II Translate the text: Chemical and Physical properties of petroleum and related substances
Petroleum is a complex mixture of gaseous, liquid, and solid hydrocarbons. In addition to the hundreds of different hydrocarbons composing petroleum, there are also other compounds which contain oxygen, nitrogen, and sulphur. Relatively small amounts of water and inorganic matter are frequently present. Chemical Composition of Petroleum
Carbon 82.2 to 87.1 per cent
Hydrogen 11.7 to 14.7 per cent
Sulphur 0.1 to 5.5 per cent
Nitrogen 0.1 to 2.4 per cent
Oxygen 0.1 to 7.4 per cent
Minerals 0.1 to 1.2 per cent
The physical and chemical properties of different samples of petroleum are not uniform because of the varying chemical composition and the presence of impurities. Petroleum and related substances occur in the physical state as a liquid (as in crude oil), as a gas (as in natural gas), and in the solid or semi-solid state (as in ozokerite and asphalt). Since petroleum in the natural reservoir occurs in most cases under pressure, some of the gaseous material and certain solid materials are dissolved in the liquid petroleum.
The physical and chemical properties of petroleum in the natural reservoir are somewhat different from those observed at the surface because of changes in temperature and pressure and loss of volatile constituents.
Colour. The colour of petroleum varies considerably. Some oils may be almost colorless, others are light yellow, red, green, brown, and black, or any intermediate shade. In thin layers colours are less intense, and in black oil the thin layers are brown in colour. The higher gravity crude oils are usually amber or green in colour. The lower gravity crudes are dark brown to black in colour.
When crude oils are observed by reflected light the colour appears to be different than that observed by transmitted light. A brown oil often appears to be green by reflected light. Ultra-violet light causes crude oils to fluoresce with characteristic colours even if they are of the same colour by transmitted and reflected light. This phenomenon is now widely used to test cores, samples, and drilling mud for oil shows.
Most of the pure hydrocarbons are colourless but may become coloured upon oxidation. Colour may be caused by the addition of oxygen to colourless leuco bases and the uniting of the leuco bases with other elements or groups to give coloured compounds.
Odour. The odour of a crude oil is determined by its composition. The paraffins and naphthenes are responsible for the agreeable odours, whereas the un-saturated hydrocarbons, certain nitrogen compounds, and sulphur cause the disagreeable odours. The crude oil has a disagreeable odour because of hydrogen sulphide and other sulphur compounds. Most of crude oils have an agreeable odour.
Specific Gravity. The specific gravity of a crude oil is the ratio of the weight of a given volume at a temperature of 60°F. to the weight of an equal volume of distilled water at the same temperature. It is an index to the value of oil. In general, the lighter oils have greater value because they contain larger quantities of gasoline and other valuable products.
Since oil is lighter than water, the specific gravity is normally expressed as a decimal or fraction. The decimal scale is used in most foreign countries, but in the United States the gravity is expressed in degrees by. whole numbers, using the A.P.I. scale. The A.P.I, scale is a variation of the Baume scale which is an Arbitrary one. It was derived by using water as 10 degrees and a modulus of 140. Specific gravity may be converted into degrees Baume and A.P.I, and vice versa by the following formulas:
Degrees
Baume =
Specific
Gravity 60°/60° F=
Degrees
A.P.I.=
Gravity
60°/60°F.=
The A.P.I. gravity of crude oil varies with changes in depth, temperature, age of the rocks, impurities, and with the geologic history of different areas. In general, the gravity of oil increases with depth and with increasing geologic age of the rocks. There' are, however, many exceptions to this general rule, The occurrence of low-gravity oil in Carboniferous limestones below light oil in Cretaceous sandstones may be due to difference in origin and geologic history. The Carboniferous oils may have been exposed to weathering during Triassic and Jurassic times.
In many oil-fields associated with faults, the deep producing formations contain oil which is lower in gravity than that in the shallow formations. In the same areas the gravity of the oil in the deep formations on unfaulted structures is higher than in shallow beds. The more volatile constituents have apparently migrated upward along the fault planes to raise the gravity of oil in higher sands.
Viscosity. Viscosity is a measure of the internal resistance to motion of a fluid by reason of the forces of cohesion between molecules or molecular groups. It is a property of great importance in the movement of oil through rocks. Viscosity is usually expressed in the time necessary for a volume of oil at a definite temperature to flow through a unit opening. Viscosities are determined by means of a number of standardized viscometers.
Crude oils vary considerably in viscosity. Those with a high A.P.I. gravity, in general, have n low viscosity whereas those with a low gravity have a high viscosity. This relationship is not always uniform, however, because two oils of the same gravity do not always have the same viscosity. The viscosity is influenced by the chemical composition.
Optical Activity. Most oils have the power of rotating the plane of polarization of polarized light. This is known as optical activity. A light ray after emerging from a Nicol prism is vibrating in one plane only. Light which is vibrating in one plane only is plane polarized light. The plane of polarized light is rotated to the right in some oils, and in others it is rotated to the left; those oils are known respectively as dextrorotatory and levulorotatory. The optical activity in most oils is caused by the presence of cholesterol, which is found in both vegetable and animal matter. It has the formula C26H45OH.
Boiling-point. The boiling-points of crude oils are quite variable and depend upon their composition. The boiling-point of an oil of a low A.P.I. gravity is usually high because of a larger proportion of hydrocarbons with a high boiling-point, whereas the boiling-point of an oil of a high A.P.I. gravity is usually low because of the presence of hydrocarbons with a low boiling-point. Therefore, the more volatile the hydrocarbons in a crude oil, the lower the boiling-point.
Fluorescence. All crude oils show a fluorescence or glow of varying degree under ultraviolet light. The slightest trace of oil will fluoresce under ultraviolet light. One part oil dissolved in 100,000 parts of carbon tetrachloride will fluoresce enough to be observed. The colour observed is usually a green-yellow. Fluorescence of crude oil is excited when subjected to ultraviolet radiation ranging between 2,000 and 3,800 angstroms. Tests showed that there is a marked difference in the fluorescence of oils from different fields and from different formations in the same field.
The fluorescent properties of crude oil may serve as a means of correlating oil sands in certain oil fields.
In the petroleum industry, fluorescence analysis may be used to determine the grade and purity of crude oil and its refine products, and to identify specific zones or horizons from which a well is producing. It is also widely used in field operations to test cores, cuttings, and drilling mud for shows of oil.
Unit 3