The Gas Index Model estimates life cycle greenhouse gas (GHG) emissions due to natural gas consumed in the contiguous U.S. The primary focus is on more accurate accounting of methane leakage from the gas system, and granular data on the life cycle methane leakage for gas consumed in various U.S. cities.
First, GIM estimates methane emissions from individual production areas, based on extensive measurements of methane leakage reported in the literature and recent gas production rates. The model represents all natural gas production in the contiguous U.S., including offshore. The model allocates a portion of the methane leakage to natural gas that is consumed by end users, based on energy content of consumer-grade natural gas produced; the remaining methane leakage is allocated to natural gas liquids and oil. For each gas-producing state, the model sums methane emissions for methane leaked from oil and gas production operations that is attributable to gas consumed by end users. Then the model calculates the methane leakage intensity for dry gas produced (quantity of methane leaked per unit of dry gas), as well as methane leakage intensity for natural gas liquids and oil produced.
Second, the model uses EIA data on flows of gas supplies across state lines to estimate the origins of the gas consumed in each state. Based on the origins of the gas in various producing states (or other nations), the model calculates an average methane leakage intensity for the gas consumed in each state.
Third, for particular cities, the model calculates the approximate distances traveled by the gas through transmission pipelines, from the production areas to cities where gas is consumed, from each of the producing states. GIM calculates an average distance gas travels to reach each city, and estimates methane leakage from the transmission system on that basis.
Fourth, based on data for the gas utilities serving particular cities, detailing the utilities’ distribution networks and customer base, the model estimates methane leakage from pipelines within cities and from customer gas meters. For cities that have had comprehensive measurements of methane leakage, or credible reports of methane leakage from gas utilities, the model adds additional methane leakage based on these measurements or reports. For gas utilities’ systems, methane leakage is allocated separately to residential and commercial customers, and to industrial and electric power customers.
Fifth, the model estimates methane leakage from buildings, known as “behind the meter” or “beyond the meter” leakage. This leakage has been found to occur from gas pipes within buildings and from gas-burning appliances (e.g., furnaces, water heaters, stoves).
The model also calculates additional citywide leakage may be occurring, for those cities that have had measurements of methane leakage and estimations of the fraction that originates from natural gas. This additional leakage is above what is estimated for citywide leakage in the model.
Finally, the model estimates changes in greenhouse gas emissions when switching residential/commercial heating from gas heaters to electric heaters.
For a complete methodology, please visit the Gas Index site.