GB2233363A

GB2233363A - Down-hole bent motor housings

Info

Publication number: GB2233363A
Application number: GB9005235A
Authority: GB
Inventors: Colin Walker
Original assignee: Baroid Technology Inc
Current assignee: Baroid Technology Inc
Priority date: 1989-06-28
Filing date: 1990-03-08
Publication date: 1991-01-09
Anticipated expiration: 2010-03-08
Also published as: GB9005235D0; GB2233363B; GB8914799D0

Abstract

A bent motor housing 1 has a top end 5 by which it is connected to the end of a drill string within a borehole 4 and a bottom end 6 on which a drill bit 3 is mounted for drilling the borehole 4. A bend 9 is provided intermediate the top and bottom ends for angularly offsetting the rotational axis 11 of the drill bit 3 relative to the longitudinal axis of the drill string to cause the drill bit 3 to engage the wail of the bore hole 4 on one side. Furthermore a longitudinal bore 2A provided for passage of a motor shaft 2 extends from the top end 5 to the bottom end 6 of the housing 1 and is concentrically disposed with respect to the top end 5 and the bottom end 6 but eccentrically disposed with respect to intermediate housing sections 7 and 8 so as to form a stand-off portion 10 in the vicinity of the bend 9 for bearing against the wall of the borehole 4 on the opposite side to that engaged by the drill bit 3 so as to cause the angle of tilt THETA of the rotational axis 11 of the drill bit 3 relative to the axis of the borehole 4 to exceed the angle of the bend 9 in the housing 1. <IMAGE>

Description

"Down-Hole Bent Motor Housings" This invention relates to down-hole bent motor housings for use in drilling a borehole along a curve.

It is well known, in the field of directional drilling of boreholes, to drill a borehole along a curve using a mud motor having a bent housing for angularly offsetting the rotational axis of the drill bit relative to the longitudinal axis of the drill string so as to cause the drill bit to engage the wall of the borehole on one side and to thereby effect drilling along a curve whose curvature is determined by the angular offset of the bend in the housing. However, the angle of the bend in the housing is generally limited to about 12 by the articulated motor shaft which extends through the housing to the drill bit.

Furthermore modern drilling methods can impose severe directional drilling demands which cannot be met by conventional bent housing mud motors. For example, in drilling a curved section of borehole in order to deflect the direction of the borehole from vertical (that is Oo inclination) to horizontal (that is 900 inclination), it may be necessary to increase the inclination of the borehole by, say, 120 for every hundred feet drilled.

This degree of curvature cannot be attained using a conventional bent housing mud motor.

It is an object of the invention to provide a down-hole bent motor housing permitting directional drilling at an increased curvature.

According to the present invention, there is provided a down-hole bent motor housing for use in drilling a borehole along a curve, the housing being elongate and having a top end by which it is to be connected to the end of a drill string within the borehole, a bottom end on which a drill bit is to be mounted for drilling the borehole, a bend intermediate the top and bottom ends for angularly offsetting the rotational axis of the drill bit relative to the longitudinal axis of the drill string to cause the drill bit to engage the wall of the borehole on one side, and a longitudinal bore extending from the top end to the bottom end of the housing and provided for passage of a motor shaft, wherein the housing has a stand-off portion in the vicinity of the bend for bearing against the wall of the borehole on the opposite side to that engaged by the drill bit so as to cause the angle of tilt of the rotational axis of the drill bit relative to the axis of the borehole to exceed the angle of the bend in the housing.

The provision of the stand-off portion enables the angle of tilt of the rotational axis of the drill bit relative to the axis of the borehole to be increased relative to the angle of tilt obtainable using a conventional down-hole bent motor housing having a bend of the same angular offset. This increases the interference between the drill bit and the side of the borehole, as well as increasing the side force exerted by the drill bit on the formation being drilled, thus enabling a greater curvature to be obtained during drilling.

In a preferred embodiment of the invention the bore comprises a first rectilinear passage extending through a top portion of the housing and a second rectilinear passage extending through a bottom portion of the housing, the two passages meeting at the bend at an angle corresponding to the angular offset of the bend, and the provision of the stand-off portion bearing against the wall of the borehole resulting in the first passage being slightly inclined relative to the axis of the borehole in use.

Furthermore it is preferred that the housing comprises a top cylindrical housing part and a bottom cylindrical housing part, the two parts meeting at the bend at an angle corresponding to the angular offset of the bend, and the first and second passages extending respectively through the top and bottom housing parts and being coaxial therewith at least in the vicinity of the top and bottom ends of the housing.

Advantageously each of the top and bottom housing parts comprises a concentric housing section remote from the bend in which the associated passage is coaxially disposed and an eccentric housing section adjacent the bend in which the passage is eccentrically disposed so as to provide a thickened region of the housing wall, constituting said stand-off portion, on the side on which the housing bears against the wall of the borehole.

The eccentricity of the passage in the eccentric housing section of each of the top and bottom housing parts is preferably such that the thickness of the housing wall is at a maximum on the side on which the housing bears against the wall of the borehole and at a minimum on the diametrically opposite side, whereas the concentric housing section of each of the top and bottom housing parts has a wall thickness which is less than the minimum wall thickness of the associated eccentric housing section. Such an arrangement provides enhanced stiffness at the bend.

It is also preferred that the housing is formed from a single piece of metal by machining. This also increases structural rigidity.

In a development of the invention the bottom housing part is rotatable with respect to the top housing part to vary the angular offset, for example by means of an arrangement as described in British Patent No. 1494273.

The invention also provides a down-hole mud motor incorporating such a motor housing.

In order that the invention may be more fully understood, a preferred embodiment of bent motor housing in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 diagrammatically shows the motor housing in use within a borehole; and Figures 2 and 3 show diagrammatic cross sectional views on an enlarged scale along the lines II-II and III-III in Figure 1.

Referring to Figure 1 the bent motor housing 1 accommodates a drive shaft 2 extending through a longitudinal bore 2A in the housing, and a drill bit 3 is attached to the drive shaft 2 in the vicinity of a bottom end of the housing and is caused by the bend in the housing to engage one side of a borehole 4 extending through a formation 12 in order to drill along a curve in generally known manner. Although not shown in the figure, the top end of the housing is connected to the end of a drill string within the borehole 4 and houses the rotor of the motor which is coupled to the drill bit 3 by the drive shaft 2. The drive shaft 2 is articulated in order to enable it to accommodate the bend in the housing.

Furthermore the drive shaft 2 is supported by thrust and radial bearings in the vicinity of the bottom end of the housing in known manner.

The housing 1 comprises a top cylindrical housing part 1A and a bottom cylindrical housing part 1B, the two parts 1A, 1B meeting at the bend 9 in the housing at an angle corresponding to the angular offset of the bend 9. Furthermore the bore 2A comprises a first rectilinear passage 2B extending through the top housing part 1A and a second rectilinear passage 2c extending through the bottom housing part 1B, the two passages 2B, 2C meeting at the bend 9 at an angle corresponding to the angular offset of the bend 9.

In addition the top housing part 1A comprises a conventionally configured concentric housing section 5 and an eccentric housing section 7, and the bottom housing part 1B comprises a conventionally configured concentric housing section 6 and an eccentric housing section 8. As shown in the cross-section of Figure 2, the passage 2B is 'coaxially disposed with respect to the housing section 5, and similarly the passage 2C is coaxially disposed with respect to the housing section 6. However, as shown by the cross-section of Figure 3, the passage 2B is eccentrically disposed with respect to the housing section 7, and similarly the passage 2C is eccentrically disposed with respect to the housing section 8.

It will be appreciated that the eccentric housing sections 7 and 8 have a greater outer diameter than the concentric housing sections 5 and 6, and furthermore that the eccentricity E of the passage 2B or 2C within the housing section 7 or 8 results in the thickness of the wall of the housing section 7 or 8 being at a maximum TmaX on the side on which the housing bears against the wall of the borehole 4 and at a minimum Tmin on the diametrically opposite side. The wall thickness t of the concentric housing section 5 or 6 is constant and is less than the minimum wall thickness Tmin of the associated eccentric housing section 7 or 8.

Thus the thickened region of the housing wall on the side on which the housing bears against the wall of the borehole 4 constitutes a stand-off portion 10 producing a lateral offset L resulting in the passage 2B being slightly inclined relative to the axis of the borehole 4 so as to increase the angle of tilt e of the rotational axis 11 of the drill bit 3 relative to the axis of the borehole 4 without increasing the angle of the bend 9 in the housing.

Thus, it will be appreciated that, if the bent motor housing 1 described above is used in place of a conventional bent motor housing of equal length and equal angular offset, the provision of the stand-off portion 10 will have the following effects: 1. Increase in the interference I between the drill bit 3 and the side of the borehole 4; 2. Increase in the side force F exerted by the drill bit 3 on the side of the borehole 4; and 3. Increase in the tilt angle 0 of the drill bit 3.

These three effects mean that it is possible to increase the degree of curvature of the borehole being drilled to, for example, 120 or even 150 for every hundred feet drilled.

In addition, as the housing sections 7 and 8 are of greater diameter than the housing sections 5 and 6 and greater wall thickness, there is an associated increase in stiffness of the housing 1 in the vicinity of the bend 9, thus ensuring that the increased bending moment produced in the vicinity of the bend 9 by an increase in the side force F does not result in a decrease in the angular offset of the bend due to the reverse bending effect.

This is important because the drive shaft 2 can be damaged in the vicinity of its articulated portion if reverse bending occurs beyond the permitted tolerances. Typically the flexural rigidity of the bend can be 50% higher than in a conventional bent motor housing.

It should be appreciated that the housing 1 is formed from a single piece of metal by machining both the inside and outside surfaces to form the housing sections 5 to 8, the bend 9 and the stand-off portion 10.

The increased stiffness at the bend is also advantageous in enabling a known control technique to be used in which a straight portion of the borehole is drilled by rotating the bent housing mud motor during drilling without overstressing the bent housing in such a way as to substantially decrease its service life. The substantial lateral forces to which the drill bit is subjected in such a technique are adequately compensated by the flexural rigidity of the bent housing. This technique avoids the time and cost which would otherwise be incurred by changing of the mud motor housing-when it is required to drill a straight portion of the borehole immediately after drilling of a curved portion.

Claims

1. A down-hole bent motor housing for use in drilling a borehole along a curve, the housing being elongate and having a top end by which it is to be connected to the end of a drill string within the borehole, a bottom end on which a drill bit is to be mounted for drilling the borehole, a bend intermediate the top and bottom ends for angularly offsetting the rotational axis of the drill bit relative to the longitudinal axis of the drill string to cause the drill bit to engage the wall of the borehole on one side, and a longitudinal bore extending from the top end to the bottom end of the housing and provided for passage of a motor shaft, wherein the housing has a stand-off portion in the vicinity of the bend for bearing against the wall of the borehole on the opposite side to that engaged by the drill bit so as to cause the angle of tilt of the rotational axis of the drill bit relative to the axis of the borehole to exceed the angle of the bend in the housing.

2. A bent motor housing acdording to claim 1, wherein the bore comprises a first rectilinear passage extending through a top portion of the housing and a second rectilinear passage extending through a bottom portion of the housing, the two passages meeting at the bend at an angle corresponding to the angular offset of the bend, and the provision of the stand-off portion bearing against the wall of the borehole resulting in the first passage being slightly inclined relative to the axis of the borehole in use.

3. A bent motor housing according to claim 2, wherein the housing comprises a top cylindrical housing part and a bottom cylindrical housing part-, the two parts meeting at the bend at an angle corresponding to the angular offset of the bend, and the first and second passages extending respectively through the top and bottom housing parts and being coaxial therewith at least in the vicinity of the top and bottom ends of the housing.

4. A bent motor housing according to claim 3, wherein each of the top and bottom housing parts comprises a concentric housing section remote from the bend in which the associated passage is coaxially disposed and an eccentric housing section adjacent the bend in which the passage is eccentrically disposed so as to provide a thickened region of the housing wall, constituting said stand-off portion, on the side on which the housing bears against the wall of the borehole.

5. A bent motor housing according to claim 4, wherein the eccentricity of the passage in the eccentric housing section of each of the top and bottom housing parts is such that the thickness of the housing wall is at a maximum on the side on which the housing bears against the wall of the borehole and at a minimum on the diametrically opposite side.

6. A bent motor housing according to claim 5, wherein the concentric housing section of each of the top and bottom housing parts has a wall thickness which is less than the minimum wall thickness of the associated eccentric housing section.

7. A bent motor housing according to any preceding claim, wherein the housing is formed from a single piece of metal by machining.

8. A bent motor housing according to any preceding claim, having a bottom housing part which is rotatable with respect to a top housing part to vary the angular offset.

9. A down-hole bent motor housing substantially as hereinbefore described with reference to the accompanying drawings.

10. A down-hole mud motor incorporating a bent motor housing according to any preceding claim.