Rod Pumping Gas Wells: Liquid Loading and the Problems to Expect

What is liquid loading in a gas well?

Liquid loading is the build-up of water or condensate in a gas well once the gas no longer flows fast enough to carry it out. The liquid column adds back pressure on the formation, which cuts gas inflow and, left alone, can stop the well. Many gas wells reach this stage at some point in their life. ¹²

Early in its life a gas well carries its liquids out as mist and droplets, because the gas moves faster than the critical velocity needed to lift them. As reservoir pressure declines, the inflow falls off or more liquid comes in, and the velocity drops below critical. Liquid then accumulates in the wellbore. ¹

That column hurts the well three ways: it raises the pressure against the producing formation, it adds weight the gas has to push up the tubing, and liquid that re-enters the reservoir can block gas flow near the wellbore. To keep inflow and ultimate recovery up, the liquid has to come out and stay out. ¹ See liquid loading in the glossary.

How to tell a gas well is loading up

A loading gas well usually shows it in its production and pressure records before it dies: the rate falls below the expected decline curve and stays there, the well starts to slug, and the gap between tubing and casing pressure widens over time. A critical-rate check against flowing wellhead pressure confirms it. ²

Signs listed for checking a well for loading: ²

• Critical velocity or rate. The gas rate is compared with the minimum rate that keeps liquid moving at the current flowing wellhead pressure. Below it, loading is predicted.
• A step off the decline curve. Production drops below its trend and doesn't recover.
• Slugging. Liquid arrives at surface in heads instead of a steady mist.
• Tubing–casing pressure difference. The difference grows as liquid builds in the tubing.

The water's source matters for the fix. Chlorides in the produced water point to formation water; no chlorides point to water condensing in the wellbore. ² Foaming agents, the usual cheap first try, work much better on water than on condensate. ²

When is a rod pump the right deliquification method?

A rod pump fits a loading gas well that makes enough liquid to need continuous removal, at low rate and low bottomhole pressure, where electricity is available and the well is not too deep. It is a poor fit where the gas-liquid ratio is high and the pump can't be kept clear of gas. ¹²

At least fifteen lift methods are used to deliquify gas wells, from soap sticks and velocity strings to plungers, gas lift and pumps. ¹ Conventional rod pumping is used when a gas well produces significant amounts of liquid, and it is a common deliquification system. ¹² One published screening matrix rates beam pumping as follows: ²

Two limits shape the choice. Depth: the same paper notes that at 10,000 ft only a few barrels per day of fluid can be produced. ² And energy: beam pumping is efficient, at 45–60% in that comparison, and for gas wells with small liquid rates slender rods and low horsepower may be enough. ² Foam is often the first attempt because it is cheap to try, and a plunger needs no outside energy where the well has enough gas and buildup pressure. ² In one published selection chart for gas wells, beam lift comes after plungers and gas lift. ² To compare rod lift with the other methods in general, see artificial lift methods.

How rod pumping a gas well works

The pump lifts liquid up the tubing and the gas flows up the casing annulus. The aim is to pull the fluid level down to, or below, the perforations while keeping casing pressure low, so the gas flows against as little back pressure as possible. ¹⁴

In a near-vertical well, the best case is a pump that lowers the liquid level near or below the perforations combined with low casing pressure, for example from compression. Then the gas flows against very little pressure and very low producing bottomhole pressures can be reached. ⁴

Placement is the lever. The standard strategies are to set the pump intake below the perforated interval, use effective gas separation below the intake, or both. ¹ A horizontal gas well loses the first option: the pump can't sit below the perforations, so separation has to do the work. ⁴

The pump itself is usually small. One published conversion of shallow gas wells (about 3,000 ft, 45–100 Mcf/d of gas, 3–5 bbl/d of water to remove) used small pumping units with a 1 1/16-in. pump, set as close to bottom as the wellbore allowed. ³

Rod pumping gas wells: the problems to expect

The main problem is gas reaching the pump. After that come over-pumping a small pump, valves that don't seat at slow speeds, wear from pumping water with no oil to lubricate it, and solids and paraffin fouling small valves. Horizontal wells add slugging that can gas-lock the pump. ¹³⁴

1. Gas interference. The challenge named first for rod-pumped gas wells is keeping gas from interfering with the pumping action. ¹ Gas in the barrel cuts fillage and can lead to gas lock. See gas interference.
2. Over-pumping. Running a small pump until the well is pumped off, then backing off, caused premature failures in one field. ³ See fluid pound for what pumping off does to the string.
3. Valves not seating. On short-stroke units run slowly, the balls didn't seat reliably; raising the speed from 8 to 10 strokes per minute solved it. ³
4. No lubrication. A pump lifting nearly 100% water gets no lubrication from oil, so it has to be specified for that duty. ³
5. Trash in small valves. Solids, paraffin, iron sulfide and asphaltenes caused numerous premature pump failures in the same field; doubled valves, acid before rigging up and a weekly solvent flush were the fixes. ³ See paraffin and scale.
6. Slugging in horizontals. Undulations along a horizontal lateral produce liquid slugs. When the last slug is pumped, the pump sees only gas and can stay gas-locked until enough liquid builds up again. ⁴
7. Sand. Sand needs special provisions on a beam-pumped gas well. ²

Setting the pump: depth, separation and speed

Set the intake below the perforations if the wellbore allows it, size any gas separator to the pump rate, and run a small pump fast enough for the valves to seat. Each choice is about keeping the barrel full of liquid at very low liquid rates. ¹³⁴

• Depth. In the shallow conversion above, wells with hundreds of feet of rathole had the pump set about 100 ft below the bottom perforation. Pumps first set 20–50 ft below were lowered to 100 ft on the first workover to improve efficiency. Wells with little rathole had the pump set as close to plugged-back depth as possible. ³
• Separation. Where the intake can't go below the perforations, a gravity separator handles about 50 bbl/d for each square inch of downflow area between the dip tube and the outer barrel. Poor-boy separators assembled from standard parts often fall short of that, and gas is pulled into the pump. ⁴ The downhole gas separator guide covers sizing.
• Speed and stroke. Short stroke and about 10 strokes per minute worked in the same field, for ball and seat action. ³ Low liquid rates mean low loads: polished rod horsepower did not exceed 1 hp there at 10 strokes per minute. ³
• Horizontal wells. The common practice is to land the pump in the near-vertical section above the kickoff, because pumps set in the curve or the lateral fail more often. ⁴

Run it from the fluid level

A rod-pumped gas well is run to keep the liquid off the formation without pumping the small pump dry. The fluid level is the measurement that tells you both: how much liquid stands over the perforations, and how much margin the pump has. ³⁴

Without a pump-off controller, one operator set percentage timers from the top down: start at a low run time with a high fluid level, then raise the run time while watching the fluid level fall, until the right setting was found. The earlier method, running at 100% until the well pumped off and then cutting back, over-pumped the small pump and caused premature failures. ³ That approach relies heavily on the pumper and on regular fluid-level readings. ³ For how a shot is taken, see shooting a fluid level; for the controller options, see pump-off controller vs AI control.

The same paper reports field gas production rising from about 700 to about 1,000 Mcf/d after the conversion. ³

Where SRABS fits

A gas well on rod pump is a fillage problem at very low rates: the pump has to stay full of liquid while the fluid level stays low. That is the pairing SRABS is built to read. SRABS Echo is "an automated acoustic edge device" that reads fluid level, fully remote, and "Brain reads every dynamometer card, motor current and Echo fluid level, and checks them against the physics of your rod string." For gassy wells, the solutions page puts it in one line: "Echo sees the gas and the fluid level, and Brain adjusts speed and idle time before the pump starts to pound." See the platform and how it works.

To estimate what fewer failures and pulls would mean on your own fleet, run the ROI calculator. Its default 100-well scenario is modelled.

The bottom line

A rod pump keeps a loading gas well alive by taking the liquid off the formation, and it works as long as the pump stays full of liquid. Set the intake below the perforations where you can, size separation to the rate, run a small pump fast enough to seat its valves, and set run time from the fluid level, not by pumping the well off.

If your gas wells keep losing pumps to gas and pump-off, read the cards against a fluid level, then test closed-loop control on one well.

FAQ

Sources

1. Dunham, C., Lea, J. F., Stephenson, G., "Development of Recommended Practices for Selection of Artificial Lift Systems for Gas Well Deliquification," Southwestern Petroleum Short Course, 2008. https://www.swpshortcourse.org/paper/development-recommended-practices-selection-artificial-lift-systems-gas-well-deliquification
2. Lea, J., Oetama, T., Fasokun, O., Lestz, R., Dunham, C., "Selection of Artificial Lift Methods for Deliquification of Gas Wells," Southwestern Petroleum Short Course, 2005. https://www.swpshortcourse.org/sites/default/files/papers/2005008.pdf
3. Pye, C., "Gas Well Deliquification Using C-25 & C-40 Pumping Units," Southwestern Petroleum Short Course, 2007. https://www.swpshortcourse.org/paper/gas-well-deliquification-using-c-25-c-40-pumping-units
4. Rowlan, L., Lea, J. F., Hein, N. W., "Beam Pumping to Dewater Horizontal Gas Wells," Southwestern Petroleum Short Course, 2011. https://www.swpshortcourse.org/paper/beam-pumping-dewater-horizontal-gas-wells