Abstract:
A downhole device, adapted for use in wireline or slickline application, is provided for incorporation into a downhole string and movement in a wellbore. The device comprises means to orient the device in a wellbore, and one or more conveying means arranged to engage the inner surface of the wellbore and reduce the frictional coefficient. It may also incorporate an eccentric weight and/or a vibrator.

Description:
This Application is the U.S. National Phase Application of PCT International Application No PCT/GB00/04622 filed Dec. 4, 2000. 
     This invention relates to a downhole device, and particularly one that is adapted for use in wireline or slickline applications. 
    
    
     DESCRIPTION OF THE RELATED ART 
     In conventional wireline and slickline operations, a workstring with different tools is lowered into casing, tubing or other tubulars in a borehole from a wire or cable spooled from a drum at surface. Wireline and slickline operations can be performed for many reasons during completion, maintenance and servicing, installation and retrieval of downhole apparatus, intervention and for well logging. Most strings of wireline tools comprise one or more devices that e.g. collect data from the well-bore concerning the characteristics of recovered fluids etc such as temperature, salinity etc, or perform other functions. In addition to suspending the string of tools, the wire or cable may also act as a conduit for power required by the tools to carry out their functions in the well-bore, and may include signal cables for conveying data gathered by the downhole sensors back to surface. 
     Wireline strings operate satisfactorily in vertical and near vertical wells, but problems arise when they are used in deviated wells, because when the well deviates beyond about 55°, the suspended string of tools no longer penetrates satisfactorily under gravity, as the frictional forces retarding it exceed the gravitational forces propelling it. Also, the string tends to snag on the tubular connections or other upsets on the inner wall of the casing. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention there is provided a downhole device for incorporation into a downhole string and movement in a well-bore, the device comprising means to orient the device in a well-bore, and one or more conveying means arranged on the device to engage the inner surface of the well-bore. 
     The device can comprise a downhole sub for incorporation into a string or tool. The means to orient the device or sub can be eccentric means such as an eccentric weight or shape to the sub or a portion thereof. Typical eccentric orientation means can be ballast means such as a weight eccentrically positioned in the sub to favour adoption by the sub of one or more orientations rather than others. Typically the orientations favoured are those in which the conveying means are in contact with the inner surface of the well-bore. However, in other embodiments of the invention, the means for orienting the sub can be an external shape of the sub or a portion thereof e.g. an egg or oval shape that favours orientation of the sub on a wider portion of the sub than on a narrow portion e.g. at the tips of the oval. In that embodiment the conveying means can be associated with the wider portion of the sub so that they are brought into contact with the well-bore surface when the sub orientates itself on the wider portions of the sub. 
     The conveyors can be wheels, rollers or similar, but other types of conveyors can equally be useful, for example skis, endless tracks etc. Anything that reduces the frictional coefficient of the device is suitable for basic embodiments of the invention. It is preferred that the conveying means are provided in pairs in a side-by-side arrangement and that several pairs of conveying means are provided on the device or tool as a whole. The wheels or other conveyors etc are typically provided on the lower surface of the sub i.e. the lower surface when it has been oriented by the ballast. 
     The device can be made up into a tool or tool string, but can be incorporated into other strings to reduce frictional resistance to movement of the tool string in the well. 
     In a second aspect the invention also provides a downhole device for incorporation into a tool or work string and movement in a well-bore, the device having friction reducing means on its outer surface to facilitate passage of the device through a well-bore. 
     The friction reducing means can comprise the conveying means of the first aspect of the invention and the sub of the second aspect can also incorporate orientation means in certain embodiments. 
     A motor can be provided optionally in a separate module in the sub body. The motor can be powered by a battery also contained within a separate module in the sub. 
     The sub, tool or string in which it is incorporated may have a vibrator or oscillator which may typically be in the form of an eccentric rotor that can be rotated by the motor. The motor can drive either or both of the vibrator and the conveyor, and need not drive both even when both are provided in a particular embodiment. The vibrator typically induces vibrations in the tool body at a desired frequency, which may optionally be varied e.g. by adjusting mass or position of weights on the rotor or its rpm, or other vibration means. 
     The sub, tool or string in which it is incorporated may have a sensor coupled to a switch for controlling the motor and/or the vibrator. The sensor may be adapted to sense one or more characteristics of the tool or its environment. One preferred characteristic that the sensor can detect is the attitude of the sub or tool. For example, in a preferred embodiment, the sensor detects the vertical attitude of the sub or tool and sends a signal to the motor and/or the rotor to function if the sub or tool body deviates more than a fixed amount beyond the vertical. This automatically switches on the motor to drive the wheels and/or the vibrator when the sub or tool encounters a deviated well which might tend to retard the progress of the sub or tool through the well-bore. A typical deviation that can be chosen to activate the sensor is approximately 50° to 75°. Thus, when the sub or tool enters a region of the well-bore which is deviated, say, by 70°, the sensor detects the deviation in the attitude, and as the sub or tool enters the deviated well-bore, the motor drives the wheels or the vibrator to reduce the friction of the sub or tool against the well-bore inner surface and avoiding or reducing the possibility of retarding the sub or tool on the inner surface of the well-bore. 
     The wheels or other conveyors are preferably disposed in side-by-side relationship and are arranged to contact the well-bore inner surface at or very near the circumference of the sub or tool. For example, in preferred embodiments, the wheels do not protrude substantially beyond the outer circumference of the body of the sub or tool, so that they substantially coincide with the outer circumference of the cylindrical body. Typically they extend through cut-away sections of the side-walls of the body, and follow the shape of the outer diameter of the body and/or the inner surface of the well-bore. The side-walls of the cylindrical body are typically formed with slots etc from which the wheels or other conveyors protrude very slightly so as to engage the inner surface of the well-bore. In preferred embodiments, the slots match the protruding portions of the wheels etc very closely, so as to avoid or minimise flat areas where the sub or tool can rest on the inner surface of the well-bore without contacting the well-bore inner surface with the wheels or other conveyors. The well-bore-engaging surfaces of the wheels are typically shaped to conform to the inner surface of the tubular through which the sub will be run. 
     In one preferred embodiment of the invention the wheels are of large diameter and are arranged to extend through the body of the sub at opposite sides, so that each wheel extends through opposing sides of the body and contacts opposing sides of the well-bore. Larger wheels can be more resistant to wear downhole, and can also reduce frictional coefficients. Typically the large wheels are positioned in pairs side-by-side, with each pair on the same axis through the sub. 
     In the above embodiments each wheel can be arranged on a central axle to extend through opposite sides of the sub, so that one large diameter wheel, or one side-by-side pair of wheels, can suffice. In some other preferred embodiments of the invention with wheels extending through opposite sides of the sub, smaller wheels are used with each wheel only extending through a single side to contact the well-bore surface only at one point, and typically wheels on opposing sides are mounted on different axles and optionally at the same axial position along the sub, so that an axial core can be provided for a power or data cable. 
     The various components of the sub or tool of the invention can optionally be provided in separate subs. 
     The sub or tool can therefore comprise a train of connected subs each having one or more wheels or rollers, batteries, motors, vibrators, sensors, knuckle or swivel joints. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings, in which: 
     FIG. 1 is side and sectional views through a first embodiment of a device; 
     FIG. 2 is a side view of a schematic arrangement of a second device; 
     FIG. 3 is an end view of a further embodiment within a pipe; 
     FIG. 4 a  is a plan view of the FIG. 3 embodiment; 
     FIGS. 4 b  and  4   c  are sectional views of the FIG. 3 embodiment; 
     FIGS. 5 a, b, c, d  and  e  are sectional and side views through a fourth embodiment; 
     FIGS. 6 a  and  b  show sectional and side views of a fifth embodiment; and 
     FIGS. 7 a  and  b  are further views of the fifth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, a downhole sub  1  has a body  5  having suitable connections  6  at either end to be attached into a string of wireline tools for well intervention or MWD etc. The connections may be conventional box and pin, or others as required. A knuckle joint and/or swivel  7  is provided at one or both ends to facilitate travel of the sub  1  around corners and for orientation in a pipe. 
     The body  5  is made up of an exterior housing  10  with several compartments for respectively containing a battery  12 , motor  14 , vibrator  15 , and sensor  16 . The housing  10  can typically be formed of steel, and can incorporate a belly portion  10   b  formed of lead or a denser material than the steel housing which eccentrically weights the body  10 . This has the effect of asymmetrically weighting the sub so that the belly portion naturally assumes the lowest position under gravity in a deviated well-bore. 
     Instead of being formed from different materials so as to produce the asymmetrically weighted housing, the sub can simply be arranged to have more of its heavier components like the motor, battery etc on the belly side  10   b  so as to concentrate the weight on that side and therefore to induce the asymmetric weighting without requiring a multi-piece housing. 
     Optionally, subs at the top and/or bottom of the sub  1  incorporate swivels enabling the sub  1  to rotate axially relative to the rest of the toolstring. Alternatively, in small toolstrings, this rotation of the tool is unnecessary and the swivel subs can be omitted. 
     The belly  10   b  of the sub  1  has two pairs of rollers  20  at each end mounted on respective axles  21  passing through the body  10  of the sub  1 . The rollers  20  are contained within the body  10  of the sub  1  and protrude through slots  22  in the belly portion  10   b  of the housing  10  so as to engage the inner surface of the casing etc. The rollers  20  can be shaped so that their end-on profile matches, as closely as possible, the circular cross-sectional profile of the housing  10 , and the rollers  20  are positioned very near to the circumference of the body  10  at the mid-line of the belly portion  10   b  so that when the sub  1  is oriented by the belly portion  10   b , the rollers  20  are brought into engagement with the inner surface of the casing etc and the contact between the rollers  20  and the casing is automatically maintained by the asymmetrically weighted housing of the belly portion  10   b.    
     The rollers  20  are typically carried on roller subs  10   r  forming part of the body  10 . 
     As the rollers  20  are always brought into contact with the inner surface of the well-bore casing in deviated wells, the sub  1  naturally runs along the lower surface of the deviated casing on the rollers  20 . This minimises resistance to travel of the sub  1  through the casing. As the rollers  20  only protrude very slightly through the slots  22  in the housing  10 , which extends flush against the sides of the protruding portions of the wheels, there are no flat areas of the sub  1  outer housing  10  where it may come to rest on the inner surface of the well-bore casing except for the area between the pairs of rollers  20 . 
     Since the asymmetrically weighted belly portion of the sub always orientates the sub  1  so that the rollers  20  engage the inner surface of the well-bore casing, this mitigates the possibility that the rollers  20  fail to engage the inner surface of the well-bore casing, e.g. by the sub  1  resting on a flat area out of contact with the rollers  20 . 
     In highly deviated wells (i.e. above 65° to 70°), an optional inclination sensor  16  activates the optional electric motor  14  to power either or both of the rollers  20  and an optional vibrator  15  connected in the toolstring. The electric motor  14  can optionally be battery powered from the onboard battery  12 , or can be powered from an electric line combined with or lowered alongside the wireline from surface. A hydraulic motor can replace the electric motor and can be supplied by fluid from a hydraulic line from surface. 
     The vibrator  15  comprises an eccentric rotor which is rotated by the motor  16  at a number of different frequencies which can be specified by an operator either from surface or by setting the required vibration frequency on the sub  1  before it is launched from surface. Vibration of the sub  1  helps to overcome frictional resistance to movement through casing, and can release the sub or string in which it is attached from snagging on upsets or casing joints etc on the inner surface of the well-bore casing. 
     The motor can alternatively, or additionally, drive the rollers  20  directly in a forward or reverse direction. A transmission system (not shown) of conventional design is typically provided for this purpose. With the wheels or rollers or other conveyors etc, the sub can penetrate well-bores deviated by up to around 75-80°. By also providing a motor and/or vibrator the sub can penetrate well-bores deviated up to around 87°. 
     FIG. 2 shows a modified embodiment in which the different components are provided in separate inter-connecting subs. 
     FIGS. 3 and 4 show end and side views of a further sub  100  that has no added ballast but is shaped ovally (see end view of FIG. 3) so that the sub  100  has a belly portion  100   b  and an opposing back portion  100   c  that are located on a wider radius of the sub  100 , and opposing side portions  100   s  that have a narrower radius. Wheels  120  are arranged in two sets of side-by-side pairs, with each pair mounted and optionally sealed via a wiper seal onto a common axle  121  that passes between the side portions  100   s  and is secured by a pin  121   p  so that the tips of the wheels  120   t  extend towards and through openings in the back and belly portions  100   b,c . Extending the wheels through the walls of the sub  100  gives two landing zones on the circumference of the sub  100  where it is balanced and stable. The wheels  120  protrude only very slightly through the openings, and are profiled to follow the shape of the internal wall of the well-bore W as described above. 
     When the sub  100  is in the well-bore W the oval shape tends to unbalance the sub  100  if it is resting on its side portions  100   s  because these have a narrower radius and therefore a smaller surface area in contact with the well-bore W than the back  100   c  and belly  100   b  portions. Therefore, should the sub  100  come to rest on either side portion  100   s , it will tend to tip over to balance on the tips  120   t  of the wheels  120  that extend through the belly and back portions  100   b,c . In that configuration, the tips  120   t  of the wheels  120  contact the inner portion of the well-bore W, and the sub  100  balances itself in that orientation, so that the wheels can drive the sub, or merely rotate to reduce the friction retarding the passage of the sub through the well-bore. 
     Note that it is not necessary for the sub  100  to be oval shaped, and good results can be achieved with other shapes that are more balanced in one orientation than in another e.g. regular shapes that have protrusions at the sides or elsewhere to create areas of low surface area for contacting the well-bore. 
     Typically the conveyors are associated with the portion of the sub that contacts the well-bore in the balanced orientation, and which typically has a larger surface area or a number of protrusions (such as conveyors like wheels) that are spaced apart and between which the sub can balance more easily. 
     Note also that it is unnecessary for the sub to be powered at all, and providing one or more dumb subs with simple overbalancing and un-powered wheels in a tool string can ease the passage of the string through less deviated wells. 
     FIG. 5 shows a fourth embodiment of a sub  200  with a substantially rounded body  205  through which 8 wheels or rollers extend. The rollers 220-227 are each arranged on a single axle and extending through a side wall at a flattened area on the top or bottom of the sub to contact the well-bore at only one position. Each roller (e.g.  220 ) has a pair of wheels and is typically (but not necessarily) arranged above or below another roller (e.g.  221 ), so that groups of 2 rollers are provided at 4 positions shown in sections  1 — 1   2 — 2 ,  3 — 3  and  4 — 4  in FIGS. 5 a ,  5   b ,  5   d  and  5   e respectively, along the axis of the sub  200 . Axially adjacent rollers are offset by 90° with respect to each other, so that the wheels at sections  1 — 1  and  3 — 3  are aligned with one another and extend respectively through the top and bottom faces of the sub  200 , and the wheels at sections  2 — 2  and  4 — 4  are similarly aligned with each other but are arranged at 900° to those at sections  1 — 1  and  3 — 3  to extend through the side faces of the sub  200 . 
     The provision of smaller wheels extending only through one face of the sub  200  allows an axial passage through the centre of the sub for a power or data cable  230 . 
     The offset wheels increase the likelihood of some of the wheels engaging the well-bore and make it less crucial to have an orientation means in this embodiment of the invention. Therefore, embodiments such as this one can have rounded sub bodies so that they can fit better into smaller bore tubulars. However, orientation means can optionally be provided with this embodiment, for example in the form of eccentric weights or shapes in or on the sub body. 
     Swivels  250  can be provided to allow the sub to rotate on its axis relative to the rest of the string S and the swivel inner body, and the swivels  250  can optionally have a hollow bore to allow passage of the cable  230 . Bearings, seals and grease ports etc can optionally be provided. An anti-torque tube  260  connects to the swivel inner body to remain static while the sub body  200  swivels around it in order to keep the cable  230  from twisting. 
     FIGS. 6 and 7 show a further embodiment of a sub  300  with an eccentric-shaped body  305  having 4 rollers  320 - 323  each mounted on a separate axle  320   a - 323   a  and each having 2 wheels extending through the flattened areas of the top and bottom walls of the sub  300 . Rollers  320  and  321  are mounted in a pair respectively at the top and bottom of the sub, as are rollers  322  and  323 , but the latter are axially spaced from rollers  320  and  321  along the sub body. This arrangement allows the passage of an anti-torque tube or mandrel  360  and cable  330  for data and/or power as described for the FIG. 5 embodiment. The sub  300  optionally also has swivels  350 , seals and bearings as described for the FIG. 5 embodiment. 
     In common with some other embodiments described, the wheels on the rollers  320 - 323  are shaped to follow closely the contour of the outer surface of the sub body  305  and the inner surface of the well-bore, and thereby assist in the orientation of the sub  300  onto the wheels at the top or bottom of the sub  300 . This also makes the sub more stable while resting on the wheels and less likely to tip over so that the wheels disengage from the inner surface of the tubing and frictional resistance to onward movement increases. 
     The axial passage can accommodate any kind of elongate member such as power or data cables, or coiled tubing, or simply control wires for tools further down the string. 
     Modifications and improvements can be incorporated without departing from the scope of the invention. For example, the vibrator or oscillator is typically a motor which rotates eccentric weights. The eccentric weight can be adjusted to account for different overall tool string weights either by using a different mass of eccentric weight, or by adjusting its position. The motor can be electrically or hydraulically powered. The vibrator or oscillator can alternatively be a hammer-type device providing an impact type of oscillation, and can similarly be electrically or hydraulically powered. The oscillations delivered to the tool can thus be axial, transverse or radial, and can be adjusted to be of a desired frequency and/or amplitude, which can optionally be altered during operation of the sub or tool. This causes static friction to reduce to dynamic friction and induces downward movement of the sub in the well-bore.