Filling U.S. natural gas storage to maximum working gas design capacity would be a lot like eating 19 slices of deep-dish pizza; although it’s theoretically possible, it’s probably not something you would want to even try.

With working gas levels already exceeding the 3 Tcf level eight weeks before the traditional end of the storage injection season, there easily could be a record amount of gas put into storage this year. However, there is widespread uncertainty about how much gas can reasonably be expected in storage at the beginning of the winter.

A new report by the Energy Information Administration calculates total U.S. working gas capacity of nearly 400 active underground storage facilities operated by 123 entities in the Lower 48 states. An estimate of aggregate working gas capacity based on the engineering design capacity of the fields can be obtained by deducting the sum of base gas in those fields — that is the gas that remains in the fields to create enough pressure to make them operational — from the sum of total design capacity for all the fields. According to EIA, the designed maximum working gas capacity for all fields as of the end of June 2006 using this method was 4,030 Bcf.

However, this engineering-based estimate ignores the large number of operational factors and individual storage decisions that come into play in the marketplace, EIA noted. For example, pipeline and local distribution company operators may hold some space in reserve for operational efficiency. Working gas levels may depend on commercial arbitrage opportunities. Some fields may operate on different cycles and peak at times other than the end of October. Other fields may have low injection capabilities that require an extended injection period. Fields may be brought down for maintenance or expansions. Some fields may be in the process of being decommissioned, while other new fields may be delayed or put into partial service because of the high cost of base gas.

A traditional rule of thumb is that the industry saves about 5% of total working gas capacity to handle load fluctuations and avoid curtailments, but even that level at about 3,829 Bcf seems too high — sort of like eating only 18.05 slices of pizza to save room for cake. The largest ever recorded volume of working gas in storage was 3,472 Bcf reported at the end of November 1990 and that level still fell 357 Bcf, or more than 10% short, of the estimated 3,829 Bcf total using the 5% rule of thumb.

That’s why a more practical number for full storage capacity is determined by summing up the peak volumes for each storage field that were reported during a recent historical period. EIA calls this the noncoincident peak volume. From 2000 to 2004, this total was about 3,609 Bcf.

“The noncoincident peak volume overstates the amount of actual storage achieved at a given point during the five-year period because it does not account for differences in timing of occurrence,” EIA said. “However it is a data-driven estimate that reflects actual operator experience. Although the field level volumes were the largest ones reported during the five-year period, each storage field may not have achieved its practical maximum, which suggests that this calculation might yield a somewhat conservative result.”

EIA also notes that some small marginal storage fields are either dormant or currently appear to be shutting down operations based on a “continuing sequence of withdrawals for at least 12 consecutive months.” As of the end of 2004, EIA said, these marginal fields had a working gas capacity of 16 Bcf.

That leaves the noncoincident peak working gas capacity in the U.S. at about 3,593 Bcf, which means that storage is currently 86% full and there is about 509 Bcf of storage space remaining with eight weeks left in the injection season. Given that estimate, the industry needs to inject only 63.6 Bcf of gas per week into storage to reach 100% full on Friday, Nov. 3.

For a copy of the EIA report, titled Estimates of Maximum Underground Working Gas Storage Capacity in the United States, go to https://www.eia.doe.gov/.

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