Emissions Big Picture: Metering Closes the Accuracy Gap
Monitoring on the ground is helping the industry shift from best estimates to hard data so it can bring the true emissions profile into focus.
The F2X unit at the Macaw Energies facility in Houston prior to being deployed for the south Texas pilot.
Source: Macaw Energies.
In business, it’s long been said that what gets measured can be managed. Naturally, the oil industry is keen to measure, and manage, greenhouse gas emissions. If it can also profit from what would otherwise be a waste stream? All the better.
But how should these gases be measured—whether they are released during completion flowback operations, through flaring of associated gas at an oil field, or via leaks?
Meters are an obvious go-to method for directly measuring emissions at the point of release, but satellites are increasingly being used for the big-picture—or, ahem, 30,000-ft view—they can provide.
The use of satellites to monitor emissions raises an important question: How accurate is that big-picture view?
That is a question Petroleum Development Oman (PDO) wanted answered as it worked to reduce its flaring volumes, and an SLB customer needed to understand it with regard to estimated emissions from completion flowback operations.
In 2020, the World Bank’s Global Flaring and Methane Reduction (GFMR) Partnership issued a global gas flaring tracker report that ranked Oman, which had committed to the World Bank’s Zero Routine Flaring Initiative in 2016, as the 10th highest flaring country in the world.
In the 2023 report, Oman had reined in its flaring levels enough to improve its standing to 13th.
PDO was reporting flaring data to the World Bank, but the data used in the reports differed from what PDO supplied, Najma Al Waily, energy and water policy advisor at PDO, said during a technical session at SPE’s Annual Technical Conference and Exhibition (ATCE) in Houston in October.
That discrepancy triggered a study to validate flare volume measurements using metered flare volume data and comparing that data to flared gas volumes estimated by Visible Infrared Imaging Radiometer Suite (VIIRS) Nightfire, which is generated from satellite-based observations.
The study, described in SPE 228079, analyzed data from four PDO facilities selected due to “noticeable” data discrepancies, which were attributed to limited data availability, data synchronization issues, metering quality concerns, and inadequate satellite readings due to low flow rates.
“We looked into the variance between the satellite data and our measurements in 2022 and 2023,” she said.
PDO monitors flow rates using Class 4 ultrasonic flowmeters and uses practices like comprehensive flare meter mapping, a flare measurement dashboard, monthly flare gap-to-potential huddles, monthly flare meter health huddles for operational reviews, valve replacement/repair, and flare reporting audits to help it reduce its flare volumes, the authors wrote.
The PDO team analyzed flowmeter readings obtained at 1-minute intervals from the flare sites and assessed the accuracy of VIIRS Nightfire data using two satellite calibration methods of those same sites in a bid to understand whether the World Bank’s flaring data for PDO assets may have been overestimated, she said.
The calibration methods in question were those developed by Cedigaz (the National Association for Natural Gas Information) and John Zink. Cedigaz uses a linear formula that assumes steady combustion efficiency and a typical hydrocarbon composition, while John Zink applies an empirical, nonlinear formula developed from controlled flare experiments, according to the paper.
The John Zink approach showed better accuracy for small flare volumes than the Cedigaz calibration.
At one facility PDO analyzed, two flare stacks were measured using an independent ultrasonic flowmeter and recorded in real time, while satellites recorded only one reading that is considered the sum of both flares (Fig. 1). The flowmeter data remained steady, while the satellite data showed “high noise.” The authors wrote that the satellite readings from March to May were higher, whereas readings from June onward were lower because of heavy cloud cover.
Fig. 1—Facility A, comparison of satellite with flowmeter.
Source: SPE 228079.
Data gathering via satellite can be limited by smaller volumes as well as weather conditions, such as clouds, Al Waily said, adding, “As an outcome, (the World Bank is) going to change the calibration method from Cedigaz to John Zink to have better accuracy.”
The Flowback Anomaly
Completions flowback is an area where estimated methane emissions may differ wildly from data derived from direct measurements.
Manasi Doshi, senior research scientist at SLB, said while presenting SPE 227951 at ATCE that the discrepancy between measured and estimated emissions can be significant and that the completions flowback process itself—when fluids return to the surface from the wellbore after hydraulic fracturing operations—is a major reason.
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