The extraction

In terms of extraction and processing, during the production stage a sufficient number of wells is drilled to maximise the exploitation of the oil field. Every day, over a period of approximately 20-30 years, a well produces between 500 and 1,000 tonnes of oil (a few thousand barrels) and a few hundred thousand cubic metres of natural gas.
Initially, the oil flows up the pipes owing to the pressure of the water and the gas contained in the field. Thus, 30% of the oil and 90% of the natural gas are extracted. An additional 10-15% can be extracted by keeping up the pressure and adding more water or gas. Finally, an additional 10-15% can be extracted by injecting emulsions, steam or solvents washing the rocks and removing more oil.
Approximately 1/5 of the world production of oil comes from the sea, a percentage bound to increase in the next years. In this case, during the first stages many wells are drilled a few meters away from each other. Then, to drain all the fields, also horizontally, the wells are deviated to reach locations up to a few kilometres away from each other. If the sea bottom is more than 400-metre deep, undersea plants are necessary and the opening of the well is on the sea bottom.
During the petroleum production phases, hydrocarbons are extracted along with large quantities of liquid wastes. These effluents must be treated adequately to avoid contamination of the environment. The liquids produced during the drilling phase consist mainly of produced water and injection water. The former is extracted together with the hydrocarbons; in fact, in deposits, oil and natural gas are associated with a large amount of water, much saltier than ocean water. Moreover, as the reservoir reaches depletion, the amount of hydrocarbons extracted decreases and the volume of produced water increases, until, at the completion of the production stages, the volume of water extracted is greater than the volume of hydrocarbons. Produced water contains organic and inorganic compounds, often toxic, that must be treated before the water is disposed of.
Injection water is the water that returns to the surface after having been pumped into the reservoir to maintain adequate pressure. In the majority of cases, the water is injected back into the well; in the case of offshore oil activities, it can be discharged into the sea, but only if it does not contain pollutants, in other cases it can be reused for other purposes,  such as agriculture.
Disposal of sulphur-containing compounds in associated gas
Associated gas that is found associated with petroleum fields can contain large amounts of sulphur compounds (mainly H2S). In this case, the associated gas is treated in specific desulphurisation plants that can eliminate up to 99.9% of the H2S present. A waste product of desulphurisation plants is solid sulphur (S8); however, after having been protected by an adequate moisture barrier, it can be reused or stored at the production site for future use.
One of the main uses of solid sulphur is the production of fertilisers, but there are other applications: for example, it can be used to make sulphur concrete. The latter is more resistant than regular concrete and has a double advantage: it uses waste material that would be taken to the dump and it reduces the consumption of fresh raw materials.

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