This report includes enhancements to the data for 2011. EIA has been monitoring the status of storage fields and confirmed that some dormant fields totaling 22 billion cubic feet could not be readily used for natural gas storage. Data on working gas design capacity for 2011 reflect the elimination of these dormant fields from inventory. Data for 2010 on working gas design capacity are shown both including and excluding the inactive fields to show the effect of the change. These data are available for download in the Natural Gas Annual Respondent Query System.

Estimates of Natural Gas Storage Capacity and Historical Maximum Storage Volumes, as of April 2010 and April 2011 (Billion cubic feet, unless otherwise noted)
	Demonstrated Peak Working Gas Capacity1		Working Gas Design Capacity2			Demonstrated Peak Working Gas Capacity as Percent of Working Gas Design Capacity		Maximum End-of-Month Working Gas Inventories3
			Including Inactive Fields4	Active Fields Only (Excluding Inactive Fields)
Region	Apr-10	Apr-11	Apr-10	Apr-10	Apr-11	Apr-10	Apr-11	Apr-10	Apr-11
East	2,196	2,205	2,281	2,276	2,288	96%	96%	2,097	2,097
Producing	1,297	1,340	1,383	1,366	1,406	94%	95%	1,224	1,263
West	556	558	688 R	688	694	81%	80%	524	524
Lower 48	4,049	4,103	4,353 R	4,331	4,388	93%	94%	3,833	3,847
Totals may not equal sum of components because of independent rounding. Note: 1) 'Demonstrated Peak Working Gas Capacity' is the sum of the highest storage inventory level of working gas observed in each facility over the prior 5-year period as reported by the operator on the Form EIA- 191M, "Monthly Underground Gas Storage Report." The timing for peaks for different facilities need not coincide. 2) 'Working Gas Design Capacity' is an estimate of a natural gas facility's working gas capacity, as reported by the operator on the Form EIA-191A "Annual Underground Gas Storage Report," and represents the sum of the result across all fields. 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. 3) 'Maximum End-of-Month Working Gas Inventories' is the maximum volume of gas reported in the Natural Gas Monthly by region or the lower 48 States. These data are the aggregation of gas in storage as reported by respondents on Form EIA-191M, "Monthly Underground Gas Storage Report" for a given month. For the 5-year period ending April 2011, the maximum working gas in storage for the East Region was reached in October 2009; November 2009 for the West Region, October 2010 for the Lower 48 States, and the maximum working gas in storage for the Producing Region was reached in November 2010. 4) Working gas design capacity estimates for 2010 includes the capacity for facilities that were recently confirmed to be inactive in 2011. Removing the inactive fields had the effect of lowering estimates of working gas design capacity by 22 Bcf for the lower-48 States. Further discussion of inactive facilities is included in the Enhancements reference. R - Revised Information about storage regions is available at "Storage Basics." Sources: Form EIA-191M, "Monthly Natural Gas Underground Storage Report" and Form EIA-191A, "Annual Natural Gas Underground Storage Report."

The demonstrated peak working gas capacity for U.S. underground working natural gas storage for the lower-48 States is estimated at 4,103 billion cubic feet (Bcf) as of April 30, 2011, an increase about 1 percent since last year at this time. This estimate of working gas capacity is based on demonstrated noncoincident peak working gas storage volumes for individual active gas storage facilities reported to EIA over the 60-month period ending in April 2011. Working gas design capacity also increased by about 1 percent, to 4,388 Bcf.

Demonstrated peak working natural gas capacity as a percent of working gas design capacity is lower in the West Region than the East and Producing Regions for several reasons. The West Region has several still-active fields whose primary role is not seasonal storage. These include fields used for pipeline load balancing and fields that are being drawn down to be taken out of service. Also, some fields in the West Region have large design capacities, but have infrastructure constraints such as limited pipeline and compression capacity that limit actual storage utilization and peak capacity.

Summary of Findings

Demonstrated peak working gas capacity in the lower-48 States increased by 54 Bcf between April 2010 and April 2011. The majority of this increase occurred in the Producing Region, where demonstrated peak working gas capacity increased by 43 Bcf. This increase in demonstrated working capacity in the Producing Region resulted in part from the increased utilization of existing storage facilities and the completion and expansion of new storage projects.

Working gas design capacity increased 57 Bcf in the lower-48 States from April 2010 through April 2011, with 40 Bcf of the increase occurring in the Producing Region. The estimates of working gas design capacity reflect downward adjustments for dormant storage fields of 17 Bcf in the Producing Region, 5 Bcf in the East Region, and totaling 22 Bcf for the lower-48 States. Capacity additions in the Producing Region were primarily high-deliverability salt dome facilities.

Demonstrated peak working gas capacity relative to design capacity has increased since April 2010. This pattern of growth occurred primarily in the Producing Region, with peak usage rates climbing by nearly 2 percent. Changes in demonstrated peak working gas capacity relative to design capacity in the other regions were less, with the East region increasing less than one percent and the West declining 1 percent.

The rate of peak demonstrated working gas storage capacity additions slowed between April 2010 and April 2011, compared with the previous 12-month period. As of April 30, 2011, demonstrated working gas peak capacity had increased 54 Bcf. This compares with net additions of 160 Bcf to working gas demonstrated peak between April 2009 and April 2010. The slowing rate of increases in demonstrated peak working gas capacity during 2011 is, in part, a result of fewer storage capacity completions during 2010-2011. Several storage capacity expansion projects are currently under construction in the lower-48 States (primarily in the Producing and West Regions), but are not expected to be completed in 2011.

Working gas stocks as of March 31, 2011, totaled 1,581 Bcf, according to the Natural Gas Monthly. Overall, working gas stocks were slightly above the 5-year (2006- 2010) average inventory level of 1,576 Bcf entering the 2011 injection season (April through October). Working gas stocks are expected to reach 3,738 Bcf by the end of October 2011, according to the September 2011 Short-Term Energy Outlook. This would leave working gas stocks about 3 percent below the record levels in storage reported in October 2010.