With the 2013 natural gas storage refill season still humming along with weekly injections that are higher than historical comparisons, it is important to remember that storage levels and available capacity for working natural gas, or natural gas that is used for withdrawal, respond to a different set of factors in each region, according to the Energy Information Administration (EIA).

In an energy brief released Wednesday, the government agency reiterated that its Short-Term Energy Outlook is projecting that U.S. working inventories will reach 3,800 Bcf by the end of October 2013 — the traditional end of the refill season — with April-October injections similar to those in 2008-2011, but much higher than in 2012, when Oct. 31 inventories reached a record 3,930 Bcf. The EIA said projected inventory levels for Oct. 31, 2013 are lower than those for the same date in 2012 because of increased withdrawals in the Producing region during a colder 2012-2013 winter, along with flattening production in 2013.

With that said, the EIA pointed out that the storage capacity utilization of the American Gas Association’s original three storage regions of the United States all behave differently due to differences in weather patterns, types of facilities, pipeline constraints, proximity to supplies and regulations.

EIA data shows that underground natural gas storage fields in the East region, which includes Illinois, Indiana, Iowa, Kentucky, Maryland, Michigan, Missouri, Nebraska, New York, Ohio, Pennsylvania, Tennessee, Virginia and West Virginia, have neared their capacity limits before each winter since 2005. “Storage field utilization in the East has ranged between 87% to 92% before November of each year despite changes to market conditions,” EIA said.

For many states in the East region, local distribution companies are legally required to purchase and store working gas to ensure sufficient inventories to meet increased winter demand, the brief said. As a result, working gas storage capacity generally tends to be full in the East by the end of October, regardless of weather and market conditions.

Turning attention to the West region, which covers facilities in California, Colorado, Minnesota, Montana, Oregon, Utah, Washington and Wyoming, EIA pointed out that much like the East, storage operators in the West use their facilities to ensure that inventories are sufficient to meet increased demand during the winter months. In addition, both interstate and intrastate natural gas pipelines use western storage facilities to support load balancing.

“Storage levels are relatively stable and generally lower than in the East and Producing regions, due largely to unused capacity in depleted wells in areas such as the Williston Basin that do not have easy access to areas of high demand, although they have increased in recent years,” EIA said. “Since 2005, end-of-October working gas storage capacity utilization has been between 59% and 72%.”

Finally, there is the Producing region, which EIA said is a bit more difficult to forecast. The Producing region, which includes Alabama, Arkansas, Kansas, Louisiana, Mississippi, New Mexico, Oklahoma and Texas, differs from the East and West regions in that its salt cavern storage facilities require lower volumes of base gas, which is natural gas required to maintain adequate pressure in the facilities. As of December 2012, EIA data showed that salt caverns accounted for 407 Bcf, or 27%, of the 1,522 Bcf total working gas design storage capacity in the Producing region.

“Since the beginning of 2012, base gas has accounted for about 41% of total storage capacity in the Producing region, versus 53% in the East region and 46% in the West region,” EIA said. “This difference in the base gas required means that a relatively greater portion of storage capacity in the Producing region can be set aside for working gas. Changes in the Producing region’s pre-winter working gas inventories have driven changes to overall inventories in the Lower 48 states.”

Because salt cavern storage operates under high pressure, facilitating a quick turnaround for injections and withdrawals, market participants in the region are able to more immediately respond to short-term price fluctuations. “Injections and withdrawals thus follow a more flexible schedule, causing end-of-season storage levels to vary,” EIA said. “In the past five years, working gas storage levels at the end of October have varied from roughly 74% of available storage capacity (2008) to 92% of capacity (2009).”