Almost a decade ago, Robert M. Parker, a well analyst for Texaco USA, wrote a World Oil article
entitled "How to Prevent Gas-Locked Sucker Rod Pumps". Mr. Parker's insight forever changed the way many
of us think about downhole pumps. He described two kinds of gas lock.
Type 1 Gas Lock occurs only if the fluid level is greater than 1/3 the total depth to the seating nipple
or if a bridge is present that gives the effect of pumping from below a packer (rare). Expanding gas
bubbles "tickle" the balls and seats (valves), and the well comes to equilibrium pumping at very low efficiency
and with an elevated fluid level. The cure for Type 1 gas lock is to use either a snubber cage or a backpressure
Type 2 Gas Lock occurs when a volume of gas is trapped between the valves in a pump. In a Type 2 gas
lock, the peak pressure of the trapped gas on the downstroke is insufficient to overcome the hydrostatic head on the
traveling valve. Then, the pressure is not reduced enough on the upstroke to allow the standing valve to open
and admit new fluid. Both valves are effectively stuck in the closed position and the pump refuses to
pump. This is essentially the opposite of the Type I gas lock, but the results appear the same. Using a
backpressure valve is not the cure for Type 2 gas locks; the practice will actually make the problem worse.
The secret to avoiding Type 2 gas locks is to configure your pumps such that they pump gas nearly as well as they
pump liquid. The key phrase is "unswept volume"...you want as little of it as possible.
Avoid "travel tube" pump designs; stationary barrel designs are inherently better with respect to compression
ratios. Traveling valves should be on the bottom of the plunger, never on top, and use a flush hex seat retainer
plug instead of the traditional type. When spacing the stroke, the traveling valve should come as close as humanly
possible to the bottom of the pump at the bottom of the downstroke. The pull rods in most pumps are too short and
should be replaced. Avoid double ball and seating (it does not accomplish what it is touted to do anyway).
When the pump closes up, there should be as little void space (unswept volume) as possible, and if you do a good job you
will have little trouble with gas lock. Such nicely designed and executed pumps are frequently referred to
as "high compression" pumps.
This is not an academic exercise. We have increased oil production from wells using this simple technology.
We have been able to pump wells from below packers without a venting scheme. In general, wells pump better if these
easy rules are followed.
In recent years, the high compression concept has stimulated the development of a number of new products.
There were a dozen or more producers of low unswept volume cages; however, most of these entities seem not to have
survived the last couple of years of low oil prices. The major pump manufacturers of course make their own
versions of various gimcracks; most of which do not perform particularly well.
But the best and most economical high compression cage is made by Evison
Manufacturing Corporation and is available from Poor Boy Supply Company on this very website.
Click here to learn about the Evison
TM Oil KingTM
"OK" HP/P High Compression Open Barrel Cages.
Also visit Robert M. Parker's
The Sucker Rod Pump Information Site.
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