Plunger Lifted Gas Well Requirements

Plunger lift is an artificial lift type used to remove liquid from natural gas wells so that natural gas can flow freely to the surface. Plunger lift systems are also deployed to keep the tubing free of paraffin/scale/salt and to lift oil to the surface in oil wells where gas pressure is sufficient.

In the US, studies show 80% to 90% of gas wells will at some point have liquid gather in the bottom of the well, impeding the flow of natural gas to the surface. As wells reach this condition, artificial lift techniques assist in the removal of liquid to improve production. Gas wells may be a candidate for plunger lift if 1) liquid is gathering in the tubing 2) sufficient gas pressure is present and 3) sufficient gas volume is available. When a well meets these basic requirements; plunger lift can be an effective, low cost and fun method to restore production to the well's pre-liquid loaded condition. Other requirements include no holes in the tubing, same tubing ID from the bottom hole spring through the well head, less than 250 barrels/day liquid to be removed and trained operators!

Determining if liquid is gathering in the bottom of the well can easily be achieved by viewing a decline curve, production chart and/or determining the well’s critical flow rate. The critical flow rate is the flow rate at which all liquid in the well bore is carried to the surface. This rate can be found by using Turner’s or Coleman’s critical flow rate graphs or equations. When the well is flowing above the Critical Flow rate, the flow rate carries all liquid in the tubing to the surface. If the well is flowing below the Critical Flow rate, then some liquid is gathering in the bottom of the well.

A decline curve depicts the natural rate at which gas production declines over time. Decline curves are best constructed and viewed from initial to current production. If the rate of production significantly drops beneath the decline curve, and casing pressure simultaneously increases; liquid could be gathering down hole, impeding the flow of gas to the surface. Viewing the natural decline curve is also an excellent method by which to set a production target for each gas well operated.

Analyzing a production chart often reveals periodic peaks and valleys in the production rate as liquid loading occurs. Often, this is caused by liquid gathering in the bottom of the well, thus restricting the flow of gas. When pressure in the well sufficiently increases, gas pushes the liquid column to the surface. With the tubing now clear, production spikes until liquid once again gathers in the bottom of the well. This process is repeated over and over until the gas pressure is insufficient to move the liquid to the surface; at which point gas production is significantly reduced or even stops.

As a rule of thumb, the pressure in a gas well is sufficient to move the liquid column to the surface when the Lift Pressure is at least double the Liquid Load (downward pressure created by the column of liquid in the tubing). With the well shut-in, the pressure created by the column of liquid in the tubing can be determined by subtracting the well’s tubing pressure from the casing pressure (assuming no packer). The Lift Pressure can be determined by subtracting the line pressure from the casing pressure. When Lift Pressure is at least double the Liquid Load, it’s likely the well will have enough pressure to move a plunger and the column of liquid above it to the surface. For a more exact estimate of the casing pressure required, the equation developed by Foss and Gaul is utilized.

Finally, a well must have sufficient gas volume to be a candidate for a plunger lift system. As a rule of thumb, the well needs at least 400 standard cubic feet of gas per barrel of liquid per 1,000 feet of depth. For an 8,000 feet deep well, removing 10 barrels of liquid a day would require at least 32,000 scf/day or 32 Mcf/day (400 scf X 10 barrels X 8000 ft / 1000 ft = 32,000 scf/day).

Based on the above requirements, many plunger lift capable wells in the US are produced using less effective, less environmentally friendly and/or more costly artificial lift means; including installing artificial lift too late; intermitting wells, periodic venting, adding foaming agents or utilizing gas lift. These decisions are often based on limited knowledge or preconceived misconceptions. Understanding the benefits and limitations of plunger lift can help ensure good decisions result in optimal production for your plunger lift capable wells.   

Shale Tec LLC offers a 2-Day training course on plunger lift. If you, or your firm, are interested in learning more about selecting plunger lift well candidates or optimizing plunger lifted gas wells, please visit www.ShaleTec.com for more information.