Natural gas is the last fossil source of energy to be used on a global scale. For over a century, when the gas was discovered in areas that were far from the places where it could be utilized, it was preferably burnt at the gas well or freed into the atmosphere, because harnessing it in a pipeline and making it travel for many kilometres was too expensive. The situation has changed over the past forty years and today natural gas ranks third in world consumption of energy and is the fossil source with the best growth prospects.
Often, albeit not always, natural gas is extracted from the same fields as oil. Just like oil, natural gas is the result of the transformation of organic substances deposited at the bottom of ancient seas and lakes (sedimentary basins). Therefore, there is no search for natural gas distinct from that of oil, but a single research activity for hydrocarbons: only after exploration wells are drilled is it possible to ascertain the nature of the deposit. The term “associated gas” is used when natural gas is dissolved into oil or makes up the top layer of the oil field; the expression “non-associated gas” is used when the field contains almost exclusively natural gas (for example, the large field of the North Sea or the Netherlands).
Extracting natural gas from underground is quite easy. Usually it is trapped together with oil under a rock layer. Due to the big pressure, as soon as drilling is finished, the gas comes out and it is necessary to “direct” it into a pipe and guide it towards its final destinations or storage centres. The latter are not tanks like the ones containing oil, but they are exhausted natural reserves that once contained natural gas, oil or water and that are used today as real “warehouses” for the gas.
If the natural gas from the field is wet, it undergoes a preliminary conditioning to separate methane from the other gaseous hydrocarbons such as propane, butane and ethane. The separation is simplified by the fact that methane is marked by a much lower critical temperature (above which gas cannot be liquefied).
The long distance transport of natural gas started in 1958 when natural gas was imported from Canada into the U.S. At present natural gas is transported in the gaseous state through gas pipelines, else by means of natural gas carriers in the liquid state (Liquefied Natural Gas).
Natural gas storage
The storage of natural gas plays an important role in regulating the supply in order to meet the great seasonal variation in demand. Consumption of natural gas is much greater in winter than in summer, while the supply of gas is relatively stable throughout the year.
From the large pipes of the national distribution network, thousands of kilometres of smaller pipes are derived to convey natural gas to industrial plants and households. In the city networks managed by the distribution firms the gas pressure is maintained at lower levels than the large transport networks for technical and safety reasons.
The distribution of gas in Italy
In Italy, imported natural gas is introduced into the national network at eight entry points, where the network connects to the import pipelines (Tarvisio, Gorizia, Passo Gries, Mazara del Vallo, Gela) and at the LNG regasification terminals (Panigaglia, Cavarzere, Livorno).
When a gas field is depleted, the decommissioning of the production facilities follows. The activities carried out during the decommissioning phase include the safe removal of the pre-treatment plant, the platform structures, the compression structures and the hydrocarbon dispatch facilities and the removal of the wellheads and the pipelines that connect to the collection points. Following the dismantling of the production facilities, there is the environmental restoration phase. The areas where the wells and the treatment facilities were located are reclaimed and restored to pre-mining conditions, with the planting of grasses and trees. As far as the decommissioning of offshore facilities is concerned, operations to safely plug and abandon the well must be carried out and the installations and pipelines that connected the platform to treatment facilities on land must be removed. These operations are very delicate and require specialised personnel in order to avoid adverse environmental impacts. Once the installations have been removed, suitable sites must be identified for materials that cannot be reused and for the disposal of potentially polluting products. An alternative to the dismantling and removal of offshore installations envisages the reuse of disused platforms in-situ as artificial barriers, for example. In fact, it has been observed that many artificial structures placed in open water are soon colonised by benthic macrofauna and by a large number of fish species that find a suitable habitat to reproduce. Another alternative is the installation of offshore wind turbines on the disused platforms. In fact, these offshore platforms can support wind turbines with the advantage that they are far from the coast, where the winds are strong and constant, and where there they do not have a negative effect on the landscape. The option of leaving disused offshore platforms in place must be carefully evaluated from an environmental and a legislative point of view.
Impacts on air
The extreme flexibility of natural gas makes it one of the fuels easier to use, whereas the low content of pollutants makes it an environmentally friendly fuel. During combustion natural gas produces carbon dioxide and nitrogen oxides (NOX), albeit to a lesser degree than other fuels.
Gas flaring and gas venting
In an oil field, oil is almost always associated with a certain quantity of natural gas: newer oil wells are equipped for the recovery of both oil well gas and crude oil and hence the gas is an additional resource of the oilfield.