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CHAPTER 9 UNDERGROUND STORAGE OF NATURAL GAS Underground gas storage may be defined as the storage in reservoirs of porous rock at various depths beneath the surface of the earth of large quantities of natural gas not native to these reservoirs. Before planning storage-field capacity and deliverability, one must have knowledge of the market requirements. The influence of the weather on sales of gas for space heating of buildings is important. When the annual storage volumes and daily delivery rates are developed for a distribution system, the facilities for storage fields may be designed to meet the need. 9.1 THE NEED FOR GAS STORAGE All types of natural gas sales, whether domestic, commercial, industrial, or space heating, present variable load factors. Probably the most difficult load for the distributor to meet is the space-heating load. It varies from a minimum of zero in the summertime to a maximum that can occur any time during the cold winter months. Probably the largest potential market for the natural gas distributor is residential space heating. The problem of the distributor, then, is to find efficient and economical methods of handling the load-factor problem in distribution of space-heating gas. o o Variation in space heating needs is measured in degree-days, using 65 F (18.3 C) mean temperature as the base temperature. A mean temperature of 45 °F corresponds to 20 degree-days. For space-heating, Figure 9.1 shows the variation in heating loads for the Detroit area, totaling 6404 degree-days in any one heating season, November through March. 9.2 BASIC CHARACTER OF A STORAGE RESERVOIR A section and a plan view of a reservoir equipped for storage are shown in Figure 9.2. The storage container is a porous solid with a caprock overhead to prevent vertical migration. Water in the storage zone underlies all or part of the gas filled sane or carbonate. Wells designated I/W, for "input and withdrawal", are completed in the storage zone. Observation wells are completed in the water-bearing porous media to permit observation of the pressure and any migrating gas. Depleted gas reservoirs are prime candidates for conversion to storage. The size of the reservoir is determined by calculation from geological data or from the production and reservoir pressures. Such calculations are relatively simple for cases with little or no water movement. The typical injection and withdrawal pattern in storage is shown by Figure 9.3. A delivery system can be installed to cover the market demand for the year, and ideally the unused gas in summer is stored for use in winter. Some flexibility is needed, since variation in weather causes varying demands. Storage fields and pipelines may require some period of reduced load in summer for testing. The storage gas is considered in two parts. The base gas provides for sufficient gas pressure to produce gas adequately at the end of withdrawal. The gas at pressures above the base pressure is termed working gas storage gas and makes up the annual turnover of gas. Figure 9.4 illustrates the pressure-gas quantity relation, showing base gas and working storage. The use of pressures above discovery, a delta pressure, gives added usage for a given container (larger than the discovery gas quantity). This practice has demonstrated large economic benefits to the storage industry for converted gas reservoirs. In gas storage, pressures in the earth may be up to 0.7 psi/ft. There are four key elements in observing ongoing gas storage operations. 1. Monitoring, 2. Inventory verification, 3. Deliverability assurance, and 4. Safety. 102 9.2.1 Monitoring Monitoring means more than taking data and making records. It is the analysis of the data that usually detects the early signs of unwanted gas movement. The system under consideration, in addition to the resrvoir, includes 1. Surface piping, 2. Wellbores, 3. Layers of rock above and below the storage zone, and 4. Surrounding area at distance of 1 to 3 miles or more. The reservoir engineer should develop a mental model of the reservoir behavior. Only then the deviations due to unwanted behavior become evident. For example, gas losses through corrosion spots or casing collars can be detected by rising annulus gas pressures, temperature, noise, and neutron logs in wells, or even lower than normal closed wellhead pressures in comparison with neighbors. The challenge is to gather large amounts of data and display them so that the reservoir engineers may see trends and note anomalies that imply that all is not according to the model in mind. 9.2.2 Inventory Verification The "inventory" is basically a thermodynamic quantity. It relates to the amount of natural gas in storage. Its verification is generally approached by two independent concepts: "volumetric" and "depletion". There are various analytical

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