Description

WORK EXTRACTION FROM DOWNHOLE PROGRESSIVE CAVITY DEVICES

Technical Field

[0001] This invention relates to techniques for extracting work from progressive cavity devices such as pumps (PCPs) and motors (PDMs). In particular, the invention relates to such techniques for use with downhole devices such as are used in wells in the oil and gas industry.

Background Art

[0002] PCPs and PDMs comprise a hollow cylindrical stator with an elongate rotor positioned therein. The stator has helical lobes formed on its inner surface, typically formed from some elastomeric material. The rotor has helical lobes formed on its outer surface. As is explained below, the number of lobes and their helical pitch are different for the rotor and stator of any pump or motor. The term 'progressive cavity device' is used here to mean any such device, whether configured as a pump or as a motor. Progressive cavity devices are used in oilfield applications in two major segments: artificial lift and drilling. They have shown a high life if designed and dimensioned properly, and have demonstrated acceptable tolerance to finer solids (such as LCM during drilling and sand during production).

[0003] In both cases, the rotor and stator axes of a progressive cavity device are eccentric to each other, leading to a rotation of the rotor inside the stator with a simultaneous nutation of the rotor centreline to the stator centreline. In the case of artificial lift, an electric motor drives the rotor through a eccentric universal joint to allow for the eccentricity of the rotor to the centre. In the case of drilling motors, the flow of drilling fluid through the device forces the rotor to rotate inside the stator, leading to a rotation of the drillbit through a double universal joint system or a flexible drive shaft. The configuration of the rotor/stator lobes is one-off (e.g. 1 :2, 3:4, 7:8) and this ratio can be used to calculate the nutation speed of the rotor around the centreline of the stator given the rotation speed of the rotor on itself. The nutation rotation direction is opposite to that of the rotor rotation.

Disclosure of Invention

[0004] A first aspect of the invention provides a progressive cavity device comprising a hollow stator having a rotor shaft positioned therein, wherein the rotor shaft is provided with rotating motion drive connections at both ends thereof.

[0005] The drive connections can be for driving the rotor shaft or for extracting drive from rotation of the rotor shaft. Preferably, at least one of the drive connections operates at a speed that is different to the rotation speed of the shaft.

[0006] The drive connections can couple to the rotation of the shaft. Such drive connections preferably comprise a shaft connecting two universal joints, or a flexible shaft. Alternatively, the drive connections can couple to the nutation of the shaft. Such drive connections can include a non-nutating connection that imposes a nutation speed on the shaft. Such a drive connection can comprise a disc mounted for rotation on a further shaft, the rotor shaft being connected eccentrically to the disc. Nutating connections can include, for example, a planetary gear system, the rotor shaft being connected to a planet gear.

[0007] The device can be configured to act as a motor or as a pump. When configured as a motor, the rotor is driven by pumping fluid through the stator, and a rotating drive connection is taken from both ends of the stator to power other devices, hi a preferred embodiment, the drive at one end is used to rotate a drill bit. The drive at the other end can be used to power a crushing device or an electricity generating device.

[0008] When configured as a pump, the rotor is driven by a drive connection from a motor at one end and a rotating drive connection is taken from the other end of the rotor to power other devices. Examples of preferred devices to be powered via the rotating drive connection include fluid air mixers, crushing devices, reaming or drilling devices and fluid mixing/shearing devices.

[0009] This invention provides methods of tapping to and extracting rotation at both ends of a progressive cavity device rotor. This rotation can be extracted at various rates and used to perform simultaneous operations with the use of only one pump or motor. Even when one end of the pump is what drives the rotor to create a fluid circulation, the other end can be used to perform additional work, without the need for another hydraulic or electric motor being added. Potential applications exist in various oilfield segments.

Brief Description of Figures in the Drawings

[0010] Figures 1 and 2 show axial and radial sections of rotor/stator combinations for progressive cavity devices;

Figure 3 shows one embodiment of a non-nutating drive connection; Figure 4 shows another embodiment of a non-nutating drive connection; and Figures 5 and 6 show embodiments of progressive cavity devices with drive connections at both ends of the rotor.

Best Mode for Carrying Out the Invention

[0011] This invention provides techniques for tapping to and extracting rotation of both ends of a progressive cavity device rotor. For example, this rotation can be used for a cuttings crusher at the entrance of the device, or for a gas/fluid agitator at the end of a downhole pump. This invention describes how different output speeds can be utilized in various oilfield applications.

[0012] The power section of a downhole motor converts hydraulic energy from the drilling fluid into mechanical energy to turn the bit. Using the reverse Moineau pump principle, the positive displacement motors operate by using the surface pumps to force the drilling fluid between a helical shaft, and a sealing sheath. The helical shaft is rotated by the fluid and is called the "rotor", while the sheath is fixed and called the "stator." The stator is connected to the rest of the drill-string (or a Coiled Tubing) via the top sub. Thus the inertia of the drill string counters the torque created by the operation of the motor. The stator will only rotate when the drill string is rotated when driven from the surface.

[0013] A relatively known and constant amount of rotation is required to pass a fixed volume of fluid through the system, so the motor rotation, or revolutions per minute (rpm), is proportional to the flow rate. A small part of the flow 'bypasses' doing rotary work as it leaks through the rotor and stator contact line from a high- pressure cavity to an adjacent low-pressure cavity.

[0014] Both rotor and stator have matching helical profiles, but the rotor has one less spiral (or lobe) than the stator. Figures 1 and 2 show axial and radial sections of rotor/stator combinations. In Figure 1, the rotor 10 has a single lobe and the stator 12 has two lobes 14 (ratio 1:2) . In figure 2, the rotor 16 has five lobes 18 and the stator 20 has six lobes 22 (ratio 5:6). The power section of such a downhole motor is designated by the ratio of its rotor/stator lobes. For example, a 4:5 power section has four lobes in the rotor and five in the stator. Motors such as the PowerPak range of motors from Schlumberger are available in 1 :2, 2:3, 3:4, 4:5,

5:6, and 7:8 lobe configurations. The ratio of the rotor/stator helical pitch is the same as the ratio of the rotor/stator lobes. For example, the stator pitch of a current commercial 5:6 lobe motor is 52.70 in, and the pitch of its rotor is (5/6) x 52.70 in, or 43.92 in.

[0015] In an assembled power section, the rotor and stator form a continuous seal at their contact points, producing cavities independent from each other. As fluid (water, mud, or air) is forced through these progressive cavities, it causes the rotor to move around inside the stator. The movement is the combination of rotation and nutation. When completing a revolution, the rotor nutates once for each of the rotor lobes. Therefore, nutation creates mechanical stresses on the motor at a much higher rate than the rotation itself, and becomes a limiting factor in many cases. As an example, a motor with 7:8 rotor/stator lobe configuration rotating at 100 rpm has a nutation speed of 700 cycles/minute.

[0016] In the case of an artificial lift pump, an electric motor is usually situated below the PCP pump and is driven via electrical cables running from the surface.

[0017] Finally, in the case of a circulation pump in a wireline powered and conveyed drilling machine, an electric motor is placed above the PC pump and drives it to create a fluid circulation to carry the cuttings. This same pump can also be used in to create a vacuum in low bottomhole pressure reservoirs for sand and debris cleanout operations.

[0018] There are a number of ways in which the concept of this invention can be implemented. In a conventional drive connection, the drive input or output connects to the rotor shaft axis such that one rotation of the shaft equates to one rotation at the drive connection. In one embodiment of the invention, instead of connecting to the rotor centre to extract or create the rotation, the connection can be via a non-nutating drive as shown in Figure 3 that rotates (or dictates) the rotor *nutation* speed. In this embodiment, the end of the rotor shaft 40 projects from the stator which comprises a metal housing 42 having an elastomeric insert defining the lobe structure 44 (the example shown here is a 1 :2 system for simplicity, other ratios are equally applicable). The non-nutating drive comprises a nutation disc 46 fixed to the end of a shaft 48 which is itself mounted in bearings 50 for rotation. The end of the rotor shaft 40 connects to the nutation disc 46 by means of a rotation bearing 52 which is offset from the disc centre. In use, where the rotor is being driven by fluid flow or a motor attached to the other

end of the rotor (not shown), the rotor shaft 40 is driven to rotate about its axis in direction ω _R whereas the nutation at the end of the rotor 40 is coupled via the disc 46 such that the shaft 48 rotates in the opposite, nutation direction ω^. Alternatively, the shaft 48 is the one that could be driven by a motor, the non- nutating connection being used to provide a drive input to the rotor, typically when acting in a pump configuration.

[0019] Using the embodiment of Figure 3, a faster spinning (and therefore more efficient) electrical motor can be used to rotate a pump at a lower rpm. In the case of a downhole motor, the rotation of the bit can be at the faster nutation rate, and allows use of faster-turning bits without the necessity for a high rotor rotation rate. Typical rotation rates for drilling bits are in 100-300 rpm. Thus, when connecting directly to the rotor shaft rotation for drive, a 200-600 rpm nutation of the rotor in the stator (of a 1 :2 motor) must be accommodated, increasing the fatigue stresses on the stator elastomer. If the nutation is used as the drive via the non-nutating connection, then for a 300 rpm rotation of the bit, only a 150 rpm rotor rotation would be required. The decreased torque penalty can be offset by providing additional (longer) motor stages for example.

[0020] In another embodiment, as shown in Figure 4, the nutation of the rotor can be used to drive (or be driven by) the planet carrier of a planetary gearbox at the rotor nutation speed. The rotation of the planet carrier with respect to the sun gear (or output/input shaft) via the planets and the relationship of the planet and sun teeth, can dictate the output shaft rotation as a fixed proportion of the nutation speed. In the embodiment of Figure 4, the nutation disc of Figure 3 is replaced by a planetary gear arrangement which comprises a non-rotating housing 60 having the ring gear 62 fixed thereto. The sun gear 64 is fixed to the shaft 48. The end of the rotor shaft 40 connects to a planet gear 66 mounted on a planet carrier 68 (further idler planet gears (not shown) may also be mounted on the carrier 68). In this case, the shaft 48 rotates in the same direction as the rotor shaft 40 rotates (due to the reversing effect of the gears), but at a rate fooN that is a function of the nutation dependent on the gear ratios in the planetary gear system.

[0021] In the embodiments of both Figure 3 and Figure 4, the shaft rotation is coaxial with the drive axis of the motor/pump.

[0022] In another embodiment of the invention, connections are provided at both ends of the rotor of a progressive cavity pump, therefore allowing work to be extracted

above and below the pump (without the need to run electrical wires and drive additional electrical motors, or the need to add two hydraulic motors). A similar benefit is the use of the upper rotation of the rotor when the lower part is attached to a driving electrical motor via a gearbox.

[0023] Apart from or in addition to the extraction methods of the nutation speed from the rotor described above, methods of torque and rotation transmission that are currently used in other technologies can also be used; such as through a dual universal joint configuration, or through a flexible shaft that can flex to accommodate the full offset of the rotor nutation. The flexible shaft is subjected to higher fatigue loads, but has neither moving nor rubbing parts as the universal joints do.

[0024] Figures 5 and 6 show embodiments using such techniques for connecting to both ends of the rotor of a progressive cavity device. In Figure 5, one end of the rotor 40 is connected to a load/motor 70 by means of a connection shaft 72 having universal joints 74 at the shaft and load /motor ends. The other end of the rotor shaft 40 is connected to a second load/motor 76 by means of a flexible shaft 78. In both cases, the loads/motors 70, 76 are driven/drive at the same rate and direction CU _R as the rotor 40. In Figure 6, the load/motor 70 is connected to the rotor 40 by means of a non-nutating connection 80 of the type described above in relation to Figure 3. The load/motor 76 is connected to the rotor 40 by means of a dual universal joint connection 82 of the type described above in relation to Figure 5. Ln this case, the load/motor 70 operates at the nutation rate, i.e. in the opposite direction to the rotor shaft rotation and at a rate GD _N , whereas the load/motor 76 operates at the same rate and direction CD _R as the rotor 40.

[0025] There are a number of ways in which the embodiments of the invention can be implemented to involve fluid flow turning the rotor and in turn a drill bit below, but with the added benefit of a rotation above the motor that can be used for grinding or for reaming for example. For a pump, an electrical motor below can create the rotor rotation, and the rotation above the pump can be used to drive a fluid/air mixer to ease dual phase reservoir fluid lift.

[0026] The ability to drive the rotor from either end and to extract rotating motion from the other (at rotation or nutation speeds) allows for a multitude of applications. Such applications include:

• for drilling applications: using a hydraulic motor to drive a drill bit at a lower end, and using upper end rotation/nutation to drive a crusher to protect the stator elastomer by reducing solids size; or using upper end [higher] nutation speed to power an alternator and create electricity to power sensors and actuators (such as for a flow bypass), without having to use power from the power system of an associated MWD tool;

• for artificial lift applications: using an electric motor below to drive the artificial lift pump, with a fluid/air mixer above to facilitate lift; or using an electric motor above to drive the pump, and a crusher below to break, or detain and grind larger rocks before they enter the pump; for wireline drilling and lateral construction applications: using an electric motor to drive a fluid circulation pump, and a cuttings crusher below to assure that only small particles enter the pump section; or using an electric motor to drive a circulation pump, and using upper rotor rotation or nutation to drive a backreamer;

• for a wired Coiled Tubing well cleaning application; using an electric motor to drive a vacuum/suction pump, and using lower rotor rotation or nutation to mobilize sand or thick muds (to break the thixotropy). These are just preferred examples of embodiments of the invention and other changes within the scope of the invention will be apparent.