|  Definitions, Sources and Explanatory Notes |
|--------------------------------------------------|
|  Category:   | Natural Gas Storage                       |
|-----------------------------|-------------------------------------------|
|                             |
|  Topic:      | Weekly Working Gas in Underground Storage |
|                         |
|-------------------------|
|   Definitions |  |
|                         |
| Key Terms               | Definition                                                                                                                                                                                                                                                                                 |
|-------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| Base Gas                | The volume of gas needed as a permanent inventory to maintain adequate reservoirpressures and deliverability rates throughout the withdrawal season. All native gas is included in the base gas volume.                                                                                  |
| Natural Gas             | A gaseous mixture of hydrocarbon compounds, the primary one being methane.                                                                                                                                                                                                                 |
| Underground Gas Storage | The use of sub-surfacefacilities for storing gas that has been transferred from its originallocation. The facilities are usually hollowed-out saltdomes, natural geological reservoirs (depleted oil or gasfields) or water-bearing sands topped by an impermeable caprock (aquifer). |
| Working Gas             | The volume of total natural gas storage capacity that contains natural gas available for withdrawal.                                                                                                                                                                                       |
| East Region             | Connecticut, Delaware, District of Columbia, Florida, Georgia, Massachusetts, Maryland, Maine, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Vermont, Virginia, and West Virginia                                                 |
| Midwest Region          | Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, Tennessee, and Wisconsin                                                                                                                                                                                                 |
| Mountain Region         | Arizona, Colorado, Idaho, Montana, Nebraska, New Mexico, Nevada, North Dakota, South Dakota, Utah, and Wyoming                                                                                                                                                                             |
| Pacific Region          | California, Oregon, and Washington                                                                                                                                                                                                                                                         |
| South Central Region    | Alabama, Arkansas, Kansas, Louisiana, Mississippi, Oklahoma, and Texas                                                                                                                                                                                                                     |
|                                                                               |
|-------------------------------------------------------------------------------|
| For definitions of related energy terms, refer to the   EIA Energy Glossary. |
|  	Sources |
|-----------------|
|  |
|--|
|  | Week ending Dec. 31, 1993: Form EIA-191, "Monthly Underground Gas Storage Report".
Weeks January 7, 1994 through February 22, 2002:ESTIMATES derived from a computation process that uses both EIA monthly survey data and American Gas Association weekly survey data.
Week ending March 8, 2002: linearly interpolated between the derived weekly estimates that end March 1 and the initial estimate from the EIA-912 on March 15.
Weeks beginning with March 15, 2002: Form EIA-912, "Weekly Underground Natural Gas Storage Report"  
- Natural Gas Survey Forms and Instructions       	Explanatory Notes      This table tracks U.S. natural gas inventories held in underground storage facilities. The weekly stocks generally are the volumes of working gas as of the report date. Changes in reported stock levels reflect all events affecting working gas in storage, including injections, withdrawals, and reclassifications between base and working gas.  
  The weekly storage estimates are based on a survey sample that does not include all companies that operate underground storage facilities. The sample was selected from the list of storage operators to achieve a target coefficient of variation of the estimate of working gas in storage which was no greater than 5 percent for each region.  EIA maintains a  historical database of estimated measures of sampling variability for published estimates of weekly stocks and their net changes dating back to April 2015.   
  For a comparison between estimates of end-of-month working gas based on weekly survey data and EIA’s monthly census of all underground storage operators, see Differences Between Monthly and Weekly Working Gas in Storage.   
  This data with supporting charts and analysis can be found in the  Weekly Natural Gas Storage Report (WNGSR).  
The Basics of Underground Natural Gas Storage.  
  Complete documentation of EIA's estimation methodology for WNGSR, which includes a description of the methodology for producing estimated measures of sampling variability for estimates of weekly stocks and their net changes, is available in the report, Methodology for EIA Weekly Underground Natural Gas Storage Estimates. In addition, EIA has provided answers to frequently asked questions  about incoporating estimated measures of sampling variability for published estimates of weekly stocks and their net changes in WNGSR.  
  In October 2003, EIA updated the estimation methodology for the Weekly Natural Gas Storage Report and revised the weekly data back to July 2003 at that time.  As a result, a portion of the net change between 6/27/03 and 7/4/03, 142 billion cubic feet, is attributable to the change in methodologies.  
  In November 2015, new regions were implemented for the WNGSR. A new history was created back to January 2010 to reflect weekly estimates for the new regions. Prior history with the former regions (east/west/producing) is available in an archive history. |