Boring or penetrating the earth – Means traveling with tool to constrain tool to bore along...
Patent
1995-02-09
1996-07-16
Bagnell, David J.
Boring or penetrating the earth
Means traveling with tool to constrain tool to bore along...
175 76, 175107, E21B 700, E21B 3412
Patent
active
055358357
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the steering of a drill bit at the end of a drill string within a borehole to selectively effect drilling along either a curved path or a substantially straight path.
It is known to drill a section of borehole along a deviated path so as to effect a change in trajectory of the borehole by use of a downhole trajectory control device which causes the drill bit located at the end of the drill string to be tilted to cause drilling at an inclined angle with respect to the immediately preceding section of the borehole. A known trajectory control device of this type comprises a mud motor which is installed in the bottomhole assembly close to the drill bit and which is arranged to angularly tilt the rotational axis of the drill bit relative to the axis of the section of borehole being drilled, so that rotation of the drill bit at the end of the borehole results in drilling along a curved path provided that the drill string is maintained at a defined axial orientation.
In one such arrangement utilising a downhole motor, the motor is supported by first and second eccentric stabilisers which are axially spaced apart along the motor housing and axially offset relative to one another so as to impart the required tilt to the rotational axis of the drill bit. By turning of the drill string to cause rotation of the motor housing to a known orientation, which is indicated to the surface by a downhole steering system, and subsequently maintaining such orientation during rotation of the drill bit by the motor, deviated drilling may be effected in the required direction. Furthermore, for sections of borehole where a straight trajectory is required, continuous rotation of the motor housing during rotation of the drill bit by the motor will result in a mean position of the axis of rotation of the drill bit which is coincident with the axis of the section of borehole being drilled. Thus, by alternating intervals of continuous housing rotation with intervals in which the orientation between the axis of rotation of the drill bit and the borehole axis is fixed, the trajectory of the borehole can be controlled as required. However changes in trajectory cause stresses to be induced in the housings of the motor and downhole steering system, and this can lead to damage or excessive wear in the associated tubular elements, threaded connections and internal components.
Normally continuous rotation of the drill string is used to effect drilling along a straight path. However, in circumstances in which the curvature of the borehole would lead to fatigue failure due to the magnitude and number of cyclic stress reversals in the drill string, continuous oriented drilling is employed with the orientation being changed by 180.degree. every few feet. This results in an extremely tortuous borehole path and therefore significantly increased drag. As a consequence, the total length of borehole which can be drilled is reduced and subsequent casing operations are made more difficult.
It is an object of the invention to provide steering apparatus which reduces the stress related problems identified above.
According to the present invention there is provided apparatus for steering a drill bit at the end of a drill string within a borehole to selectively effect drilling along either a curved path or a substantially straight path, the apparatus comprising a first downhole motor assembly for coupling to the drill bit and operable to rotate the drill bit to effect drilling, the first motor assembly being arranged to angularly tilt the rotational axis of the drill bit relative to the axis of the section of borehole being drilled, a second downhole motor assembly for coupling the first motor assembly to the drill string and operable to rotate the first motor assembly, and actuating means for selectively (i) effecting rotation of the drill bit by the first motor assembly while the first motor assembly is maintained at a defined axial orientation so as to cause drilling of the borehole along a curved path in the direction of tilt of t
REFERENCES:
patent: 4143722 (1979-03-01), Driver
patent: 4492276 (1985-01-01), Kamp
patent: 4667751 (1987-05-01), Geczy et al.
patent: 5311953 (1994-05-01), Walker
Bagnell David J.
Baroid Technology Inc.
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