Note: Descriptions are shown in the official language in which they were submitted.
CA 02433627 2003-06-26
ROTATING TAG BAR ASSEMBLY
FOR PROGRESSIVE CAVITY PUMP
FIELD OF THE INVENTION
The present invention relates to tag bar apparatus for progressive cavity
pumps for oil wells,
and relates in particular to rotating tag bar assemblies.
BACKGROUND OF THE INVENTION
Installation of an oil well typically involves drilling a wellbore down to an
oil-bearing
subsurface formation, lining the wellbore with a cylindrical casing, and then
installing a string of
production tubing inside the casing. The casing is perforated in the region
where it penetrates the
oil-bearing formation, allowing crude oil from the formation to enter the
casing through the
perforations. A pump of suitable type is then installed to lift the crude oil
to the surface through the
production tubing.
Progressive cavity pumps are commonly used to pump heavy oil (i.e., highly
viscous crude
oil) from oil wells. Figure 1 illustrates a typical prior art progressive
cavity pump. The main
components of a progressive cavity pump (also known as a "PC pump" or,
alternatively, a "screw
pump") are a cylindrical stator section having an inner surface contoured in a
dual helix
configuration, and an elongate rotor formed in the shape of a single helix.
When the rotor is inserted
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into the stator, a series of sealed cavities are formed between the rotor and
the stator. The stator is
connected to the bottom of the production tubing string, and is typically of
the same diameter as the
tubing. The bottom of the stator is open, or is provided with openings, to
allow oil to enter the pump
and the sealed cavities. The rotor is suspended from a rod string which is
rotated by a surface drive
unit. Due to the helical configuration of the rotor and stator, rotation of
the rotor effectively causes
the sealed cavities to progress upward around the rotor. As a result, oil in
the sealed cavities travels
upward through the pump into the tubing and up to the surface for recovery and
processing.
PC pumps for oil wells are typically 5 to 10 meters in length, with the rotor
usually being
slightly longer than the stator. It is important for the rotor to be properly
positioned inside the stator,
so means are required for accurately positioning the rotor in a PC pump
installed in a well that is
typically hundreds or even thousands of meters deep. This is commonly
accomplished by providing
a "tag bar" in the pump assembly at a selected location below the stator. The
tag bar is typically a
steel pin extending transversely across a tubular section near the bottom of
the pump assembly, and
a selected distance below where the bottom of the rotor is intended to be when
the pump is in
operation. The tag bar does not interfere with pump operation because oil is
free to flow upward into
the pump on either side of the tag bar.
With the tag bar in place, the stator is connected to the bottom of the tubing
string, which is
then lowered into the well to a desired location in an oil-bearing formation.
The rotor, suspended
from the rod string, is then lowered inside the casing such that it passes
partially through the stator
until it hits (or "tags") the tag bar, which prevents further downward
movement of the rotor. It is
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then a simple matter for an operator at the surface to accurate raise the
rotor a specific distance
(commonly around half a meter) to its desired operating position inside the
stator.
This procedure also provides a buffer space between the tag bar and the bottom
of the rotor
S to allow for stretching of the rod string during operation of the PC pump.
As pumped oil rises inside
the production tubing, a considerable portion of its weight is exerted on and
must be borne both by
the rotor and by the rod string supporting the rotor. The resultant axial
tension forces in the rod
string cause elastic elongation of the rods, thus causing the rotor to drop to
a position somewhat
lower than when installed. The location of the tag bar is determined with this
in mind, so that there
should always be free space between the tag bar and the bottom of the rotor,
thus preventing
undesirable metal-on-metal rotational contact between the rotor and the tag
bar.
It should be noted that the term "tag bar" is also commonly used to refer to
the assembly in
which the tag bar (as described above) is housed, with the intended meaning
usually be clear from
the context of the reference. For purposes of this patent document, and unless
the context clearly
requires otherwise, the term "tag bar" refers to the specific element that is
provided for locating the
bottom of the rotor as described in the preceding paragraphs, and the term
"tag bar assembly" refers
to a larger component or assembly that includes or houses a tag bar.
PC pumps of the type generally described work satisfactorily in many
situations, including
wells into heavy-oil-bearing formations having a high sand content. The sand
particles will
commonly be mixed in with the heavy oil, and PC pumps have proved to be very
effective in
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pumping such oil-and-sand mixtures. However, problems can arise when the oil-
and-sand mixture
contains clumps of sand or perhaps clay lumps, of such size or prevalence that
the oil-and-sand
mixture cannot flow into the pump. If that happens, the pump will be clogged,
and oil production
will cease. It will typically then be necessary to withdraw the pump so that
the bottom of the tubing
can be cleaned out, after which a new pump can be installed.
What is needed is a method and apparatus that would break up clumped masses of
sand in
a heavy oil well so as to maintain flow into and through the PC pump, without
needing to withdraw
the pump and clean out the tubing. The present invention is directed to this
need.
BRIEF DESCRIPTION OF THE INVENTION
In general terms, the present invention is a rotating tag bar assembly having
a cylindrical
sleeve with one or more sand-disturbance elements protruding outwardly from
the outer surface of
the cylindrical sleeve, plus one or more openings through the wall of the
cylindrical sleeve for entry
of oil into the interior of the assembly. The diameter of the cylindrical
sleeve is approximately the
same as or slightly smaller than the outer diameter of the stator of the PC
pump to which the
assembly is to be mounted. A tag bar is provided in the lower section of the
assembly, extending
transversely across the cylindrical sleeve. The assembly is mounted, coaxially
and rotatably, to the
bottom of the PC pump stator, using a ring bearing adapted to carrying axial
tension loads from the
weight of the rotating tag bar assembly, while providing passage for crude oil
into the stator from
the tag bar assembly.
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The sand-disturbance elements may be flexible or rigid, and their particular
shape is not
critical to the invention. To give only two examples, the sand-disturbance
elements could take the
form of helical vanes, or they could be axially-oriented vanes or paddles.
Alternatively, the sand-
disturbance elements could be radially-extending lugs or pins, or flexible
elongate blades or wires.
The oil-entry openings are located in spaces between the sand-disturbance
elements. In
preferred embodiments, the oil-entry openings will be elongate slots oriented
at an angle relative to
the axis of the assembly, as schematically illustrated in Figure 2. This
angled slot configuration
provides for optimal flow into the tag bar assembly as it rotates clockwise
(looking downward). In
other embodiments, the oil-entry openings may be longitudinally-oriented
slots, or they could be
circular openings. However, the specific shape and orientation of the oil-enhy
openings are not
critical to the invention.
Near the top of the cylindrical sleeve, means are provided for engaging the
rotor such that
rotation of the rotor will cause rotation of the tag bar assembly, while still
allowing for relative
vertical movement of the rotor (i.e., for setting the rotor position and
allowing for rod stretch), and
also allowing for flow of oil from the interior of the cylindrical sleeve into
the stator of the PC pump.
In one embodiment, this is accomplished by providing the rotor with a
flattened lower section of
uniform blade-like cross-section, and providing the cylindrical sleeve with
arotor-engaging member
having an opening adapted to receive the flattened section of the rotor. The
opening in the rotor-
engaging member is of the same general shape as the flattened section of the
rotor but is slightly
larger, such that the flattened section can slide freely through the opening,
but will cause the
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cylindrical sleeve to rotate when the rotor is rotated. The rotor-engaging
member is transversely
mounted inside the cylindrical sleeve at a point near the upper end thereof,
and is sufficiently narrow
in width that there is still at least one passageway for oil to flow upward
withing the sleeve and past
the rotor-engaging member.
Alternatively, the lower portion of the rotor could be fabricated with a
square or hexagonal
cross-section, and the opening in the rotor-engaging member would be of a
complementary shape.
Other shapes for the lower portion of the rotor are possible without departing
from the essential
principles of the invention.
In the preferred embodiment, the invention will also include guide means, for
guiding the
modified lower section of the rotor into the corresponding opening in the
rotor-engaging member,
thus preventing the rotor from hanging up on the rotor-engaging member during
insertion.
The operation of the present invention can now be readily understood. The PC
pump is
connected to the bottom of the tubing string, with the rotating tag bar
assembly attached to the
bottom of the stator. The rotor is then inserted through the stator and
through the rotating tag bar
assembly, with the modified lower section of the rotor sliding through the
opening in the rotor-
engaging member until the bottom of the rotor hits the tag bar. The rotor is
raised to its preferred
height, in accordance with usual procedures, but with at least a portion of
the modified lower section
of the rotor extending into the opening in the rotor-engaging member. When the
rotor is rotated, the
rotating tag bar assembly, along with the sand-disturbance elements, will
rotate at the same rate. The
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rotating sand-disturbance elements engage and break up the clumps into smaller
particles and mix
them in with the crude oil, such that the mixture can flow freely through the
oil-entry openings in
the cylindrical sleeve.
In an alternative embodiment, the tag bar assembly of the present invention
does not have
a tag bar in its lower section as previously described. Instead, the purpose
and function of the tag
bar are served by the rotor-engaging member. In this embodiment, the rotor is
formed or provided
with a shoulder at a selected location point at or near the top of the
specially-shaped lower portion
of the rotor that passes through the opening in the rotor-engaging member.
When the rotor is
inserted through the opening in the rotor-engaging member, the shoulder will
"tag" the rotor-
engaging member, thus locating the rotor within the pump. In this way, the
rotor-engaging member
serves as the tag bar, so is it not necessary to provide a separate tag bar
that is tagged by the bottom
of the rotor.
In a further alternative embodiment of the invention, the rotating cylindrical
sleeve is
enclosed within a cylindrical shroud that is non-rotatingly and coaxially
mounted to the lower end
of the PC pump stator. The inner diameter of the shroud is necessarily larger
than the outer diameter
of the sleeve, and is selected to provide an annular space of desired width
between the shroud and
the sleeve. Oil-entry openings are provided in the cylindrical wall of the
shroud. These openings
may be, but do not necessarily have to be, similar in shape and orientation as
the oil-entry openings
of the rotating cylindrical sleeve. In this shrouded embodiment, the sleeve
and sand-disturbance
elements rotate with the rotor as in non-shrouded embodiments. The rotating
sand-disturbance
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elements will operate to break up sand clumps or oil/sand clumps passing
through the oil-entry
openings in the shroud wall.
One particular advantage ofthe shrouded embodiment is that it prevents contact
between the
sand-disturbance elements and the inner surfaces of the well casing, which can
cause undesirable
wear to the sand-disturbance elements and the casing. In unshrouded
embodiments, such contact
may be unavoidable due to the tendency of a PC pump to wobble or "chatter" due
to operational
torque, given that the radial space between the PC pump and the casing is
commonly as little as 25
millimetres.
The bottom of the shroud may be lower than the bottom of the rotating sleeve,
such that the
sleeve rotates freely within the shroud. In preferred embodiments, however, a
second ring bearing
is provided near the bottom of the shroud for rotatably receiving the lower
end of the rotating sleeve.
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