and one such project in Kankakee County, Ill., with an anticipated cost of $36 million and an estimated capacity of 90.000.000 M cubic feet is under consideration at present. Gas also may be stored in depleted, partly depleted, or even active oil fields. A good example of this type of storage is the Playa del Rey field in California, which did not contain a gas cap originally; but part of it is being used today for storing natural gas. In the Bammel field. Harris County, Tex., natural gas is being stored in a field that is currently producing liquefied petroleum products through a cycling plant. The stored gas will help to maintain the pressure in the gas-distillate reservoir. In Michigan. New York. Texas, and perhaps some other States, liquefied petroleum gases (butane and propane) are being stored in artificially created caverns in massive salt formations. The cavities are formed by mining or by dissolving the salt in water circulated through wells drilled from the surface. In at least one field, the brine solution displaced by the liquefied petroleum gases is stored in a sand formation, from whence it can be recovered and used to maintain the pressure in the cavern when the liquefied petroleum gases are withdrawn. This method may be applicable to the storage of natural gas. It is not in use now, however, and its possibilities are not known. REQUIREMENTS OF A GAS-STORAGE FIELD It has been stated previously that a storage field should be near the market it serves and large enough and capable of producing gas rapidly enough to meet the demands caused by equipment failures or seasonal variations in temperature. A field may meet all of the above requirements, however, and still be useless as a storage field-if it is not gas-tight. It is self-evident that most depleted gas fields were gas-tight before they were drilled; otherwise, gas could not have accumulated in them. Gas, which tends to migrate upward, has been prevented from so doing by an impermeable rock formation. In some fields the impermeable formation overlies a separate permeable formation, and the gas is trapped in anticlines, domes, and other structures created by folds in the rock strata. In other fields the gas is trapped in formations that have variable permeability, and the impermeable portions of the formation prevent migration of the gas from permeable lenses. Both types of fields are being used for storage in the Appalachian region. Idealized cross sections showing the relationship of the permeable and impermeable formations in the two types of fields are shown in figure 2." Whether or not a field is gas-tight after it has been depleted depends upon the condition of the wells that exist in the field. In some depleted fields, casing (pipe) has been pulled from the wells and salvaged, leaving the open hole unprotected. Other wells have been plugged with lead or wooden plugs or with cement; some have merely been filled with dirt and debris. In addition, there may not be any surface indication of the old wells. Cultivated fields, highways, houses, barns, and other structures may be located directly over them. Unless the history of a depleted field is known and the wells can be found and reconditioned if needed, the risks occasioned by improperly abandoned wells may make the field economically unsuitable for consideration. A storage field must be gastight. Murdy, R. J.. The Underground Storage of Gas: Mine Inspectors Inst. America, WilkesBarre, Pa., June 9, 10, and 11, 1952. The capacity of a field is determined by its areal extent, the thickness of the formation in which gas can be stored, and the effective porosity of the formation. Contrary to some popular opinion, the formation in which gas occurs may not contain large cavities or caverns. In many fields, gas is found in sandstone resembling that used in constructing buildings; the gas is present in the millions of interstices between the grains of sand. Part of the space is filled with a cementing material which holds the grains together, and this material may isolate some of the interstices so that there is no passageway between them. Only the interstices that are interconnected are effective in producing gas, and they are the only ones in which gas can be stored. They constitute the effective porosity of the formation. Where information regarding the porosity and thickness of the formation and the areal extent of a field is available, its capacity to store gas at various pressures can be calculated. If these factors are not known, they can be determined by laboratory analysis of rock samples from the formation, electrical well-logging devices, and engineering appraisals. Another important characteristic is the deliverability of the field. In other words, how rapidly can gas be produced from it, or conversely, how rapidly can gas be injected into it without building up an excessive pressure at the wells? This can be determined in part by the number of wells drilled in the field and the pressure at which gas is stored. The rate at which gas will flow through the formation itself, however, depends directly upon the permeability of the formation-that property that is determined by the size and arrangement of the passageways between interstices. If the passageways will accommodate a high rate of flow with a relatively low pressure drop between interstices, the permeability of the formation is said to be high. The deliverability of wells drilled into such formations is correspondingly high, and fewer wells are required. Not only will each well produce gas at a higher rate, but each will produce gas from a larger surrounding area in a highly permeable formation than it will in a less permeable one. A field with small capacity and high deliverability is suitable for some purposes; a large field with less deliverability is suitable for others but a field that is not gas-tight is suitable for none. Thus, ability to hold gas under pressure is the principal requisite of a storage field. If it will leak, the field is useless to a company that wishes to store gas. Of course, a leaky field also may be hazardous to nearby coal mines: whether or not it is depends on how the gas leaks and where the gas goes. Neither a gas company nor a coal-mine owner wants or can afford a leaky storage field. DEVELOPMENT OF A GAS-STORAGE FIELD LEGAL RIGHT TO STORE GAS MUST BE OBTAINED If a gas company does not already have the legal right to store gas in an underground formation, that right must be obtained. Many leases were obtained from landowners long before the storage of gas underground was contemplated, and storage rights are not provided therein. Usually, however. landowners are willing to extend these rights and thus prolong the receipt of income from a source that otherwise would be depleted. In Illinois, Oklahoma, Kansas, and New Jersey, the benefit of underground gas storage to the general public has been recognized, and laws have been passed extending the right of eminent domain to underground gas-storage fields. Thus, storage rights can be obtained by condemnation proceedings if necessary to serve the best interests of the public; however, such proceedings rarely are employed. FINDING AND RECONDITIONING OLD WELLS The finding and reconditioning of old wells in a depleted field are vital factors in determining whether or not it will be successful in its new role. An undiscovered or improperly completed well might leak gas from the storage field and cause a loss to the gas company as well as a hazard to nearby coal mines. Consequently, these operations receive thorough consideration and study by geologists, petroleum and natural-gas engineers, and other skilled personnel of the gas companies. Reworking old wells is costly, but when the success or failure of a multimillion dollar project is at stake, no practice short of the best that is known can be afforded. Locating abandoned wells is sometimes difficult, when little or no surface trace of the wells remains. Old maps, well-log files, and other old records may provide valuable clues. Royalty owners, lease employees, retired drillers, and other old inhabitants of an area usually are sources of helpful information. Commercial pipe detectors, wartime mine detectors, dip needles, and magnetic compasses are particularly useful. With this equipment, engineers can find the exact location of many abandoned wells and the approximate location of others. Some wells can be found only by extensive excavation with a bulldozer, and gas companies have dug up several acres of land in some instances to find abandoned wells; the gas company's engineers must be satisfied that all of the wells in a field have been found before they proceed to develop it for gas storage. After the wells are found, their mechanical condition is determined; if necessary, they are plugged or reconditioned. This is a costly endeavor but is fully justified. A well in an active gas field must be completed to last for the life of the field, whereas a well in a gas-storage field must be completed to last indefinitely. Completion practices vary in the Appalachian region according to conditions encountered in the wells. Figure 3 illustrates typical casing and cementing methods used by one company." Both the surface and production strings of casing are cemented to the surface, providing a double string of cemented pipe through coal measures. This method affords excellent protection against the loss of gas from storage into coal measures, water sands, or elsewhere. Completion methods used by other companies vary somewhat from that shown in figure 3, but all must protect against leakage. The problems of drilling and completing oil and gas wells through coal beds are not new. The Director of the Bureau of Mines called a meeting of coal operators and oil and gas operators in 1913 to discuss precautionary methods to be taken when drilling oil or gas wells through workable coal beds.12 13 Later, in April 1928, the American Institute of Mining and Metallurgical Engineers made a report on the subject. In November 1929, the Bureau of Mines issued .an Information Circular" describing, in part, regulations governing the drilling of oil or gas wells through workable coal beds in several States, notably West Virginia. It can be seen, therefore, that the subject has been exposed to much discussion by competent authorities, and regulation by several States. Presentday drilling and completion practices are the results of many years of experience. Pluging wells that are unsuitable for one reason or another for use in a storage field is a kind of reconditioning. As a matter of fact, some wells that are ultimately plugged require considerable reconditioning-redrilling and cleaning out-before they can be plugged. Figure 4 illustrates a method by which one gas well was plugged for abandonment; 15 however, this is not the only way that a well can be plugged effectively, and it must not be considered a standard for the region. NEW CONSTRUCTION Although finding a suitable field and reconditioning old wells are primary requisites of an underground gas-storage project, they are not the only factors to be considered. Pipelines must be built, in many instances, to serve as connecting links between the storage field and the distribution system. Compressor stations to boost the gas pressure are frequently needed. Gas-dehydration plants may be required to remove water vapor from the gas and thus prevent formation of hydrates in the pipelines. Also, additional wells usually are needed to provide adequate deliverability of gas during peak loads. All of these factors add to the cost of the project. The gas industry cannot afford these expenditures without reasonable assurance that the storage will be gas-tight. The drilling of new wells in the vicinity of coal mines is of vital concern to coal operators. In drilling these wells, the gas companies are not restricted, as they might be in reconditioning old wells, as to the size of the hole drilled and the resultant casing programs. New wells in gas-storage fields embody all of the best practices and materials known to the industry. 11 Work cited in footnote 10. 12 Hood, O. P., and Heggem, A. G., Proposed Regulations for the Drilling of Gas and Oil Wells: Bureau of Mines Technical Paper 53, 1913, 28 pp. 13 Rice, G. S., Hood, O. P., and others, Oil and Gas Wells Through Workable Coal Beds; papers and discussions: Bureau of Mines bull. 65, 1913, 101 pp., 1 pl.. 11 figs. 14 Herbert, C. A., Notes on Precautions To Be Taken When Drilling Oil or Gas Wells Through Workable Coal Beds or Through Mine Workings: Bureau of Mines information circular 6195, 1929, 8 pp. 15 Work cited in footnote 10. |