Digital control architecture optimizes plunger lift wells

DUSTIN SANDIDGE, ChampionX Artificial Lift 

Operators are increasingly monetizing the benefits of digital technology on their producing assets. Because of strong initial productivities and the critical importance of keeping high-rate wells operating at peak output, digital automation and optimization tools have largely been focused on new drills at the earliest phases of the decline curve. That makes perfect sense. Why wouldn’t an operator direct the impact of digital technology as close as possible to the point of maximum return on the multimillion-dollar drilling and completion investment? 

However, top-tier producing assets tend to be relatively few and far between in most oil and gas companies’ portfolios. After all, the decline curve is relentless. A new well coming online today will become a marginal well tomorrow, as depletion exacts its toll, ultimately moving it to the opposite end of the production spectrum. Once there, it will be in good company. According to the U.S. Energy Information Administration (EIA), nearly nine out of every 10 wells active in 2022 yielded fewer than 50 barrels of oil equivalent (boe) (gas and liquids) per day, Fig. 1. 

Fig. 1. Total U.S. wells by daily production rate.

Moving farther down the decline curve, 77% of the total 912,962 producing wells in the United States in 2022 produced 15 boed or less. Moreover, the majority averaged less than four boed and the largest single category of wells (nearly 300,000 of them) plotted all the way to the right-hand side of the production-versus-time curve at one boed. 

The numbers are clear: marginal wells matter. Although they may make small individual contributions, they collectively represent a critical share of baseline production, contributing steady sales volumes and bankable revenue streams. However, the lower a well’s daily output, the greater the fiscal pressures and the less wiggle room to navigate the tightrope between the profit/loss columns. 

Among the most marginal barrels of them all are those produced via plunger lift. By nature, wells appropriate for plunger are gassier (the rule of thumb is at least 400 ft³ of gas/bbl of fluid/1,000 ft), and they are often legacy producers in the latter stages of the lifecycle. In some wells, plunger lift may be the only economic artificial lift method, including mature gas wells producing liquids that must be removed to flow the formation and low-volume oil wells with increasing gas-to-oil ratios. 

LIFE-SAVING SOLUTION 

Operating cost is at the center of the marginal plunger lift world, and efficiency reigns supreme. It may seem counterintuitive at first glance, but the tighter the margin, the greater and more immediate the impact of deploying technology to improve productivity and bottom-line performance can be. That’s why the strategic application of digital automation and optimization can be such a lifesaver for plunger lift wells on the far, back end of the decline curve. 

However, there’s a dichotomy. Historically, the steep adoption cost curve associated with applying advanced digital solutions has put the technology out of reach for the very assets it could benefit most: marginal wells at the highest risk of becoming uneconomic, should anything disrupt the cost/revenue balance. Even so, the good news is that a new branch in digital control architectures is resetting the “dollars and sense” rationale of equipping low-output plunger lift wells with sophisticated digital intelligence. 

The SMARTEN™ Unify control system offers a fresh alternative to existing controller designs, which tend to be either higher-cost, full-function supervisory control and data acquisition (SCADA) systems, with proprietary communications, or low-cost, limited-function standalone devices that lack remote visibility and logic-based optimization. Importantly, this new control system was engineered expressly to align technical and business requirements around state-of-the-art workflows and operational knowledge, to grow the margins on marginal plunger lift wells, giving operators the ability to balance optimizing production, on one hand, with holding the line on CAPEX and OPEX on the other, Fig 2. 

Fig. 2. The SMARTEN™ Unify control system is a new type of digital control and optimization architecture for plunger lift wells. It was engineered expressly to align technical and business requirements around state-of-the-art workflows and operational knowledge to grow the margins on marginal plunger lift wells.

With this new platform, the plunger lift assets relegated to the most marginal classification in the portfolio—those wells that have tended to be the last in line for digital investments—may just become some of the smartest and most consistently returning of the bunch. 

WHY PLUNGER LIFT? 

Because it is a highly cost-effective form of artificial lift, plunger lift is the method of choice for many operators of low-rate oil, condensate and gas wells. It uses natural reservoir pressure (formation gas) to surface fluids without a pump, motor, external power source or injectant, minimizing both upfront capital and recurring operating expenditures. Originally developed to deliquefy gas wells, the plunger, as an added bonus, also keeps the tubing swept of paraffin, scale, asphaltene, etc. as it ascends and descends the well. Plunger lift can also be used in tandem with gas lift, to produce both higher-rate oil wells and aging gas wells with exhausted natural reservoir drive (both of which are gas-assisted plunger lift). 

A plunger lift system has five fundamental components: 

• The plunger 
• A bottom-hole spring at the end of the tubing string, to cushion the plunger’s landing 
• A lubricator/catcher on top of the wellhead, to receive the plunger  
• An arrival sensor 
• A digital controller 

The plunger travels through the tubing string at speeds as high as 1,000 ft or more per minute, depending on plunger type and style. Plungers are sized with just enough dimensional clearance to allow them to fall freely through the tubing, while still being able to mechanically seal against the inside diameter of the tubing to prevent liquids from falling back as the plunger rises toward the surface. 

There are two basic plunger types: 

• Continuous bypass (flow-thru), which features a valve mechanism to allow fluid to pass through the plunger body as it falls, without requiring shut-in time; 
• Conventional designs, encompassing several styles of plungers, all of which require closing the well for a certain time, to allow the plunger to fall against production flow. 

A cycle is one complete round trip. It begins with the plunger on bottom and the surface valve in the closed position, allowing pressure to build in annulus between the tubing and casing. When annular pressure reaches a defined setpoint, the controller opens the surface valve and gas enters the tubing below the plunger, providing the lifting force to propel the plunger and fluid column to the surface. 

As the plunger arrives, fluids are pushed through a wellhead outlet into the sales line and the plunger is caught in a spring-loaded receiver in the lubricator. The gas trapped below the plunger then evacuates into a flowline. 

OPTIMIZING PLUNGER CYCLES 

Mechanically, plunger lift sounds simple enough: reservoir pressure builds to the point where it can carry a plunger to surface, taking fluid along for the ride. Operationally, however, things get complicated in a hurry, with several interrelated factors influencing how well the plunger can do its job. The trick is getting cycling exactly right, to ensure efficient plunger operation without either too much or too little liquid buildup in the tubing. 

Keeping fluids lifted off bottom minimizes back pressure on the perforated interval, allowing more reservoir inflow into the tubing. Under-cycling can allow too much fluid on top of the plunger, making it difficult for the plunger to lift the load to surface as well as restricting inflow. Under this scenario, excess wear and tear to equipment is avoided, but it comes at the expense of daily production output.  

On the other hand, over-cycling creates several operational headaches and can also seriously degrade equipment performance and durability. It results in insufficient fluid build between trips, which can lead to excessive plunger velocities that degrade system performance and cause premature wear and damage to mechanical components. Experience indicates that plunger wear is the number-one root cause of plunger lift well failures. 

The fundamental goal is to keep the variables controlled, to sustain the maximum production target, using the fewest roundtrip cycles. This maintains an ideal pressure profile to promote gas and fluid inflow influx into the tubing, without subjecting the well to the effects of either over- or under-cycling. This is where the controller comes in. 

Regardless of the type of application or style of plunger, the controller is the brains of the operation. It regulates cycling, using tubing and casing pressures, and it controls the motor valve. The simplest controllers are basic on/off devices, similar to rod pump-off controllers. These shut in the well for a predetermined time, to allow the plunger to fall to bottom and allow for fluids to accumulate in the tubing before turning the well back on. 

However, optimizing plunger operations is anything but a “set it and forget it” proposition. Each cycle is a little different. Continuously maintaining the ideal plunger lift performance means adjusting to whatever conditions the well is presenting at any given moment. That requires having data generated at a high enough frequency to capture conditional dynamics as they occur, the logic to be able to make sense of it all, and the automated controls to act in a timely enough manner to matter. Again, the plunger might be whistling along at 1,000 ft per minute, so optimization must be done continuously within real-time operational parameters. 

INTO THE LIGHT 

The hard-and-fast reality check on implementing digital technology in plunger lift wells is simple: is it in sync with the operational, technical and economic realities of producing low-volume assets? The Unify control system was developed with that alignment in mind. This system delivers next-generation digital functionalities at a cost of entry that is appropriate for even the most marginal plunger lift wells. It centers the full force of logic-based diagnostics and optimization on the marginal plunger lift wellsite and seamlessly connects it to digital-enabled offsite data management and analysis workflows. 

The platform harnesses the latest A.I., IoT and wireless technologies for edge device automated control and around-the-clock situational visibility. Specific features include: 

• Wi-Fi connectivity, with no proprietary radio requirements  
• Full remote visibility and operational control  
• Unprecedented data granularity, to provide critical insights into plunger cycling  
• Expert logic-based analysis, to continuously optimize well operations  
• Actionable edge analysis and 360^o situational visibility 
• Standard APIs for data hosting models.  

Any one of these features can be a powerful enabler in its own right. Together, they create a new digital key to unlock exponential increases in efficiency and optimization, through total-view well intelligence, real-time diagnostics, remote monitoring and full-range automated control—all incorporated within a plug-and-play platform scaled to the economic profile of the typical plunger lift well and easily integrated into existing SCADA networks. 

The high-frequency data are generated at one-second intervals, providing the up-close granularity to optimize individual plunger cycles on a 24/7 basis, as well as the richness to delineate trends and identify upside opportunities across a large grouping of wells. With Unify, operators can connect their “wish list” of plunger lift automation capabilities with the real-world fiscal constraints of operating marginal properties and the practical need for IT simplicity. 

No longer must valuable cashflow-generating plunger lift assets be left outside looking in at the unfolding digital revolution. They can finally be brought fully into the digital light, to realize late-stage production value, boost bottom-line profitability, recover full reserves potential, capture upside value and extend productive life. 

CASE HISTORY  

The technology was field-trialed in a range of plunger lift application environments. One installation was a vertical natural gas well running a conventional plunger in the Piceance basin of western Colorado. In this case, the Unify control system’s live data capture proved critical to diagnosing an operational issue and determining the root cause. 

Liquids output abruptly declined in conjunction with a dramatic build in tubing pressure (TEP). While these not-so-subtle developments were readily observed, without high-quality data and troubleshooting logic, the plunger lift well operator’s biggest challenge in this situation was typically figuring out not what had happened, but rather, figuring out why it had happened. 

With multiple potential explanations for the well’s behavior, delineating the exact cause may have consisted of investigating and eliminating any number of possibilities—from a hole in the tubing to a frac hit or equipment failure. 

Fortunately, Unify’s high-resolution data instantly detected the upset condition in TEP and provided the diagnostic insights to determine the root cause of the pressure build. In Fig. 3, blue indicates the trend line for TEP, and red shows the casing pressure (CEP). Each successive TEP peak and valley represents one complete roundtrip cycle. The difference between TEP and CEP at the peak of each cycle correlates to the amount of fluid accumulated in the tubing during plunger off time. 

Fig. 3. Tubing pressure spike following failure of standing valve.

TEP behaved, as expected, during the first several plunger cycles (see left part of Fig. 3), but it instantaneously jumped during the third cycle from right and remained elevated at nearly the same level as CEP during the next two cycles. What the data show in this instance was the precise moment of failure for a standing valve (tubing stop) located immediately below the bumper spring. 

The plunger was on the bottom, awaiting a sufficient TEP build to surface, when the valve failed. With nothing preventing fluid from evacuating out the end of tubing, the plunger’s descent pushed liquids out of the bottom of the production string into the surrounding casing and left the plunger with little available load during the two subsequent cycles, creating the TEP anomaly. The operator was able to quickly recognize the malfunctioning standing valve and replace it, so production operations could resume with minimal downtime. 

The mechanical operation of a plunger lift system experiences numerous conditions during normal cycling that can make problems difficult to discern and even harder to accurately diagnose. As this example illustrates, the high-resolution data are critical to understanding what is occurring at any point in time and what actions are necessary to maintain optimal plunger cycling, to keep production flowing to the sales line. 

 About the author

DUSTIN SANDIDGE is Automation Product Line manager at ChampionX Artificial Lift, focused on PCS Ferguson automation and control solutions for plunger lift and gas lift applications, at ChampionX’s Parachute, Colo., facility. Mr. Sandidge has more than two decades of experience in remote monitoring, automation and expert logic-based optimization of plunger lift, gas lift and PALG and GAPL wells.