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It is generally accepted that sucker rods
operating in normal conditions do not go into compression on
the downstroke and only go into compression when excessive
fluid pump friction occurs or well condition problems cause
opposing forces. Pump friction, which is often
misunderstood, comes about from friction between the plunger
and barrel interface and can be adjusted by the plunger and
barrel fit. A too close clearance will create excessive
friction. Another source is the friction of the fluid
passing through the traveling valve and the plunger on the
downstroke. It is more common with heavier oil and can be
remedied by changing the pump design. Rod deflection is
mostly due to tight clearances, high linear speed, fluid
pound, gas pound, (Figure 3) wellbore deviations and
particulates trapped between the plunger and barrel. The
larger the plunger, the more likely bucking can occur,
especially in deviated wells, so using the smaller diameter
plunger and longer stroke length is usually recommended. To
counteract this force, sinker bars can be added until the
sucker rods are in tension again.
Gas lock occurs when the pump has pumped
down to the pump-chamber decompressing limit and the
pressure drop inside the pump chamber at the top of the
stroke is less than the pump intake pressure. Proper pump
design can help to give the pump the highest pump
compression and decompression ratio by eliminating
uncompressible space in the pump chamber. Adjusting the
pumping
unit to have a longer stroke will allow the pump to lower
the fluid level because the pump decompression limit will
increase. Adjusting the rod string to minimize downhole
stroke loss is also recommended.
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Reducing
Electricity Costs
The cost of electricity is a significant
part of lifting fluids and producing oil and gas. Rod pumps,
compressors, submerged electrical pumps and various surface
pumps all consume power. There are two ways to reduce the
cost of electricity: use less or pay less. Ron Turner with
Oxy described seven specific areas that, with the assistance
of a detailed electrical model, could be analyzed for
potential reductions in use and improvement in reliability
(which in turn reduces downtime and maintenance costs). They
are:
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Install digital relays and recloser
controls to record and identify electrical faults and to
quickly recover from faults and outages.
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Reclosers, fuses, and relays must be
properly coordinated so that a problem in one area does
not cause tripping in another.
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Install capacitors in conjunction with
oilfield motors to reduce the inherent reactive current
created by the motors, which does no work but the power
company bills for it.
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Evaluate the need for voltage regulation, particularly
at the end of the service line to protect equipment from
load fluctuations and poor power quality.
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Evaluate and install effective grounding protection from
lightning.
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Perform a cost-benefit analysis to
determine the value in replacing inefficient motor,
transformers and other power-consuming devices with more
efficient models.
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Evaluate the potential savings of
operating more efficiently by utilizing adjustable speed
drives for those devices that have varying speeds and
power demands.
The
other side of the cost is the price paid for electricity.
While several states have deregulated power (including
Texas, Pennsylvania, and Ohio), even those that have not
usually have options. Generally, the highest price is to buy
firm retail service from the local utility. It usually has a
demand component proportional to the highest usage day for
the utility to have firm generating capacity reserved for
the customer, plus the costs of the electricity itself based
on the load characteristics of the operator. The operator
must become familiar with the local tariffs to determine the
optimum rate for the service required. At minimum there
would be an interruptible rate that would have less demand
charge. The tariff would define the conditions under which
they would interrupt and the utility could tell the historic
frequency. Another option could be to aggregate meters to
make a more stable, hence less expensive, load pattern.
Frequently there are other options between firm and
infinitely interruptible.
Toni Holliman
(Pioneer) presented the wider array of options available in
a deregulated market, where the operator can choose from
other suppliers and use the local utility to deliver it. A
sample of those, in addition to the utility-provided power
include (1) a fixed-price option, where the buyer locks in
future prices, (2) pricing in blocks (on-peak, off-peak,
around-the-clock), (3) heat-rate option in which the price
varies with the cost of fuel, (4) spot pricing, with an
option to convert to any of the first three, (5) load acting
as a resource, where the buyer has the option to self
interrupt and get paid a market-based price for avoided
usage, and (6) several variations of an interruptible rate.
Considerable savings can be achieved if operators have
flexibility on their load. |
State-of-the-Art
Summary