Description:
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?
