ANTI-VIBRATION SYSTEM FOR PC PUMPS

Description

FIELD

The present invention relates to a wellbore tool and, in particular, to a progressing cavity (PC) pump (PCP), a support for reducing vibration and a system.

BACKGROUND

Over the years PC pumps have gotten much larger, such as both longer and larger in diameter to pump more fluids.

PC pumps are inherently unbalanced since the rotor moves around inside the stator on a radius equal to the eccentricity of the rotor. The rate of oscillation of the rotor inside the stator is equal to twice the rotation speed of the rotor.

To pump more fluids, it is desirable that PC pump speeds up to 500 rpm are sometimes used but generally companies like to keep the speeds under 400 rpm due to experiences with shorter mean times between failure of the pump systems. Failures are often caused by vibrations of the rotor at frequencies close to one of the natural frequencies or harmonics thereof of the stator and the production tubing connected to the stator.

Traditionally, the only support point of the PCP stator has been the torque anchor below the pump, which is below the tag bar. With this support only below the pump, the stator and tubing vibrate and move around until they sometimes reach the casing walls. This can cause impacts and failures.

The present applicant has been providing torque anchors with centralizing means and tubing supports with centralizing means. The industry has been adopting this strategy since they have generally found longer mean run times between failure.

SUMMARY

In accordance with a broad aspect of the present invention, there is provided a centralizer for installation on a well structure, the centralizer comprising: a body with a central longitudinal opening, drag blocks retained on the body and a rubber spring biasing each drag block radially outwardly from the body.

In accordance with another broad aspect of the present invention, there is provided a PC pump assembly comprising: a PC pump including: a rotor, a stator in which the rotor moves, the stator including an upper end, a lower end and an outer diameter, and a centralizer on the stator, the centralizer including centralizing blocks that are normally outwardly biased and resiliently collapsible, the centralizing blocks having an outer diameter greater than the stator outer diameter.

In accordance with another broad aspect of the present invention, there is provided a method for addressing downhole vibration damage of a PC pump in a well, the method comprising: installing a wellbore centralizer on a stator of the PC pump, wherein the wellbore centralizer includes centralizing blocks that are normally outwardly biased and resiliently collapsible, the centralizing blocks defining a centralizer outer diameter greater than a stator outer diameter; moving the PC pump and the wellbore centralizer into the well including compressing the centralizer blocks radially inwardly to be constrained within an inner diameter of the well; moving the PC pump into position within the well; and operating the PC pump while the wellbore centralizer resists deflection of the stator

It is to be understood that other aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein various embodiments of the invention are shown and described by way of illustration. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all within the present invention. Furthermore, the various embodiments described may be combined, mutatis mutandis, with other embodiments described herein. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings, several aspects of the present invention are illustrated by way of example, and not by way of limitation, in detail in the figures, wherein:

FIG. 1A is a side elevation of a PC pump assembly according to the present invention;

FIG. 1B is an enlargement of block A of FIG. 1A showing a centralizer attached to a stator of the PC pump of FIG. 1A;

FIG. 2A is a perspective view of a centralizer useful in the present invention;

FIG. 2B is an end elevation view of the centralizer of FIG. 2A;

FIG. 2C is a section along line B-B of FIG. 2B;

FIG. 2D is an enlargement of block C of FIG. 2C;

FIG. 2E is a perspective view of the centralizer of FIG. 2A but with a drag block shown exploded;

FIG. 3A is an exploded, perspective view of a centralizer according to the present invention;

FIG. 3B is the centralizer of FIG. 3A assembled and installed on a stator of a PC pump;

FIG. 4 is a perspective view of another centralizer according to the present invention;

FIG. 5 is a perspective view of another centralizer according to the present invention; and

FIGS. 6 to 8 are graphs showing FEA analysis as described in Example II.

DETAILED DESCRIPTION OF EMBODIMENTS

The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments contemplated by the inventor. The detailed description includes specific details for providing a comprehensive understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.

A system has been invented wherein a PC pump stator is stabilized against the wellbore wall, for example the casing inner wall. This stabilization reduces motion of the pump stator during use and mitigates the problems noted above with respect to vibration. This system mitigates damage due to harmonic oscillation of PCP systems by reducing vibration at the pump, which is the source of the vibration, rather than chasing the vibration problems up the tubing.

In one embodiment, the system increases the first harmonic speed to above 500 rpm. By adding stabilizing supports onto the PC stator, it is possible to keep the fundamental frequency above 500 rpm, so rotor vibrations do not fall near a natural frequency.

Stabilization is achieved by installing one or more centralizers on the PC pump stator. The centralizers hold the PC stator centrally in the well and reduce or eliminate problematic vibrations. Each of the one or more centralizers is installed to encircle the stator. For example, the centralizer is an annular member that is installed on the stator in a position concentrically about the stator. The centralizer creates an enlargement on the PC pump that effectively increases the outer diameter of the PC pump at the location of the centralizer. As such, the centralizer holds the pump stator substantially centrally within the well. The centralizer may be selected to fit snugly around the stator OD and to have an effective outer diameter that is about the same or slightly larger than the inner diameter of the well in which the centralizer is to be used.

FIGS. 1A and 1B illustrate a PC pump stator assembly including a stator 13 and a plurality of centralizers 1a, 1b, 1c. FIG. 8 illustrates another PC pump stator assembly including a stator 13 and a single centralizer 1.

A PC pump stator has an upper end 13a, a lower end 13b and an outer wall that defines an outer diameter of the pump stator. A PC pump stator has a cylindrical outer wall such as formed of steel. The wall houses, or has integrally therein, an inner wall defining an elongate passage (not shown) extending from upper end 13a to lower end 13b. The inner wall may be formed of an elastomer, but is sometimes also formed of metal. The PC pump further includes a helically formed rotor 15 (shown in phantom) that is rotatable in the elongate passage of the stator. The inner wall of the stator has a helical recessed surface open to the elongate passage such that when the pump rotor rotates therein, a sequence of small, discrete cavities form between the stator and the rotor and fluids in the cavities are moved through the pump.

In a PC pump, there is a tag bar 13c at or just below the lower-most end of the stator. The upper end of the stator is configured for connection into a production string of tubulars and the lower end accommodates or, as shown, is configured for connection to a sub containing tag bar 13c. Therethrough, lower end 13b is coupled to a production apparatus extending down below the PC pump. The stator ends 13a, 13b are generally configured with threaded connections such as with threaded box or pin ends.

The one or more centralizers 1, 1a, 1b, 1c are coupled to the stator 13 and encircle its outer surface. The number of centralizing supports needed on a PC pump depends on the degree to which the vibration is to be dampened and what other centralizing structures are in the string.

In one embodiment, as shown in FIG. 8, there is at least one centralizer 1 coupled directly on the PC pump stator 13. Even just one centralizer installed on the stator may assist with vibration dampening. In some embodiments, also shown in FIG. 8, there may be centralizing structures, such as centralizers or tubing anchors, installed close to but not on the stator. Such centralizers may be installed on tubing segments close to one or both of the ends 13a, 13b of the PC pump stator 13. In such an arrangement, even one centralizer installed on the pump stator damps to some degree the problematic vibration in the pump string. This one centralizer may be positioned relatively centrally between the ends of the stator encircling the stator outer diameter. If there are two or more centralizers on the stator, they may be positioned adjacent each other or spaced apart.

In another embodiment such as shown in FIGS. 1A and 1B, there are centralizers 1a, 1b installed encircling the outer diameter and positioned adjacent the upper end 13a and adjacent the lower end 13b, respectively, of the stator. Possibly, there may be one or more further centralizers 1c spaced apart along the stator length between the end centralizers 1a, 1b. While there may be various configurations, generally up to five centralizers are used on very long stators: one on each end and three spaced apart along the stator length. The number and position of the centralizers depends mostly on the length and to a lesser extent on the diameter and/or wall thickness of the stator.

Some pumps approach 45 feet in length. The smaller the diameter of the stator, the less its stiffness. As an example, therefore, a 45 foot stator with a 3¾ inch OD may require up to five centralizers installed thereon. Most pumps can be adequately supported with one centralizer at or directly adjacent each end and one or two centralizers near a mid point between the end centralizers. The end centralizers may be positioned on the stator or on a tubing string structure adjacent the stator ends.

The stiffness of the PC pump centralizers requires consideration. In particular, a useful centralizer has to have some lateral resiliency. It should be stiff enough to reduce vibration, but not so stiff that it requires excessive force to push it into the hole. This is especially relevant to long reach horizontal wells.

Therefore, entirely rigid, inflexible centralizers are not useful since they must have clearance to the casing inside diameter and an inflexible centralizer can generate problematic wear on the PC pump. Further, impacts with the casing can be damaging to the tubing as well as other well components. It is noted that spacing due to clearance to the casing inside diameter means that the spring constant for the centralizer starts at a low value until the centralizer contacts the casing after which time the stiffness becomes much higher to provide centralizing support.

If the one or more centralizers deflect substantially, vibration in the pump may not be adequately mitigated. For example, if the one or more centralizers are not stiff enough, that lowers the fundamental frequency, and it may not be possible to keep the fundamental frequency above 500 rpm.

One useful centralizer may be the AVS™ centralizer from Evolution Oil Tools Inc. of Calgary, Alberta Canada. With reference to FIGS. 2A to 2E, the AVS™ centralizer is made up of a body 1 with three or more, such as four, recesses, each recess of which accommodates a drag block 2. The drag blocks are pushed radially outwardly by springs 4. Springs 4 are, for example, leaf springs, formed of steel or metal spring alloy. The drag blocks are retained in their recesses by pins 3. The biasing force on the drag blocks drive the drag blocks out to an effective outer diameter sized to bear against the casing wall when installed on a stator and tripped into the well. The body has an open inner diameter through which it can be installed on the stator outer diameter. While the body has threaded ends, the ends or the body 1 may have to be reconfigured to accommodate installation over a stator as discussed herein below.

While the centralizer of FIGS. 2A to 2E may be useful for installation on a stator, centralizers constructed entirely from steel or other spring alloys, as in FIGS. 2A to 2E, may have limitations in terms of delivering a very high spring stiffness and yet being flexible enough to slide easily with the PC pump into the casing while exerting a very high force against additional deflection due to vibration. In particular, springs 4 in the centralizer of FIG. 2A to 2E are constructed of spring steel or alloy, for example, and therefore may not be best suited for anti-vibration in a PC pump system.

In one embodiment, a particularly useful centralizer includes biasing members formed from a rubber elastomer instead of spring steel/metal alloys.

With reference to FIGS. 3A to 5, three example centralizers are shown. Using the centralizer of FIG. 3A as an example, these centralizers all include a body 5, drag blocks 2 and retention pins 3. Drag blocks 2 and retention pins 3 are similar to the structures of FIG. 2. However, each drag block 2 is biased outwardly from the body by a rubber spring 6. Thus, each steel spring 4 in FIG. 2 is replaced by rubber spring 6 in the centralizers of FIGS. 3A to 5.

The rubber springs 6 are constructed of a rubber elastomer. Rubber elastomers have a progressive spring stiffness. In other words, the spring stiffness of rubber starts quite low because the compressed area of the rubber is initially small, then as forces increase, the compressed area of the rubber increases and the spring stiffness increases. A metal spring on the other hand has a more or less constant spring stiffness. In one embodiment, the rubber biasing members of the centralizers are each elongate in shape and have a length similar to the length of the drag that they are configured to fit behind. They may be faceted or round in cross sectional shape and are generally solid. The biasing members are constructed of elastomers such as nitrile or saturated nitrile (i.e. nitrile rubber is also known as nitrile butadiene rubber, NBR, Buna-N, and acrylonitrile butadiene rubber) or a fluorocarbon rubber, such as one known as Viton™ available from The Chemours Company. The rubber spring may be installed in compression. The rubber may be initially compressed almost negligibly and is held in place: compressed between the drag block and the body by the drag block retaining pins. Once the centralizer is installed into the casing, the centralizer drag blocks are constrained to the inner diameter of the well. This inward radial movement of the drag blocks causes the rubber to compress further. The amount of compression relates to the amount of compressive force. This compression may be slight, but the amount of compression can be adjusted by adding a metal shim against the rubber spring. If employed, a shim can be installed between the stabilizer body and the spring. A centralizer constructed with a centralizing member biased with rubber in compression has a desirable progressive nature of the spring characteristic.

The body 5 has an open inner diameter through which it can be installed on the stator outer diameter. There are various configurations through which the body can be installed on a PC pump stator. While prior centralizers sometimes were configured to be threaded into a tubing string, an installation on a PC pump stator requires a new approach, as the stator may typically be an elongate tubular without threaded connections except on its ends 13a, 13b. Therefore, the body of the centralizer may be configured for installation without threading into a tubing connection.

In one embodiment, such as that shown in FIGS. 3A and 3B, body 5 is configured as a split body. The body has two body pieces, each approximately semi-annular. The body pieces are clamped together by bolts 7 to form an annular body around the open inner diameter. The body pieces 5a, 5b are clamped encircling and against the PC pump stator 13 using bolts 7 (FIG. 3B).

With reference to FIG. 4, a second centralizer suitable for installation on a PC pump stator is shown. This centralizer includes four body segments 8 connected by links 9. The links are connected to the body segments bolts 10. This centralizer can use existing drag blocks 2 and retention pins 3. When mounted on the PC pump stator set screws 11 are threaded through the holes on each body segment. Set screws 11 are driven against the stator to prevent the assembly from sliding up or down the stator.

FIG. 5 shows another centralizer, which is installed by slipping onto the stator. The body 12 has an open inner diameter with a size large enough to fit over any end enlargements of the stator. As such, the body can be slipped onto the stator of the pump. Body 12 is locked into place with set screws 11 to prevent sliding up or down the stator. It uses the same drag blocks 2 and retention pins 3 as the centralizers of FIGS. 2 to 4. The springs that bias the drag blocks out may be metal but in one embodiment are constructed of rubber, as shown in FIG. 3.

The centralizers of FIG. 3A to 5 with rubber springs for biasing the drag blocks, while described above with reference to installation on a pump stator, are also useful for mounting elsewhere on other well structures such as elsewhere along a tubing string. The centralizers can each be constructed with an ID sized to mount anywhere along a conventional tubing joint.

A PC pump assembly including a PC pump with one or more centralizers coupled to its stator may be employed in a method for addressing downhole vibration damage of a PC pump in a well. The method includes: installing a wellbore centralizer on a stator of the PC pump, wherein the wellbore centralizer includes centralizing blocks that are normally outwardly biased and resiliently collapsible, the centralizing blocks defining a centralizer outer diameter greater than a stator outer diameter; moving the PC pump and the wellbore centralizer into the well including compressing the centralizer blocks radially inwardly to be constrained within an inner diameter of the well; moving the PC pump into position within the well; and operating the PC pump while the wellbore centralizer resists deflection of the stator. The pump assembly may include further centralizers on the stator or above or below the stator. The centralizer on the stator and the further centralizers may be any centralizer including any of the centralizers in FIGS. 2A to 5. Any centralizers on the stator may have rubber springs, selected for enhanced spring properties over centralizers with metal springs. A centralizer below the stator may be a torque anchor.

EXAMPLES

Example I

Under identical test conditions, a centralizer including three steel leaf springs provided a spring constant K value of 2,400 pounds per inch and a centralizer with a compressed rubber spring provided a K value of 16,500 pounds per inch.

Example II

Finite element analysis (FEA) was used to study deflections of a PC pump stator. The analysis was conducted using a simulated pump stator of 348 inches long and a 3¾ inch OD. The analysis employed various centralizer configurations and multiple numbers of tubing joints attached to the stator. The FEA was used to calculate deflections, stresses and fundamental natural frequency of the PCP stator, its supports and the production tubing.

With reference to FIG. 6, FEA analysis for a prior art system is shown graphically. In the prior art system, a tool that has a centralizing effect is installed below the PC pump: on a tubing string segment below the lower end of the PC pump. In a PC pump, the tag bar is just below the lower-most end of the stator. The centralizer is a CATA™ Torque Anchor from Evolution Oil Tools, Inc.

The distance between the top of the stator and the middle of the torque anchor is 366 inches. The length of the stator is 348 inches.

The deflections are shown based on the pump stator lying horizontal supported on the torque anchor. The amount of deflection can be directly converted to the base natural frequency of the stator on its supports. It will be appreciated that the deflection of three inches, which is present at the upper end, can create significant damage to the pump, the tubing string and the casing. This vibration can create waves of destructive vibration that is transferred up through the tubing string toward surface.

Calculation of fundamental rotor speed based on the calculated deflection is 122 rpm. The second harmonic is 2×122=244 rpm. Therefore, the fundamental and second harmonic would likely present vibration problems. FIG. 6 represents a typical PCP installation with a CATA mounted below the pump. Note that the pump deflects substantially even under its own weight indicating a very low fundamental frequency

With reference to FIG. 7, another analysis is shown where a centralizer is installed both close above and close below the PC pump. One centralizer is installed on a tubing string segment below the lower end of the PC pump and a second centralizer is installed on a tubing string segment connected above the upper end of the PC pump. The lower centralizer is a CATA Torque Anchor from Evolution Oil Tools, Inc. and the upper centralizer is an AVS centralizer according to FIG. 2A.

Regarding FIG. 7, static displacement is shown in inches and that is used to calculate fundamental or base natural frequency. The zero axis is displaced as marked by the zero on the chart. The deflection at the centralizer is very small at both ends and is only due to the flexibility of the centralizer springs. This illustrates that some problematic vibration, with deflections of 1-1.5 inches, continues to be generated mid-way between the two centralizers. By way of explanation, this curve is a calculation based on static weight of the stator resting on the centralizing supports. At a certain speed, the rotating eccentric weight of the rotor would induce this same characteristic deflection of the stator.

Calculation of fundamental rotor speed based on the calculated deflection is 194 rpm. The second harmonic is 2×194=388 rpm. Therefore, the fundamental rotor speed and the first harmonic would likely present vibration problems

With reference to FIG. 8, another analysis is shown where a centralizer is installed both close above and close below the PC pump and another centralizer 1 is installed centrally directly on the stator. The lower centralizer is a CATA Torque Anchor from Evolution Oil Tools, Inc. and the upper centralizer is an AVS centralizer according to FIG. 2A. The middle centralizer 1 is according to FIG. 3A.

The two end centralizers are installed on a first tubing string tubulars connected at the ends of the PC pump, as in FIG. 7. The middle centralizer is installed approximately mid-way between the ends of the stator. The simulation selected the middle centralizer as having a rubber spring. The middle centralizer is spaced 185 inches from each of the upper centralizer and the lower centralizer.

Static displacement is shown in inches. This illustrates that maximum deflection was reduced to less than 0.03 inches.

Calculation of fundamental rotor speed based on the calculated deflection is 1240 rpm. This is well above the maximum running speed of 500 rpm so vibrations up the tubing are largely eliminated by adding one centralizer support at the center of the stator.

This demonstrates the inherent flexibility of long stators and illustrates that stator supports are an effective method of solving vibration problems associated with PC pumps.

Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention and is not intended to limit the scope of the invention as defined in the claims that follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.