Note: Descriptions are shown in the official language in which they were submitted.
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Centralizer for Sucker Rod Strings
Field of the Invention
The invention relates to a centralizer apparatus for sucker rod
strings. In particular, the invention relates to a centralizer for
rotating rod strings driving a downhole rotary pump.
Background of the Invention
Wells for the production of fluid hydrocarbons, such as oil, in
the vast majority of cases require the use of a downhole pump. The
use of a downhole pump is essential in situations where the fluid
is of high viscosity, or where there is relatively low well
pressure. Once a well has been drilled, a production tubing string
is directed down the well as a conduit for the removal of the heavy
oil from the reservoir. This tubing string consists of successive
lengths of metal tubing which are lowered into the borehole and
threaded together to form a continuous tubing.
Down hole pumps are generally either mounted in or attached to
the bottom end of the production tubing. The pumps are operated by
a sucker rod string extending through the tubing. The sucker rod
string, like the tubing string, is made up of successive lengths,
called sucker rods, which can be connected end to end. Each sucker
rod is typically a solid cylindrical shaft of about 25 - 30 feet in
length, and has a diameter of about 0.75 inches. Unlike the tubing
string sections, the sucker rods have dual threaded pin ends. In
order to interconnect two sucker rods end to end, the industry
standard is to use a sucker rod coupling. This coupling is a
cylindrical internally threaded member, with an outside diameter
greater than the outside diameter of the sucker rods. A typical
sucker rod coupling has an outside diameter of 1.875 inches.
The role of the sucker rod string is to actuate the downhole
pump. Two principal approaches to actuation of the downhole pump
using a sucker rod string are known: a reciprocating motion of the
rod string or a rotating motion. In both types of operation, but
particularly in the latter, solid material, e.g. sand, can be
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removed from the well suspended in the fluid being pumped. The
particulate material leads to wear on the surfaces to which it is
exposed, i.e. the outside of the sucker rod string, and the
interior of the tubing string. This wear problem is compounded in
deviated wells with well bores of angular, curved or deflected
orientation.
Current development of heavy oil fields or reservoirs commonly
involves several wells placed close together from a single "pad".
Since several wells are typically drilled from this central "pad",
a number of the wells must be bored in and produced from different
directions. Those wells with angled well bores are called angular
or deviated wells. Especially in angular wells, the sucker rod
string often assumes a curvilinear shape inside the tubing string,
and as a result of the tension in the sucker rods, tends to be
pulled into the inside curve of the tubing string. As a result of
this bowing, the movement of the sucker rod string to actuate the
downhole pump leads to rubbing or slapping of the sucker rod string
against the tubing string. This leads to excessive wear of both
components. Hard particulates, such as sand, in the pumped fluid
can then get trapped between the sucker rod string and the tubing
string leading to abrasion of these parts. As the sucker rod
coupling has a larger outside diameter than the sucker rods, the
wear is concentrated at those points in the tubing string
corresponding to the position of the sucker rod coupling in the
sucker rod string. The wear on the tubing string can lead to
erosion of the wall of the tubing string with a consequential
detrimental impact on the production from the well, and the need
for costly repairs.
This wear problem is well known in the industry and commonly a
centralizes device is used to keep the sucker rod string coupling
spaced apart from the wall of the tubing string. U.S. Patent No.
4,913,230 by Rivas et al. discloses a centralizes for a
reciprocating sucker rod, which centralizes includes internally
threaded box ends for connection with the externally threaded pin
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ends of the sucker rods. U.S. Patent No. 5,247,990 by Sudol et al.
describes centralizer sleeves for mounting directly onto a sucker
rod either by sliding thereon or by installing it on the rod in the
field, whereby the centralizer is manufactured in two or more
pieces for field assembly. These centralizers are for use with
reciprocating sucker rods and are mounted on the sucker rods
themselves rather than a sucker rod coupling. The disadvantage of
such centralizers is that any wear is directly on the sucker rod
body component, leading to premature weakness of the sucker rod
string.
One way in which the disadvantage of wear on the sucker rod has
been overcome is to provide a sucker rod centralizer which is
placed at the connections of the sucker rods. U.S. Patent No.
4,905,760 by Kenneth Gray, shows a common sucker rod coupling which
is machined to have a decreased external diameter and is provided
with an external, fusion bonded, resinous coating to impart
abrasion resistance and extend the service life of the coupling.
U.S. Patent No. 4,757,861 by Albert A. Klyne discloses a sucker rod
coupling centralizer assembly. The Klyne patent discloses a
centralizer shaft with double pin ends and a tubular centralizer
sleeve which can slide over the centralizer shaft. The centralizer
shaft is connected to the sucker rod string using a sucker rod
coupling at each end of the centralizer shaft, such that the sucker
rod coupling outer diameter is greater than the outer diameter of
the centralizer shaft and the inner diameter of the centralizer
sleeve. This increases cost, since two rod couplings are needed for
each sucker rod connection rather than one as in conventional
assemblies. Klyne also teaches an improvement over the basic design
wherein the end surfaces of the rod couplings engaging the
centralizer sleeve and the centralizer body are coated with an
abrasion resistant material to improve the service life of the
centralizer. U.S. Patent No. 4,919,202 of Carl Clintberg describes
a similar centralizer arrangement wherein the only difference is
the presence of torque washers between the centralizer sleeve and
the sucker rod couplings, which washers act as sacrificial wear
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surfaces. Bearing shoulders for the torque transfer washers are
created by practically recessing the centralizer sleeve into the
centralized shaft. U.S. Patent No. 5,261,498 of Steinkamp et al.
discloses a drill string centralizer which is basically of the same
principle construction as the sucker rod centralizers of Klyne and
Clintberg. All of these centralizers have the disadvantage that the
centralizer shaft is of reduced cross-section at the connection
with the sucker rod pin ends, creating a weak point.
It is now an object of the current invention to provide a
centralizer assembly which overcomes the problems encountered with
the prior art centralizers, combines the centralizer and rod
coupling functions into one centralizer and which allows the
centralizer shaft to be of smaller diameter thereby facilitating
centralizer sleeves of larger shell thickness which can be field
mounted.
Summary of the Invention
Accordingly, the present invention provides a sucker rod
centralizer for coupling and centralizing sucker rods wherein the
coupling has a pair of terminal box ends internally threaded for
attachment to a threaded pin end of a sucker rod string component,
a shaft extending between the box ends and having a smaller outside
diameter than the outer diameter of the terminal box ends, and a
centralizer sleeve for fittingly and slidingly engaging an outer
surface of the shaft, the outer diameter of the sleeve being larger
than the outer diameter of the box ends.
In a preferred embodiment, the centralizer coupling has a
single dumbell-shaped body over which the centralizer sleeve can be
placed in the field.
Brief Description of the Drawings
Fig. 1 is a side view of a preferred embodiment in accordance
with the invention.
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Fig. 2 is a semi-isometric view of the embodiment shown in
Figure 1.
Fig. 3 is an exploded view of the embodiment shown in Figure 1.
Fig. 4 is an end view of the embodiment shown in Figure 1.
Fig. 5 is a semi isometric view of a second preferred
embodiment including a slitted centralizer sleeve.
Detailed description of the Preferred Embodiment
The preferred embodiment of a centralizer coupling according to
the invention is shown in Figure 1. In this embodiment the
centralizer 10 is of a dumbell shape including a pair of
cylindrical box ends 12, a shaft 14 extending therebetween and a
centralizer sleeve 11 mounted in its operational position on the
shaft. Each box end 12 of the centralizer 10 has a greater outside
diameter than both the shaft 14 and the sucker rod to which it will
be attached as further described below. A centralizer sleeve stop
shoulder 15 is created at each juncture of the shaft 14 with the
box ends 12 due to the smaller outer diameter of the shaft 14
relative to the ends.
Figure 2 more clearly illustrates the structure of the integral
box ends 12. Each coupling end 12 is provided with an axial
internally threaded bore 16 for engagement with the threaded pin
end of a sucker rod (not shown). The dimensions of the thread of
the internally threaded bore 16 are matched to those of the sucker
rod pin ends. To increase the strength of the centralizer, the box
ends 12 are longer than the internally threaded bore 16 which
extends only partially into each box end 12. Furthermore, the
length of the bore 16 is greater than the maximum insertion depth
of the sucker rod pin ends. The unitary design of the box ends 12
at the shaft 14 provides a centralizer shaft of greater strength
and a centralizer which can be manufactured at a lower cost than
conventional double pin shaft couplings.
In a preferred embodiment, the shaft 14 is approximately the
same length as the box ends 12. The diameter and length of the box
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ends 12 are the same as that of an API standard "regular" sucker
rod coupling for the size of sucker rods to which they will be
attached. The box ends 12 are bored and tapped to ~ the length.
The outside diameter of the shaft 14 is less than the outside
diameter of the box ends 12. It can be even less than the diameter
of the internally threaded bore 16 in which case the solid,
unthreaded portion of the box ends 12 is extended in axial
direction to provide added strength at the juncture of the shaft
with the box ends. It is over this shaft 14, and between the end
shoulders 15 of the box ends that the centralizer sleeve is
positioned. The reduced diameter of the shaft 14 allows the use of
a centralizer sleeve 11 of greater shell thickness which in turn
translates into greater centralizer sleeve strength.
The exploded view of Figure 3 illustrates how the centralizer
sleeve 11 fits over the box ends 12 in the preferred embodiment of
the invention. In the embodiment illustrated, the centralizer
sleeve 11 is split along an axial plane into a pair of sleeve
portions 17 and 18, which in this embodiment are made of a hard
wearing material such as Kevlar°. The sleeve portions are provided
with interlocking structures for connecting them. Preferably, the
first sleeve portion 17 has a locking tongue 19 running the length
of each of the lateral edges of the first portion 17. This locking
tongue 19 is complementary in shape to a longitudinal locking
groove 20 which runs the length of each of the lateral edges of the
second portion 18. In operation, the first and second centralizer
sleeve portions 17, 18 are interlocked by the complementary tongues
19 and grooves 20 to form a cylindrical centralizer sleeve 11 that
fits over the shaft 14. The inner diameter of the centralizer
sleeve 11 is less than the outer diameter of the coupling ends 12
to prevent the centralizer sleeve sliding off the shaft 14 over one
of the coupling ends. The sleeve 11 preferably slidably closely
fits over the shaft 14 to prevent wobble of the sleeve relative to
the shaft during rotation of the sucker rod string. The outer
diameter of the centralizer sleeve 11 is larger than the outer
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diameter of the box ends 12 to prevent contact thereof with the
production tubing which would result in wear of the box ends and
the tubing.
In the preferred embodiment, the centralizer sleeve 11 consists
of a tubular body 21 which in the assembled condition continuously
surrounds the shaft 14. The wall thickness of the tubular body 21
is a principle determinant of the strength of the sleeve 11.
Integral to the centralizer sleeve 11, and on the outer surface
thereof, are a series of spaced apart spacer fins 22 which maintain
the necessary spacing between the centralizer sleeve and the
production tubing to permit passage of the well fluid being pumped.
The height of the spacer fins 22 is substantially equal to the
distance between the surface of the body 21 and the inner diameter
of the production tubing. Thus, the height of the spacer fins 22 is
determined by the diameter of the tubing, the diameter of the
sleeve, and the desired volume of the annular space between the
sleeve body and the tubing.
The spacer fins 22 in the illustrated embodiment are straight
and extend parallel to the axis of the sleeve 11. However, for
applications of the centralizer with reciprocating strings, they
may also be positioned at an angle to the axis or may be curved.
The centralizer sleeve remains stationary and the sucker rod string
rotates inside of it. Therefore, the centralizer assembly operates
as a bearing. Curved spacer fins provide slightly better stand-off,
but the advantage is slight and the disadvantage is a higher
pressure drop.
The extension of the spacer fins 22 above the outer diameter of
the box ends 12 is more clearly illustrated in Figure 4. The spacer
fins 22 are preferably of trapezoid cross-section to reduce wear on
the production tube but can be of rectangular, square or rounded
cross-section.
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In another preferred embodiment, the centralizer sleeve 11
consists of a single part. As illustrated in Figure 5, the single-
piece centralizer sleeve 11 includes a slit 24 which runs the
length of the centralizer sleeve 11 and allows the sleeve 11 to be
forced open for sliding over the shaft 14 or one of the box ends 12
for installation onto the shaft. The slit is preferably placed at
an angle to the axis of the sleeve to provide the sleeve with
greater flexibility, but can be positioned parallel thereto or of
helical shape. The sleeve 11 is preferably made of a flexible
plastic material, such as Nylon°. Materials which are flexible and
have good abrasion resistance may also be advantageously used, for
example Teflon~. In both of the above embodiments the centralizer
sleeve 11 can be replaced while the centralizer coupling 10 is
incorporated in a sucker rod string (not illustrated).
In order to reduce wear of the stop shoulders 15, the shaft 14
and the centralizer sleeve 11, the wear surfaces of the stop
shoulders 15 and the shaft 14 are chrome plated. The preferred
materials used for the centralizer sleeves are usually resistant to
abrasion and therefore it is generally the shaft 14 which is
subjected to the most wear if not protected by a hard surface. Thus
the shaft 14 is preferably provided with a hardened or hard coated
surface. A hard facing of the shaft 14 is achieved, for example, by
chrome plating or by plasma spraying with harder materials, for
example, tungsten carbide or ceramic materials.
Although in a preferred embodiment of the invention as
described above, a two-piece centralizer sleeve in which one piece
has a locking tongue, and the other has a complementary locking
groove is described, other complementary locking means can be used.
Further, centralizer sleeves can be composed of more than two
pieces and need not be cylindrical. For example, a centralizer in
the shape of a helically wound band (not shown) can also be used.
In addition, while the preferred embodiments above describe the use
of a single centralizer sleeve on. each coupling, it will be readily
understood that two or more centralizer sleeves can be used.
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While only specific embodiments of the invention have been
described, it is apparent that additions and modifications can be
made thereto, and various alternatives can be selected. It is,
therefore, the intention in the appended claims to cover all such
additions, modifications and alternatives as may fall within the
true scope of the invention.
