Acoustic Flow Monitoring Helps Optimize Swabbing Program for Shallow Gas Wells

Abstract

Technology Update Conducting economically effective swabbing programs in fields of unmetered shallow natural gas wells is challenging at today’s low gas prices. Producers have long been aware that not all wells respond positively to interventions such as swabbing. At higher gas prices, knowledge of individual well responses to interventions is not critical, as it makes economic sense to swab wells in groups if the increase at the group meter justifies the cost. As gas prices decline, producers have decreased expenditures on swabbing programs as returns became harder to realize. However, producers can maximize returns on interventions such as swabbing by using acoustic technology to locate high-performing wells and then focusing on maximizing returns from those wells. Acoustic Flow Measurement Acoustic technology is employed in a number of applications in the oil and gas industry, including pipeline monitoring and downhole leak detection. In this type of application, acoustics are used to measure the flow of natural gas in shallow wells. Turbulence in the pipe is interpreted into a flow measurement by means of an acoustic sensor that is inexpensive, unobtrusive from an operational perspective, and easy to install. With this device, producers can detect the response to an intervention, decide if the well rates as a high performer and then, if appropriate, move the device to another well. The following four stages of detection, processing, transmission, and representation describe the monitoring process. The nonintrusive acoustic sensor is mounted to the (typically 2-in.) flowline to detect the signal. Where many flow measurement methods employ straighteners to reduce turbulence, it is turbulence in the flow that creates the strongest acoustic signal. Locations close to rigid objects such as support pilings, which restrict pipe movement and dampen vibration, are avoided whenever possible. Using a magnetic tip to ensure a strong coupling between the pipe surface and the sensor diaphragm, the sensor converts the pipe’s mechanical vibration energy into an acoustic signal. The sound is then focused and amplified mechanically by an acoustic chamber. Finally, a microphone in the chamber converts the signal into a voltage for processing. In the processing stage, the controller tests the signal for frequencies below 15 kHz. A microprocessor handles the Fourier transform to perform on-site frequency analysis powered by the unit’s built-in solar panel and battery. If the signal is weak, weather-related or other noises such as chattering check valves or loose label tags clanging against the pipe can interfere with frequencies in the range of interest. Filtering may be employed to improve the signal-to-noise ratio by removing as much nongas related noise as possible. The controller then repeatedly samples the acoustic signal to build hourly averages.