Do standing valves improve production?
Standing valves are comprised of a ball and seat positioned in the bottom-hole spring assembly at the bottom of the tubing string. The valve opens as gas enters the tubing. When the gas flow stops, the valve closes, trapping liquid in the tubing. As the plunger falls through the liquid, the standing valve prevents liquid from exiting the end of tubing. On the next run, the plunger is able to lift all the liquid in the tubing to the surface, enabling gas to flow freely.
Some believe standing valves limit the ability to push tubing liquid into the well bore when necessary. When pulling a bottom-hole spring with a wire line, the pressure created by a large column of liquid above the spring can add substantial weight. Pressurizing the tubing will push the liquid out of the end of the tubing, allowing the bottom-hole spring to be pulled, inspected, refurbished and re-installed. Some operators believe standing valves restrict this practice. However, many manufacturers offer a spring-loaded relief valve positioned under the ball and seat, allowing sufficiently pressurized liquid to be pushed out the end of the tubing.
With this objection addressed, there is little reason not to use standing valves. Some wells will respond better than others. Vertical wells in which the end of tubing is above the perforations draw liquid into the tubing when the well is flowing and allow liquid to fall out the end of the tubing when the well is closed. Horizontal wells with bottom-hole springs positioned in the curve function in a similar fashion. In both cases, liquid cycles in and out of the tubing, impeding the flow of gas to the surface. Installing standing valves in these type wells will generally improve production.
Observing casing and tubing pressure build rates (See Fig 1) when a plunger lift well is closed can also signal the need for a standing valve – or a shorter shut-in period. The difference between casing and tubing pressure represents the pressure created by the column of liquid in the tubing. If this differential pressure decreases during the off cycle (casing and tubing pressure equalize), then liquid is being pushed out of the tubing. Again, adding a standing valve will generally improve production (See Fig 2).
Fig 1: Liquid pushed out of tubing as casing and tubing pressure equalize
Fig 2: Production improvement with standing valve