Down-hole motor and method for directional drilling of boreholes.

The invention relates to a down-hole motor and a method for directional drilling of boreholes in underground formations.

During drilling of a borehole in an underground formation it is frequently required to vary or to adjust the direction of drilling. Such adjustment of the drilling direction is commonly carried out by a kick-off procedure during which procedure a smoothly curved borehole section is drilled to bring the borehole at the desired course.

Various tools are known in the art for carrying out kick-off procedures. A suitable kick-off tool is disclosed in U.S. patent specification 3,260,318. This known tool consists of a down-hole drilling motor of the Moineau type. The stator housing of this motor is bent such that in the operative position of the motor in a borehole a drill bit being connected to the output shaft thereof rotates about an axis of rotation that is inclined with respect to the local borehole direction. During drilling by means of said motor the drill string is kept immobile and hence a curved borehole extension is drilled. This known motor, however, is not designed for drilling straight borehole sections and after each kick-off operation the motor has thus to be removed from the drill string which requires a time consuming roundtrip procedure.

A down-hole drilling motor for alternately drilling straight and curved borehole sections is disclosed in U.S. patent specification 3,667,556. This known motor is described in the prior art portion of claim 1. In this known motor the bearing assembly that supports the output shaft is connected in a pivotable manner to the motor housing such that the output shaft can be positioned in an inclined position relative to the housing. By varying the angle of deflection between the housing and the output shaft straight and curved borehole sections can be drilled at will. Major disadvantages of this known motor reside in the fragility of the pivots between bearing and housing and in the complexity of the remotely controlled system for adjusting the angle of deflection.

An object of the present invention is remedy these drawbacks in order to provide a directional drilling tool that forms a simple and reliable means for directional drilling of a borehole.

A further object of the present invention is to provide a simple and reliable method of drilling straight and curved borehole sections -at will by manipulating the drill string by means of the rotary table at the drilling floor.

The motor according to the invention is characterized in that the housing is provided with stabilizer means for centralizing the longitudinal axis of the motor in a borehole and in that the bearing unit is mounted in such a manner in the housing that the central axis of the output shaft intersects the longitudinal axis of the motor at a point of intersection located outside the housing.

It is to be understood that in this specification and in the claims the term "longitudinal axis of the motor" refers to the central axis of the surface of revolution that envelopes the outer surface of the stabilizer means.

It is further to be understood that in this specification and in the claims the expression "a point of intersection located outside the housing" indicates that the point of intersection lies on that part of the longitudinal axis of the motor that protrudes fran the lower end of the housing.

In an attractive embodiment of the present invention the point of intersection between the longitudinal axis and the central axis of the output shaft is located near the face of a drill bit being connected to the output shaft.

Various types of down-hole drilling motors may embody the invention such as electrical motors and hydraulic motors actuated by the mud flow through the drill string. Suitable hydraulic motors are turbines, vane motors and Moineau motors.

The method for directional drilling of a borehole with the down-hole motor of the present invention comprises the steps of (a) connecting a drill bit to the output shaft of the motor and connectjng the motor to the lower end of a drill string, and subsequently lowering the drill string, motor and bit in a borehole, (b) actuating the motor and applying weight on bit, and (c) simultaneously with step (b) rotating the drill string over periods that are preceded and succeeded by selected periods in which the drill string is not rotated.

The invention will now be explained in more detail by way of example with reference to the accompanying drawings, wherein <ul id="ul0001" list-style="none"><li>Figure 1 shows a longitudinal section of the lower part of a down-hole motor according to the invention;</li><li>Figure 2 shows at a smaller scale than Figure 1 a side view of the motor of Figure 1 in the operative position thereof during drilling of a straight borehole section;</li><li>Figure 3 shows a side view of the motor of Figures 1 and 2 but now in the operative position thereof during drilling of a curved borehole section.</li></ul>

In Figures 1, 2 and 3 similar reference characters designate similar parts of the drilling assembly.

Reference is now made to Figure 1 showing in detail the lower part of a down-hole motor according to the invention. The motor is a hydraulic motor of the Moineau type consisting of a stator motor housing 1 within which a rotor 2 is rotatably arranged. A connecting rod 4 is connected to the lower end of rotor 2 by means of a universal joint 6 and the lower end of connecting rod 4 is connected to an output shaft 5 by means of another universal joint 7. As the construction and operation of a Moineau motor are known per se no detailed description of the motor parts and their operation is given in this specification.

The housing 1 includes an upper housing part lA and a lower housing part 1B, which parts are secured to each other by a screw thread connection 9. The upper housing partly is provided with two stabilizers 10A and 10B (see also Figures 2 and 3), each stabilizer comprising four radially extending stabilizer blades for centralizing the motor in a borehole. The shape of the stabilizers 10A and 10B is such that a cylindrical surface of revolution (not shown) may envelope the outer surfaces thereof. The central axis of said surface of revolution forms the longitudinal motor axis I. The lower housing part 1B includes a bearing unit 11 comprising suitable thrust and radial bearings for supporting the output shaft 5 such that the shaft 5 is rotatable about its central axis II. A drill bit 12 is detachably mounted on the lower end of the output shaft 5 by means of a screw thread connection 14. The shaft 5 and bit 12 comprise inner cavities 15 and 16 communicating with each other for passing drilling liquid to the bit face 19. The upper end of shaft 5 is provided with a port 17 through which drilling liquid that is discharged from the interior of the housing 1 may enter the cavity 15.

In the down-hole drilling motor assembly the bearing unit 11 is arranged in an inclined position in the housing 1, such that the central axis II of the output shaft 5 intersects the longitudinal axis I of the housing 1 at an acute angle A at a point of intersection 20 located outside the housing 1.

The purpose of the location of the point of intersection 20 outside the housing 1 will now be explained together with the method for directional drilling with reference to Figures 2 and 3.

Reference is first made to Figure 2 showing a side view of the down-hole drilling motor of Figure 1 in the operative position thereof during drilling of a straight borehole section 22 (see arrow VI) in an underground formation 23.

Before starting the drilling operation a drilling assembly has been composed at the surface by connecting the drill bit 12 to the output shaft 5 and by connecting the upper end of the motor housing 1 to the lower end 27 of a drill string 26. The drilling assembly has subsequently been lowered in the borehole 22 until the bit face 19 engages the bottom of the borehole 22 at a predetermined weight on bit.

Liquid is then pumped through the interior of the drill string 26 into the motor housing 1 for actuating the rotor 2 (see Figure 1) to rotate the output shaft 5 and the bit 12 about the central axis II (see arrow V). The drilling liquid is discharged from the housing 1 via the inner cavities 15 and 16 (see Figure 1) in the shaft 5 and the bit body 12 to the bit face 19 for cooling and cleaning the cutters thereof and for lifting drill cuttings fran the borehole 22.

During drilling the stabilizers 10A and 10B laterally support the motor in the borehole 22 such that the longitudinal axis I of the motor housing 1 substantially coincides with the longitudinal axis of the borehole 22. Drilling of a straight borehole section (see arrow VI) is now performed by rotating the motor housing 1 about the longitudinal axis I (see arrow III) thereof by rotating the drill string by means of the rotary table (not shown) at the drilling floor and by rotating simultaneously therewith the shaft 5 and the bit 12 relative to the housing about the central axis II of the shaft 5. The drill bit 12 consequently rotates in the borehole 22 about both axes I and _II, thereby describing an orbital movement around the longitudinal axis I. Due to this orbital movement the drill bit 12 will deepen the borehole 22 in the direction of the longitudinal axis I, and as a result thereof a straight borehole section will be drilled (see arrow VI). As the point 20 of intersection of the axes I and II (which point 20 forms the centre of the rotation of the bit 12) is located close to the bit face 19 creation of an oversized or spiralling borehole is avoided.

Reference is now made to Figure 3 for explaining the manner in which a curved borehole section is drilled.

Contrary to drilling of a straight borehole section, during which the drill string 26 (and consequently the motor housing 1) is rotated, the drill string rotation is stopped during drilling of a curved borehole section 33. The drill bit 12 is now solely driven by the down-hole motor and the bit 12 rotates solely (see arrow V) about the central axis II of the output shaft 5. As the axis II is inclined at an angle A with respect to the longitudinal motor axis I, the drilling direction deviates from the direction of the lower end of the borehole, hence a curved borehole section 33 is being drilled. On drilling the curved extension of this section 33 (see arrow VII) the lower stabilizer 10A and subsequently the upper stabilizer 10B enter this extension whereby the tilt of the motor housing 1 is gradually increased, as a result whereof the curvature of the borehole extension will further increase.

When the borehole is found to be directed along. the desired course, and drilling the hole should be continued in a straight line, the drill string 26 is actuated again (by rotating the rotary table at the drilling floor) to rotate the motor housing 1. A straight borehole section will then be drilled in the way as explained hereinbefore with reference to Figure 2.

To reach a target area in the subsurface formation 23 the drilling operator may repeat the above described procedure for alternately drilling straight and curved borehole sections at will. A sequence of straight and curved borehole sections is then drilled by actuating the drill bit 12 to rotate by means of the down-hole motor and simultaneously therewith rotating the drill string 26 (by rotating the rotary table) over periods that are preceded and succeeded by selected periods over which the drill string 26 is not rotated (and the rotary table is locked).

Each time when a curved section is to be drilled, the drill string rotation is stopped and the motor housing 1 is oriented in the borehole so as to allow the drill bit 12 to deepen the borehole in the desired deviated direction. The orienting procedure may be carried out either by rotating the entire drill string 26 over a finite angle by means of the rotary table or by varying the drill string twist by adjusting the reaction torque of the motor housing 1 on the lower drill string end 27 either by adjusting the weight-on-bit, or by adjusting the pressure of the drilling liquid that actuates the down-hole motor. When the motor housing 1 has been oriented in the desired position drilling proceeds whilst the drill string 26 is locked against rotation in the way as explained hereinbefore with reference to Figure 2.

Optionally the drilling assembly is provided with suitable logging and telemetering equipment to provide the drilling operator with data on the actual borehole direction and motor orientation. Such equipment is known per se and does not require a detailed description thereof.

The invention is not restricted to the use of the moineau motor shown in the drawing. Any type of down-hole motor known in the art may be used such as a vane motor, a hydraulic turbine or an electric motor. If desired the rotor may be axially aligned with the output shaft (and co-axial to the axis II of the output shaft), instead of being co-axial to the longitudinal axis I of the housing.

Furthermore the invention is not restricted to the use of the type of stabilizer means shown in the drawings. Any type of stabilizer means may be used such as a series of stabilizers with either straight or helically shaped blades, or a series of stabilizers that are either eccentric or concentric to the motor housing. If desired, one of the stabilizers may be mounted on the lower end of the drill string. The use of a series stabi_- lizers mounted on the motor housing may even be avoided by using a down hole motor wherein a motor housing is provided with large radius sections that are shaped for performance thereof as stabilizer wings. Such a motor is disclosed in applicant's British patent application 7932750 (filed: September 21, 1979).

It is observed, that the angle of inclination A between the longitudinal motor axis I and the central axis II of the output shaft is chosen such that the drill bit (that may be any type of rotary drill bit known in the art) is able to drill straight and curved borehole sections in the way as explained with reference to Figures 2 and 3. Depending on properties of the formation rock and the bit geometry the angle of inclination A may be up to 5°. Most types of drill bits known in the art will be found to be suitable for use in combination with the down-hole drilling motor according to the invention, by selecting the angle of inclination A between 0.25° and 2.5°. By locating the point of the intersection of the axes I and II close to the bit face, these bits will be able to drill straight and curved borehole sections at will, without creating an oversized or spiralling borehole.