Boring or penetrating the earth – With below-ground tool drive prime mover – With positive connection between tool and support shaft for...
Patent
1996-09-19
1998-12-29
Bagnell, David J.
Boring or penetrating the earth
With below-ground tool drive prime mover
With positive connection between tool and support shaft for...
1753252, E21B 1710, E21B 1716
Patent
active
058530533
DESCRIPTION:
BRIEF SUMMARY
The invention is concerned with improvements in or relating to stabilisation devices for drill motors, particularly but not exclusively for use with downhole motors for operation in a bore.
Down-hole motors, which may be positive displacement motors, turbo-drills or any suitable motor arrangement for operation within a bore or other confined passage to drive the drill bit, are conventionally fitted with stabiliser devices to guide the motor body, shaft and drill bit in the bore. It will be understood that an un-stabilised arrangement working at operational speeds can partake of a whirling action producing a spiralling motion which can seriously reduce the drilling rate, the effect being particularly severe in the case of certain types of geological formations in which the bore is being formed.
The stabilisers conventionally used comprise a series of vanes machined with or secured to the motor body and usually the bit itself. The gaps between the vanes permit the passage of drilling fluid between the motor and the drilling area.
The motor body is connected to the drilling string which extends back to the surface and a motor shaft is connected to the drill bit itself. The direction of formation of the well bore formed by the rotating bit may be controlled in a number of ways, for example by means of a so-called deviating device, a bend and/or by an eccentric stabiliser kick pad incorporated in the assembly at a suitable position.
During normal drilling the drill string is rotated at a lower speed than that of the drill bit, thereby eliminating the directional influence of the deviating device. Orientated drilling is achieved by aligning the deviating device in the desired direction and then drilling progresses with the drill string held stationary and the bit rotated by the downhole motor.
The rate of change of well bore angle is affected by a number of factors. However, one of the most important parameters is the so-called bit overhang, i.e. the distance from the lower bearing stabiliser on the motor body housing to the operating faces of the drill bit. In general this distance needs to be as short as possible in order to maximise the rate of angle change of the deviating assembly. However, obtaining a smooth well bore profile is important to ensure the drilling assembly can progress in the bore hole at an optimal rate of penetration.
To achieve a smooth bore hole profile the bit must be adequately stabilised usually by means of a stabilising section on the bit gauge. Conventionally a gauge length at least equal to the bit diameter is required, but in general the greater the overhang the larger the gauge length needs to be. Clearly the requirements for a smooth wellbore profile conflict with the requirements for rate of angle change sometimes referred to as the dogleg severity. In practice the construction of the downhole motor and the method of joining the drill bit to the motor shaft conflict with the need to minimise the bit overhang length. Consequently, the bit stabilisation has generally been considered as secondary to the need to achieve the required dogleg severity. Consequently, the performance of motors (ROP) in the orientating mode has been substantially below that achieved when the drilling string is rotated.
Some of the performance reduction is due to the difference in static and dynamic friction coefficients between the drill string and the wellbore. However, a large proportion of the reduction in ROP is due to poor wellbore profile. In a conventional drilling assembly, the motor shaft extending from the motor body is machined internally with a standard bit-box thread. Sufficient length must be allowed for a proper grip of the shaft portion extending from the body, i.e. the internally threaded portion, by a rig tong device for engaging or dis-engaging the drill bit. In addition the bit itself is normally required to allow gripping with a bit breaker.
It will be appreciated that these requirements require a length penalty that conflicts with the desirable dimensions for attaining the maximum ra
REFERENCES:
patent: 3807513 (1974-04-01), Kern et al.
patent: 4261425 (1981-04-01), Bodine
patent: 5452772 (1995-09-01), Van Den Bergh
Downie Andrew McPherson
Gilchrist David Andrew
Bagnell David J.
Neyrfor-Weir Limited
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