Reservoir rocks usually contain saline water and hydrocarbons. The hydrocarbons, liquid and gas, are believed to come from organic material such as plant and microscopic animal remains. The liquid is known as crude oil. These hydrocarbons are made of carbon and hydrogen atoms, and their characteristics depend on their composition. Samples are collected for analysis through reservoir fluid sampling.
Paraffins: The Simplest Hydrocarbons
The simplest hydrocarbon is methane (CH₄), a central carbon atom surrounded by four hydrogen atoms. The alkane group of hydrocarbons is known as the paraffins. A crude oil made mostly of these compounds is called paraffinic.
| Hydrocarbon | Formula | State at atmospheric conditions |
|---|---|---|
| Methane | CH₄ | Gas |
| Ethane | C₂H₆ | Gas |
| Propane | C₃H₈ | Gas |
| Butane | C₄H₁₀ | Gas |
| C₅ and above, up to about C₂₀ | — | Liquids |
| Larger molecules | — | Viscous liquids or solids |
Under increasing temperature and pressure, even the light paraffins can turn to liquid.
Hydrocarbons Inside the Reservoir
Within reservoir rocks, hydrocarbons may exist as a liquid with the lighter gaseous components in solution, or as crude oil with free gas co-existing. As the number of carbon atoms increases, the possible structures become more abundant and the fluid becomes more complex. For C₅ and heavier hydrocarbons, the molecule can take the form of a ring structure, such as cyclohexane. Formation pressure controls whether the gas stays dissolved or separates.
Naphthenes and Aromatics
- Naphthenes (cycloparaffins) are saturated ring hydrocarbons such as cyclohexane. Crudes rich in them are called naphthenic or asphaltic.
- Aromatics are unsaturated ring hydrocarbons, such as benzene. A hydrocarbon molecule that does not have its full complement of hydrogen atoms is unsaturated. Aromatics have distinct smells.
Non-Hydrocarbon Elements
Crude oil can also contain inorganic elements, mainly sulphur, nitrogen, oxygen, vanadium, sodium and potassium.
How Fluids Are Distributed and Why Composition Matters
Overall, gases are made up of the lighter, smaller-chain molecules, while crude oil composition can be paraffinic or asphaltic (for example naphthenes). In a reservoir, the fluids separate by density: gas on top, oil in the middle and saline water at the bottom. The crude oil composition has a large bearing on the production characteristics and on the problems that can be met in a production system.
Frequently Asked Questions
What are paraffins?
The alkane group of hydrocarbons, with the general formula CₙH₂ₙ₊₂, starting with methane.
What is the difference between naphthenes and aromatics?
Both have ring structures. Naphthenes are saturated; aromatics, like benzene, are unsaturated.
Key Takeaways
- Hydrocarbons are chains and rings of carbon and hydrogen.
- C₁ to C₄ are gases at atmospheric conditions; heavier paraffins are liquids or solids.
- Crude oil can be paraffinic, naphthenic (asphaltic) or mixed, and composition affects production problems.
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