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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 feet.

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 behavior.

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).

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 US underground natural gas storage, April 1997-March 2000 Source: International Gas Consulting Inc. volume of ng given in billions of cubic feet (Gcf) week Gcf 1 859 2 834 3 832 4 862 5 906 6 986 7 1033 8 1111 9 1199 10 1301 11 1395 12 1500 13 1561 14 1660 15 1746 16 1798 17 1862 18 1928 19 1986 20 2058 21 2127 22 2215 23 2309 24 2395 25 2467 26 2563 27 2643 28 2718 29 2781 30 2809 31 2812 32 2817 33 2754 34 2660 35 2599 36 2525 37 2403 38 2260 39 2177 40 2047 41 1978 42 1840 43 1691 44 1605 45 1522 46 1442 47 1351 48 1309 49 1238 50 1116 51 1053 52 1009 53 1070 54 1070 55 1133 56 1192 57 1274 58 1380 59 1459 60 1553 61 1650 62 1734 63 1831 64 1927 65 1995 66 2063 67 2155 68 2227 69 2310 70 2368 71 2436 72 2518 73 2575 74 2647 75 2688 76 2741 77 2800 78 2836 79 2866 80 2908 81 2975 82 3014 83 3055 84 3096 85 3051 86 3037 87 3037 88 3063 89 3018 90 2928 91 2751 92 2613 93 2394 94 2196 95 2093 96 2025 97 1930 98 1878 99 1790 100 1655 101 1600 102 1445 103 1374 104 1334 105 1343 106 1374 107 1380 108 1380 109 1423 110 1489 111 1563 112 1640 113 1700 114 1791 115 1866 116 1943 117 2029 118 2089 119 2163 120 2257 121 2309 122 2320 123 2371 124 2414 125 2463 126 2529 127 2589 128 2671 129 2743 130 2826 131 2886 132 2914 133 2971 134 2986 135 2986 136 3010 137 3015 138 2995 139 3000 140 2929 141 2857 142 2780 143 2586 144 2437 145 2314 146 2214 147 2029 148 1780 149 1560 150 1409 151 1277 152 1197 153 1171 154 1120 155 1080 156 1034
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