1.3. Petroleum States

Whether petroleum exists as a liquid, solid, semisolid, or gas depends on the particular combinations of hydrogen and carbon atoms present in a given deposit, and the temperature and pressure surrounding that deposit. Commonly, the liquid state of petroleum is loosely called "oil" or "crude oil," and the gaseous state of pe­troleum is loosely called "natural gas. An example of a semisolid or solid state pe­troleum is "tar" or "asphalt." Petroleum may change states, i.e., change from a liq­uid to a gas, just as ordinary water may change states, from liquid water to gaseous steam.

As you know, ordinary water changes states depending on temperature and pressure. At normal atmospheric pressure, which is about 14.7 pounds per square inch (psi) at sea level, water that is about 32°F will freeze to a solid state: ice. On the other hand, if the water is hot enough, or about 212°F at atmospheric pressure, the water will vaporize or boil and will revert to the gaseous state: steam.

The freezing and boiling temperatures, or points, of water depend on the pressure held against the water. If water is heated on Pike's Peak where the atmos­pheric pressure is lower than it is at sea level, the water will boil at a temperature lower than 212°F. Conversely, if water is heated in Death Valley, which is below sea level, the water will boil at a temperature that is higher than 212°F. For any particular pressure, water has a unique boiling temperature.

Fig. 1-4 if a state or phase diagram. It approximates the boiling temperature of water at various common pressures. The figure shows that showing boiling wa­ter (vapor state) is dependent upon pressure. The boiling point of water is 212°F only at normal atmospheric pressure (14.7 psi). On top of a 7,000-ft mountain, the atmospheric pressure may be only about 11 psi. From the figure, we see that at 11 psi the boiling point of water is about 200°F, 12°F lower than at sea level. That if why food takes longer to cook at higher elevations; the food is cooking at a lower temperature.

Fig 1-4 Phase diagram for water.

Petroleum states are similar to water slates. However, the petroleum state depends on the particular composition of the petroleum as well as the temperature and pressure. Just as the state of water can change, the state of petroleum can also change. Petroleum that is liquid at normal temperature and pressure could be solid at a lower temperature. Likewise, it could be gaseous at a higher temperature. High pressure tends to make petroleum more dense. Thus, gaseous petroleum at low pressure could be a liquid at higher pressure.

Crude Oil

When the chemical composition of petroleum is such that it is in a liquid state at normal temperature and pressure, it is commonly referred to as crude oil. Crude oil, or simply oil, may vary in consistency from a very light, thin liquid like motor gasoline to a very heavy, thick liquid such as asphalt. In color, the crude oil may be as clear as water or any combination of yellow, green, brown, or black. Most (but not all) liquid petroleum weighs less than water. Therefore, most oil floats on water. Oils that are close to the same weight as water, or oils that are heavier than water, are called heavy oils.