Characterising the distribution of methane and carbon dioxide emissions from the natural gas supply chain

The use of natural gas in global energy systems has increased substantially over the last decade as countries are driven toward decarbonisation. Natural gas exhibits lower combustion emissions than coal or oil, but life cycle supply chain emissions are significant and highly variable. In particular, methane is the main constituent of natural gas and is itself a very strong greenhouse gas. Results from methane emissions studies are often conflicting and vary widely, rendering average emission values unrepresentative. A knowledge of the emissions range, and the distribution within that range, is vital in developing representative average estimates, estimating regional or national emissions, or determining the impact of a change in policy or technology. Many factors affect supply chain emissions of methane and carbon dioxide, such as the reservoir type, supply chain route, equipment and operational practises, or regional regulation. However, there remains a lack of understanding of the contribution of each factor and why. 

This study defines the distribution of total methane and CO2 emissions from different natural gas supply chain routes, identifying the contribution from each stage and quantifying the effect of key parameters on emissions. The study uses recent high-resolution emissions measurements with estimates of parameter distributions to build a probabilistic emissions model and Monte Carlo simulation for a variety of technological supply chain scenarios.

The distribution of emissions is extremely heavily skewed, resembling a log-log-logistic distribution for most supply chain scenarios: median estimates which represent typical facilities are modest at 18 - 24 g CO2 eq./ MJ HHV, but mean estimates which account for the heavy tail are 22 - 107 g CO2 eq./ MJ HHV. To place these values into context, emissions associated with natural gas combustion (e.g. for heat) are approximately 55 g CO2/ MJ HHV. Thus, some supply chain scenarios are major contributors to total greenhouse gas emissions from natural gas. For methane-only emissions, median estimates are 0.8 - 2.2% of total methane production, with mean emissions of 1.6 - 5.5%. The heavy tail distribution is the signature of the disproportionately large emitting equipment known as super-emitters, which appear at all stages of the supply chain. The study analyses the impact of different technological options and identifies a set of best technological option (BTO) scenarios. This suggests that emissions-minimising technology can reduce supply chain emissions significantly, with this study estimating median emissions of 0.9% of production. However, even with the emissions-minimising technologies, evidence suggests that the influence of the super-emitters remains. Therefore, emissions-minimising technology is only part of the solution: reducing the impact of super emitters requires more effective detection and rectification, as well as pre-emptive maintenance processes.