Читать книгу Synthesis Gas - James Speight G., James G. Speight - Страница 19

Just like the term oil sand (tar sand in the United States), the term oil shale is a misnomer since the mineral does not contain oil nor is it always shale. The organic material is chiefly kerogen and the shale is usually a relatively hard rock, called marl. Oil shale is a complex and intimate mixture of organic and inorganic materials that vary widely in composition and properties. In general terms, oil shale is a fine-grained sedimentary rock that is rich inorganic matter and yields oil when heated. Some oil shale is genuine shale but others have been misclassified and are actually siltstones, impure limestone, or even impure coal. Oil shale does not contain oil and only produces oil when it is heated to approximately 500^oC (approximately 930^oF), when some of the organic material is transformed into a distillate similar to crude oil (Lee, 1990; Scouten, 1990; Lee, 1991; Speight, 2008, 2012).

Thus, when properly processed, kerogen can be converted into a substance somewhat similar to crude oil which is often better than the lowest grade of oil produced from conventional oil reservoirs but of lower quality than conventional light oil. Shale oil (retort oil) is the liquid oil condensed from the effluent in oil shale retorting and typically contains appreciable amounts of water and solids, as well as having an irrepressible tendency to form sediments. Oil shale is an inorganic, non-porous sedimentary marlstone rock containing various amounts of solid organic material (known as kerogen) that yields hydrocarbon derivatives, along with non-hydrocarbon derivatives, and a variety of solid products, when subjected to pyrolysis (a treatment that consists of heating the rock at high temperature) (Lee, 1990; Scouten, 1990; Lee, 1991; Speight, 2008, 2012).

Oil production potential from oil shale is measured by a laboratory pyrolysis method called Fischer Assay (Speight, 1994, 2008, 2012) and is reported in barrels per ton (42 US gallons per barrel, approximately 35 Imperial gallons per barrel). Rich oil shale zones can yield more than 40 US gallons per ton, while most shale zones produce 10 to 25 US gallons per ton. Yields of shale oil in excess of 25 US gallons per ton are generally viewed as the most economically attractive, and hence, the most favorable for initial development. Thus, oil shale has, though, a definite potential for meeting energy demand in an environmentally acceptable manner (Lee, 1990; Scouten, 1990; Lee, 1991; Bartis et al., 2005; Andrews, 2006; Speight, 2008; 2012).

The oil shale deposits in the western United States contain approximately 15% w/w organic material (kerogen) (Lee, 1990; Scouten, 1990; Lee, 1991; Speight, 2012). By heating oil shale to high temperatures (>500^oC, >930^oF), the kerogen is decomposed and converted to a volatile liquid product. However, shale oil is sufficiently different from crude oil and refining processing shale oil presents some unusual problems but, nevertheless, shale oil can be refined into a variety of liquid fuels, gases, and high-value products for the petrochemical industry.

The United States has vast known oil shale resources that could translate into as much as 2.2 trillion barrels of known oil-in-place. Oil shale deposits in the United States are concentrated mainly in the Green River Formation in the states of Colorado, Wyoming and Utah, which account for nearly three-quarters of this potential (Lee, 1990; Scouten, 1990; Lee, 1991; Bartis et al., 2005; Andrews, 2006; Speight, 2008; 2012). Because of the abundance and geographic concentration of the known resource, oil shale has been recognized as a potentially valuable United States energy resource since as early as 1859, the same year Colonel Drake completed his first oil well in Titusville, Pennsylvania (Chapter 1). Early products derived from shale oil included kerosene and lamp oil, paraffin, fuel oil, lubricating oil and grease, naphtha, illuminating gas, and ammonium sulfate fertilizer.

Since the beginning of the 20^th century, when the United States Navy converted its ships from coal to fuel oil, and the economy of the United States was transformed by gasoline-fueled automobiles and diesel-fueled trucks and trains, concerns have been raised related to assuring adequate supplies of liquid fuels at affordable prices to meet the growing needs of the nation and its consumers. Thus, it is not surprising that the abundant resources of oil shale in the United States were given consideration as a major source for these fuels. In fact, the Mineral Leasing Act of 1920 made crude oil and oil shale resources on federal lands available for development under the terms of federal mineral leases. This enthusiasm for oil shale resources was mitigated to a large extent by the discoveries of more economically producible and easy-to-refine liquid crude oil in commercial quantities, which caused the interest in oil shale to decline markedly.

However, the interest in oil shale resumed after World War II, when military fuel demand and domestic fuel rationing and rising fuel prices made the economic and strategic importance of the oil shale resource more apparent. After the war, the booming postwar economy drove demand for fuels ever higher, starting with the commencement of the development, in 1946, of the Anvil Point, Colorado, oil shale demonstration project by the United States Bureau of Mines. Significant investments were made by commercial companies to define and develop the US oil shale resource and to develop commercially viable technologies and processes to mine, produce, retort, and upgrade oil shale into viable refinery feedstocks and by-products. Once again, however, major crude oil discoveries in the lower-48 United States, off-shore, and in Alaska, as well as other parts of the world, reduced the foreseeable need for shale oil and interest and associated activities again diminished.

By 1970, oil discoveries were slowing, demand was rising, and crude oil imports into the United States, largely from the Middle Eastern oil-producing nations, were rising to meet demand. Global oil prices, while still relatively low, were also rising, reflecting the changing market conditions. Ongoing oil shale research and testing projects were reenergized and new projects were envisioned by numerous energy companies seeking alternative fuel feedstocks (Speight, 2008, 2011c). These efforts were significantly amplified by the impact of the 1973 Arab oil embargo which demonstrated the vulnerability of the oil-consuming nations, particularly the United States, to oil import supply disruptions, and were underscored by a new supply disruption associated with the 1979 Iranian Revolution.

By 1982, however, technology advances and new discoveries of offshore oil resources in the North Sea and other bodies of water provided new sources for oil imports into the United States. Thus, despite significant investments by energy companies, numerous variations and advances in mining, restoration, retorting, and in-situ processes, the costs of oil shale production relative to current and foreseeable oil prices, made continuation of most commercial efforts impractical.

Despite the huge resources, oil shale is an underutilized energy resource. In fact, one of the issues that arise when dealing with fuels from oil shale is the start-stop-start episodic nature of the various projects. The projects have varied in time and economic investment and viability. The reasons comprise competition from cheaper energy sources, heavy front-end investments and, of late, an unfavorable environmental record. Oil shale has, though, a definite potential for meeting energy demand in an environmentally acceptable manner (Bartis et al., 2005; Andrews, 2006; Speight, 2020).