Methodology

Demonstrated Peak Working Gas Capacity Estimates: We base estimates on aggregation of the non-coincident peak levels of working gas inventories at individual storage fields as reported monthly over a 60-month period on Form EIA-191, Monthly Natural Gas Underground Storage Report. This data-driven estimate reflects actual operator experience. However, the timing of peaks for different fields need not coincide. Actual available maximum capacity for any storage facility may exceed its reported maximum storage level over the last five years and is virtually certain to do so in the case of newly commissioned or expanded facilities. Therefore, this measure provides a conservative indicator of capacity and may understate the amount that can be stored.

Data from Form EIA-191, Monthly Natural Gas Underground Storage Report, are collected from storage operators on a field-level basis. These data reflect inventory levels as of the last day of the report month, and a facility may have reached a higher inventory on a different day of the report month, which would not be reported on Form EIA-191.

Working Gas Design Capacity Estimates: We base estimates on the aggregation of the working gas design capacity of individual storage fields as reported on Form EIA-191, Monthly Natural Gas Underground Storage Report. Facilities are required to report changes in their design capacities on Form EIA-191, and new facilities must begin reporting as soon as they come on line.

Working gas design capacity is a measure based on the physical characteristics of the reservoir, installed equipment, and operating procedures particular to the site that is often certified by federal or state regulators. However, logistical, operational, and practical considerations may preclude attainment of the maximum design capacity of a storage field, so the sum of design capacities may exceed actual available maximum storage capacity.

Inactive Fields: A field is classified as inactive if all the following conditions are true:

1. Working gas has been depleted.
2. No injections of working gas into the field were reported during the report year on EIA-191.
3. The respondent to the EIA-191 confirmed that no injections of working gas into field are expected to occur during the upcoming calendar year, or the field is being decommissioned or plugged, or both.

If an inactive field is restored to service and working gas is injected into the field, the status will be changed to active.