In plunger-lift systems the sensing of the plunger is one of the most critical, yet least advanced, aspects of the system. Missed arrivals and false plunger detections lead to poor system performance and undesired events such as well shutdowns. Even worse, catastrophic events such as the breach of a lubricator can occur, causing damage to equipment and operator injury or death.

Coil-induction systems

Until recently, it has simply been accepted that the only way to detect a plunger is by using current induced in a coil as the plunger passes by the sensor. This current is used to close a switch. The switch’s open/closed status is communicated to a nearby control system, which then actuates one or more pneumatic valves to control the flow of gas and fluids. Plunger detection also is used frequently in various optimization methods.

The problems with coil-based technology are numerous. To get the necessary amount of current, the plunger must be made of ferrous materials and must travel by the sensor at a minimum speed. If these conditions are not met, an arrival can easily be missed.

False detections also are frequent with coil-based technology. With no way to determine differences between plungers and outside influences such as a nearby compressor, sensors easily can falsely detect an arrival, throwing off optimization routines.

Coil-based sensors present a number of additional challenges such as variations in coil manufacturing, shorter product life, and high power consumption.

Magnetic sensing technology

The oil and gas industry is adopting a new sensor architecture that uses a magnetic integrated circuit to detect the magnetic field and translate it into the digital domain.

This solution not only reduces power consumption but also allows more detailed processing to occur. A range of digital filters and algorithms can easily be applied to ensure accurate interpretation of the magnetic field as opposed to simply relying on a spike in current coming off a coil. Not only are the issues surrounding slow-moving plungers eliminated, but the ability to adjust sensitivity is added, thus allowing for more accurate detection of a wider range of plunger materials and types and at virtually any velocity.

The lower sensitivity settings are used with ferrous plungers that generate a large-magnitude change. This allows the sensor to ignore small fluctuations and eliminates the problems associated with false detects from vibrations and electrical interference from surrounding equipment. Conversely, sensitivity can be increased when dealing with smaller or nonferrous plungers that generate a smaller magnitude signal, which eliminates missed arrivals.

This fundamental shift in the design of plunger arrival sensors also allows a number of other benefits. These include, but are not limited to, less dependence on operational voltage, a wider range of interfaces, lower power consumption, and more visibility of the signal in the magnetic domain. This increased visibility allows streaming and logging of real-time data that show the exact behavior of the signal as a plunger enters the field of view. This allows a greater understanding of what a plunger looks like in the magnetic domain, how to interpret what is and is not a plunger, and how to adjust the settings to give plunger-lift operators and technicians the reliable detection they seek. The establishment of this platform enables development of enhancements that were not previously possible. The next wave of this innovation includes interpreting these waveforms to derive an accurate velocity of the plunger at surface. This not only provides the operator critical safety information, but also allows prevention of costly and dangerous events that are commonplace in plunger lift. It also provides more accurate information for corrective measures and enhanced optimization algorithms.

Field success

An oil and gas optimization service provider based out of Sylvan Lake, Alberta, Canada, had a customer experiencing issues with detecting arrivals of the plunger. This particular well used 2-3/8-in. tubing with a 2-7/8-in. flanged wellhead swaged to a 2-3/8-in. lubricator. Due to heavy wellhead materials, the original coil-based sensor could only be mounted on the lubricator as it was not sensitive enough to detect through the wellhead.

The plunger was originally cycling 12 times per day, given it was sensed on every arrival, with production output of 8,000 cu m/d (283,000 cf/d). As the well depleted, velocities were not adequate to ensure the plunger would travel through the larger wellhead to the sensor. As soon as the plunger came to surface, it would slow down quickly and stop moving upward. Many times it did not reach the sensor or would slow down substantially before passing the sensor.

A coil-based sensor requires the plunger to move completely past the sensor at a fast enough speed. As in most plunger control systems, if the plunger did not appear to arrive, the well would be shut in longer.

For this well the coil-based sensor was detecting arrivals approximately two out of three times, leading to an unnecessary trip and additional close time. In addition, the extra trips meant there was not as much water on the well, leading to a faster rise of the plunger, which is potentially unsafe.

To try to achieve more consistent arrivals on the case study well, the plunger style and parameters within the control system were changed. This was successful but at the cost of production because of more downtime. Production decreased to 4,500 cu m/d (159,000 cf/d).

A Cyclops magnetic sensor was installed directly on the wellhead, original cycle times were achievable immediately, and production was restored to its original rate. The magnetic sensor was not only able to detect the plunger through the thick material of the wellhead, but also was not reliant on plunger velocity and did not require the plunger to completely pass by it to detect.