Natural gas has the same origin as oil. It is like a mixture of hydrocarbons whose main component (between 70 and 95%) is methane (CH4), its density being 0.55 compared to air.

One of the main qualities of natural gas is its relative cleanliness. Burning it only releases water and carbon dioxide, in proportions 2 to 3 times less than coal and 1.5 to 2 times less than oil. The environmental impact of natural gas is much lower than that of coal or oil, with almost no contribution to the emission of oxides of sulfur and nitrogen.

As shown in Figure below, the gas chain consists of five main branches.

The first two branches, exploration and production, are similar to those of petroleum, as natural gas has been formed by the same process and is sometimes located in the same area as an oil field. In that case it is called associated gas, which can be either dissolved in the oil or separated.

Natural gas presents stronger exploitation constraints than oil. While the latter can be transported and stored in a relatively simple way thanks to its liquid form, with a low specific volume, natural gas, in gaseous form under standard conditions, needs to be packaged in a special way so as to be transported under safe conditions.

To transport natural gas thousands of miles from the production site to the place of consumption, requires the use of complex and expensive technologies.

Today, two main routes are used:

• transport by pipeline;

• transport by LNG (liquefied natural gas) tankers.

With pipelines, the gas is transported under pressure in steel tubular conduits with a diameter of 0.20 to 1.4 m, which are to be butt-welded, with recompression stations about every 80 km. This heavy and very expensive infrastructure has the advantage that large flows can be routed in gaseous form.

LNG transport in liquid form also represents a technical feat. To liquefy the gas at atmospheric pressure, it must be cooled to -160 °C, which reduces its specific volume by a factor of 600.

LNG tankers with a capacity of 75,000 to 160,000 m3 bring LNG to port.

The complete LNG chain includes:

• a pipeline, which connects the place of production to the port area;

• a liquefaction terminal and a storage unit of liquefied gas;

• a fleet of tankers;

• a regasification terminal;

• a network of pipelines to ensure gas distribution.

Note that the natural gas liquefaction consumes a significant amount of energy, and that losses occur at different stages of the LNG chain. The resulting consumption is about 13% .

In addition to these transportation facilities, storage in aquifers or salt cavern is necessary to adapt supply to demand, which varies greatly, especially between winter and summer.

Gas consumption is indeed likely to fluctuate significantly and abruptly, the main cause of these fluctuations being the change in the demand for heating needs. A pipeline network or a LNG chain cannot adapt to the gas demand load variation between summer and winter, which varies in a ratio 1 to 4.

As the user cannot store natural gas, the distributor has to do so. Two techniques are mainly used for the storage of large quantities, including inter-seasonal storage:

• deep aquifer storage, between 400 m and 1200 m, which uses a configuration consisting of a favorable geological anticline or bowl whose upper part is formed of impermeable ground (clay). The technique is simple, the gas pressure driving the water, but it leads to the formation of an unrecoverable gas bubble;

• salt cavern storage, between 900 and 1400 m deep, for large capacity storage, is achieved by creating an artificial cavity in solid salt, capable of withstanding the mechanical load, by dissolving the salt through the circulation of freshwater. The gas is then injected or pumped as needed ;

• a third possibility is storing LNG in tanks buried near the regasification terminals. This technique, which requires continuous cooling tanks to compensate for the heat loss, is limited to smaller quantities.

The distribution network consists of pipes of varied sizes (8 cm to 1 m), operating at medium pressure (50 mbar - 4 bar) and low pressure (20 mbar).