About natural gas storage:
Natural gas is an important fuel source in the
United States, produced by wells drilled into rocks in the Gulf Coast
states, in California, and along the continental shelf (among other
locales). Natural gas consumption is currently running about 20 trillion
cubic feet annually and is expected to increase another 10 Tcf over
the next few decades as consumers switch from oil and electric heat
to natural gas, and as new demands for natural gas crop up (see below).
Demand for natural gas for heating increases during
the US winter, however it's not possible to increase production from
gas wells to match the increasing demand; production remains more-or-less
constant. Therefore, gas produced during the low demand summer months
must be stored until needed, which presents a big challenge to the
gas industry; natural gas, even condensed into LNG, takes up a lot
of volume. Above ground storage tanks are useful and convenient on
a small scale, but most natural gas is stored underground. Underground
reservoirs for gas include oil fields that have been depleted of their
petroleum (the extraction wells can be easily reversed and turned
into injection wells) and abandoned salt mines (common in the Gulf
Coast states where natural gas is produced from nearby formations).
The current US underground storage capacity is over 3 trillion cubic
The data show the variation in natural gas storage
in the US (almost all underground) with time (on a weekly basis) from
April 1997 to March 2000, covering three gas years. The data are quite
cyclical, as expected. Stored gas increases throughout the summer
months at a fairly constant rate until about September, when the storage
rate begins to decline. Volume stored reaches a peak around New Years
Day, when wellhead production can no longer meet demand, and net extraction
from reservoirs begins to dominate.
The student can model the cyclical nature of gas
storage using a sinusoidal function, and will discover that the data
are not symmetric about the Y-axis. The student can calculate the
residuals from the model, and speculate on the origin of this asymmetric
Gas storage underground has traditionally been
used to attenuate the yearly demand cycle, however natural gas is
changing as a commodity in many ways. For example, gas fired turbines
that generate electricity are becoming more common, and some of that
electric demand will occur in the summer, to drive cooling systems.
Cooling demand for gas-fired electricity will run counter cyclical
to the heating demand.
Source: American Gas Association's report, The Evolution of Underground
Natural Gas Storage: Changes in Utilization Patterns, prepared
by International Gas Consulting, Inc. (whose website contains a condensed
version of this report).