Latest update: December 31, 2008

Note: 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 standard error of the estimate of working gas in storage which was no greater than 5 percent for each region. Based on a comparison of weekly estimates and monthly data from May 2002 through October 2008, estimated total working gas stocks have exhibited an average absolute error of 18 billion cubic feet, or 0.8 percent.

Background

The Energy Information Administration (EIA) provides weekly estimates of working gas volumes held in underground storage facilities at the national and regional levels. These are estimated from volume data provided by a sample of operators that report on Form EIA-912, "Weekly Underground Natural Gas Storage Report." The EIA first released its estimates of underground storage for the week ending May 3, 2002, on May 9, 2002, in the Weekly Natural Gas Storage Report (WNGSR). EIA also publishes data for the end-of-month working gas volumes that it collects from the form, EIA-191, "Monthly Underground Gas Storage Report." While these surveys both result in estimates of working gas in storage, some differences between the two series occur, which are useful to understand when comparing current weekly data estimates with values in prior years or to the corresponding data from the monthly series.

Table 1 presents the difference between estimates of working gas in underground storage as reported at the end of each month on Form EIA-191 (a monthly census of all underground storage operators) and end of month estimates interpolated from the weekly estimates (based on a sample of storage operators reporting on Form EIA-912).

Table 1. Monthly-Weekly Differences in Underground Storage Estimates (Billion cubic feet)

Download History (2002 to Present)

	Monthly From Weekly Values				Natural Gas Monthly				Difference
Month End	East Region	Producing Region	West Region	Total Lower 48	East Region	Producing Region	West Region	Total Lower 48	East Region	Producing Region	West Region	Total Lower 48
Jan-07	1,366	762	283	2,411	1,326	767	285	2,379	40	-5	-2	32
Feb-07	842	588	230	1,660	823	594	231	1,649	19	-6	-1	11
Mar-07	703	631	239	1,572	715	649	239	1,603	-12	-18	0	-31
Apr-07	742	682	268	1,692	757	696	267	1,720	-15	-14	1	-28
May-07	1,027	792	329	2,147	1,042	811	326	2,179	-15	-19	3	-32
Jun-07	1,288	871	377	2,536	1,309	899	372	2,580	-21	-28	5	-44
Jul-07	1,530	930	404	2,864	1,549	944	400	2,894	-19	-14	4	-30
Aug-07	1,696	903	406	3,005	1,709	903	405	3,017	-13	0	1	-12
Sep-07	1,876	972	436	3,284	1,898	979	438	3,316	-22	-7	-2	-32
Oct-07	2,013	1,058	463	3,535	2,032	1,070	465	3,567	-19	-12	-2	-32
Nov-07	1,932	1,045	463	3,440	1,939	1,055	462	3,456	-7	-10	1	-16
Dec-07	1,564	892	386	2,848	1,586	909	384	2,879	-22	-17	2	-31
Jan-08	1,155	677	258	2,091	1,139	660	256	2,055	16	17	2	36
Feb-08	760	535	189	1,484	749	524	191	1,465	11	11	-2	19
Mar-08	570	498	174	1,242	574	497	176	1,247	-4	1	-2	-5
Apr-08	679	544	194	1,417	689	549	197	1,436	-10	-5	-3	-19
May-08	913	651	253	1,817	924	657	256	1,836	-11	-6	-3	-19
Jun-08	1,142	709	305	2,157	1,157	705	310	2,171	-15	4	-5	-14
Jul-08	1,410	746	353	2,509	1,415	745	356	2,516	-5	1	-3	-7
Aug-08	1,689	794	381	2,864	1,693	791	382	2,867	-4	3	-1	-3
Sep-08	1,880	852	428	3,160	1,887	845	431	3,163	-7	7	-3	-3
Oct-08	2,010	939	463	3,412	2,009	932	458	3,399	1	7	5	13

Sources: Monthly from weekly values-linearly interpolated values based on volumes available with the Weekly Natural Gas Storage Report System; Natural Gas Monthly.

Note: Totals may not equal sum of components because of independent rounding.

Sources of Differences Between the Monthly and the Weekly Series

EIA determined a number of factors that can contribute to the implied differences:

• Inventory reports by respondents to both the monthly and weekly systems can differ somewhat, sometimes simply because activities did not occur in the schedule implied by the linear interpolation of the weekly data. However, these differences historically have tended to be less than 1 Bcf in absolute value per respondent on average.
• Reclassification of base gas to working gas contributed to differences between the weekly and monthly series, owing to the 2-month lag between the two series.
• The estimation methodology may produce weekly volume estimates for the nonsample operators that differ from the reported monthly values.

EIA has used a number of estimation methodologies, all of which have been based on using attributes of the reported information from the operators in the sample to estimate working gas volumes for the non-sample companies. There have been periodic changes to the methodology or sample since 2002 that have impacted the resulting estimates. The initial methodology relied on fixed ratios and a fixed sample that may not have captured a change in the relative significance of sample operators and nonsample operators in total inventory in a given period. In October 2003, EIA implemented several adjustments to the weekly series that affected the observed differences beginning in July 2003. These included: a revised estimation methodology, an expanded sample, and revisions for the 17-week period from July 4, 2003 through October 24, 2003. In August 2005, EIA implemented a change to the basic methodology and revised the sample (see Methodology). In conjunction with those changes, revised estimates for the period from mid June 2005 to August were released.

To some extent, differences between the two data series likely will continue. EIA will provide periodic updates of the information in the tables above as an ongoing resource for reference or for those who may want to modify their own comparative assessment of weekly or monthly storage trends.