Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 237

The term bitumen (also, on occasion, referred to as native asphalt, and extra heavy oil) includes a wide variety of reddish-brown to black materials of semisolid, viscous to brittle character that can exist in nature with no mineral impurity or with mineral matter contents that exceed 50% by weight. Bitumen is frequently found filling pores and crevices of sandstone, limestone, or argillaceous sediments, in which case the organic and associated mineral matrix is known as rock asphalt.

Bitumen is a naturally-occurring material that is found in deposits where the permeability is low and passage of fluids through the deposit can only be achieved by prior application of fracturing techniques. Tar sand bitumen is a high-boiling material with little, if any, material boiling below 350°C (660°F), and the boiling range approximates the boiling range of an atmospheric residuum.

In order to define bitumen, extra heavy oil, heavy oil, and conventional crude oil, the use of a single physical parameter such as viscosity is not sufficient. Physical properties such as API gravity, elemental analysis, and composition fall short of giving an adequate definition. It is the properties of the bulk deposit and, most of all, the necessary recovery methods that form the basis of the definition of these materials. Only then is it possible to classify crude oil, heavy crude oil, extra heavy crude oil, and tar sand bitumen. For example, tar sands have been defined in the United States (FE-76-4) as:

…the several rock types that contain an extremely viscous hydrocarbon which is not recoverable in its natural state by conventional oil well production methods including currently used enhanced recovery techniques. The hydrocarbon-bearing rocks are variously known as bitumen-rocks oil, impregnated rocks, oil sands, and rock asphalt.

The recovery of the bitumen depends to a large degree on the composition and construction of the sands. Generally, the bitumen found in tar sand deposits is an extremely viscous material that is immobile under reservoir conditions and cannot be recovered through a well by the application of secondary or enhanced recovery techniques.

The expression tar sand is commonly used in the crude oil industry to describe sandstone reservoirs that are impregnated with a heavy, viscous black crude oil that cannot be retrieved through a well by conventional production techniques (FE-76-4, above). However, the term tar sand is actually a misnomer; more correctly, the name tar is usually applied to the heavy product remaining after the destructive distillation of coal or other organic matter. Thus, alternative names, such as bituminous sand or oil sand, are gradually finding usage, with the former name (bituminous sands) more technically correct. The term oil sand is also used in the same way as the term tar sand, and these terms are often used interchangeably.

On an international note, the bitumen in tar sand deposits represents a potentially large supply of energy. However, many of the reserves are available only with some difficulty and that optional refinery scenarios will be necessary for conversion of these materials to liquid products because of the substantial differences in character between conventional crude oil and tar sand bitumen.

Because of the diversity of available information and the continuing attempts to delineate the various world tar sand deposits, it is virtually impossible to present accurate numbers that reflect the extent of the reserves in terms of the barrel unit. Indeed, investigations into the extent of many of the world’s deposits are continuing at such a rate that the numbers vary from one year to the next.

The term bitumen, as used in oil shale technology is the lower molecular weight soluble, organic component of oil shale. The amount of bitumen is low, usually on the order of 0.5 to 5% w/w of the total weight of the oil shale. Thus, oil shale organic matter can be characterized as two materials, kerogen and bitumen, the latter being benzene soluble. Bitumen is also the product produced by the high molecular weight material thermal decomposition of kerogen.

The yield of bitumen (extractable material) increases with (i) increasing extraction temperature, (ii) with increasing polarity of the extraction solvent, and (iii) with the chemical reactivity of the solvent. Moreover, bitumen is generally richer in hydrogen (H/C may be on the order of approximately 1.6 with a molecular weight of approximately 1,200) and nitrogen, while it is lower in the proportion of aromatic constituents (and consequently richer in aliphatic constituents) than the corresponding kerogen.

Although the thermal decomposition kinetics is complex, the thermal decomposition of the bitumen (either extracted or produced from the thermal decomposition of kerogen) is often simply described (like the thermal decomposition of kerogen) by a kinetic mechanism involving a first order mechanism which involves the production of thermal bitumen: