Underground Gas Storage Facilities: Design and Implementation

By Orin Flanigan

You get information needed to evaluate a reservoir, determine the particular requirements of the job, and design a storage facility that will operate at its full potential.

Underground Gas Storage Facilities combines background information with a systematic approach for examining a specific reservoir to determine the most appropriate day-to-day method of operation. It presents a thorough discussion of topics such as estimating customer requirements, types of storage, sizing of surface facilities, and estimating deliverability. Of particular interest is the section on the economics of storage design, which examines the specific cost factors involved and presents examples to determine an economically optimum design.

• Information and technical tools to evaluate a reservoir
• Determine the particular requirements of the job at hand
• Design a storage facility that will operate at its full potential

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 7, 1995
• ISBN: 9780080543390

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Preface

Most books on the underground storage of natural gas are written from the viewpoint of the petroleum engineer or the reservoir engineer. This is natural because these two disciplines are heavily involved in evaluating the reservoir for the prospective project. This book assumes that the petroleum engineer and the reservoir engineer have already done their job. One or more reservoirs have been selected, their physical properties such as porosity and permeability have been determined, and their aerial extent, depth, and thickness have been measured. The security of the reservoirs has been evaluated and a judgment made that they are sufficiently secure to proceed.

At this point the pipeline engineer and the planner enter the scene. These individuals are fully familiar with the transmission facilities and their operation as well as the characteristics of the various gas supplies and the customer loads. They are also familiar with the economic and operating philosophies peculiar to their particular company. The purpose of this book is to provide these individuals with the information and technical tools necessary to interact with the petroleum engineer and the reservoir engineer, to evaluate each reservoir, and to integrate this information into the design of a storage facility.

CHAPTER 1

Weather Patterns

Energy is a vital part of practically all industrial concerns. Without it, most companies would cease to function. This energy can originate from many basic sources including:

Electricity is missing from this list because electricity is a secondary source of power. All of the above sources of fuel can be converted into electricity.

Nuclear power is so regulated and so financially demanding that it is practical only for the production of power in very large amounts. It is not viable for the ordinary medium-sized industrial organization. Water power is widely used for the production of electricity, but it is no longer used to any extent as a source of industrial power by itself. Wind power and solar power have had some small development in special situations, but they are not practical for the average fuel user. Geothermal power is severely limited by its geographic occurrence. This leaves coal, oil, and gas as the primary sources of fuel used by industry.

Fuel Storage

Any fuel requires storage to some degree. Coal is delivered from the mine to the point of usage by rail or truck. Since these deliveries are not continuous, coal must be stored to accommodate usage between deliveries. This can be done in bins for moderate amounts or in open stock piles for larger amounts. The coal piles are stable, are not unduly affected by the weather, and the storage effort is reasonably economical. Fuel oil (and other liquid fuels) is similar to coal in some of these aspects. Much of the oil is delivered by truck or rail. This requires that storage be available to accommodate usage between deliveries. Fuel oil can be delivered to larger industrial users by pipeline. Even in this case, however, the pipeline is not dedicated to delivering oil alone. Usually the pipeline transports batches of different products. These hydrocarbon products may vary from gasoline on one end of the spectrum to heavy fuel oil on the other. Because of this, the shipments of oil by pipeline are periodic, and storage must be provided for the periods of usage between shipments. This storage may take the form of atmospheric pressure tanks.

Natural gas is the only one of these commonly used fuels that does not require storage by the user. Natural gas is delivered to the customer by pipelines. These pipelines are dedicated to the customers they serve, and they transport only the one product. The user always has a supply of natural gas available for use by simply turning on a valve. Unfortunately, this is not the end of the storage story.

The transmission pipelines that carry the natural gas to the customer have some limit on their capacity. The pipelines also have a cost associated with them. As long as the loads that the pipeline serves are constant throughout the year, the pipelines can be economically designed to handle the service. When a large portion of the customer loads are temperature sensitive, however, a problem arises. Many residential, commercial, and industrial customers use gas for environmental heating. This is heating to counteract the low ambient temperatures that occur in winter. The colder the atmospheric temperature becomes, the greater is the amount of gas required to heat the customer structures. The coldest days of the winter only occur a few days each year. If a large portion of a pipeline’s load is heat sensitive, economically designing facilities to supply this load on a year-round basis can be difficult. If the limitation on service during the very cold periods is due to pipeline capacity limitations, gas storage located near the consumption area is necessary. If the limitation on service is due to a supply limitation, the gas storage location is not as critical.

Storage techniques for the three primary fuels differ due to the differing nature of the three fuels. A standard cubic foot of each of the three fuels contains the following heat content in Btus:

Natural gas is the most difficult to store; because it is a gas, it must be contained in a container that is leak-proof under pressure. Neither coal nor oil has this restriction. Because of its lower heating value per unit volume at atmospheric pressure, gas requires either much larger atmospheric pressure storage vessels or pressurized storage vessels.

Fuel gas has been stored in atmospheric pressure storage vessels. These vessels, called gas holders, were used in the early part of this century to store manufactured gas in large cities. The holders had telescoping sections that rose vertically as gas was introduced into the vessels. The pressure in the holders was about 10 to 15 inches of water column. The largest of these gas holders had a capacity of about 1,000,000 standard cubic feet and were roughly 100 feet in diameter and about 100 feet high when full. These storage devices worked well with the relatively low volumes of relatively high-priced gas that were consumed in the area. When larger volumes of natural gas were introduced into the cities and the prices declined, the older gas holders were no longer adequate or economical to continue to use.

Pressure vessels have also been used for gas storage. Because of the economics and logistics, these storage efforts have been limited to relatively small volumes and applied to special cases where the higher cost could be justified. Mixtures of propane and air have been used to supplement natural gas supplies on very cold days. This mixture is considerably more expensive than natural gas, so that its application has been restricted to special situations.

The underground storage of natural gas has evolved as the preferred means of supplementing natural gas supplies for temperature-sensitive loads on very cold days. In most cases a former producing field is used and transformed into a storage facility. In some cases storage facilities have been developed in aquifers that did not previously hold gas. In either case the reservoir and the supporting equipment must be tailored to the needs of the pipeline company.

Historical Weather Data

For heat-sensitive gas usage, accurate load forecasting requires accurate weather information. At the present time, accurate long-range weather forecasting is not feasible. It is possible, however, to determine what temperature pattern should be expected for a normal winter period. This normal pattern can serve as a basis for a load forecasting procedure.

The U.S. Weather Bureau keeps extensive historical temperature records at each of their many weather stations. This information is processed to yield a normal temperature for each day of the year for each station. Unfortunately, this weather bureau normal is not adequate for gas load forecasting.

The procedure used by the weather bureau is to take the high temperature and low temperature for each day and average these to yield a daily average temperature for that location. These daily average temperatures are then averaged for each day of the year for some long period, typically 30 years, to yield a normal temperature for that date.

Table 1-1 illustrates this process, although it is an oversimplification of the Weather Bureau averaging process. The Weather Bureau effectively plots the data for each month, smoothes the data, and uses the smoothed data to obtain the averages. Table 1-1 does, however, illustrate the problem that this processed data presents for the load forecaster.

Table 1-1

Hypothetical February Temperature History—Raw Data

Table 1-1 contains the February temperature history for five consecutive years for a given location. Both the temperature history and the location are hypothetical, and the five years represent no particular period. The average monthly temperatures for each of the five years are similar, and the number of degree days for each of the five February months varies less than 10 percent. The five monthly periods do not represent extremes, but they are quite similar. Each of the five months contains a low temperature of 20°F and a high temperature of 50°F.

An examination of the five-year temperature average in the right-hand column, however, shows that the lowest temperature in this column is 28.2°F. Similarly, the highest temperature is 41.6°F. This is in contrast to the low temperature of 20°F that occurred in each of the five individual years. This is because the 20°F temperature occurred on a different date in each of the five years. The same is true of the 50°F temperature in each year. Thus the average temperature data in the right-hand column of Table 1-1 would be fine for estimating the monthly gas consumption for a heat-sensitive load, but it would be quite inaccurate in showing the extremes of load that could occur on given days.

In order to alleviate this problem, a different processing procedure must be used for the raw data. This can be accomplished by averaging the data for the coldest day in each month, the second coldest day in each month, the third coldest day in each month, and so forth. Table 1-2 illustrates the intermediate step in this process. The temperature data for the month for year 1 is sorted by ascending temperature. The same is done for years 2 through 5. The left-hand column in Table 1-2 no longer represents the day of the month, it now is the number of days that the temperature is as cold or colder than the indicated temperature. The right-hand column is the average of the sorted temperature data.

Table 1-2

Hypothetical February Temperature History—Sorted