The Future of Energy Gases

O

ver the course of human history, the energy sources that people have used have become more efficient and cleaner, from biomass (wood or peat) to coal to oil. The continuation of that trend can be seen in the increased global use of gas, a fuel with environmental and economic benefits, although coal and oil will almost certainly continue to be major sources of energy well into the 21st century. The trend will probably continue with the development of nonpolluting, renewable energy sources, such as solar energy, and various forms of nuclear energy. Before societies can achieve the goal of developing cleaner, more efficient sources of energy, however, it is likely that the role of natural gas in the energy mix will increase.

use of gaseous fuels. Gaseous fuels may be the energy of the future, whether as a natural gas or as an energy carrier for a gas such as hydrogen. Increasing the use of natural gas today could begin development of the infrastructure necessary to carry, store, and use gaseous fuels. This use might the pave way for the ultimate conversion of the present liquid- and solid-based fuel system to a gasbased fuel system. Although many people are optimistic about future supplies of natural gas, the amount of that supply is still uncertain. The current production supply of natural gas is running low; in order to maintain production, there must be a significant increase in

the number of exploratory and development wells drilled. Such an increase will produce a dilemma, however.

Increased drilling will increase reserves and could therefore satisfy demand, but drilling will not increase significantly until the price of gas goes up; higher prices could then diminish demand. Also, use of natural gas is intended to reduce the effects of environmental damage caused by fuel use, but numerous wells, many in environmentally sensitive areas, will need to be drilled to get adequate supplies of natural gas.

Understanding the nature and occurrence of energy gases, therefore, is important for two critical issues facing our Nation-the economy and the environment. The U.S. Geological Survey (USGS) is actively involved in research on the origin, abundance, and availability of energy gases. This research is pertinent to discussions of the future energy mix, global warming, the dependence of the Nation on imported fossil fuels, and the size of gas reserves, all of which are concerns faced daily at local, State, and national levels.

What Is Natural Gas?

Na

atural gas usually refers to methane, although small amounts of other hydrocarbon gases such as ethane, propane, and butane are also present in natural gas mixtures. Methane is the simplest of the hydrocarbon molecules-one carbon atom surrounded by four hydrogen atoms-and exists in variable amounts in the Earth's atmosphere, water, and rock layers.

Natural processes create methane in three different ways. Biogenic gas is expelled from microorganisms during the digestion of organic compounds; thermogenic gas results from the decomposition of organic matter when heat and pressure are applied; abiogenic gas is created deep within the Earth's crust when crustal gases react with minerals or when hydrogen- and carbon-rich primordial gases seep from the Earth's interior. Most of the methane that is extracted from large natural-gas accumulations found in the Earth's crust is believed to be either biogenic or thermogenic in origin, although abiogenic methane may also be present.

29

Naturally occurring gases in the Earth's crust are collectively referred to as natural gas. Some of these gases are combustible and give off heat energy when burned; they are collectively called energy gases. Energy gases typically make up more than 75 percent of all natural gas and are mostly hydrocarbon gases, which are compounds composed of hydrogen and carbon atoms only. The flame represents the typical proportions of energy gases found in natural gas, shown in order of abundance. Methane constitutes 80 to 100 percent of the volume of energy gases. A household flame contains nearly 100 percent methane.

Commercial quantities of methane are present in many geologic environments, and the modern era of exploration for natural gas has stimulated the development of new technologies and strategies that expand capabilities to find gas accumulations. The most common accumulations of gas are associated with oil. Gas often forms a pocket between the denser oil below and an impermeable cap rock above. Where no natural-gas distribution system is available, gas produced in association with oil is often considered a nuisance. In many countries, if gas cannot be used as a fuel source for field operations, it is often vented or flared. In the United States, gas is almost never flared but is reinjected to maintain reservoir pressure.

Other natural-gas accumulations are termed unconventional, because gas in these accumulations either is more expensive to extract than gas found in conventional

settings associated with oil or requires new technologies for extraction. For instance, gas that accumulates where reservoir rock permeability is low is referred to as tight gas. In this type of reservoir, there may be large pore spaces to hold gas, but, because there is little if any connection between the pore spaces, the flow of gas to the well bore is greatly restricted. Because tight gas is widely disseminated, usually without well-defined boundaries, and is difficult to extract, its ultimate resource potential remains uncertain.

Coal-bed gas is another unconventional resource. Coal is carbon-rich, woody terrestrial plant material that has been compressed and transformed. A byproduct of this transformation is methane, and its presence in coal beds is one of the greatest dangers to underground mining-at high concentrations in mines, methane causes explosions. By extracting coal-bed gas before mining, the potential danger is reduced and an energy resource gained.

Another source of unconventional natural gas that has attracted attention is deep gas, which is located in rocks at depths greater than 4,000 meters and may represent a potential supply. Methane remains stable at depth, whereas oil tends to concentrate in more shallow parts of the crust, either because it has been pushed upward by the gas or because it

breaks down at the high temperatures found at depths greater than about 5 kilometers. Also, as pressure increases, gas compresses; compression results in more energy per unit volume and, therefore, may enhance the desirability of deep exploration.

Gas is also trapped in media other than rock. In shallow crustal horizons, enormous volumes of gas can be dissolved in brines. Although most scientists think brine gas is uneconomic, like the vast quantities of gold dissolved in sea water, local circumstances and ingenuity may combine to give such limited economic potential.

gas

Gas trapped in the atomic lattice of water ice is another unexplored, unconventional resource. These so-called hydrates look like dry ice, but hydrates will burn. Although huge quantities of gas exist in hydrate form, the commercial value of this gas source is yet to be demonstrated. Recent studies suggest that more than 100 million trillion cubic feet of methane exists in gas hydrates worldwide; the potential resource may be as much as 700,000 trillion cubic feet. This amount represents about 10,000 times more gas than the global natural-gas consumption in 1990, a truly immense quantity. There is, therefore, great incentive to learn more about methane in gas hydrates and the technology required to develop it.

States comes from burning fossil fuels. The remainder comes from nuclear and hydroelectric power and small amounts of geothermal, wind, and solar power. When fossil fuels burn, the carbon-hydrogen bonds are broken, and heat is released. Carbon combines with atmospheric oxygen to form carbon dioxide (CO2), which is released to the atmosphere, while the hydrogen atoms combine with oxygen to form water vapor. Because of its lower carbon content, methane combustion produces roughly one-half the CO2 emissions of coal and about two-thirds as much CO2 as oil while delivering the same amount of thermal energy.

Carbon dioxide and water vapor are the two most abundant greenhouse gases. These gases, which also include methane, ozone, nitrogen oxides (NOx), carbon monoxide, and chlorofluorocarbons (CFC's), linger in the atmosphere, where they can affect the temperature of the Earth's surface as well as cause other effects. Short-wavelength energy from the Sun passes through the atmosphere and reaches the Earth, but the greenhouse

In 1993, the U.S. Geological Survey assembled a team of senior scientists for a major study on the role of energy gases in the Nation's energy mix. The "Energy Gases Team" produced a series of three publications under the umbrella title The Future of Energy Gases, describing the origin, distribution, development, and use of natural gases. The series consists of Professional Paper 1570, which is technical in content; the colorfully illustrated Circular 1115, written in nontechnical language; and a 30-minute video appropriate for general audiences. The results of the study are summarized in this article.