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Introduction to Natural Gas from Oil Wells
Crude oil contains natural gas both in solution with the crude oil, and as free gas in the oil reservoir rock pores. Once the oil and gas is brought up to the production platform, the modern oil production platforms separate the natural gas from the crude. This gas is then compressed and sent ashore through sub-sea pipelines for further processing at a natural gas refinery.
When learning about the chemical composition of natural gas from oil wells, we begin with an overview of how the oil reservoir was formed and the crude oil and natural gas extraction, before moving on to the composition of the natural gas.
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Overview of the Formation of Crude Oil Reservoirs, Extraction and Refining Methods
Crude oil reservoirs were formed millions of years ago during Carboniferous Period in the Paleozoic Era, through a build-up of creatures and plants that died and sunk to the bottom of a mass of water. These remains were covered in silt, mud and sand being compressed under heat and eventually formed a sedimentary rock wherein the crude oil was created.
The newly created oil was then squeezed into reservoir rock due to temperature and pressure. If this rock was capped by an impermeable layer of rock known as cap-rock, then a reservoir was formed.
Oil reservoir geological surveyors carry out a seismic survey and once an oil reservoir has been discovered several other surveys are carried out to confirm the location. A drilling rig is anchored over the location and several small cores extracted from the new reservoir rock formation.
Once these confirm the presence and capacity of the oil reservoir, a well is sunk into the reservoir and capped to await the arrival of a production platform. This platform has all the facilities aboard to extract the oil, processing it and separating the natural gas from the crude oil, before both hydrocarbons are sent ashore via subsea pipelines.
At the oil refinery the oil is further processed into the petroleum products such as petroleum, diesel and we all use, the natural gas being processed and normally used to supply heat to the numerous oil processing systems. The surplus natural gas is then further processed to Liquid Petroleum Gas (LPG) and butane.
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Natural Gas from Oil Wells- Chemical Composition
As we have seen, natural gas is found in solution with the crude oil as associated gas, also being present in the reservoir rock pores as free gas.
The chemical composition of the natural gas from oil wells varies with the location of the oil reservoirs, for example gas from a North Sea Oil Platform and that from a Saudi Arabian oil reservoir will have different chemical compositions; therefore, for the purpose of this article we shall give the major components of natural gas from oil wells in general.
The main constituents of natural gas from oil wells are as follows:
1. Hydrocarbon Main Components
There are two main components:
- Methane CH4
Methane is a colorless, odorless, flammable gas and is the main component (75%) of natural gas.
- Ethane C2H6
Ethane is a liquefied, colorless odorless, flammable gas and is around 15% in natural gas.
2. Hydrocarbon Secondary Components
There are five secondary components:
- Propane C3H8
Propane can be liquid or gas and is odorless, colorless and flammable.
- Butane C4H10
Butane is highly flammable, colorless and odorless being easily liquefied.
- Hexane C6H14
Hexane is a colorless, very odorous gas and can be toxic, inhalation should therefore be avoided.
- Pentane C5H12
Pentane is the most volatile of the hydrocarbon with the same properties as butane.
3. Other Non-Hydrocarbon Components
There are a number of other components:
- Carbon Dioxide CO2
Carbon Dioxide is a chemical compound, odorless in small quantities, but highly toxic causing dizziness and asphyxiation, it is also non-flammable.
- Hydrogen Sulfide H2S
Hydrogen Sulfide is a poisonous, colorless, flammable gas with the obnoxious smell of rotten eggs, inhalation should be avoided.
- Helium He
Helium is a chemical element and is an odorless, tasteless, inert and non-toxic gas.
- Nitrogen N2
Nitrogen is a chemical element and is a colorless, odorless, tasteless inert gas.
When the natural gas from oil wells is processed; only the methane (CH4) and a few traces of the other components remain in the gas.
The remaining components that make up around 10% of the natural gas are further processed, being used for a variety of purposes in industry.