The golden age of gas
written by Andrea Bellati
The International Energy Agency (IEA) is an international organisation that represents one of the points of reference in the world energy industry. Every year IEA publishes on its website a great quantity of data and analyses divided by energy source and country. In 2011 the Agency experts worked on a study called “Some gold for gas?” that presents an alternative scenario that depicts how the contribution of natural gas in satisfying energy requirements might evolve in the future up to 2035. According to this document, in the coming years we might witness a substantial increase in the use of natural gas, as a response to the world's growing demand for clean energy and also because of the considerable supplies of this source worldwide. According to data published on World Oil and Gas Review (a magazine regarding oil and gas in the world) in 2010 world natural gas consumption was about 3,000 billion cubic metres, equivalent to about 21% of primary energy consumption. The world's natural gas reserves today are about 193,000 billion cubic metres. But these are just a part of the reserves of gas present in the subsoil. In fact, this number refers to “proven” reserves, i.e. those gas reserves that can be recovered at the current market prices and utilising existing technology. The majority of these reserves is represented by “conventional” gas. However, if we add “unconventional” gases such as shale gas, tight gas and coal bed methane, the quantity that can be supplied for future consumption increases sharply and reaches about 400,000 billion cubic metres.
This quantity of gas would be enough to satisfy the current consumption for over 250 years.
The blue hydrocarbon
Natural gas is the cleanest of hydrocarbons, the one with the lowest local and global impact. The combustion of natural gas produces about half the carbon dioxide and nitrogen oxides produced when burning oil and coal, and small quantities of carbon monoxide and dusts. Moreover, unlike coal and oil, the combustion of natural gas does not produce sulphur oxides, the main source of acid rain. Hence, until we are able to achieve a greater contribution of renewable sources of energy, natural gas could guarantee environmental sustainability and could satisfy the growing energy demand. IEA pinpoints other four factors that could make natural gas the protagonist of the energy future: an increased share of gas in China's energy mix, the decline of nuclear power after the accident in Fukushima, a growth in the number of gas-powered vehicles and a greater exploitation of unconventional gas.
Unconventional gas: shale gas
The technology required to extract natural gas from the subsoil was developed many years ago: the rock has to be drilled until the gas reservoir is reached, then the difference in pressure created allows the collection of gas that rises up the pipes naturally from the drilled deposit.
Unconventional gas, instead, refers to a gas that until quite recently was difficult to extract because it was technologically complicated or too expensive. Among the most promising unconventional sources there is “shale gas”, i.e. gas that is trapped in rocks formed from layers of clay at a depth of 2000 to 4000 metres.
The main difference between a conventional well and a shale lies in the kind of rock that contains the gas and in the techniques used to produce it. In the case of conventional gas, the reservoir is made up of porous and permeable rock into which gas has migrated from the source rocks rich in organic material where it was generated. Shale gas, on the contrary, is trapped in the same rock where it was formed, usually consolidated clay, that is porous and rich in gas, but not permeable. Hence, unlike conventional gas, once drilled, these rocks do not allow the gas to flow to the surface. However big a conventional gas deposit is, it is limited when compared to shale formations that can extend for hundreds of kilometres.
Another difference between the two kinds of deposits regards the amount of gas extracted in time. The production rates of shale gas decline rapidly after an initial peak but the reservoir lasts longer than traditional deposits whose production is more uniform but does not last long. Moreover, shale gas has a lower productivity respect to conventional gas. In fact, up to 70% of the gas contained in a convenional natural gas deposit can be recovered, while shale rocks have a maximum recovery factor of 30%. Due to the low productivity, a large number of shale gas wells have to be drilled: in the United States, for example, there are about 1000 active drilling rigs that can drill 8-10 thousand extraction wells a year. The cost of perforation and completion of a well depends mainly on its depth and the number of hydraulic fractures required to extract the gas: in the United States costs can range from 4 to 10 million dollars.
Since shale gas is trapped in rock, the latter must be “stimulated” to promote the migration of gas towards the well and up to the surface. Horizontal drilling and hydraulic fracturing are the main techniques utilised to increase the productivity of shale gas. Hydraulic fracturing involves injecting a fluid down a well bore at high pressure. This process brings about the formation of new microfractures in the rock and the connection of existing fractures, creating escape routes for the gas towards the well. To prevent the fractures from closing again, the fluid utilised, made up mainly of water, contains grains of sand or ceramic. Hydraulic fracturing takes place in different stages and can entail the use of up to 20-30,000 cubic metres of water per well.
Even horizontal drilling reaches the objective of increasing the production of shale gas by intersecting natural fractures in the rock. Horizontal drilling has been used for several years: the drill is gradually deviated, exploiting the elasticity of steel, in order to obtain a wide curve that positions the drill bit parallel to the surface of the ground.
Shale gas in the world
Even though unconventional gas has taken on great importance and popularity only in recent years, in the United States this gas has been produced for over thirty years. However, it is the rapid exploitation of this gas that has brought about the expansion of shale gas production in the USA. In the decade 2000-2010 its production grew from 10 to 140 billion cubic metres (in Italy annual consumption is slightly more than 80 billion cubic metres) and was sufficient to satisfy about 23% of the annual natural gas requirements of the United States.
In addition to the USA, important shale gas deposits can be found in Canada, Europe, Asia (especially China, one of the countries with a growing energy demand), even though production in these areas has not yet got off the ground.
The exploitation of shale gas has some drawbacks. The main worry regards the risk of contaminating the water-bearing strata during hydraulic fracturing. In actual fact, the deposits are much deeper than the water-bearing layers used by man and the potential risk of leakage in the more superficial parts of the wells is improbable because they are totally encased in cement. Along with the water contamination issue, recently there has been much debate regarding the alleged contribution of shale gas to the greenhouse effect. The worries arise from the fact that during the initial stages of production, a small part of the gas extracted is released into the atmosphere. Currently, specific technologies are being developed to curb these leakages which do not, however, exceed 1% of the total production of a well.
Not only shale: tight gas and coal bed methane
Tight gas is contained in reservoirs in many ways similar to those of conventional gas, but much more compact and hence with low permeability. The extraction techniques utilised are the same as those used for shale gas, i.e. hydraulic fracturing and horizontal drilling.
Coal Bed Methane (CBM) is a natural gas trapped in coal seams present in the subsoil. In these reservoirs, methane is chemically bonded to coal and in order to release it all the water must be pumped out of the coal seam.
For more information, the Oil and Gas Review can be consulted, an online publication full of articles in English. Another important source of data is the IEA website: in the pages dedicated to publications, the reports mentioned in this special can be found in English