Crude oil and natural gas

Crude oil

Crude oil is a complex mixture consisting of 200 or more different organic compounds, mostly alkanes (single bond hydrocarbons on the form C_nH_2n+2) and smaller fraction aromatics (six-ring molecules such as benzene C₆H₆)

Different crude contains different combinations and concentrations of these various compounds. The API (American Petroleum Institute) gravity of a particular crude is merely a measure of its specific gravity or density. The higher the API number expressed as degrees API, the less dense (lighter, thinner) the crude. Simply put, this means that the lower the degrees API, the denser (heavier, thicker) the crude. Crude from different fields and from different formations within a field can be similar in composition or significantly different.

In addition to API grade and hydrocarbons, crude is characterized for other undesired elements like sulfur, which is regulated and needs to be removed.

Crude oil API gravities typically range from 7 to 52, corresponding to about 970 kg/m³ to 750 kg/m³ , but most fall in the 20 to 45 API gravity range. Although light crude (i.e., 40-45 degrees API) is considered the best, lighter crude (i.e., 46 degree API and above) is generally no better for a typical refinery. As the crude gets lighter than 40-45 degrees API, it contains shorter molecules, which means a lower carbon number. This also means it contains less of the molecules useful as high octane gasoline and diesel fuel, the production of which most refiners try to maximize. If a crude is heavier than 35 degrees API, it contains longer and bigger molecules that are not useful as high octane gasoline and diesel fuel without further processing.

For crude that has undergone detailed physical and chemical property analysis, the API gravity can be used as a rough index of the quality of crudes of similar composition as they naturally occur (that is, without adulteration, mixing, blending, etc.). When crudes of a different type and quality are mixed, or when different petroleum components are mixed, API gravity cannot be used meaningfully for anything other than a measure of fluid density.

For instance, consider a barrel of tar that is dissolved in 3 barrels of naphtha (lighter fluid) to produce 4 barrels of a 40 degrees API mixture. When this 4-barrel mixture is fed to a distillation column at the inlet to a refinery, one barrel of tar plus 3 barrels of naphtha is all that will come out of the still. On the other hand, 4 barrels of a naturally occurring 40 degrees API crude, fed to the distillation column at the refinery could come out of the still as 1.4 barrels of gasoline and naphtha (typically C₈H₁₈), 0.6 barrels of kerosene (jet fuel C_12-15 ), 0.7 barrels of diesel fuel (average C₁₂H₂₆), 0.5 barrels of heavy distillate (C_20-70), 0.3 barrels of lubricating stock, and 0.5 barrels of residue (bitumen, mainly poly-cyclic aromatics).

The previous figure illustrates weight percent distributions of three different hypothetical petroleum stocks that could be fed to a refinery with catalytic cracking capacity. The chemical composition is generalized by the carbon number which is the number of carbon atoms in each molecule - CnH2n+2. A medium blend is desired because it has the composition that will yield the highest output of high octane gasoline and diesel fuel in the cracking refinery. Though the heavy stock and the light stock could be mixed to produce a blend with the same API gravity as the medium stock, the composition of the blend would be very different from the medium stock, as the figure indicates. Heavy crude can be processed in a refinery by cracking and reforming that reduces the carbon number to increase the high value fuel yield.

Natural gas

The natural gas used by consumers is composed almost entirely of methane. However, natural gas found at the wellhead, though still composed primarily of methane, is not pure. Raw natural gas comes from three types of wells: oil wells, gas wells, and condensate wells.

Natural gas that comes from oil wells is typically termed “associated gas.” This gas can exist separately from oil in the formation (free gas) or dissolved in the crude oil (dissolved gas). Natural gas from gas and condensate wells in which there is little or no crude oil, is termed “non-associated gas.”

Gas wells typically produce only raw natural gas. However, condensate wells produce free natural gas along with a semi-liquid hydrocarbon condensate. Whatever the source of the natural gas, once separated from crude oil (if present), it commonly exists in mixtures with other hydrocarbons, principally ethane, propane, butane, and pentanes. In addition, raw natural gas contains water vapor, hydrogen sulfide (H₂S), carbon dioxide, helium, nitrogen, and other compounds.

Condensates

While the ethane, propane, butane, and pentanes must be removed from natural gas, this does not mean that they are all waste products. In fact, associated hydrocarbons, known as natural gas liquids (NGL), can be very valuable by-products of natural gas processing. NGLs include ethane, propane, butane, iso-butane, and natural gasoline. These are sold separately and have a variety of different uses such as raw materials for oil refineries or petrochemical plants, as sources of energy, and for enhancing oil recovery in oil wells. Condensates are also useful as diluents for heavy crude.