A world first partnership for long-term, real-time monitoring of a large underground gas storage field.
When California, USA-based utility company Pacific Gas and Electric (PG&E) required an accurate and reliable system for monitoring a large underground gas storage (UGS) field, it partnered with advanced sensing technology company, Paulsson, to seek research funding from California Energy Commission (CEC) and to develop and field test a real-time 24/7 monitoring system. The CEC approved the proposal resulting in a three-year research project, which started in mid-2020.
Paulsson is known for inventing, designing, manufacturing and deploying all-optical sensors into deep, high-temperature and high-pressure boreholes. In this instance, its solution was an acoustic and ultra-large aperture antenna based on fibre optic sensing technology, using distributed acoustic sensing (DAS) technology from Fotech, a bp Launchpad company.
Since installation, the system has achieved real-time gas flow detection and monitoring both in the well and in the formation, which is a world first for this type of application. The system has also recorded numerous small and large earthquakes from nearby faults and those as far away as Japan and Peru, providing valuable information about the UGS formation properties using these natural near and far seismic sources.
A Need For Accurate Monitoring
PG&E is one of the largest public utility companies in the US, owning and maintaining more than 6,000 miles of transmission pipelines, 42,000 miles of distribution pipelines and three natural gas storage facilities.
PG&E’s largest gas storage field is based at McDonald Island in the Sacramento-San Joaquin River Delta in California. It has a maximum capacity of 82 BCF and can provide 25% of Northern California’s winter peak-day gas demand. There are 87 total wells, 81 of which operate for injection and withdrawal; the other six operate as observation wells. When gas demand is low, gas is injected into underground storage facilities. When demand is highest during winter months, dry hydro-drought years, and a hot summer’s AC peak, gas is withdrawn to generate electric power.
Monitoring of gas storage fields is vital to ensure the safety of the local community and environment, to potentially optimise operations and diversify gas resources, and to protect shareholders and ratepayers from the high cost of disruptions and accidents. The Californian utility company recognised it needed a robust and accurate way of monitoring its field and partnered with Paulsson to provide and test a solution.
Paulsson offers extremely accurate sub-surface surveying and monitoring solutions using fibreoptic acoustic and vector seismic sensors as well as distributed temperature sensors (DTS) and distributed strain sensor (DSS) technologies. The company has many years of experience developing, deploying and processing data from high-pressure and temperature non-electronic all-optical sensor arrays capable of operating at pressures up to 25,000 psi and at temperatures up to 600°F (320°C) in both vertical and horizontal boreholes.
After receiving a competitive US$1.5 million grant from the California Energy Commission in partnership with PG&E and Fotech, Paulsson set about designing, building, installing and monitoring a sensor array in the PG&E McDonald Island UGS field. Central to the system is Rayleigh-scattering-based interrogation technologies from Fotech using an enhanced distributed acoustic sensor array (EDAS) designed by Paulsson.
Dr Björn Paulsson, CEO at Paulsson, says, “Fotech is trusted for its DAS technology. I knew that its equipment would provide the best possible monitoring and detection for this application. The DAS system has performed extremely well and has provided robust, reliable and accurate data 24/7 for a period of, thus far, eight months, recording over 400TB of transformative data.”
How DAS Works
Fotech’s Helios DAS solution essentially turns a fibreoptic cable running alongside horizontal gas pipelines or vertical gas or geothermal wells into thousands of vibration sensors, able to detect any disturbances along the length of the pipeline or the well casing.
The technology sends thousands of pulses of light along the fibre optic cable every second and monitors the fine pattern of light scattered back. When acoustic or vibrational energy – such as that created by gas flow in the well or the formation – creates a minute strain on the optical fibre, this changes the back-scattered light pattern. By using advanced algorithms and processing techniques, DAS records these changes to identify and categorise any disturbance in real-time, thus allowing immediate processing and interpretation – and, if needed – corrective action to be taken.
Steve Cammish, chief technology officer at Fotech, says, “Each type of disturbance has its own signature, and the technology can tell an operator, in real-time, what happened, exactly where it happened and when it happened. What’s more, DAS provides continuous monitoring 24/7 and over long distances of up to 100km along pipelines or along and near the deepest wells drilled.”
The solution for PG&E saw a fibreoptic cable attached to the 4.5in OD production tubulars, which were lowered down a wellbore inside the 8 5/8” casing for approximately 2km. Using DAS, the array effectively consists of approximately 2,500 sensors, providing accurate real-time monitoring that makes real-time processing, interpretation and action possible.
Having this volume of sensors means Paulsson was able to effectively create a large aperture acoustic antenna so the flow in the gas field wells and in the geological formation where the gas is stored, as well as any other events, can be detected and pinpointed to within 1m. DAS has also enabled Paulsson to differentiate between the high permeability sandstone layers and the low permeability shale so a dynamic model of the gas storage field formation can be built.
Paulsson says, “This is a very exciting step forward for the technology as we have been able to detect gas flow in a reservoir away from the well for the very first time. Thanks to DAS’s exceptional accuracy and continuous monitoring capability, it doesn’t just provide us with static data; it allows us to see how the field reacts to the injection of fluids, both gas and liquids, in real-time. This means the data from DAS will allow us to create a 3D image of the ground 1-2km around the borehole, providing complete formation and dynamic flow visibility to our customer, PG&E.”
Traditionally, geophones spatially separated by approximately 15m (50ft), or more, have been used in such applications, but the conditions in boreholes are extremely demanding, with high pressures and high temperatures. Another critical aspect is the need to be able to record the high seismic frequencies found in the sub-surface below the weathering zone.
“Using fibreoptics and DAS is better suited to boreholes’ challenging conditions as they can withstand the extreme environment, even at geothermal temperatures. The DAS technology allows for the first-time large arrays of many 1,000s of sensors to be installed, creating large aperture high-frequency sensor antennas with close spatial sensor sampling using the optical technologies,” Paulsson continues.
A Solution That Provides Total Visibility
It is not only gas flow that Paulsson has been able to detect and monitor, but also earthquake events on the other side of the Pacific Ocean in Fukushima, Japan, and Peru, South America. Paulsson says, “The sensor array has picked up acoustic signals from earthquakes over 8,000kms away, in Japan and Fiji, which is amazing. Being able to detect other large-scale seismic events based on tectonics means we can provide a clear picture of what is happening in the many far and near fault zones in the pacific basin. PG&E is pleased with the results our monitoring solutions have provided.”