Various techniques can be used to estimate methane emissions. For instance, methane concentrations can be measured locally in ambient air with gas analyzers (e.g., Maazallahi et al. 2023; MacMullin and Rongère 2023; Kumar et al. 2021) or regionally in the atmosphere using satellite, drone, and other imaged-based techniques (Pandey et al. 2023; Duren et al. 2019). Another strategy can be followed in the very specific case of natural gas network utilities. These consist of both transmission networks and distribution networks. Immediately after production at the field, the gas produced is purified and prepared for pumping. Pumped by compressor stations to high pressures, it enters the transmission network designed to transport substantial volumes of gas across extensive distances. Gas flows into the high-pressure pipeline network via a number of entry sites, delivered either via pipelines from offshore or onshore gas fields, or via international transit pipelines and, in some cases, LNG imports. At all entry points, the gas must be measured with great accuracy for fiscal reasons. At some point, the gas leaves the gas transmission networks and enters the distribution networks. The interface between both systems requires accurate gas metering. The distribution networks, which are adjusted to gradually reduce pressure, then transport gas from transmission networks to end consumers. Again, gas metering is essential for billing purposes. The imbalance between gas entering the network and gas exiting is unaccounted for gas (UFG).
In what follows, the aim is to estimate UFG volumes in the world's networks, based on data compiled in a database by the CEDIGAZ association. This international non-profit association was founded in 1961 and is widely recognized as a reliable source of information by gas analysts. Over the years, it has collected a large amount of data from natural gas network operators and created a database covering more than 120 countries.
We define UFG as (CEDIGAZ 2023):
UFG = Leakage + Own use gas + Theft
“Leakage” refers to gas losses due to daily operations, venting, accidentally damaged pipes or metering errors. Leaks can occur on various devices such as compressors. “Own use gas” (OUG) is the gas used by the network utility to deliver the gas it transports to the customers. It is then used to support network operations, in the form of preheating and fuel for compressor stations. For example, gas must be heated prior to pressure reduction, through the preheating process, to mitigate the negative effect of a temperature drop on equipment during pressure reduction. Besides, “Theft” represents the gas withdrawn from the network by unauthorized third parties.
Leaks from high-pressure gas transmission pipelines are generally very rare and very low, as the high-pressure gas transmission network consists of welded steel high-pressure pipelines, which do not normally leak. Under normal circumstances, the percentage of UFG associated with gas transport systems is therefore quite low, typically less than 0.5% of total flows. On the other hand, UFG levels in distribution networks can be much higher, usually in the order of 1 to 5%.
The evaluation of UFG volumes calls for data on UFG percentages, gas demand and pipeline network sizes. Demand data were extracted from the 2021 CEDIGAZ database. Pipeline lengths were sourced from transmission system operators (TSO) or the Central Intelligence Agency world factbook. The UFG percentages used at national level, defined as the ratios of UFG volumes to national gas demands, were those provided by national regulators, governments or TSOs, where available and reliable. Otherwise, there were two possible cases. First, UFG percentages were calculated as a weighted average of the UFG percentages reported by individual TSOs and distribution system operators (DSOs). Second, in the absence of any data, they were estimated on the basis of the age and physical characteristics of the pipeline network and the number of customers. The CEDIGAZ database lists 108 countries with gas demand above 0, ranging from 0.05 billion m3 to 868 billion m3 (USA). This list has been reduced to 82 countries based on the following criteria: annual gas demand greater than 1 billion m3; existing gas transmission/distribution networks; data of sufficient quality.
Table 1. UFG volume, mean UFG percentage, and resulting methane emissions per world regions in 2021 (CEDIGAZ 2023).
Region
|
Mean UFG (%)
|
Total UFG (bcm)
|
Resulting methane emissions (Mt)
|
Asia Oceania
|
2.04
|
11.77
|
3.02
|
Middle East
|
1.64
|
9.14
|
2.85
|
Africa
|
1.35
|
2.29
|
0.72
|
Central & South America
|
1.32
|
1.72
|
0.54
|
Europe
|
0.52
|
2.25
|
0.70
|
North America
|
1.88
|
19.87
|
6.20
|
CIS and Ukraine
|
1.95
|
12.68
|
3.95
|
World
|
1.67
|
59.72
|
17.98
|
UFG percentages by countries are displayed in Figure 1. They range from 0.01% to 15%, with an average value of 1.7%. The countries with the highest UFG percentages are Myanmar (15%), Syria (13.8%), Pakistan (11.7%), South Africa (8%) and Australia (4%). However, UFG percentages must be set against UFG volumes, which depend on the size and maturity of the natural gas market. For example, Myanmar's UFG percentage and volume are 15% and 0.615 billion m3 respectively, compared with 2.10% and 17.997 billion m3 for the USA, and 0.75% and 0.325 billion m3 for France. A 0.1 decrease in the percentage of UFG can result in a significant drop in UFG volume, depending on the country.
At the regional level (Table 1), the largest UFG percentages are in Asia-Oceania (2.04%) and the Commonwealth of Independent States (CIS) (1.95%), followed by North America (1.88%), the Middle East (1.64%), Africa (1.35%), Central and South America (1.32%), and Europe (0.52%). The UFG percentages for CIS and North America are above the world mean. They are also the two leading producers of natural gas with the oldest and largest natural gas networks.