional drilling apparatus for use in drilling a deviated bore, the apparatus comprising: means for mounting on a drill string for offsetting a drill bit in a bore and including an eccentric mass rotatably mounted on a mandrel, said mandrel for coupling to a drill string, and said mass tending to maintain a desired offset; and a relatively flexible string portion for coupling to a drill string adjacent said offsetting means and for coupling to said mandrel.
2. The apparatus of claim 1, wherein the relatively flexible string portion is a flex joint.
3. The apparatus of claim 1 or 2, wherein the eccentric mass is mounted between two larger diameter string elements.
4. The apparatus of claim 3, wherein said larger diameter string elements are stabilisers.
5. The apparatus of any of claims 1 to 4, wherein the offsetting means includes a stabiliser including an offset element.
6. The apparatus of claim 5, wherein said stabiliser comprises a non-rotating portion defining an offset outer profile, the non-rotating portion being rotatably mounted on a mandrel.
7. The apparatus of claim 6, wherein the offset mass and the stabiliser portion share a common mandrel.
8. The apparatus of claim 6 or 7, wherein the nonrotating portion of the stabiliser comprises inner and outer parts, each part defining an offset bore, whereby by varying the relative orientation of the offset bores it is possible vary the offset of the mandrel relative to the outer circumference of the stabiliser.
9. The apparatus of claim 6,7 or 8, wherein the nonrotating stabiliser portion is coupled to the eccentric mass, the relative angular orientation of the mass and stabiliser portion being selected to achieve a desired deviation of the bit.
10. The apparatus of any of claims 5 to 9, wherein a downhole adjustable coupling is provided between the mass and the stabiliser.
11. The apparatus of claim 10, wherein said coupling includes drive means for rotating the mass relative to the stabiliser.
12. The apparatus of claim 11, wherein the drive means comprises a motor on one of the mass and stabiliser for engaging a gear on the other of the mass and stabiliser.
13. The apparatus of claim 12, wherein an electric motor is axially mounted in the mass and has a drive pinion engaging gearing associated with a stabiliser mandrel.
14. The apparatus of claim 11,12 or 13, wherein said drive means is a battery powered electric motor and the apparatus further comprises an electrical generator utilises the relative rotation of the parts of the apparatus in use to provide a trickle charge for charging a battery.
15. Directional drilling apparatus for use in drilling a deviated bore, the apparatus comprising: offsetting means for mounting on a drill string and said means including an offset portion for offsetting a drill bit in a bore and an eccentric mass for rotatably mounting on a drill string and for maintaining the offset portion at a desired orientation in a bore; and downhole drive means for rotating the eccentric mass in a bore and changing the relative orientation of the mass and the offset portion.
16. Directional drilling apparatus for use in drilling a deviated bore, the apparatus comprising: a stabiliser for mounting on a drill string and including a non-rotating offset portion for offsetting a drill bit in a bore; an eccentric mass for rotatable mounting on a drill string and being operatively associated with the offset portion of the stabiliser for maintaining said portion at a desired orientation in a bore; and downhole drive means for rotating the eccentric mass relative to the offset portion.
17. Directional drilling apparatus substantially as described herein and as illustrated in Figures 1 to 5 or Figure 6 to 10 of the accompanying drawings.