Methodology

Demonstrated Peak Working Gas Capacity Estimates: Estimates are based on aggregation of the noncoincident peak levels of working gas inventories at individual storage fields as reported monthly over a 60-month period ending in April 2012 on Form EIA-191M, "Monthly Natural Gas Underground Storage Report." This data-driven estimate reflects actual operator experience. However, the timing for peaks for different fields need not coincide. Also, actual available maximum capacity for any storage facility may exceed its reported maximum storage level over the last 5 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 that may understate the amount that can actually be stored.

Data from Form EIA-191M, "Monthly Natural Gas Underground Storage Report," are collected from storage operators on a field-level basis. Operators can report field-level data either on a per reservoir basis or on an aggregated reservoir basis. It is possible that if all operators reported on a per reservoir basis, then the demonstrated peak working gas capacity would be larger. In addition, 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 recorded on Form EIA-191M.

Working Gas Design Capacity Estimates: Estimates are based on the aggregation of the design working gas capacity of individual storage fields as reported on Form EIA-191A, "Annual Natural Gas Underground Storage Report." Facilities are required to submit Form EIA-191A when changes occur in their design capacities or when new facilities come online. These estimates represent an aggregation of the most recent Form EIA-191 capacities on file per facility. It 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. There were no reported injections of working gas into the field during the report year on EIA-191A, and
3. The respondent to the EIA-191A confirmed that no injections of working gas into field are expected to occur during the upcoming calendar year.

Until the 2011 data collection of the EIA-191A reporting natural gas storage capacity, EIA maintained fields satisfying the first two conditions in the inventory of natural gas storage fields. These dormant storage fields were maintained in the inventory of storage fields and included in the calculation of working gas design capacity, because they represented potential sources natural gas and natural gas storage that could be used. However, outreach with respondents to the EIA-191 indicates that these dormant fields could not be readily used for natural gas storage. EIA will continue to monitor the status of these fields. If an inactive field is restored to service, and working gas is injected into the field, the status will be changed to active.