Methodology

Demonstrated Peak Working Gas Capacity Estimates: We calculate these estimates by aggregating 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 past five years, and it 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 are collected from storage operators on a field-level basis. These data reflect inventory 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 calculate estimates by summing the working gas design capacity of individual storage fields as reported on Form EIA-191, Monthly Natural Gas Underground Storage Report. Facilities must report changes in their design capacities on Form EIA-191, and new facilities must begin reporting as soon as they come online.

Working gas design capacity is a measure of the physical characteristics of the reservoir, installed equipment, and operating procedures particular to the site—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: Fields designated as inactive must meet all the following conditions:

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

By restoring service, inactive fields can be reclassified as active.