About cumulative 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 (such as
gas fired electric generators).
Gas produced during the low demand summer months
must be stored until needed, which presents a big challenge to the
gas industry. 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 rock formations).
The current US underground storage capacity is over 3 trillion cubic
feet, as can be seen from the data.
The data show the variation in cumulative natural
gas storage capacity in the US (almost all underground) from 1932
to 2000. New storage capacity is added to the previous year's total
to create an ever increasing series of values, though not at a constant
rate. Much of today's storage capacity came online from approximately
1950-1980, as reflected in the steeper portion of the S-shaped distribution
of values. The shape of the cumulative storage capacity curve reflects
the variability in new storage additions with time. Students could
be asked to sketch this other curve (time versus additions) based
on the shape of the cumulative curve.
The student can analyze the data using a logistic
model, a form of exponential growth where the rate of growth decreases
linearly with increasing time (in this case). Trial and error may
be appropriate to finding a well-behaved model, or the student may
have a logistic regression algorithm on their technology of choice.
What is the projected limit to gas storage according to the model?
Logistic growth is common in many natural organisms; their size or
abundance is often limited to physiology or environmental conditions
(a fixed food supply). Is there really some "physical limit"
to underground gas storage capacity, and if so, what might that be?
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).