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Penny
Pinching
Pump-Offs
Several options exist for producers caught
between a need to improve rod-pumped well production
efficiency and an inability to finance the large capital
investment required for the more sophisticated monitoring and
control systems. One of these is the "Penny Pincher" pump-off
control from D-JAX Corporation of Midland, TX. This POC
operates on a very simple principle: the speed of the pump
plunger will increase slightly if the barrel is empty rather
than full, and small differences in measured
pump-strokes-per-minute can indicate when the well is pumped
off downhole. According to Allen Lindsey, General Manager of
D-JAX, "The sensor wand mounted on the gearbox pedestal reacts
to a magnetic strip on the counterweight and sends a signal to
a microprocessor that determines the speed of the rods and
compares it to the speed measured at the beginning of a cycle,
when the pump is full. When the difference indicates a pumped
off condition, the controller shuts down the pump." This is
the only sensor and the only measurement utilized by the POC.
"Its simplicity makes it reliable, easy to install, easy to
calibrate and easy for pumpers to operate," adds Lindsey. A
very simple and easily understandable key pad allows each
controller to be adjusted or re-calibrated.
The advantage this product offers over a
simple timer is of course the fact that an actual measurement
of the well’s condition is used to control the pump. When a
well is run on a timer, adjusting the cycle time (usually in
increments of 15 minutes) is a matter of trial and error. With
low-productivity wells the operator typically adjusts the run
time and then monitors well production. By iteratively
decreasing or increasing the run time, the operator can
eventually determine the maximum production corresponding to
the minimum run time. If, however, the well is producing in a
waterflood with varying injection rates, the well's pumped-off
fluid level may not remain static over time and an
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optimal cycle run time may never be
realized, even with this time-consuming trial and error
method.
According to Lindsey, the D-JAX pump off
controller reacts to the difference in pump speed rather than
an absolute measurement of well condition, and this is an
advantage if well conditions change. Says Lindsey, "Paraffin,
gas content, and other variables do not affect the
controller's ability to identify a pumped-off condition."
For example, a pumping unit running at ten
strokes per minute with the pump barrel filled completely on
each stroke, would have a single stroke speed of 6000
milliseconds per stroke (msps). As the downhole fluid level
drops off and the pump barrel no longer fills completely, the
stroke speed increases, dropping the msps value until it
reaches a minimum when the well is pumped off, say for example
5982 msps, a "delta" of 18 ms. While this increase in speed is
imperceptible to the eye, the sensor installed between the
gearbox and crank arm can detect and quantify the difference,
and thereby determine when the well has pumped off.
The second parameter used by
the Penny Pincher is the period between pumping cycles that is
required for the fluid level to return to its maximum: the
downtime. This parameter is generally set through trial and
error, with the help of a dynamometer to determine exactly
when the pump is once again filling completely. Of course it
is important to accurately determine both the downtime and the
delta parameters to optimize the well's pumping performance.
The controller also features an
option whereby the operator can program the unit to run for
only a portion of the pumping time during a certain number of
pumping cycles. For example, setting the controller for five
cycles at 95% will force the pump to run for 95% of the
established pumping time for five cycles, after which, on the
sixth cycle, the unit would run until the delta indicated the
well was pumped
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off. This feature is designed
to ensure that fluid pound strokes are eliminated, for
example, when fiberglass rods are at risk.
The Penny Pincher has been
around for 12 years, but D-Jax recently introduced new
internal electrical surge protection for the controller. The
standard system of thermistors, resistors and varistors proved
effective in mitigating damage from nearby lightening strikes
but could not prevent catastrophic failures in cases of direct
strikes. While rare, these events were costly for the
operator.
A solution was found in
isolating the POC logic board from the power supply board
using fiber optic couplings. The simple fiber optic circuit D-Jax
uses between the power supply and the logic board consists of
an emitter (transmitter), which converts normal electrical
signals to light and sends them through the optical fiber, and
a receiver that converts the light signal back to an
electrical signal. The isolation begins with the 110 volt
power supply which feeds the logic board 12 VAC, reducing the
potential for flashover and collateral damage. Another
potential source of surges is the sensor circuit coming from
the gearbox pedestal. The sensor circuit is conventional
copper wire to the power supply and fiber optic from the power
supply to the logic board. Any surges brought in by the sensor
circuit will stop at the power supply. While this
configuration cannot completely eliminate all damage from
lightning strikes or other powerful electrical surges, it does
minimize the economic impact of damage when such events occur.
By isolating the logic board from the power supply board,
catastrophic damage will occur only to the power supply, which
is about one-fourth as costly to replace as the logic board.
The system has proven effective thus far. D-Jax has installed
approximately 550 fiber optic-based controls, many in areas
where lightning has been a problem, and has yet to see a
failure attributable to lightning or other electrical events. |
State-of-the-Art
Summary