Natural gas is not “produced” per se. It is rather an extraction followed by the refining of a raw material available in nature. Thus, there are several essential steps before obtaining natural gas usable by individuals.
Find natural gas deposits
The search for natural gas and the estimation of the volumes contained are based on seismology: special sensors are mobilized to record data relating to the propagation of artificial shock waves, making it possible to obtain information on the geological structures present. Subsoil mapping is based on these data to assess the presence of natural gas.
Several types of basins appear during explorations, they are differentiated according to:
Their hydrocarbon consultation
- prolific basins that are rich in hydrocarbons such as the southern Gulf of Guinea for oil, the Nile delta for gas or the Niger Delta, some of which are rich in oil and gas,
- the sterile basins whose hydrocarbon concentration does not allow a profitable exploitation;
The state of progress of their exploitation
- there are pools that have been discovered for a long time and that have already been drilled several times; these are the mature basins whose exploitation continues but with very little chance of meeting new giant deposits,
- the poorly mature basins have not been the subject of many drilling and are the preferred prey of oil companies,
- Finally the virgin basins have been very little exploited, never explored, because of the geography that makes them less accessible.
Extraction of natural gas
Natural gas extraction infrastructure
When the natural gas deposit is deemed sufficiently certain in terms of its volumes and economic operating conditions, the facilities required for its operation may be constructed: extraction wells and pipelines connecting them to the collecting networks. The extraction operation consists of bringing the natural gas trapped in the subsoil to the surface by digging a hole, sometimes up to 6000 m deep. A drill derrick is installed; it is a metal tower up to 30 meters high that will insert vertically drill rods. The first rod is equipped with a drill bit that will be used for digging, in case the rock is very hard the teeth may be diamond so that they do not break. In many cases, natural gas will exit wells without outside intervention due to pressure, but sometimes pumping equipment is required.
The expansion of the head of the natural gas well allows the condensation of a portion of the recovered product: it is natural gas well liquids, from which are produced valuable substances, such as naphtha. The rest of the products, in the gaseous state, are transported by pipeline to the natural gas processing unit.
Offshore natural gas extraction infrastructures are more expensive than those deployed to exploit onshore gas fields. Offshore requires drilling platforms and / or special vessels.
Once the gas is extracted, the pressure of the circuit is reduced to that provided for the transport before injection of the gas into the transport pipeline.
Lifespan of a natural gas field
The life of a deposit is very variable. It usually varies from 15 to 30 years. Some gigantic deposits can be exploited for 50 years. On the other hand, deep-sea deposits, whose operating costs are much higher, make the life of the deposit much shorter (between 5 and 10 years). The exploitation of a deposit consists of 3 stages:
- A first phase of “startup” is during the first 3 years. Wells are drilled progressively which increases the production of natural gas;
- a period of plateau during which production is stable: its duration may vary according to the size of the basin;
- Finally, a period of decay that will precede the closure of the site: during this phase the production of hydrocarbon falls between 1 and 10% per year.
End of a deposit
The exploitation of a deposit ends when the cost of extraction becomes higher than the profit produced by the sale of the gas:
- because the residual water that rises with the hydrocarbon is more and more important and the separation of water (dehydration) is then much too expensive,
- The enhanced recovery processes are too costly to continue operating profitably.
Reserves are never fully depleted, so the gas field recovery rate reaches 60 to 80%, which is relatively higher than that of oil deposits that does not exceed 50%.
Natural gas treatment
Natural gas facilities may be located more or less upstream of the value chain, near natural gas extraction sites or areas of consumption. The treatment of natural gas is to dehydrate it by dew point and to separate its components:
- liquefied petroleum gas (LPG, not to be confused with liquefied natural gas LNG),
- carbon dioxide (CO2) released into the atmosphere, sequestered or transmitted to a nearby industrial user,
- acid gas sold to the chemical industry or sequestered,
- Helium, sold if it is present in sufficient quantities (which allow creating a significant source of income for the deposit).
The remainder of the gas, which accounts for the vast majority of the natural gas extracted and consists mainly of methane (CH4), is injected into the gas transmission networks to the places of consumption.
Natural gas condensates (recovered in the liquid state at the well exit) and liquefied petroleum gas is the two products with the highest market value. Some gas operations have only been set up to extract these products, and reinjection of the rest of the natural gas into the deposit instead of exploiting it commercially.