Note: Descriptions are shown in the official language in which they were submitted.
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U.S. PROVISIONAL PATENT
Docket No. 12386-001 ETY
SINGLE STRING RECIPROCATING PUMP SYSTEM
FIELD OF THE INVENTION
The present invention relates to a single string pumping system to produce
underground
fluids and crude oil in particular. In this invention, a single hollow rod
tube is provided in place
of a conventional solid sucker rod and tubing structure where the oil is
produced in the annular
space between the rod and the tubing.
BACKGROUND OF THE INVENTION
Sucker rod pumping is widely used in the production of underground fluids such
as crude oil and water. In a conventional sucker rod pumping system, a
downhole barrel/plunger
pump is driven by a surface walking beam pump jack. An oil well is usually
lined with a well
casing and also includes a tubing string and a solid sucker rod string which
reciprocates the pump
plunger vertically within a pump barrel. The pump barrel is secured to the end
of the tubing. The
crude oil is pumped to the well head through the annular space between the rod
and tubing. In
this system, the tubing cost can be a significant portion of the total capital
cost, especially in deep
well installations.
Conventional pumping systems of this type suffer from low efficiency. Pump
efficiency
averages less than 50% due to elasticity in the tubing length which occurs
when switching
between the up and down strokes. Frequent pump servicing is usually required
to remove
paraffin wax deposition from the tubing or sand which has settled on top of
the barrel and
plunger. When the rod pump is used in a heavy oil well, rod falling
difficulties during the down
stroke are usually encountered due to the high density of the crude oil and
high viscous friction
W:\ETYFILES\Wang,J\Jame-Ted ju121.doc
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between the sucker rod and the crude oil. Furthermore, a large amount of
energy is utilized in
the pumping action in just lifting the rod string due to the weight of the rod
string and due to
friction between the sucker rod and the crude oil.
Another significant disadvantage of conventional tubing/sucker rod
configurations is
encountered when servicing the pump. In order to pull the pump out of the
hole, two separate
trips are required to pull the tubing string and the rod string out of the
hole.
It is known to use hollow sucker rods in place of conventional solid rods,
however, the
hollow bore of the rod is typically used to deliver liquid downhole from the
surface. In U. S.
Patent No. 4,089,626, hollow rods are used to inject fluid into the bottom of
the pump. No
suggestion is made that fluid may be produced through the hollow rod.
Therefore, the tubing
string has not been eliminated in this patent. In U.S. Patent No. 4,984,003, a
crude sample
recovery method is provided which uses the hollow rod center for injecting
chemicals such as
surfactants which may help the flow of viscous fluids. However, a conventional
rod/tubing
structure is also used in this patent.
In Chinese Patent No. 95-104622.5, a hollow rod oil production system is
disclosed. A single rod string substitutes for the rod/tubing concentric
structure. In one example,
the well head uses flexible hose as the oil outlet port. This arrangement
automatically increases
safety and environmental concerns, due to the possible high pressure, repeated
bending, and
severe ambient conditions which may cause the flexible hose to deteriorate and
fail. In the other
example, a rigid well head is disclosed, however, the casing annular opening
is blocked,
restricting any access from the outside. This is not acceptable because the
casing vent is
necessary for many essential well service operations, such as reversed well
wash circulation,
releasing casing gas, injection of lifting gas and dynamic fluid level
measuring. As well, this
invention requires two separate trips to set the dual slip packer and
pump/piston in position, as is
the case with other conventional pump systems.
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It is also known to use electric heating to prevent solid formation in crude
oil producing
wells and to reduce viscosity of the crude oil. An electrically heated sucker
rod was disclosed in
U.S. Patent No. 3,859,503, which has electric resistance heating elements
installed in each rod
joint. The heating element is electrically insulated in the bore hole. This
type of arrangement of
heating element in series will restrict the practical running depth (and
effective working length)
due to the voltage gradient drop. On the other hand, if an elevated voltage is
used to try to solve
the voltage drop, the electric insulation becomes difficult. The hydraulic
sealing and electric
insulation of the rod system escalates the cost of manufacture.
In U.S. Patent No. 4,716,960, various means for introducing electric current
into the well
were disclosed. The tubing can be heated by passing electric current to it
using different means.
The system is still uses the concentric rod/tubing structure when a rod pump
is used (annular
space is used for the flow avenue), therefore all drawbacks of the rod /
tubing structure remain in
the system. Furthermore, in the rod pump case, electric insulated conduction
is accomplished
through a fiberglass rod, then by silver soldering welded to the metal rod and
finally through a
wheeled connection to the tubing, a complicated and high-cost system. Sand
deposition will be
even more severe in such a system because the density and viscosity of the oil
in the annular
space is reduced, even though the velocity and cross sectioned area remain
unchanged.
In Chinese Patent 9216143.7, an electrically heated rod is disclosed in which
the electric
cable is placed in the center of the hollow rod. The hollow center of the rod
is sealed from the
underground fluid. The concentric rod/tubing structure is still the basic
structure in this patent.
The above arrangement, the same as in most of the plunger/rod pumping systems,
must use the
annular space between the tubing and the hollow rod as its fluid delivering
channel.
Therefore, there is a need in the art for a pumping system which may mitigate
the
disadvantages of the prior art and allow for:
(a) reduction of the well completion costs due to the elimination of the
tubing string;
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(b) higher pump efficiency due to the elimination of the elastic elongation of
the tubing
string which occurs during reciprocation of the rod string; and due to the
elimination of
the fluid friction resistance during the up stroke;
(c) reduction of the rod string elastic elongation because of a thinner,
lighter fluid column
and the higher rigidity of the larger cross-section;
(d) reduction in solids accumulation due to the higher fluid velocity in a
smaller cross-
section;
(e) less heat loss from the fluid to the surroundings which will move wax
precipitation point
upward;
(f) reduction of the work-over time and labor effort due to saving one trip of
tubing
pulling/running;
(g) mitigation of rod falling difficulties as a result of reduced friction
resulting from the
temperature/viscosity effect;
(h) the ability to install the apparatus in slim holes or deformed holes due
to the smaller size
that can be used; and
(i) use of a rigid well head instead of a flexible hose while maintaining an
open casing
annular vent.
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SUMMARY OF THE INVENTION
The present invention is directed to a novel hollow sucker rod pumping system
which
allows for production of fluid within the hollow rod string, a rigid wellhead
which allows access
to the casing annular space, one-trip running and setting of the pump with the
hollow rod string
and optionally, electric heating of the rod string using Kelvin's skin effect
and the proximity
effect.
Additional advantages may be realized as follow. Due to the smaller cross-
sectional area
of the hollow rod as compared to the annular space in a conventional tubing
system, a higher
fluid flow velocity is maintained which provides a higher solid carrying
capacity. Also, there
will be less heat loss of the underground fluid to the surrounding formation,
and therefore wax
precipitation is likely to be reduced. The tubing string becomes unnecessary.
One work-over
trip is saved because the pump may be run into the hole in the trip and
retrieved in one trip. Load
on the walking beam of the surface pump jack is reduced because a thinner
liquid column is
lifted. In addition, the load on the pump jack during the up stroke is reduced
because the fluid
viscous friction is removed. The fluid column is lifted together with the
hollow rod, therefore
there is no relative fluid moving to relative to the wall of the rod tube,
which is unavoidable in
the rod/tubing pumping system. Another advantage of the system is the fact
that the liquid
column simply "stays in" the rod tube all the time thereby eliminating the
hydrostatic pressure
against the plunger/barrel gap. The benefits obtained from heating the string
include lower oil
viscosity which improves the pump efficiency. Heating the invented string is
optional and may
be an alternative measure for the heavy oil or high wax well case. This pump
system may be
designed for a deformed casing well where normal tubing and pump size are
restricted, or for use
in a slim hole.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of an exemplary embodiment with
reference to the accompanying simplified, diagrammatic, not-to-scale drawings.
In the drawings:
Figure 1 is a schematic representation of the pumping system of the present
invention.
Figure 2 is a cross-sectional detail view of a preferred embodiment of a
wellhead
of the present invention.
Figure 3 is a cross-sectional view of a preferred embodiment of the cross-over
joint.
Figure 4 is a view of the pump barrel, pump barrel slip, pump barrel hunger
and
barrel retrieval / actuation.
Figure 5 is a schematic view of the of the spread J slot /pin and S slot/pin.
Figure 6 is a view of the shear pin window nipple and drop bar.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides for a novel hollow sucker rod pumping system.
All terms
not defined herein have their common art-recognized meanings. As used herein,
the term "single
string" or "hollow rod string" or "rod string" refers to a hollow sucker rod
string which drives the
down hole plunger reciprocally and delivers the produced fluid through its
central bore. As used
herein, the term "stationary string" refers to the joints and pump barrel
which engage the well
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casing and remain stationary while the pump plunger or piston is reciprocated
by the single
string.
In general terms, as shown schematically in Figure 1, the pumping system of
the present
invention combines a rigid well head (5) having a casing vent (7), a hollow
rod string ( 11 ) and
stationary string (60). The combination of the hollow rod string and the
stationary string allows
for the running in and retrieval of the pump barrel in one trip. The tubing
string is eliminated as
the fluid is produced within the hollow rod string (11). The hollow rod (11)
may be electrically
heated as an option for heavy oil application or for wells with a high wax
presence.
A. The Electric System
Referring now to Fig 1, a transformer ( 1 ) converts the source power into the
desired
single phase AC, for instance to 440V, 60 HZ AC power. The power is then
transferred to the
control panel (2). An electric conduit (3) which can be flexibly hung on the
walking beam of the
surface jack pump (not shown) by ordinary means, is introduced into the rod
conduit joint (4).
The conduit (3) is then introduced into the hollow polished rod (23) and
passed through the rod
cross-over joint (21), and goes out of the cross-over joint (21) from the
lower end through a port
(35) as shown in Fig. 3. The electric conduit (3) is then secured by any
suitable means (9) such as
a clamp to the outside surface of the hollow rod (11). The conduit (3) is
electrically connected to
the hollow rod (11) at a pre-selected depth (sufficient for the depth of the
solid formation) by a
suitable conducting connector (12). The well head (5) is grounded (8), as is
the power unit.
Therefore the electric current circuit is completed.
There is enough of a gap between the conduit clamp (9) and the separating
sleeve
(10) to avoid substantial contact and friction wear since the hollow rod (11),
together with the
clamp (9) and conduit (3), are moving up and down within the separating sleeve
(10). Conduit
(3) is also electrically insulated and designed for the working environment of
downhole
conditions. For example, the maximum temperature limit of the conduit should
preferably not
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be less than 150° C. Once suitable AC electric current is applied to
the system, heat will be
generated by the hollow rod (11). The heat is generated by Kelvin's skin
effect which is the
phenomenon of current only traveling on the inner surface of the tube, which
leaves only a low
voltage on the outer surface of the rod tube. A test indicated that under AC
voltage of 390 V and
a current of 120 A, the outer surface of the hollow rod only carries 0.09 V
for a 9 meter joint and
about 9 V for an 800 meter connected string. The voltage at the wellhead at
the surface will be
zero because of the energy consumption through the rod (the voltage gradient
being towards the
wellhead) and because the wellhead is grounded. The system is therefore
electrically safe.
In another embodiment of the electrical heating system the conduit (3) may be
placed inside of the tube rod (not shown). Due to the concentric
configuration, proximity effect
may increase the apparent electric resistance of the tube rod therefore make
the generation of
heating faster and to a higher temperature level. In this embodiment, a means
of centralizing the
conduit may be required to ensure that the conduit is in the center of the
tube rod. The
centralizing means should also have fluid passing openings to allow the fluid
to flow through
them.
B. The Heating System
The conduit (3) functions only as the AC current transporting medium. There is
no heating element except the hollow rod associated with the system. Heat on
the hollow rod is
generated through Kelvin's skin effect and through the proximity effect which
is the
phenomenon of apparent resistance increase in the turns of a coil type
conductor when high
frequency AC current is applied. The increased apparent resistance of the
hollow rod is a major
contributor to heat generation.
The temperature of the rod may be controlled through a feed back system. In a
preferred embodiment, a temperature sensor (not shown) may be installed within
the wellhead
(5) to provide a feed back signal to a controlling system in the control panel
(2). The feedback
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signal may start or shut down the power supply at a pre-designated temperature
range or may
adjust the power output level to maintain the oil flow from the oil outlet
port (6) at a desired
temperature. The crude oil clouding point or wax/paraffin solidification point
is typically in the
range of 20 to 70° C. At temperatures less than 100° C, most
carbon metal mechanical
properties, such as minimum yield stress, strain or elastic Young's modulus,
are not
meaningfully affected from a practical engineering standpoint. The hollow rod
is constructed of
an industry accepted material such as 35CrMo or API N80 steel. Therefore,
elevated
temperatures at the 100° C level will not raise any practical concerns
for the mechanical
performance of the rod while still being above the wax solidification point.
C. The Well Head, Separating Sleeve & Cross-over Joint
Referring to Fig. 1, a section of a conventional solid rod (22) is connected
at its upper end
to the walking beam of the surface pump jack (not shown). The rod connector
(4) has a hollow
center (4a) and a side port (4b) is connected to the lower end of the solid
rod (22). The hollow
polished rod (23) is connected to the rod connector (4) and forms an open
avenue for the
placement of conduit (3). Conduit wire seals (37) are provided at the lower
end of the polished
hollow rod (23) to prevent any fluid from entering the hollow center, which is
shown in Fig. 3.
As shown in Fig. 1 and Fig.3, the cross-over joint (21) has a hollow chamber
and several
open slots (33) in its top cone-shaped section which allow the fluid to flow
from the hollow rod
center into the oil outlet port (6) as indicated by Fig. 1 and 2. The cross-
over joint (21) has a
larger diameter sliding piston section which matches the inner diameter of the
separating sleeve
(10) so that no fluid above may leak back to the well. The separating sleeve
(10) is designed
with sufficient length, i.e. the same length as the piston stroke in the pump
barrel, to enable the
cross-over joint (21) to travel up and down within the sleeve (10). Both
surfaces of the cross-over
joint (21) and the separating sleeve (10) are treated to withstand constant
friction wear over long
periods, as is well-known in the art. With this piston and barrel mechanism,
the fluid produced
from the hollow rod (11) can be delivered to the outlet port (6) exclusively.
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Referring to Fig. 3, the polished rod (23) is connected to the cross-over
joint (21). An
open port (32) is arranged at the top of the joint (21 ) with seals (37) at
both end of port (32). The
electric conduit (3) runs through the port of the polished rod (23) and then
through the chamber
of the cross-over joint (21), through the seals (37). Therefore no fluid is
allowed to enter the
polished hollow rod (23). At the lower end of the cross-over joint (21), the
conduit wire (3)
passes through the port (35) which has connectors (34) and (36) to secure the
wire in place. The
cross-over joint (21) preferably has four ports or slots (33) evenly
distributed around the cone
shaped shoulder section on its upper part, which are the flowing channels for
fluid to flow from
the hollow rod center into the well head outlet port (6) and then to the
outside of the well. The
cross-over joint (21 ) also has a larger diameter body below the cone shaped
shoulder section that
leaves a gap above the cross-over joint (21) and the separating sleeve (10) as
a fluid flowing
channel. The inner diameter of the separating sleeve (10) matches with the
outer diameter of the
cross-over joint (21) in such a manner that the cross-over joint slides within
the sleeve (10) in a
fluid-tight manner.
As shown in Fig. 2, the separating sleeve (10) connects with a cone shaped
hanger (27)
which is sealed to the casing joint (29). The sleeve hanger (27) also has a
seal ring (26) which
matches the oil outlet joint (25) and "O" ring seal (28) which matches with
casing joint (29). The
seal ring (26) is made of a softer metal than the sleeve hanger (27) and the
outlet joint (25), such
that when compression stress is applied by bolting the outlet joint (25) to
the casing joint (29),
the seal ring (26) will act as a metal seal. The seal ring (26) should
preferably be replaced each
time the well head is disassembled. The outside diameter ("OD") of the
separating sleeve (10) is
designed of such size as to leave a gap between itself and the inner diameter
("ID") of the casing
joint (29). For instance, in the case of 5" casing (19) with wall thickness of
10.36mm and ID of
106.3 mm, the OD of the separating sleeve can be designed the same as that of
API standard
2.875" tubing which has an OD of 73.025 mm, and ID of 57.4mm. That will give a
gap between
the separating sleeve (10) and the casing (19) of 16.64 mm. The above gap
arrangement is large
enough for normal casing vent operation. A larger gap can be arranged by
adopting a larger ID
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casing joint if desired. In the above example, the separating sleeve (10) has
a ID of 57.4 mm,
and the cross over joint may be designed with ID of 40 mm which has a larger
flowing area than
the main string. That will give more than 8.5 mm a wall thickness of the cross
over joint, which
will satisfies the mechanical strength requirement with the opening port on
it. In the above
example, an OD of less than 40 mm hollow rod is then allow to be connected
with the lower end
of cross over joint (21) through commonly known means.
As shown by Fig. 3, the cross-over joint (21) has a port (35) leading the
conduit (3 ) from the
chamber of the cross-over joint (21) to the outside of the cross-over joint
(21), and the conduit is
then clamped to the outer surface of the hollow rod (11 ) by means of the
clamp (9). At both ends
of the port (35), the wire passes through seals (34) and (36). This
arrangement satisfies the need
for introducing the conduit (3) to the desired depth of the hollow rod from
the outside without
interrupting the sliding seal between the crossover joint (21) and the
separating sleeve (10). The
clamp (9) is designed in such a way that the outer maximum diameter is smaller
than the OD of
the cross-over joint 21 in order to avoid any possible contact with the
separating sleeve (10).
D. The Single String and the Stationary String
Figure 1 schematically illustrates the stationary string (60) after it has
been run into the hole
and set to engage the casing (19). When running into and out of the hole, the
stationary string
(60) engages and is locked to the hollow rod string ( 11 ). Once in position,
the hollow rod string
disengages the stationary string and may reciprocate to produce the pumping
action.
As shown in Fig 4, a hanger setting/flshing pin or a "J" pin (13) is secured
on the exterior of
the hollow rod string (11). A pump plunger or piston (38) is secured to the
lower end of the
hollow rod string (11). The pump piston (38) has a hollow chamber and includes
a traveling
valve ( 17) at its lower end.
The stationary string comprises, in order from top to bottom, a "J" slot joint
(20), a "S" slot
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joint (39), a slip joint (41) and a pump barrel (16). These parts are shown in
partly disassembled
form in Fig. 4. The "J" slot joint (20) has a J type slot (20a) for the
receiving the J pin (13). The
S slot joint is secured to the bottom of the J slot joint (20) and has a S
slot for receiving the S pin
(40) which is secured to the slip joint (41). The S slot joint (39) slidingly
engages the slip joint
(41) and includes a slip engaging cone (42) which activates the slips (43) on
the slip joint (41).
The cone (42) is rotatably mounted to the S slot joint. This arrangement
allows the cone to
be pushed in or pulled out from the slips (43) without taking any rotation
torque during the
releasing operation of the slip. A set of slips (43) through high elastic
steel belts (44a) are
secured to the slip joint (41) at a suitable position. The home position of
the slips is a position
that the slips are not engaged as shown in Fig. 4.
A set of friction belts (15) are secured to the slip joint (41) around the
circumference of the
slip joint. The friction belts are designed and assembled in such a way to
create frictional
resistance to either vertical or rotational movement. However, the level of
resistance is not so
high that manipulating the hollow rod string or stationary string is
difficult. The friction belts
also serve to centralize the stationary string within the well bore. The pump
barrel (16) is then
connected to bottom of the slip joint. There is a stationing check valve (18)
secured to the bottom
of the pump barrel and a gas relief pin (49) and mounting bracket (50) is also
provided. An anti-
wear seal sleeve (45) receives the pump plunger (38) within the pump barrel.
The method of running the string into and out of the hole will now be
explained with
reference to Fig. 5. Which shows the spread view of the J slot joint (20) and
The S slot joint (39).
Shadowed dot point shown on the view indicating the initial running in hole
position of the J pin
and the S pins of the assembled string.
1. During the running-in-hole trip, the stationary string is in the position
depicted in Fig 4
and Fig 5. The J pin (13) is in position "a" of the J slot (20a) while the S
pin (40) is in
position "d" in the S slot (39a) as the hollow rod string (11)/stationary
string (60)
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combination is lowered to the desired depth. The J pin pushes downward on the
stationary string which is slightly resisted by the frictional resistance
provided by the
friction belts (15). The "d" position of S pin locks the slip in a home
position to prevent
any vertical movement of the cone (42). The string may be moved vertically and
rotated
counter-clockwise without the risk of pre-setting the slips.
2. To release the slip from its home or travelling position when the pump
barrel is at the
desired depth, the hollow rod string is first rotated clockwise. Rotation of
the hollow rod
string obviously rotates the J pin, thereby moving the J slot joint. As a
result, the S slot
joint rotates relative to the S pin, which remains stationary because of the
friction belts on
the slip joint. The S pin will then occupy position "e". The J pin remains at
position "a".
This unlocks the slip joint from the S slot joint. Next, the hollow rod string
(11) is
slacked and lowered down. With the friction resistance from bottom, the J pin
will remain
in position "a" while the S pin will then be moved to upward in the S slot to
position "~'.
This action will push the slip cone (42) downward to the engaged position
where the cone
causes the slips to bulge outward and engage the casing wall.
3. The hollow rod string is then rotated clockwise again in order to lock the
slips in the
engaged position. Clockwise rotation will move the S slot joint such that the
S pin now
occupies position "g". The J pin (13) still remains in the position "a".
4. The hollow rod string must now be disconnected from the stationary string
by releasing
the J pin from the J slot. This is accomplished by pulling upward and rotated
clockwise
at the same time. While the S pin remains in "g" locking position, the J pin
will be pulled
and moved out of the J slot through channel "c". The J pin (13) needs to be
pulled to a
pre-designed distance away from the J slot for safety and practical reason.
This pre-
designed distance is the same safety gap distance from the bottom of the
traveling valve
(17) to the top of the stationing valve (18). The polished rod (23) and the
solid penny rod
(22) shown in Fig. 1, at the surface is then connected to the horse head of
the pump jack
beam (not shown). During this connection, both the surface horse head (not
shown) and
the downhole pump piston are maintained at their bottommost position. The pump
and
string are all prepared for pumping operation.
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During the down stroke of the hollow rod string during normal pumping,
stationary valve
(18) is forced to close, the traveling valve (17) opens and fluid in the pump
barrel (16) is
squeezed into the piston chamber (38). During the upstroke, the traveling
valve (17) closes so
that the fluid column remains within the hollow rod string and is lifted
upwards.
The operation of retrieve the pump barrel is just the opposite of the setting
operation as
described above. The string (11) is first lowered and the J pin (13) is then
caught by the J slot
(20) and placed in the position of "a". Rotating the string (11) from surface
counter-clockwise
will release the S pin from locking position "g" to "f'. Pulling the string
(11) will pull the slip
cone (42) out of the slips and the slips will move back to the home position
by the force of the
elastic belt shape steel plate (44a). Rotating the string (11) further counter
clockwise will then
lock the slips in the home position again. The string is then pulled straight
upward. The J pin
moves to position "b" and pulls the stationary string out of the hole.
In oil wells with a high gas content, the pump may sometimes suffer from gas
lock in the
pump barrel. This problem may be alleviated by adapting a commercially
available gas-releasing
pin (49) to the top of the stationary valve (18), as is well known in the art.
It is also desirable to provide a shear pin window nipple within the hollow
rod string as is
shown in Fig. 6 to release the fluid column within the hollow rod string when
pulling the string
out of the hole. A drop bar (49) is released in the bore of the hollow rod
string which reaches the
window nipple and shears the pin (48). This opens window nipple windows (46a)
to permit fluid
to drain out as the string is being pulled out of the hole.
The above mechanism allows the hollow rod string to work as flowing channel
and deliver
reciprocal driven force to the pump plunger at the same time. Another
important function of the
above mechanism is the pump barrel slip anchors may be run into the hole with
the hollow rod
string, set and locked in the working position, and then be later retrieved by
the same hollow rod
string with only one trip.
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