Nature of Crude Oils and Their Products

Petroleum is a complex mixture of hundreds of hydrocarbon compounds of different types which is stored in sedimentary rocks in the form of gas or liquid called reservoir fluid. The ratio of gas to oil in a reservoir fluid is called the gas-to-oil ratio (GOR) and is an important characteristic of reservoir fluids. Once a reservoir fluid is produced from a production well at a surface facility, it is brought to surface conditions (usually at 1 atm and 60°F) through multistage separators. Produced liquid is generally referred to as crude oil, and the gas is referred to as associate gas. The general composition of a reservoir fluid with produced liquid and gas from a petroleum reservoir in Kuwait is shown in Table 2.1 (ASTM MNL50). The C₇₊ fraction represents all compounds with carbon number >7.

As shown in this Table 2.1 a crude oil is mainly a mixture of hydrocarbons with some heteroatoms such as sulfur (S), nitrogen (N), oxygen (O), and metals. The

TABLE 2.1

Composition of a Reservoir Fluid and Flashed Gas and Produced Crude Oil from a Reservoir in the Middle East

Note: Numbers represent mol% of each component.

composition (wt%) of the elements depends on the source of the crude and varies within narrow limits as:

Carbon (C), 83.0-87.0%

Hydrogen (H), 10.0-14.0%

Nitrogen (N). 0.1-2.0%

Oxygen (O). 0.05-1.5%

Sulfur (S), 0.05-6.0%

Metals (nickel, vanadium, and copper), <1000 ppm (0.1%)

Generally in heavier oils, amounts of carbon and heteroatoms increase with increase in density (lower API gravity). Hydrocarbons found in petroleum oils are mainly from four families or groups: (1) paraffins, (2) olefines, (3) naphthenes, and (4) aromatics. Paraffins, olefins, and naphthenes are sometimes called aliphatic versus aromatic compounds. Naphthenes and aromatics are cyclic compounds, while paraffins and olefins are non-cyclic. The simplest hydrocarbon compound is methane from the paraffin group. Paraffins are divided into two groups of normal paraffin (also called n-alkanes) and iso-paraffins (or branched paraffins). The general closed formulae for these groups are: C_nH_2n+2 (paraffins), C„H_2|1 (olefins), C„H₂„ (naphthenes), and C_nH_2n_₆ (where n>6) for aromatics. Structures of some sample compounds are shown below:

n-Butane (n-C₄H_K) or simply shown as n-C4) is an example of an n-paraffin or n-alkane compound.

Iso-octane (2-methylheptane) is an example of a branched paraffinic compound (C₈H_I8).

Compounds with one double bond (also called mono-olefins) or alkenes such as ethene also named ethylene (CH₂=CH₂) and propene or propylene (CH₂=CH-CH,) are from olefinic compounds. Since olefins are generally unstable compounds, usually most petroleum products are olefin-free.

A few sample compounds from the naphthenic group are shown here:

Some examples of aromatic compounds are:

Compounds with methyl groups have an attachment of CH, and the cycles with double bonds are attached by CH. Heavier oils have higher amounts of polynuclear aromatic hydrocarbons fPAHs). Sulfur and nitrogen are usually present in PAHs such as:

Sulfur also may exist in non-cyclic compounds such as mercaptans (R-S-H) and sulfides (R-S-R') where R and R' are alkyl groups. Asphaltene is an aromatic compound (PAH) that may exist in the residues of some heavy oils and has a molecular weight of 1,000-5,000 g/mol and density of 1.1-1.2 g/ml. A sample molecule of asphaltene is shown in Figure 2.1 (Speight, 1998).

FIGURE 2.1 An example of an asphaltene molecule.

TABLE 2.2

Some Petroleum Fractions Produced from Distillation Columns

Crude oil is fed into an oil refinery where it first goes through atmospheric and then vacuum distillation columns. Some products of crude oil distillation with their carbon number and boiling ranges are given in Table 2.2 (Riazi, 2007).

Based on the API gravity and viscosity of oil, a general classification of crude oil is given in Table 2.3. According to this classification, the term heavy oil refers to oils with API degrees between 10 and 22, while oils with API gravity of greater than 22 are referred to as light oil. If the API gravity is less than 10 but the oil under reservoir conditions is mobile it is referred to as extra heavy oil, and if immobile (viscosity greater than 10,000 cSt) it is referred to as bitumen or oil sand.

TABLE 2.3

Classification of Oils and Definition of Heavy Oil, Extra Heavy Oil and Bitumen