Self cleaning pistons

A technique facilitates tool operation with mobile pistons submitted to differential pressure. The technique utilizes a mobile piston slidably mounted in a corresponding piston passage. The piston passage is defined by a passage wall surface, and the piston may be moved linearly along the piston passage under the influence of an actuating fluid or to pump a fluid. The exterior surface of the piston and/or the passage wall surface have a groove or a plurality of grooves located and arranged to collect particulates from the fluid. Removal of the particulates facilitates actuator piston function by reducing, for example, third body abrasion and jamming of the piston.

BACKGROUND

In many hydrocarbon well applications and other applications, pistons are employed to actuate a variety of tools and systems. In some applications, the pistons are actuated by a working or actuating fluid. In other applications, the pistons are used to move fluid: this is the case in a piston pump. During their usage, these pistons are submitted to a difference of fluid pressure between their two extremity surfaces. To contain this difference of pressure, many pistons employ seals, e.g. elastomeric dynamic seals or metal piston rings, to exclude particulates in the actuating fluid from the piston/cylinder bore interface. In some applications, e.g. wellbore drilling applications using drilling mud (either as actuating fluid or as the fluid being moved), the fluid may contain a substantial amount of hard particulates. Even with seals, the particulates can enter the clearance between the piston and the surrounding wall which often is a cylindrical wall. As a result, the particulates can damage the piston components via third body abrasion and/or completely jam the motion of the piston as the particulates embed in the piston and/or surrounding cylinder surfaces.

SUMMARY

In general, a system and methodology are provided for facilitating tool operation, e.g. actuation, with mobile pistons submitted to differential pressure. The technique utilizes a mobile, e.g. actuating, piston slidably mounted in a corresponding piston passage. The piston passage is defined by a passage wall surface, and the piston may be moved linearly along the piston passage with an actuating fluid. The exterior surface of the piston and/or the passage wall surface have a groove or a plurality of grooves located and arranged to collect particulates from the actuating fluid. In some applications, the grooves are formed in a hardened material. Removal of the particulates facilitates actuator piston function by reducing, for example, third body abrasion and jamming of the piston. With a desired layout of the grooves, the accumulated particles may pass from one groove to the next groove while the piston performs its reciprocating movement. This progressive movement of the particles allows clean-up of the grooves so that lengthy operation of the piston can be achieved with fluid containing particles.

DETAILED DESCRIPTION

The present disclosure generally relates to a system and methodology for facilitating tool actuation by improving the reliability of actuator pistons. The technique effectively removes particles from the interface, e.g. clearance, between an actuator piston and the surrounding wall of the bore. The bore can be cylindrical or another suitable shape, such as a section of a toroid. This system can be associated with a piston and bore involving tight clearance which serves to limit the fluid leakage if no conventional seals are installed along the clearance. However, the groove system can also be used with a piston involving a conventional seal or seals. The particulates are removed to grooves to reduce detrimental effects, such as third body abrasion and jamming of the piston motion.

With proper lay-out of the grooves, the piston can facilitate particular removal well-being moved along a main piston axis. In some applications, a difference of fluid pressure may be applied between both extremities of the piston while a leak rate is maintained at a low value. The groove shape can be optimized to enhance the self-cleaning of the groove in a variety of applications. Also, additional features may be added in the groove to improve the cleaning of the groove. In the case of a piston which may perform short strokes, a mobile sleeve with similar grooves may be added between the piston and the wall. This mobile sleeve is constructed to perform longer stroke for ensuring the progressive movement of particles to enhance clean-up.

In some applications, the grooves are formed in a hardened material to further reduce the potentially deleterious effects of particulates in the piston actuating fluid. This approach may be used in a variety of applications, including downhole well applications in which the actuating fluid may contain particulates or is susceptible to particulate contamination. For example, the system and methodology may be used for downhole piston actuators operated by actuating fluid in the form of drilling mud. The system also can be used in pumping applications involving moving pistons and plungers Such a pumping design can be applied on the surface of an actuating rod passing through a wall wetted by fluid where fluid is contained from one side of the wall versus the other side of the wall.

In applications involving either actuating or pumping pistons, differential pressure may be applied to the piston (between the two extremities of the piston). This differential pressure has a tendency to push the fluid into the tight clearance between the piston and the bore which increases the risk of entraining particles in this narrow clearance. In the case of an actuating application, the force delivered by the piston is generated by the differential pressure of the actuating fluid; and the force, the differential pressure, and the displacement of the piston are in the same direction. In the case of a pump application, the force applied on the piston is in the opposite direction of the differential pressure applied onto the piston. When the force is applied in the opposite direction, the configuration of the system may change, e.g. an orientation of some features of the grooves may change.

In some embodiments, the technique employs at least one actuating piston slidably mounted in a corresponding piston passage. The piston passage is defined by a passage wall surface, and the piston may be moved linearly along the piston passage with an actuating fluid, e.g. drilling mud or another suitable actuating fluid. The exterior surface of the piston and/or the passage wall surface have a groove or a plurality of grooves located and arranged to act as chambers for collecting particulates from the fluid that is present. Removal of the particulates facilitates the piston (or rod and shaft) function by reducing, for example, third body abrasion and jamming of the piston (or rod and shaft).

In some applications, the grooves may be formed in a hardened material, e.g. polycrystalline diamond, silicon-bonded diamond, tungsten carbide, ceramic, stellite or other hard materials, to further limit abrasion and other potentially deleterious effects caused by the particulates. Depending on the application, the outer surface of the piston and the surrounding piston passage surface may be formed of hard material with closely spaced tolerances to limit leakage flow around the piston. In this manner, the piston actuator may be operated without using dynamic seals in a variety of applications

Referring generally toFIG. 1, an example of a system20, e.g. a borehole drilling system, is illustrated as having a tool string22deployed in a wellbore24. The tool string22comprises a plurality of tools26which are actuatable via pistons28. In this example, the pistons28and their corresponding tools26are actuatable via a suitable actuating fluid30. The actuating fluid30may comprise a downhole well fluid, such as a drilling mud, other actuating fluid pumped downhole, or fluid occurring naturally downhole. For example, the actuating fluid30day comprise fluids occurring naturally downhole under sufficient pressure to serve as actuating fluid30.

By way of example, tool string22may comprise a drill string having a drill bit32which is rotated to drill the wellbore24in a desired formation34. In this example, the pistons28may be part of piston-type actuators36used for steering the drill bit32along a desired trajectory. Other pistons28may be in the form of internal pistons38, e.g. piston sleeves, for actuating a variety of sliding sleeves, valves, and/or other tool string components. However, many types of pistons28and actuating fluids30may be used in a variety of borehole drilling applications and other applications in which the drilling fluid30contains particulates or is susceptible to contamination by particulates.

Referring generally toFIG. 2, an example of an actuator system40comprising piston28is illustrated. In this example, piston28is slidably received in a piston passage42of a surrounding structure which may be referred to as a stator44. The piston passage42is defined by a passage wall surface46. In many applications, the piston28is generally circular in cross-section and the piston passage42is generally cylindrical in shape. As illustrated, the piston passage wall surface46has a plurality of passage cross grooves48which may be arranged circumferentially around the piston passage42. In some applications, the passage cross grooves48comprise a plurality of separate grooves although the plurality of passage cross grooves48can be connected together in, for example, a helical pattern or other suitable pattern.

The piston28is received in piston passage42for linear movement along a linear axis50. The piston28also comprises an exterior surface52having a plurality of piston cross grooves54which may be arranged circumferentially around the piston28. In some applications, the piston cross grooves54comprise a plurality of separate grooves although the plurality of piston cross grooves54can be connected together in, for example, a helical pattern or other suitable pattern. The piston cross grooves54and the passage cross grooves48slide past each other during movement of the piston along the linear axis50and provide collection regions for collecting particulates that may be in actuating fluid30. The movement of piston28along an axis50is caused by the pressure of the actuating fluid30acting against an end of the piston28as represented by actuating fluid arrows30inFIG. 2.

In the embodiment illustrated, the passage cross grooves48are formed in a hardened material56. By way of example, the hardened material56may comprise polycrystalline diamond, silicon bonded diamond, tungsten carbide, ceramic, stellite or another suitable hard material which protects against abrasion from particulates in actuating fluid30. In the example illustrated, the hardened material56is constructed as a sleeve58mounted within a body60of stator44. However, the hardened material56also may be applied as a coating or insert, or the entire stator44may be formed of hard material56.

In this example, the piston cross grooves54also may be formed in a hardened material56. By way of example, the hardened material56may again comprise diamond carbide, tungsten carbide, or another suitable hard material which protects against abrasion from particulates in actuating fluid30. The hardened material56may be used on piston28and/or stator44depending on the application. Additionally, the hardened material56may be the same type of material on both piston28and stator44or the hardened material56may be different between these two components. In the example illustrated, the hardened material56of piston28is constructed as a sleeve62mounted along an interior body64of piston28. However, the hardened material56also may be applied as a coating or insert along piston body64, or the entire piston28may be formed of hard material56.

Depending on the application, both the passage cross grooves48and the piston cross grooves54may be formed as timed grooves. The timed grooves48,54enable each edge-edge pair of corresponding grooves48,54to come into contact sequentially and also for particles in actuating fluid30to be driven progressively into successive chambers/grooves in the direction of the pressure gradient and thus out of the interface between piston28and the stator44. As illustrated, the interface has a running clearance66and grooves48,54provide chambers for receiving the particulates as they are driven out of this interface, thus maintaining the running clearance66. The effectiveness of the timed grooves48,54may be further enhanced by selecting an appropriate groove pitch68for passage surface46and an appropriate corresponding groove pitch70for the exterior surface52of piston28.

The flow in the working clearance can be estimated. For this description, we refer initially to the sequence illustrated inFIG. 3. In this example, fluid is forced into running clearance66and becomes a leak. The leak rate depends on the difference of pressure applied onto the piston and also the fluid properties (e.g. rheology and density). Geometrical elements also may play a role in the determination of the leak rate. As a simplified hydraulic model of the leak, the fluid can be considered as being forced into the narrow flow slit having the following characteristics:its thickness is the working clearance66;its perpendicular extent is the circumference of the piston28,its axial extend is the sum of the tight overlaps between the piston28and the bore in which the piston28moves. This axial extent is represented as71aand71binFIG. 3.

An example of a set of parameters for this geometry and flow is as follows:The working clearance is small and may be in the range of 10 to 100 microns, e.g. 20 microns is considered for this example. The flow is mostly laminar.The circumference of the piston may be in the range of 25 to 200 mm In the following example, the circumference is 120 mm (piston diameter is 38.2 mm)The sum of the tight overlap may be in the range of 5 to 50 mm: In the following example, this length is is 25 mm (71a+71b).The fluid viscosity may be in the range of 1 to 100 cenitPoise. In the following example, the viscosity is 10 centiPoise (0.01 Pa S).

The flow in a narrow slit is given by:

Using the values of this particular example, Q=1.6 10−18m3/s (7 10−12GPM)

For this low flow rate, the fluid velocity in the working clearance is in the range of 0.66 10−9m/s.

The Reynolds number is :

This confirms that the flow in the working gap is laminar, so that the formula (a) is adequate.

With such narrow working clearance66, some larger particles of the fluid cannot pass in the clearance. They are illustrated as particles72accumulated at an entrance to clearance66in Position1as shown inFIG. 3. When considering drilling mud, lost circulation material (LCM) particles and even some barite can form this accumulation. These large particles may represent up to 10% or even 20% or even 30% of the fluid volume. With such large amounts, the accumulated volume may form a ring of particles72, as illustrated at Position1. When the piston moves to certain positions, the accumulated particles72can suddenly enter in the grooves48.

When the piston28moves from Position1to Position2(the other end of the stroke) via the displacement D1, the particles72accumulated below the piston28can then move into the groove48aas illustrated. Then, when the piston28reaches the Position3after the backwards displacement D2, the particles72in the groove48acan jump into the piston groove54is illustrated at Position3. The piston28then moves forward in its next displacement D3and this allows the particles of the groove54to jump into the groove48bas illustrated in Position4. The piston28then makes another displacement D4and returns to its retracted position. This allows the particles72to jump from groove48binto the fluid outside the piston28as illustrated by arrows74upon return of piston28to Position1. With this groove configuration, particles72may pass form the internal side of the piston28(where the pressure30is applied) to the external fluid volume76after the piston makes two complete strokes.

The groove should be able to accommodate the largest particles72accumulated below piston28is illustrated at Position1. The size of these particles is defined by the filter through which the fluid passes before reaching the piston28. The characteristic dimensions of the grooves48,54should be larger than the filter size. For example, if the filter mesh allows the passage of particles of250microns, the characteristic dimensions of groups48,54should be larger than 250 microns. For example, the groove dimensions, e.g. height (E) and depth (D) may be 500 Microns. The approximate volume of that groove would be:

If, for example, the piston makes120stroke per minutes (2 strokes per second), the maximum corresponding rate of particles would be 6 10−9m3/s

In relation with the leak rate defined above (Q=1.6 10−18m3/s), the concentration of particles could be up to:
(6 10−9m3/s)/(1.6 10−18m3/s)˜3 109

However, this is not possible as the maximum concentration can be 1. Accordingly, this demonstrates that the groove volume is quite sufficient to allow the transfer of accumulated particles on the pressure side of the piston. Very small grooves in this example are quite sufficient to insure the particle transfer rate; however the grove should allow the largest particles to enter inside the groove.

Referring again to the examples illustrated inFIGS. 2 and 3, the passage cross grooves48and the piston cross grooves54may be oriented circumferentially along a generally flat plane. Thus, as the piston28translates along piston passage42, there is full edge contact, i.e. 360° contact, between the edges of passage cross grooves48and those of piston cross grooves54. As a result, the particulates are sometimes sliced and moved into the grooves48,54rather than being simply moved into the grooves. In some applications, the passage cross grooves48and/or the piston cross grooves54may be provided with a curved, e.g. undulating, edge so that a small number of point contacts occur during movement of piston28and during the resulting interaction between the edges of grooves48and54. In some applications, one of the passage cross grooves48or piston cross grooves54is provided with a curved edge while the other remains generally flat or in another dissimilar configuration. The curve tends to drive obstructions, e.g. particulates, laterally and this movement is helpful in deflecting the particles into the grooves rather than ingesting the particulates in the material of piston28and/or stator44. In the event of piston jamming, the effort available to drive the piston28is concentrated and the crushing/cutting affect is amplified on the particulates72trapped in the interface/clearance66.

An example of grooves having a curved configuration is illustrated in the embodiment ofFIG. 4. In this embodiment, the piston cross grooves54are curved in an undulating manner as they encircle piston28. The passage cross grooves48, on the other hand, are generally planar and provide generally straight edged grooves. However, this arrangement of undulating grooves54and straight grooves48can be reversed. Additionally, both the piston cross grooves54and the passage cross grooves48can be curved with, for example, mismatching curved configurations.

Referring generally toFIG. 5, another embodiment of actuator40and grooves48,54is illustrated. In this embodiment, at least one set of the grooves48,54is arranged in a zig-zag or sawtooth configuration. The sawtooth configuration may be arranged so that the piston cross grooves54have a sawtooth or wave form which creates successive annular chambers in the passage cross grooves48of stator44that interact directly at the peaks and valleys of the piston cross grooves54.

Because of the timing effect of the different groove pitches68,70, the grooves48,54may be positioned, as illustrated, to enable a short pulse of concentrated leakage at certain groove communication points. This concentrated leakage drives particles up and out of the interface/running clearance region66. In the example illustrated, the flow path between passage cross grooves48(labeled C and D) is normally across the edges of the groove and through a restricted flow gap.

However, when the piston cross grooves54line up such that the points labeled A and B are engaging the passage cross grooves48labeled C and D, respectively, then there is a preferential flow path directly between C and D for a brief moment of piston travel. The pressure in the passage cross groove48labeled C drives fluid and particles up to the passage cross groove48labeled D. This pattern is repeated as piston28is moved along piston passage42so as to continually encourage the removal of particulates from the interface/running clearance region66.

In some applications, sufficient protection against particulates may be provided simply by forming passage wall surface46and piston exterior surface52with hardened material56so as to provide hard contact surfaces, as illustrated in the embodiment ofFIG. 6. The piston28and stator44may be formed of the hard material56, or sleeves58,62of the hardened material may be mounted in the stator44and on the piston28, respectively. In some applications, the hardened material56may be applied as a coating, e.g. applied as a coating through a high velocity oxygen fuel (HVOF) procedure. In this example, the piston28and stator44are protected by the hard material instead of placing grooves along the piston28and/or stator44. Sometimes, the embodiment ofFIG. 6may be improved for certain applications. For example, sometimes the surfaces46and52may have rough surface where the roughness results primarily from discontinuous circumferential scratches. These circumferential scratches act as grooves previously described inFIGS. 2 and 3. If sufficiently sized, the scratches are able to accommodate particles (as would the grooves illustrated inFIGS. 2 and 3). This embodiment is fully applicable for small particles or particles which may be shaved (sheared) in small enough elements.

The embodiments described above enable maintenance of tight tolerances between the piston28and surrounding stator44. In a variety of applications, tight tolerances along interface/clearance region66may be used to prevent excessive leakage. This was explained above by formula (a). Otherwise, excessive leakage (or the creation of excessive leakage through erosion of components) can have deleterious effects by reducing performance and/or by encouraging increased component wear.

As explained above, there are situations where particles may be “sheared” at the edge of the groove48of the piston passage and the edge of the groove54of the piston28. To limit the occurrence of this shearing of particles, the groove may have chamfers (78and80) as shown inFIGS. 7 and 8. With respect to movement of piston28, the difference of pressure30pushes the particles via the chamfer When the piston28is making the displacement D1, for example, the particles72stuck at chamfer80are then subjected to a force F1. Force F1pushes those particles72into the next groove which is the groove54of the piston28(seeFIG. 7). When the piston28is making the displacement D2, the particles72stuck at chamfer78are then subjected to a force F2. Force F2pushes those particles72into the next groove which is the groove48of the stator44containing the piston passage (seeFIG. 8).

The grooves48,54also may be equipped with mobile or deformable features to insure the cleaning of the groove, as illustrated in the “ball-cleaning” embodiment ofFIGS. 9 and 10. In the “ball-cleaning” system, the piston groove54is rounded and mobile elements82, e.g. balls, are installed in that groove54. The balls82adequately match the groove54. The grooves48of the stator44are rounded and shallower than the piston grooves54. When the piston28moves and undergoes vibration, the balls82shake accumulated particles into the piston grooves54so that the accumulated particles do not stick to the piston28. This process helps the progressive movement of the particles72into the next groove48of the stator44.

When the groove54is aligned with groove48, the balls or other mobile elements82can partially enter in the groove48to shake the accumulation of particles and to facilitate their transport into the next groove. When the grooves become misaligned, the balls82are pushed back into the groove54of the piston28due to the rounded shallow pattern of the groove48. An additional cleaning effect can be obtained by employing a deformable ring84installed at the bottom of the groove54, as illustrated inFIG. 10. In this embodiment, the balls82are pushed radially by the deformable ring84against the bore of the stator44. This pushing helps the cleaning effect of the balls82in the groove48of the stator44. By way of example, the deformable ring84may be made of rubber. Other mobile and deformable items also can be installed in the groove(s)54of the piston28. In some applications, the mobile and deformable items can be installed in groove48of the stator44. In these applications, the balls or mobile elements may be solid polycrystalline diamond, silicon bonded diamond, tungsten carbide, ceramic or stellite. The balls or mobile elements82also can be made of metal coated with hard layer such by an HVFO process. In some embodiments, the balls or mobile elements82may have deformable elements84which can be installed in the groove48of the stator44. A cleaning effect analogous to that explained above is similarly provided.

As explained above, the working clearance is selected so as to have enough length (shown as71a+71binFIG. 3). The stroke of the piston28may be set forth as follows:
n×(71b+E)

n being the number of grooves in the piston28; and71band E being as shown inFIG. 3.

In this example,71bis long enough to limit the leak-rate in the working clearance. The length71bmay be a few millimeters to even 10 mm or even 15 mm Dimension E is large enough to easily accommodate the largest particles72reaching the side of piston28experiencing pressure30. Dimension E may be between 100 Microns and 250 Microns, or even between 250 and 500 microns, or even up to 2 mm.

With E=500 microns and71b=5 mm, for example, the piston stroke may be 5.5 mm to ensure proper particles transfer as explained with reference toFIG. 3. In some applications, the piston displacement may be small or variable between strokes. This is the case for an actuating piston of a rotary steerable system. The stroke may be very small in some cases. In such cases, for example, an additional sleeve86may be added, as illustrated inFIGS. 11 and 12. The sleeve86slides between the stator44and the piston28. This sleeve86has openings88which act with the grooves48and54to allow accumulation and transport of particles72via the sleeve displacement. As illustrated, the sleeve86may be pushed backwards towards the pressure30side by a spring90. On the discharge side of the leak, the sleeve86ends at a slot92defined in length by a dog94. A mechanical stop96limits the backwards displacement of the sleeve86.

Referring generally toFIGS. 13 and 14, the position of the piston28and the sleeve86is illustrated at two extreme positions (retracted and extended). As illustrated, the displacement D4of the sleeve86represents the distance of sleeve movement between the two extreme positions. Movement D4is substantially larger than the displacement D3of piston28. Due to grooves/openings88of the sleeve86, particles72can perform the progressive movement from the inner chamber, where the pressure30may be present, to the outside space76.

Another example is illustrated inFIG. 15in which the piston28may be an element of a positive displacement pump. The piston28can be pushed by a cam98attached to a rotary shaft100. During the forward movement (pushed by the cam98), a discharge valve102opens by compressing a spring104and fluid in a pump chamber106is transferred into a discharge line108. During that phase, a suction valve110is closed due to a spring112. This spring112is compressed by a spring support114. During the backwards movement, the piston28undergoes a backwards movement which may be induced by a spring116. During this phase, the discharge valve102closes due to the spring104, while the suction valve110opens to let fluid from a suction chamber118enter into the pump chamber106. During these movements (especially the forward discharge phase), the differential pressure30has a tendency to push fluid and particles into the working clearance66. The piston28may be equipped with groove54and the piston passage/stator44may be equipped with grooves48to help the evacuation of particles from the working clearance in the same way as for the case of an actuating piston.

Depending on the application, system20may have a variety of configurations comprising other and/or additional components. For example, the shape and structure of drilling system components, steering components, and/or other components of the overall system20may vary in size and configuration depending on the parameters of a given application and environment. Additionally, many types of pistons28and corresponding stators44may be used depending on the application carried out by the overall system. The actuator pistons may be used in many types of drill strings, other types of well strings, and other tools actuated by fluid carrying particulates or susceptible to particulates. The actuator pistons may be used to provide steering inputs, to open or close valves and other devices, and/or for a variety of other applications. Additionally, the actuator pistons may be used in a variety of non-well applications.