Depletion of reserves in gas fields in the region is one thing, but the more pressing problem for now is deliverability, or the ability of aging producing wells to produce enough to deliver the amount of gas needed, particularly in peak demand periods of winter.

Until recently the gas wells in Southcentral Alaska have been prolific enough that producing companies have only had to open the valves to flow more gas when cold weather hit.

The wells can't do that anymore. As the fields age the gas just doesn't come up the wells as quickly. Pressure in the underground reservoirs, which drives the gas up the wells, is declining.

There are other problems, too, like water creeping into the rocks that hold the gas.

“You can switch these wells on and off like an electric light,” said Mitch Little, Marathon Oil Co.'s Alaska gas production manager. Little was part of a panel discussing the problem at a recent conference on railbelt energy problems that was sponsored by the Alaska Energy Authority.

All of this translates into deliverability for Enstar Natural Gas and the electric utilities, which generate power with gas. Data presented to the energy conference by Tony Izzo, former manager of Enstar, starkly illustrated the problem.

Izzo compared gas that was available and delivered on two cold winter nights in Southcentral Alaska 10 years apart.

On Feb. 3, 1999, the temperature dropped to minus 19 degrees Fahrenheit and gas production was cycled up to 744 million cubic feet to meet the peak in demand for heating and power, Izzo said.

On Jan. 9, 2007, temperatures dropped to minus 10 degrees, he said. However, the gas producing fields were only able to supply 531 million cubic feet. It wasn't enough.

To prevent Enstar Natural Gas from having to cut off gas to customers, the gas supply to the LNG plant was cut 35 percent, from 244 million cubic feet to 150 million cubic feet, and gas supplied to the Tesoro refinery was cut by half, from 13 million cubic feet to 6 million cubic feet, Izzo told the conference.

Gas deliverability is the most urgent problem on the table but the underlying issue is the depletion of gas reserves in the producing fields. Citing data from state Department of Natural Resources annual reports, Izzo said gas fields in Southcentral Alaska held 3.28 trillion cubic feet (tcf) of proven reserves in 1997, according to DNR reports for that year. By 2006 this had dropped to 1.68 tcf, a 49 percent decrease, according to the 2006 DNR report.

“These (reserve) numbers will change because there will be additions to reserves but the bottom line is that the overall trend is down,” Izzo said.

Demand for natural gas, however, is increasing, because of population growth. The demand for gas by utilities in the region increased by about10 percent from 37.7 billion cubic feet in 1997 to 41.4 billion cubic feet in 2006, Izzo said in his presentation.

Marathon's Little said that four large gas fields that supply most of Southcentral's gas supply were all discovered before 1965 and it is getting more difficult to throttle gas down and back up again to meet seasonal peaks.

Some of the changes in demand can be almost instantaneous, brought by a sudden plunge in winter temperatures, Little said.

Gas wells in Southcentral Alaska are aging and when production is ramped down, it takes longer to get their production back up, and the amount of gas produced is less.

Little showed the conference graphics displaying how several years ago a well could be shut in and then restarted and reach the same peak volume of output within hours.

A year later, the same well would take several weeks to come back to the same volume. Several years later, the same well took months to come back up, and then it reached about half the volume of gas the well originally produced.

The best way to keep gas, or even oil wells, flowing at maximum efficiency is to maintain them constantly at peak production although that production gradually declines over time.