Physical and Chemical Properties of Oil

Oil is described by the physical properties of density, color, viscosity, thermal expansion and other properties related to the number of carbon atoms in the molecules.

Petroleum can be of different colors. Oil colors vary in a very wide range from oilfield to oilfield: from pale yellow, yellow and even colourless to dark grey, green and dark brown shades.

The SI unit is kg/m3 at a reference temperature, typically 15 C. Knowledge of density is required for quantity calculations. In the USA and some other countries the density of petroleum products is defined in terms of API gravity. This is an arbitrary scale adopted by the American Petroleum Institute for expressing the relative density of oils. The API gravity scale is read "backwards". The higher the API number, expressed as degrees API, the less dense (lighter) the oil is. Conversely, the lower the degrees API, the more dense (heavier) is the oil.

Density of oils range from 0.65 to 1.0 gr/cm and more at 20 C. According to density, oils may be light, medium and heavy. Light oil is characterized by the density of 0.5- 0.87; medium oil: 0.871- 0.910 and heavy oil is described being as 0.910 - 1.05 gr/cm3.

Viscosity is a property of fluids that indicates their resistance to flow, defined as ratio of shear stress to shear rate. Crude oils range in consistency from water-like to tar-like solids. Fluid with a high viscosity such as syrup deforms more slowly than fluid with a low viscosity such as water. Absolute viscosity is measured in Poise. The oil specific viscosity is usually defined as ratio of absolute viscosity of a given fluid to absolute viscosity of water at the same temperature. The viscosity of oil is dependent upon temperature, pressure and shear rate. Viscosity decreases at temperature increases because molecules vibrate and interact less. Conversely, the viscosity of oil increases at temperature decreases and it can become grease-like at very low temperature.

The volume of given oil mass increases with temperature, therefore, its density decreases. The degree of expansion is expressed as the coefficient of thermal expansion. Thermal expansion is useful to determine the size of container needed when the oil is heated. Thermal expansion is expressed as the ratio of volume change to initial volume after heating 10 °C.

Crude oils are complex mixtures containing hundreds of different hydrocarbon compounds that vary in appearance and composition from oil field to oil field, therefore, in various oil fields the oil composition can vary significantly. All hydrocarbons are divided into two groups: saturated hydrocarbons and unsaturated hydrocarbons. Saturated hydrocarbons are not capable of attaching atoms and molecules while unsaturated hydrocarbons are capable of attaching atoms and molecules. The latter take part in chemical reactions easier. Hydrocarbons can be as simple as methane, but many are highly complex molecules and can occur as gases, liquids or solids. An "average" crude oil contains about 84% carbon, 14% hydrogen, 1-5% sulfur, and less than 1% of nitrogen, oxygen, metals and salts. In the refinery, most of these non-hydrocarbon substances are removed and the oil is broken down into various compounds and blended into useful products.

EXERCISE 10 Read and translate the text "Finding Oil", using the words after the text.

Finding Oil

The task of finding oil is assigned to geologists, whether employed directly by an oil company or under a contract from a private firm. Their task is to find the right conditions for an oil trap - the right source rock, reservoir rock and entrapment. Many years ago, geologists interpreted surface features, surface rock and soil types, and, perhaps some small core samples obtained by shallow drilling. Modern oil geologists also examine surface rocks and terrain, with the additional help of satellite images.

However, they also use a variety of methods to find oil. They can use sensitive gravity meters to measure tiny changes in the Earth's gravitational

field that could indicate flowing oil, as well as sensitive magnetometers to measure tiny changes in the Earth's magnetic field caused by flowing oil. They can detect the smell of hydrocarbons using sensitive electronic noses called sniffers. Finally, and most commonly, they use seismology, creating shock waves that pass through hidden rock layers and interpreting the waves that are reflected back to the surface.

The shock waves travel beneath the surface of the Earth and are reflected back by the various rock layers. The reflections travel at different speeds depending upon the type or density of rock layers through which they must pass. The reflections of the shock waves are detected by sensitive microphones or vibration detectors - hydrophones over water, seismometers over land. The readings are interpreted by seismologists for signs of oil and gas traps.

The words to be memorized:

core samples –образцы керна

to detect – обнаруживать, замечать

density – плотность

entrapment – захват, улавливание

hydrophone – гидрофон

to indicate – указывать, показывать

to interpret – расшифровывать

gravity meter – гравиметр

magnetometer – магнитометр

to measure – отмерять, измерять, расчитывать

to obtain – приобретать, добывать

reflect back – отражать

sniffer – газоанализатор

seismology – сейсмология

seismometer – сейсмограф

terrain – район, территория; местность

EXERCISE 11 Answers to the following questions, using the information from the text "Finding Oil".

1. Who usually finds oil?

2. What is the task of this kind of specialist?

3. How do modern specialists examine surface rocks?

4. What for do geologists use new methods to find oil?

5. What does seismology deal with?

6. Where and how do the shock waves travel?

7. By what are the reflections of the shock waves detected?

8. By whom are the readings interpreted?

EXERCISE 12 Read and translate the text "How to Find Oil".

How to Find Oil

Photographs from aircraft and satellites are used to begin the onshore search for oil and gas which is underground. This cuts down the time spent searching on the surface. The photographs are studied very carefully for the structures where oil might be found. If an area shows promise, then teams are sent to find out more about the rocks.

Geologists and geophysicists work closely together using a variety of methods. All the information is carefully considered, with the help of computer analysis, before any decisions to drill are made. A geologist collects small samples of rock. Sometimes the samples of rock are dug out by hand or cylindrical cores are drilled to give samples which can be cut and studied under a microscope. These help them to find out where the rocks have come from, what they are made of and how the rocks are arranged in strata.

Geologists also find out about the physical and chemical properties of the rocks and the fossil record from ancient times. All these clues give information to build up a picture of the area being surveyed. A geophysicist adds to the information of a geologist by studying the physics of the Earth. Surveys are made of the magnetic field, the gravity and how waves travel through the layers.

Magnetometers measure very small changes in the strength of the Earth's magnetic field. Sedimentary rocks are nearly non-magnetic and igneous rocks have a stronger magnetic effect. Because of these different effects on the magnetic field, measurements can be made to work out the thickness of the sedimentary layers which may contain oil.

Gravitometers measure the strength of the Earth's gravitational pull. This is not the same all over the Earth because of the different densities of the rocks. Igneous rocks like granite are denser than sedimentary rocks. Granite near the surface will have a stronger pull than the same lump deeper down, so measurements help to build up more information about the layers of rock.

Shock waves or seismic waves are used to help give a picture of deep rock structures. The idea is to make artificial shock waves and record how they travel through the Earth. The shock wave travels through the water and strikes the sea bed. Some of the energy of the wave is reflected back to the hydrophones. The rest of the wave carries on until it reaches another rock layer.

The time taken for the waves to travel from the source to the hydrophones is used to calculate the distance traveled- hence the thickness of the rock layers. The amplitude of the wave gives information about the density of the reflecting rock. A survey using artificial shock waves is called a seismic survey. The data from a survey is recorded and displayed by computer as a pattern of lines, called a seismograph.

Sometimes, surveys show that a structure is present which may contain oil and gas. If so, an exploratory well or wildcat well is drilled. Very few exploration wells find oil. Even in areas like the North Sea, where we know a great deal about the geology, only one in every eight wells which are drilled will find oil or gas in quantities worth developing.

Drilling is a very expensive activity, with each well on average costing several million dollars. Even with today's technology, there is still a low probability that oil or gas will be found. Most oil wells are between 900 and 5,000 metres deep, but it is now possible to drill 8 km below the surface, an achievement made possible by skilled operators using powerful equipment and advanced technology. However, the costs of drilling can double or treble when in very deep water, hostile environments and when high pressure or temperature is encountered.

The rock is drilled with a rotating drill bit, similar to those that are used to drill a hole in wood. The drill bit is attached to a string of steel pipes, each approximately 9 metres long. The derrick, the structure that stands above the hole, must be strong, as the drill pipe and bit are suspended from it. Only a small proportion of the total weight of the drilling string is allowed to bear on the drill bit. This proportion will vary depending on the rock formation being drilled. The derrick must also be tall enough to enable the individual lengths of drill pipe to be added to or removed from the string.

The drilling process is lubricated and cooled by a carefully constituted mud. This passes down inside the pipes to the drill bit and then returns to the top of the hole between the pipe and the sides of the hole, bearing rock debris with it. This provides the geologists with rock samples to indicate the kind of rock the drill is passing through.

The weight of the mud also prevents the escape of oil or gas if it is found. Usually the gas or oil is under pressure in the ground. To stop wasteful and dangerous gushers, a set of valves called a Christmas tree is fitted to the well head to control the flow of fluids from the well.