Energy Options Fossil Energy

The literature contains several publications that present a description of the energy sources that are available or are expected to be available during the 21st century. Energy options known today include fossil fuels, nuclear energy, solar energy, renewable energy and alternative sources. These energy options are briefly described in the chapters that follow.

Origin of Fossil Fuels

Fossil fuels are sources of energy that were formed by the death, decay, and transformation, or diagenesis, of life. The term diagenesis encompasses physical and chemical changes that are associated with lithification and compaction. Sediment can be lithified, or made rock-like, by the movement of minerals into sedimentary pore spaces.

The minerals can form cement that binds grains of sediments together into a rock-like structure that has less porosity than the original sediment. Porosity is the fraction of void space between the grains in the material. The volume of oil or gas in the rock depends on the porosity of the rock because the fluids are stored in the rock pore space. Two theories of the origin of fossil fuels are considered here: the biogenic theory, and the abiogenic theory.

Biogenic Theory

The biogenic theory is the mainstream scientific view of the origin of fossil fuels. In the biogenic theory, a type of biochemical precipitation called organic sedimentation forms coal, oil and gas. When vegetation dies and decays in aqueous environments such as swamps, it can form a carbon rich organic material called peat. If peat is buried by subsequent geological activity, the buried peat is subjected to increasing temperature and pressure. Peat can eventually be transformed into coal by the process of diagenesis. A similar diagenetic process is thought to be the origin of oil and gas.

Oil and gas are petroleum fluids. A petroleum fluid is a mixture of hydrocarbon molecules and inorganic impurities, such as nitrogen, carbon dioxide, and hydrogen sulfide. Petroleum can exist in solid, liquid or gas form depending on its composition and the temperature and pressure of its surroundings. Natural gas is typically methane with lesser amounts of heavier hydrocarbon molecules like ethane and propane. The elemental mass content of petroleum fluids ranges from approximately 84% to 87% carbon and 11 % to 14% hydrogen. These percentages are comparable to the carbon and hydrogen content of life. This is one piece of evidence that supports the idea that petroleum was formed from biological sources.

Sea level

Sea level

A. Death and B. Bacterial and c- Migration

Burial Geologic Action and Trapping

Figure 2-1. Biogenic Origin of Oil and Gas

A. Death and B. Bacterial and c- Migration

Burial Geologic Action and Trapping

Figure 2-1. Biogenic Origin of Oil and Gas

The biochemical process for the formation of petroleum is illustrated in Figure 2-1. It begins with the death of microscopic organisms such as algae and bacteria. The remains of the organisms settle into the sediments at the base of an aqueous environment as organic debris. Lake-beds and seabeds are examples of favorable sedimentary environments. Subsequent sedimentation buries the organic debris. As burial continues, the organic material is subjected to increasing temperature and pressure, and is transformed by bacterial action into oil and gas. Petroleum fluids are usually less dense than water and will migrate upwards until they encounter impermeable barriers and are collected in traps. The accumulation of hydrocarbon fluid in a geologic trap forms a petroleum reservoir.

Abiogenic Theory

In the biogenic theory, the origin of oil and gas begins with the death of organisms that live on or near the surface of the Earth. An alternative hypothesis called the abiogenic theory says that processes deep inside the Earth, in the Earth's mantle, form petroleum. Thomas Gold, an advocate for the abiogenic theory, pointed out that the biogenic theory was adopted in the 1870's. At the time, scientists thought that the Earth was formed from molten rock that was originally part of the Sun. In 1846, Lord Kelvin, a.k.a. William Thomson, estimated the age of the Earth from the rate of cooling of molten rock to be about 100 million years old. A more accurate estimate of the age of the Earth could be made after French physicist Antoine Henri Becquerel discovered radioactivity in 1896. In 1905, Ernest Rutherford proposed using radioactivity to measure the age of the Earth from the concentration of long-lived radioactive materials in rock. The Earth is now believed to be over four billion years old.

Scientists now believe that the Earth was formed by the accumulation and compression of cold nebular material, including simple organic molecules. Gold argues that simple inorganic and organic molecules in the forming Earth were subjected to increasing heat and pressure, and eventually formed more complex molecules. These complex molecules eventually became the simplest forms of life.

Gold's view is consistent with the hypothesis put forward by Russian biochemist A.I. Oparin and British geneticist J.D.S. Haldane that life emerged from inanimate material under the prevailing conditions of the primitive Earth. A classic experiment that supported the Oparin-Haldane hypothesis was performed in 1953 by two American chemists, Stanley Miller and Harold Urey. Their experiment showed that simple inorganic molecules could combine to form some basic molecules of life under conditions that may have been present on the primitive Earth. The interior of the Earth is viewed by proponents of the abiogenic theory as the crucible for forming life. The Miller-Urey experiment can be considered a model of the conditions that existed in the Earth's mantle.

Gold [1999] refutes challenges to the abiogenic theory and presents several pieces of evidence in support of the abiogenic theory, and the possible existence of a biological community deep inside the Earth. Some of Gold's evidence includes the existence of microbial populations that can thrive in extreme heat. These microbes, notably bacteria and archaea, grow at hot, deep ocean vents and can feed on hydrogen, hydrogen sulfide, and methane. Gold considers life forms at deep ocean vents transitional life forms that exist at the interface between two biospheres.

Basement Rock Sea level

Hydrocarbon

A. Deep, Hot Biosphere

Hydrocarbon

B. Bacterial and Geologic Action

C. Migration and Trapping

Figure 2-2. Abiogenic Origin of Oil and Gas

A. Deep, Hot Biosphere

B. Bacterial and Geologic Action

C. Migration and Trapping

Figure 2-2. Abiogenic Origin of Oil and Gas

One biosphere is the surface biosphere and includes life that lives on the continents and in the seas on the crust of the Earth. Gold postulates that a second biosphere exists in the mantle of the Earth. He calls the second biosphere the deep biosphere. The surface biosphere uses chemical energy extracted from solar energy, while the deep biosphere feeds directly on chemical energy. Oxygen is a requirement in both biospheres. Gold's deep biosphere is the source of life that eventually forms hydrocarbon mixtures (petroleum) in the Earth's mantle. Crustal oil and gas reservoirs are formed by the upward migration of petroleum fluid until the fluid is stopped by impermeable barriers and accumulates in geological traps. The abiogenic theory is illustrated in Figure 2-2.

Point to Ponder: Why does it matter whether the biogenic theory or the abiogenic theory is right?

Two of the arguments driving a transition from fossil energy to other forms of energy are the belief that the Earth contains a finite amount of fossil fuels, and that fossil fuels are not produced by natural processes fast enough to allow fossil fuels to be used as an inexhaustible source of energy. These arguments are based on the assumption that the biogenic theory is correct. If the abiogenic theory is correct, existing estimates of the volume of petroleum and the rate at which it is renewed could be significantly understated. There is still an argument for reducing our dependence on fossil fuels: global warming. Global warming is considered in more detail in Chapter 9.

Fossil Fuels

Fossil fuels are the dominant energy source in the modern global economy. They include coal, oil and natural gas. Each of these is discussed below.

Coal

Coal is formed from organic debris by a process known as coalification. When some types of organic materials are heated and compressed over time, they can form water, gas and coal. In some cases, a high-molecular weight, waxy oil is also formed. For example, swamp vegetation may be buried under anaerobic conditions and become peat. Peat is an unconsolidated deposit of partially carbonized vegetable matter in a water-saturated environment such as a bog. If peat is overlain by rock and subjected to increasing temperature and pressure, it can form coal.

Organisms that form coal when subjected to coalifica-tion include algae, phytoplankton and zooplankton. Coal can also be formed by the bacterial decay of plants and, to a lesser extent, animals. Organic debris is composed primarily of carbon, hydrogen, and oxygen. It may also contain minor amounts of other elements such as nitrogen and sulfur. The organic origin of coal provides an explanation for the elemental composition of coal, which ranges from pure carbon to a compound of such elements as carbon, hydrogen, oxygen, and sulfur.

Coals are classified by rank. Rank is a measure of the degree of coalification or maturation of the coal. The lowest rank coal is lignite, followed in order by sub-bituminous coal, bituminous coal, anthracite and graphite. Coal rank is correlated to the maturity, or age, of the coal. As a coal matures, the ratio of hydrogen to carbon atoms decreases and the ratio of oxygen to carbon atoms decreases. The highest rank coal, graphite, approaches 100% carbon. Coal becomes darker and denser with increasing rank.

Coals burn better if they are relatively rich in hydrogen; this includes lower rank coals with higher hydrogen to carbon ratios. The percentage of volatile materials in the coal decreases as coal matures. Volatile materials include water, carbon dioxide and methane. Coal gas is gas ab sorbed in the coal. It is primarily methane with lesser amounts of carbon dioxide. The amount of gas that can be absorbed by the coal depends on the rank. As rank increases, the amount of methane in the coal increases because the molecular structure of higher rank coals has a greater capacity to absorb gas and therefore can contain more gas.

Figure 2-3 shows an idealized representation of the physical structure of a coal seam. A coal seam is the stratum or bed of coal. It is a collection of coal matrix blocks bounded by natural fractures. The fracture network in coalbeds consists of microfractures called "cleats." An inter-connected network of cleats allows coal gas to flow from the coal matrix blocks when the pressure in the fracture declines. This is an important mechanism for coalbed methane production.

Cleats

Cleats

Overuse Coal

Coal is usually produced by extraction from coal beds. Mining is the most common extraction method. There are several types of mining techniques. Some of the more important coal mining techniques are strip mining, drift mining, deep mining, and longwall mining. Strip mining is also known as surface mining. Coal on the surface of the Earth is extracted by scraping. Drift mines are used to extract coal from coal seams that are exposed by the slope of a mountain. Drift mines typically have a horizontal tunnel entrance into the coal seam. Deep mining extracts coal from beneath the surface of the Earth. In the case of deep mining, coal is extracted by mining the coal seam and leaving the bounding overburden layers and underburden layers undisturbed.

Figure 2-4. Coal Trains in Canada

Coal is transported to consumers by ground transportation, especially by trains and, to a lesser extent, ships (Figure 2-4). A relatively inexpensive means of transporting coal is the coal slurry pipeline. Coal slurry is a mixture of water and finely crushed coal. Coal slurry pipelines are not widely used because it is often difficult to obtain rights of way for coal slurry pipelines that extend over long distances, particularly in areas where a coal slurry pipeline would compete with an existing railroad right of way.

Coalbed Methane

Environmental concerns are motivating a change from fossil fuels to an energy supply that is clean. Clean energy refers to energy that has little or no detrimental impact on the environment. Natural gas is a source of relatively clean energy. Oil and gas fields are considered conventional sources of natural gas. Two non-conventional sources of natural gas are coalbed methane, and gas hydrates.

Coalbeds are an abundant source of methane. Coal-bed methane exists as a monomolecular layer (a layer that is one molecule thick) on the internal surface of the coal matrix. Its composition is predominately methane, but can also include other constituents, such as ethane, carbon dioxide, nitrogen and hydrogen. The out-gassing of gas from coal is well known to coal miners as a safety hazard, and occurs when the pressure in the cleat system declines. The methane in the microscopic pore structure of the coalbed may be a safety hazard to miners, but it can also be a source of natural gas. Coal gas is able to diffuse into the natural fracture network when a pressure difference exists between the matrix and the fracture network.

Gas recovery from coalbeds depends on three processes. Gas recovery begins with desorption of gas from the monomolecular layer of gas on the coal surface to the coal micropores. The gas then diffuses through the coal micropores into the cleats. Finally, gas flows through the cleats to the production well. The flow rate depends, in part, on the pressure gradient in the cleats and the density and distribution of cleats. The controlling mechanisms for gas production from coalbeds are the rate of desorption from the coal surface to the coal matrix, the rate of diffusion from the coal matrix to the cleats, and the rate of flow of gas through the cleats.

Petroleum

Petroleum is a mixture of hydrocarbon molecules. Table 2-1 summarizes the composition of petroleum fluids for the most common elements. The actual elemental composition of a petroleum fluid depends on such factors as the composition of its source, reservoir temperature and reservoir pressure.

Hydrocarbon molecules in petroleum fluids are organic molecules. We expect the molecules in petroleum fluids to be relatively non-reactive and stable because they have been present in the fluid mixture for millions of years. If they were reactive or unstable, it is likely that they would have reacted or decomposed at some point in time and their products would be present in the petroleum fluid.

The volume of a petroleum mixture depends on changes in composition as well as changes in temperature and pressure. Fluids that are one phase at reservoir conditions often become two-phase fluids by the time they flow up the wellbore and reach the surface. Natural gas is a petroleum fluid in the gaseous state at surface conditions. Oil is a petroleum fluid in the liquid state at surface conditions. Heavy oils do not contain much gas in solution and have a relatively large molecular weight. By contrast, light oils typically contain a large amount of gas in solution and have a relatively small molecular weight.

Table 2-1

Elemental Composition of Petroleum Fluids

Element

Composition (% by mass)

Carbon

84% - 87%

Hydrogen

11 %- 14%

Sulfur

0.6% - 8 %

Nitrogen

0.02% -1.7%

Oxygen

0.08% - 1.8%

Metals

0% - 0.14%

Petroleum fluids are usually found in the pore space of sedimentary rocks. Igneous and metamorphic rocks originated in high pressure and temperature conditions that did not favor the formation or retention of petroleum fluids. Any petroleum fluid that might have occupied the pores of a metamorphic rock is usually cooked away by heat and pressure.

Several key factors must be present to allow the development of a hydrocarbon reservoir:

1. A source for the hydrocarbon must be present. For example, one source of oil and gas is thought to be the decay of single celled aquatic life. Shales formed by the heating and compression of silts and clays are often good source rocks. Oil and gas can form when the remains of an organism are subjected to increasing pressure and temperature.

2. A flow path must exist between the source rock and reservoir rock.

3. Once hydrocarbon fluid has migrated to a suitable reservoir rock, a trapping mechanism becomes important. If the hydrocarbon fluid is not stopped from migrating, buoyancy and other forces will cause it to move towards the surface.

4. Overriding all of these factors is timing. A source rock can provide large volumes of oil or gas to a reservoir, but the trap must exist at the time oil or gas enters the reservoir.

The stages in the life of a reservoir begin when the first discovery well is drilled. Prior to the discovery well, the reservoir is an exploration target. After the discovery well, the reservoir is a resource that may or may not be economic. The production life of the reservoir begins when fluid is withdrawn from the reservoir. Reservoir boundaries are established by seismic surveys and delineation wells. Delineation wells are wells that are originally drilled to define the size of the reservoir, but can also be used for production or injection later in the life of the reservoir. Production can begin immediately after the discovery well is drilled, or years later after several delineation wells have been drilled. The number of wells used to develop the field, the location of the wells, and their flow characteristics are among the many issues that must be addressed by reservoir management.

Electric Connections Over Head

A production system can be thought of as a collection of subsystems illustrated in Figure 2-5. Fluids are taken from the reservoir using wells. Wells must be drilled and com pleted. The performance of the well depends on the properties of the reservoir rock, the interaction between the rock and fluids in the reservoir, and properties of the fluids in the reservoir. Reservoir fluids include the fluids originally contained in the reservoir as well as fluids that may be introduced as part of the reservoir management process described below. Well performance also depends on the properties of the well itself, such as its cross-section, length, trajectory, and completion. The completion of the well establishes the connection between the well and the reservoir. A completion can be as simple as an open-hole completion where fluids are allowed to drain into the wellbore from consolidated reservoir rock, to completions that require the use of tubing with holes punched through the walls of the tubing using perforating guns to allow fluid to flow between the tubing and the reservoir.

Surface facilities are needed to drill, complete and operate wells. Drilling rigs may be moved from one location to another on trucks, ships, or offshore platforms (Figure 26); or drilling rigs may be permanently installed at specified locations. The facilities may be located in desert climates in the Middle East, stormy offshore environments in the North Sea, arctic climates in Alaska and Siberia, and deepwater environments in the Gulf of Mexico and off the coast of West Africa.

Figure 2-6. Offshore Platform in Dry Dock, Galveston, Texas

Produced fluids must be recovered, processed and transported to storage facilities and eventually to the consumer. Processing can begin at the well site where the produced wellstream is separated into oil, water and gas phases. Further processing at refineries separates the hydrocarbon fluid into marketable products, such as gasoline and diesel fuel (Figure 2-7). Transportation of oil and gas may be by a variety of means, including pipelines, tanker trucks, double hulled tankers, and ships capable of carrying liquefied natural gas.

Figure 2-7. A South Texas Refinery

Gas Hydrates

Gas hydrates are chemical complexes that are formed when one type of molecule completely encloses another type of molecule in a lattice. In the case of gas hydrates, hydrogen-bonded water molecules form a cage-like structure in which mobile molecules of gas are absorbed or bound. Although gas hydrates can be found throughout the world, difficulties in cost-effective production have hampered development of the resource. Gas hydrates are generally considered troublesome for oil and gas field operations, but the commercial potential of methane hydrates as a relatively clean energy resource is changing the industry perception of gas hydrates.

Methane hydrates contain a relatively large volume of methane in the hydrate complex. The hydrate complex contains about 85 mole percent water and approximately 15 mole percent guests, where a guest molecule is methane or some other relatively low molecular weight hydrocarbon. Methane hydrates can be found throughout the world. They exist on land in sub-Arctic sediments and on seabeds where the water is near freezing. Difficulties in cost-effective production of methane hydrates have hampered the production of methane from hydrates.

Tight Gas Sands and Shale Gas

Non-conventional gas resources include coalbed methane, tight gas sands and fractured gas shales. Coalbed methane was discussed above. Tight gas sands and gas shales are characterized by low permeabilities, that is, permeabilities that are a fraction of a millidarcy (less than 10"15 m2). The low permeability associated with non-conventional gas resources makes it more difficult to produce the gas at economical rates.

Economic production of gas from a gas shale or tight gas sand often requires the creation of fractures by a process known as hydraulic fracturing. In this process, a fluid is injected into the formation at a pressure that exceeds the fracture pressure of the formation. Once fractures have been created in the formation, a proppant such as coarse grain sand or manmade pellets are injected into the fracture to prevent the fracture from closing, or healing, when the injec tion pressure is removed. The proppant provides a higher permeability flow path for gas to flow to the production well. Non-conventional low permeability gas sands and shales often require more wells per unit area than conventional higher permeability gas reservoirs. The key to managing a non-conventional gas resource is to develop the resource with enough wells to maximize gas recovery without drilling unnecessary wells.

Shale Oil and Tar Sands

Shale oil is contained in porous, low permeability shale. Sand grains that are cemented together by tar or asphalt are called tar sands. Tar and asphalt are highly viscous, plastic or solid hydrocarbons. Extensive shale oil and tar sand deposits are found throughout the Rocky Mountain region of North America, as well as in other parts of the world. Although difficult to produce, the volume of hydrocarbon in tar sands has stimulated efforts to develop production techniques.

The hydrocarbon in shale oil and tar sands can be extracted by mining when oil shales and tar sands are close enough to the surface. Tar pits have been found around the world and have been the source of many fossilized dinosaur bones. In locations where oil shales and tar sands are too deep to mine, it is necessary to increase the mobility of the hydrocarbon.

An increase in permeability or a decrease in viscosity can increase mobility. Increasing the temperature of shale oil, tar or asphalt can significantly reduce viscosity. If there is enough permeability to allow injection, steam or hot water can be used to increase formation temperature and reduce hydrocarbon viscosity. In many cases, however, permeability is too low to allow significant injection of a heated fluid. An alternative to fluid injection is electromagnetic heating. Radio frequency heating has been used in Canada, and electromagnetic heating techniques are being developed for other parts of the world.

Point to Ponder: How does oil price affect recovery?

We have stated before that many experts believe we are running out of oil. It is becoming increasingly difficult to discover new reservoirs that contain large volumes of oil and gas. Much of the exploration effort is focusing on less hospitable climates, such as arctic conditions in Siberia and deep water, offshore regions near West Africa. Yet we already know where large volumes of oil remain: in the reservoirs we have already discovered and developed. Current development techniques have recovered approximately one third of the oil in known fields. That means roughly two thirds remains in the ground where we found it. [Fanchi, 2004, Exercise 6-9]

The efficiency of oil recovery depends on cost. We can produce much more oil from existing reservoirs if we are willing to pay for it. Most oil producing companies choose to seek and produce less expensive oil so they can compete in the international marketplace. Table 2-2 illustrates the sensitivity of oil producing techniques to the price of oil. The table shows that more sophisticated technologies can be justified as the price of oil increases. It also includes a price estimate for alternative energy sources, such as wind and solar. In some cases there is overlap between one technology and another. For example, steam flooding is an Enhanced Oil Recovery (EOR) process that can compete with conventional oil recovery techniques such as waterflooding, while chemical flooding is an EOR process that can be as expensive as many alternative energy sources.

Table 2-2. Sensitivity of Oil Recovery Technology to Oil

Price

Oil Recovery Technology

Oil Price (US$ per barrel in year 2000 US$)

Conventional

10-30

Enhanced Oil Recovery (EOR)

20-40

Extra Heavy Oil (e.g. tar sands)

25-45

Alternative Energy Sources

40 +

Point to Ponder: How high can oil prices go?

In addition to relating recovery technology to oil price, Table 2-2 contains another important point: the price of oil cannot rise to an arbitrarily high price without encountering competition from other energy options. For the data given in the table, we see that alternative energy sources become cost competitive when the price of oil rises above US$40 per barrel. If the price of oil stays at US$40 per barrel or higher for an extended period of time, energy consumers will begin to switch to less expensive energy sources. This switch has already begun in some countries. For example, consumers in European countries pay much more for gasoline than consumers in the United States. Countries such as Denmark, Germany and Holland are rapidly developing wind energy as an alternative to fossil fuels. France has opted to rely on nuclear fission energy.

Historically, we have seen oil exporting countries try to maximize their income and minimize competition from alternative energy and expensive oil recovery technologies by supplying just enough oil to keep the price at around US$25 to US$35 per barrel. Oil importing countries can attempt to minimize their dependence on imported oil by developing technologies that reduce the cost of alternative energy. If an oil importing country contains mature oil reservoirs, the development of relatively inexpensive technologies for producing oil remaining in mature reservoirs or the imposition of economic incentives to encourage domestic oil production can be used to reduce the country's dependence on imported oil.

Fossil Energy and Combustion

The chemical energy in fossil fuels is released by the process of combustion. The environmental impact of increased emission of combustion byproducts must also be considered. When a carbon based fuel burns, the carbon can react with oxygen to form carbon dioxide or carbon monoxide. If hydrogen is present, as it would be when a hydrocarbon is burned, hydrogen reacts with oxygen to form water.

These are exothermic reactions, that is, reactions that have a net release of energy. An exothermic reaction between two reactants A, B has the form

A + B -ยป products + energy Although the exothermic reaction releases energy, it may actually require energy to initiate the reaction. The energy that must be added to a system to initiate a reaction is called activation energy. An exothermic reaction will form products with a net release of energy. Fossil fuels release a relatively large amount of energy during the combustion process. An endothermic reaction does not release energy. Instead, it requires a net input of energy to form the products of the chemical reaction.

Point to Ponder: Why is fossil fuel combustion considered a problem?

Fossil fuel combustion provided a new source of energy that helped prevent deforestation and now supports a relatively high quality of life in industrialized nations.

Unfortunately, fossil fuel combustion also releases a large amount of carbon dioxide. Carbon dioxide is known as a greenhouse gas. The accumulation of carbon dioxide in the atmosphere tends to trap heat energy in the atmosphere. Many scientists believe that the additional heat is increasing the temperature of the atmosphere and is causing global warming. Global warming is discussed in more detail in Chapter 9.

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