Note: Descriptions are shown in the official language in which they were submitted.
CA 02309523 2000-OS-26
1 PUMP JACK DYNAMOMETER
2
3 FIELD OF THE INVENTION
4 This invention relates to method and apparatus for monitoring the
operation of sucker rod pumping units and more particularly for determining
6 conditions of the polish rod, which are indicative of the status of a
downhole
7 pump.
8
9 BACKGROUND OF THE INVENTION
Apparatus for monitoring sucker rod well pumping units are
11 commonly referred to as "sucker rod pump dynamometers". A typical
12 conventional sucker rod pump dynamometer has a load cell mounted on the
13 polish rod, between the bottom polish rod clamp and the carrier bar and a
14 position sensor situated on the ground below the load cell and attached to
it by a
steel cable. The load cell and a position sensor are connected by electrical
cable
16 to electronic conditioning interfacing circuits placed nearby. Electrical
cable may
17 be used to connect conditioning circuits to a computer in a service vehicle
some
18 distance away.
19 As the pump jack reciprocates during its normal pumping action it
operates the position sensor by pulling the steel cable which rotates a spring-
21 loaded drum. The drum is connected to a potentiometer, which converts
position
22 to an electrical value for output to the electronic conditioning interface
circuits.
23 The computer calculates, stores and displays a plot of load on the sucker
rod
24 versus the position of the rod in its cycle.
CA 02309523 2000-OS-26
1 One problem with the conventional equipment is that the electrical
2 cables are subject to damage including that resulting from the rough
3 environment generally or from becoming entangled in pumping equipment. If
4 entangled, the cables are either pulled apart by the pump jack movement or
the
cable pulls the load cell right off of the polish rod. Further, repeated
connecting
6 and disconnecting of cable connectors causes wear which is an additional and
7 frequent cause of failure. The relatively low signal levels are conducted
along
8 relatively long cables resulting in an analog signal which is susceptible to
radio
9 frequency noise and can cause incorrect readings. Further, the low-level
signal
severely restricts the distance that any of the signals may be carried.
11 Yet another, and potentially more significant problem associated
12 with the conventional apparatus is the use of the steel cable with the
position
13 sensor. The steel cable can become tangled or kinked thereby causing
failure or
14 generate misinformation. Also, if the steel cable is ever allowed to return
onto
the drum in an uncontrolled manner (e.g. becomes disconnected), the
16 potentiometer will be destroyed. Further, the location of the mechanical
position
17 sensor makes it susceptible to becoming inoperative due to being buried in
18 snow.
19
2
CA 02309523 2000-OS-26
1 SUMMARY OF THE INVENTION
2 In a preferred form of the invention, a sucker rod pump
3 dynamometer has a movable signal device secured to the polish rod. A polish
4 rod mounted controller directs the movable signal device to send a first
signal to
a fixed signal receiver either on the ground or the wellhead. This signal can
be
6 an ultrasonic signal or a faster triggering signal. The fixed signal
receiver returns
7 an upward signal to the movable signal device. At least one of the first or
return
8 signals is wireless for providing a distance or rangefinding function. The
9 controller monitors the time which elapses between sending the first signal
and
receiving the return signal as to enable determination of the distance between
11 the movable and fixed signal devices, indicative of the polish rod position
and
12 can be extended to calculate rod velocity. In this way, the cable and drum
type
13 potentiometer is replaced. If one of either the first signal or the return
signal is a
14 near speed-of-light signal and the other is ultrasonic, then the travel
time for the
faster signal is negligible compared to that of the slower signal, the elapsed
time
16 being substantially the same as the travel time of the slower signal.
Preferably,
17 polish rod position information is combined with polish rod load for
transmission
18 to a computer. Wireless data modems enable the computer to be located
19 remote from the pump jack for total elimination of electrical cables.
In a broad aspect of the invention then, apparatus is provided for
21 monitoring a pump jack comprising: a first device movable with the jack's
22 reciprocating polish rod for transmitting a first signal, a second fixed
device for
23 receiving the first signal and returning a second wireless signal to the
first
24 device, possible merely a reflection of the first signal but more
preferably an
3
CA 02309523 2000-OS-26
1 independent signal, and a controller for establishing the elapsed time
between
2 the triggering signal and receipt of return signal and emitting signals
indicative of
3 the relative position of the polish rod with respect to the second device.
The
4 controller transmits data indicative of the polish rod position to a remote
computer. Coupled with the input from other devices, the controller can
transmit
6 rod position, load and environmental conditions to the computer.
7 Preferably the first signal is an ultrasonic signal. In response, the
8 fixed device reflects the first signal or sends a return wireless signal to
the
9 movable device, thereby determining its instantaneous position. More
preferably, a fast signal such as a radio transmission or light is utilized as
one or
11 the other of the first or return signals.
12 More preferably, by producing and analyzing multiple return signals
13 of predetermined timed intervals, the polish rod velocity is also
determinable.
14 The above apparatus lends itself to a novel process for monitoring
a sucker rod downhole pump, comprising the steps of:
16 (a) moving a first signal device up and down with the polish rod
17 (b) initiating a first signal from the first signal device, preferably
18 being ultrasonic or a near-speed-of-light signal such as radio
19 waves or light, and directing it to a fixed signal source for
reflecting or triggering the return of a wireless return signal
21 having a known speed of travel;
22 (c) initiating a timer with initiation of the first signal;
4
CA 02309523 2000-OS-26
1 (d) receiving the wireless return signal at the first signal device
2 and terminating the timer thereupon for determining an
3 elapsed time indicative of the travel time between initiating
4 of the first signal and the receipt of the wireless return signal
at the first signal device; and finally
6 (e) calculating the relative position of the first signal device
7 based upon the elapsed time and the travel speeds of the
8 first and wireless return signals, the position being related to
9 the polish rod position.
As stated, while the fixed signal source or device could merely
11 reflect the first signal as the wireless return signal, it is more
preferable that the
12 fixed signal source be a second device which, upon receipt of the first
triggering
13 signal, sends a wireless second signal back to the first device.
Advantageously,
14 if both the first and return signals are ultrasonic, then the second device
can
introduce a predetermined time delay prior to sending the wireless return
signal
16 and thereby provide a stronger return signal and allow reflected ultrasonic
17 returns to die out before transmitting the new signal. More advantageously,
if
18 the return signal is light-speed, i.e. infrared, then as soon as an
ultrasonic first
19 signal is detected, an infrared signals is instantaneously returned,
defining the
end of the elapsed time and enabling calculation of polish rod position.
21 More preferably, the second device can send multiple return
22 signals at pre-determined intervals for enabling determination of the
polish rod
23 speed. Polish rod load can be measured and transmitted with polish rod
position
24 information to a remote computer.
5
CA 02309523 2000-OS-26
1 BRIEF DESCRIPTION OF THE DRAWINGS
2 Figure 1 is a schematic illustration of a pump jack incorporating an
3 apparatus according to the present invention.
4 Figures 2a-2e are schematic representations illustrative of the first
and second signals in five situations, related at left to a partial view of a
pump
6 jack and wellhead, more particularly:
7 Figure 2a illustrates a downward ultrasonic signal and a reflected
8 ultrasonic upward signal, spurious reflections also being shown;
9 Figure 2b illustrates a downward ultrasonic signal which is received
by a second device for implementing a time delay and which then returns an
11 ultrasonic signal;
12 Figure 2c illustrates an ultrasonic downward signal which is
13 received by a second device for instantaneously returning a near speed-of-
light
14 signal;
Figure 2d illustrates a near speed-of-light downward signal which is
16 received by a second device for instantaneously returning an ultrasonic
signal;
17 and
18 Figure 2e illustrates a near speed-of-light downward signal which is
19 received by a second device for initiating a near speed-of-light return
signal.
6
CA 02309523 2000-OS-26
1 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
2 Having reference to Fig. 1, a conventional pump jack 10 is shown
3 comprising a horsehead 12, carrier bar 14 and a cable 16 connecting the
carrier
4 bar 14 to the horsehead 12. A polish rod 18 is suspended from the carrier
bar
14 and extends down through a wellhead 20 to the sucker rod string (not shown)
6 and hence to the downhole pump (also not shown).
7 Drive 22 causes a rocking motion in the pump jack 10 which results
8 in the pump head 12 following an arcuate path 24. The cable 16 converts the
9 arcuate movement of the horsehead 12 into reciprocating vertical movement 26
of the polish rod 18. The polish rod reciprocates between a bottom position
Pbtm
11 and a top position Plop relative to a position origin Po.
12 A sucker rod pump dynamometer 28 according to the present
13 invention is provided which eliminates the need for the prior art cable and
drum
14 type position sensor. The novel dynamometer 28 comprises a load cell 30
connected to and moveable with the polish rod 18 for measuring load on the
16 sucker rod string and polish rod 18. The load cell 30 is typically situate
between
17 the carrier bar 14 and a bottom polish rod clamp (not shown).
18 A movable first signal device 32 is also secured to the polish rod 18
19 adjacent the carrier bar 14. The movable signal device 32 moves vertically
with
the polish rod 18. The movable signal device 32 is capable of sending a first
21 signal 34 and receiving a return signal 36.
22 A controller 38 is connected to the movable signal device 32. The
23 controller 38 is also secured to the carrier bar 14. Such a controller 38
is an Intel
24 87C196 microcomputer having onboard timer and TTL ports. The controller 38
7
CA 02309523 2000-OS-26
1 uses a TTL port to direct the movable signal device 32 to emit the first
signal 34
2 and also initiates its internal timer, starting at time to.
3 The controller 38 is conveniently included in the electronics
4 associated with the load cell 38 and in this way is movable with the polish
rod 18.
The operator can remove the load cell from the other components. The
6 calibration information for the load cell 30 is contained in an EERPOM (not
7 shown) contained within the load cell. Periodic removal of the load cell and
8 plugging it into an into an external power source (such as 12V DC or AC
9 adapter) allows an internal battery to be charged for powering the load cell
30
and controller 38.
11 In response to the first signal 34 a second and wireless
12 rangefinding return signal 36, such as an ultrasonic or infrared pulse, is
13 generated from a fixed base position or source 40, returned and
subsequently
14 received by the controller 38 at time t~. In order for the distance or
polish rod
position to determined, at least one of the first or return signals must be
wireless
16 and thus the travel time is related to distance.
17 The controller timer records the elapsed time Ot between signals
18 34 and 36 (t~-to). For the purposes of this description, two additional
time periods
19 are defined: the elapsed time for the first signal 34 to travel down to the
fixed
source 40 is ~tdoWn; and the elapsed time for the return signal 36 arrive at
the
21 controller 38 is ~tup.
22 The instantaneous distance d between the movable signal device
23 32 and the origin Po or fixed source 40 is determinable knowing the speed
of
24 transmission of the rangefinding signals 34 or 36 and the elapsed time Ot.
More
8
CA 02309523 2000-OS-26
1 accurate load cell, signal velocity and polish rod position calibration is
possible
2 by adding a pressure sensor and a temperature sensor (not shown).
3 It is understood that all that is needed to complete the numerical
4 calculation of instantaneous position is to ascertain the time for a
rangefinding
signal pass one-way between the device 32 and the fixed source 40 or vice
6 versa. If the first signal 34 is very much faster than the return signal 36
then its
7 time is substantially negligible and all that is required is to determine
time Otup for
8 the rangefinding return signal 36 to reach device 32 and the controller 38.
The
9 controller 38 must be able to relate the time t~, the return signal 36 is
received, to
the time to it was initiated or triggered.
11 Time ~tdoWn is substantially zero in the case of certain signals
12 including: a hard-wired transistor-transistor logic (TTL) signal, radio or
a light
13 signal. Time OtdoWn could be much longer (e.g. 17 ms over 6 m) if the first
signal
14 34 is a downwardly directed ultrasonic signal.
Controller 38 also monitors load cell 32 and receives input related
16 to the strain on the polish rod 18.
17 Having collected elapsed time 0t and knowing the timing and
18 speed of the first and second signals and load information, the controller
38 then
19 has data necessary to determine polish rod load and the position of the
controller
38 relative to the fixed source 40. Having calibrated the position of the
controller
21 to the base position Po,42, the polish rod position Pmeas (between Pbtm ...
Ptop)
22 can be calculated.
23 The controller 38 is connected to a data radio 42 which wirelessly
24 transmits a packet of data of polish rod position and load to a receiving
data
9
CA 02309523 2000-OS-26
1 radio 44. The data packet can include temperature and pressure data, load
cell
2 calibration data or computed information. The data is converted to digital
3 information and then transmitted to a remote computer 46. Depending on the
4 complexity of the controller 38, it can either pre-process inputs or merely
transmit
raw data to the remote computer 46. Computation is preferably distributed
6 between the remote computer and the controller, with the remote computer 46
7 calculating polish rod position.
8 The remote computer 46 may be a located in a service vehicle (not
9 shown). One suitable data radio is the Phantom Wireless Datalink radio
modem, model WLB-232 by Murandi Communications Ltd., of Calgary, Alberta,
11 Canada.
12 As shown in Fig. 2a, in a first embodiment, the first signal 34 is a
13 downwardly directed ultrasonic signal and the return signal 36 is a
reflection of
14 the downwardly directed signal off the fixed source 40 such as the ground
or a
specific reflector. Such an ultrasonic sensor is model HU-US33 sensor by
16 Ahernian Proffer of Tulsa, Oklahoma, used both in the movable signal device
32
17 and the fixed source 40.
18 Controller 38 determines the time interval ~t between the time to
19 the downwardly directed signal 34 is sent and receipt at time t~ of a
return signal
36.
21 Unfortunately, unlike a narrow laser beam, an ultrasonic signal has
22 an ever widening cone shape which may impinge on the wellhead 20 and
23 surrounding area. Due to the variable height of the first device 32 from
the
24 ground, the contact of the cone varies in width, presenting significant
reflective
CA 02309523 2000-OS-26
1 clutter and it is often difficult to separate spurious reflections
36',36",36"' from
2 the actual return reflection signal 36. Accordingly, there could be a need
to
3 introduce a predetermined delay, ~tdelay, built into the elapsed time Ot for
4 providing sufficient time for any spurious reflections 36',36",36"' to
dissipate,
such as are likely to occur when the fixed source 40 is mounted to the ground
or
6 the wellhead. Accordingly, between time t~ and to there is: the time for the
first
7 signal 34 to reach the base position OtdoW~; the delay time Otdelay~ and the
time
8 Ot~P.
9 In the embodiment shown in Fig. 2b, where the first signal 34 is an
ultrasonic signal, the implementation of a time delay Otdeiay m the return
signal 36
11 is accomplished by providing a second fixed signal device 48 as the source
40.
12 This second fixed device sends an independent return signal 36 in response
to
13 the received first signal 34. The advantage of providing a second fixed
signal
14 device 48 is that the return signal 36 received by the movable signal
device is
very strong and easy to detect accurately. The second device 48 is mounted so
16 as to remain independent and stationary relative to the polish rod 18.
Mounting
17 the second device 48 to the ground would suffice.
18 The second or fixed signal device 48 adds greater flexibility to the
19 form of the first signal 34 which further can include other signals such as
radio or
light which travel at or near the speed of light. The advantage of using such
a
21 fast signal is that it removes the necessity to correct polish rod position
22 calculations to account for the time it takes the signal 34 to reach the
fixed signal
23 device 48. Further, it obviates the need for a built in time delay and it
allows the
11
CA 02309523 2000-OS-26
1 sampling rate to be increased as less time is wasted for triggering of the
second
2 device 48.
3 In the case of a hard-wired first signal 34, a normally slack
4 electrical cable can connect the controller 38 to device 48 for transmitting
a 0-5 V
signal and triggering the return signal 36. In the case of a radio or light
first
6 signal 34, the second device 48 receives the first signal 34 and triggers
and
7 transmits the return signal 36.
8 Having reference to Fig. 2c, an ultrasonic signal is transmitted to
9 second device 48. Upon receipt of the first signal 34, the second device 48
immediately sends a near speed-of-light signal 36 (i.e. infrared) upwards to
the
11 first device 32. Time ~tdown is relatively long; in the order of 17 ms for
distances
12 of about 6 m. The time Otup for the infrared signal 36 to return to the
first device
13 32 is negligible (perhaps a travel time of about 50 ns). No delay Otdelay
IS
14 required as the first device receives the infrared signal and is unaware
any
reflected ultrasound. Time ~tdown is substantially larger than OtuP and
elapsed
16 time 0t is substantially equal to OtdoW~.
17 In another embodiment, as shown in Fig. 2d, a radio or infrared
18 light signal is transmitted to the second device 48. Time Otdown is
negligible. No
19 delay Otdelay is required. Where the second device is an ultrasonic
transducer 48
then time otUp is substantially larger than 4tdown~ and elapsed time is
substantially
21 equal to ~tUp.
22 In Fig. 2e, fast signals are used both up and down. The elapsed
23 time Ot is very small and ~tdoW~ is substantially equal to Ot~P.
12
CA 02309523 2000-OS-26
1 Providing repeated return signals 36, or analyzing multiple signals
2 36, can reveal additional data of interest such as polish rod velocity or
pumping
3 state.
4 Velocity of the polish rod is determined from the analysis of closely
spaced return signals. By analyzing two return signals 36,36 at pre-determined
6 intervals, the data collected by the controller 38 is sufficient to
calculate position
7 and speed based on the time interval between receipt of the two return
signals.
8 Specifically, the controller 38 can repeatedly pulse or cycle the triggering
and
9 receiving of return signals 36. The change of position of the polish rod 18
during
the time between pulses enables determination of polish rod velocity.
11 Further, the state of the stroke of the polish rod can be determined
12 from the relative velocity of the rod. As calculation of rod velocity is a
vector,
13 including direction, the sign of the velocity states whether the rod is
moving up or
14 down.
According to the method of the present invention, the movable
16 signal device 32 is caused by the controller 38 to send a signal 34 either
to the
17 ground 42 or a fixed signal device 48. A return signal 36, that is either
an echo of
18 the signal 34 or an independent signal 36, is emitted by the fixed signal
device
19 48 and is sent back to the controller 38. The data collected by the
controller 38 is
used to calculate a polish rod position based on the time interval between the
21 signals 38 and 40.
22 The controller 38 also monitors the load cell 32 to determine strain
23 on the polish rod 18 at the calculated polish rod position Pmeas. The
controller 38
13
CA 02309523 2000-OS-26
1 wirelessly transmits both strain and position data to a computer 46 via a
data
2 radios 42,44.
3 The controller 38 may additionally calculate load or some other
4 function of force on the polish rod 18 based on input from the load cell 32.
This
will of course depend on the nature of position versus load curve which is to
be
6 plotted.
7 The velocity of sound is an additional parameter used to improve
8 the accuracy of the calculation of the polish rod position. Accordingly, the
9 atmospheric pressure and temperature, both of which affect the velocity of
sound, are measured. Another method for correcting for these parameters is to
11 predetermine the length of the pump stroke. When the length of the pump
stroke
12 is known then the velocity correction can be easily calculated after the
pump has
13 made one full cycle. Acoustic velocity corrections can also be made on an
14 ongoing basis using this method.
It will be appreciated that the present invention eliminates the need
16 for wire cable, drum and potentiometer arrangement for determining polish
rod
17 position. Further, the present invention provides an alternative to
electrical cable
18 hookup between the pump jack and a computer or between pump jack controller
19 and the ground.
The above is intended in an illustrative rather than a restrictive
21 sense. Variations to the exact description above may be apparent to persons
22 skilled in such apparatus without departing from the spirit and scope of
the
23 invention as defined by the claims set out below.
14
