Deposits and extraction of Natural Gas

Deposits of natural gas

Estimated to total 511 thousand trillion cubic meters, the total reserves of natural gas are expected to last up to 200 years.

Natural gas reserves are divided into proved, probable, and potential.

Proved reserves of natural gas, which are economically extractable with currently available technologies, amount to 164 thousand billion cubic meters and are expected to last until 2060 at the present extraction rate.

The worldwide reserves or natural gas have key importance for the long-term prospects of gas use. At the end of 1960s, there was a prevalent opinion that natural gas is only a temporary energy source whose reserves would be rapidly exhausted. Today, it is certain that natural gas is and will be the fuel for the 21st century. This ambitious statement is supported by the current state of natural gas reserves and the history of their development. Deposits of fossil fuels and minerals are usually classified into several groups based on certain criteria. The classification methodology often differs based on the processing company or the purpose for which materials are used. For example, geologists use classification that is far more detailed than mining companies or institutions that compile statistics.

At the beginning of 1970s, when the first concepts were drafted for transporting natural gas from the former Soviet Union to Czechoslovakia and Western Europe, proved reserves of natural gas worldwide amounted to no more than 39,443 billion cubic meters; 12,806 billion cubic meters of which was located in USSR. Rapid advances in geological surveying on land and continental shelves resulted in the discovery and acquisition of vast deposits of natural gas, which reached some 164,600 billion cubic meters in 2000.

It is interesting to note that 71.7% of these deposits are located on land and 28.3% in oceans on continental shelves. In addition to size, the lifetime of natural gas reserves is an important factor for the long-term prospects of natural gas. The so-called static lifetime is the ratio of presently known reserves and current extraction expressed in years.

Probable reserves are reserves discovered in deposits that show high probability of being extractable under economic and technical conditions similar to those that exist with regard to proved reserves. No technical facilities have been built at deposits of this type. In addition to proved deposits, the likeliness that probable reserves will be used is very high. The shift of a certain volume of probable reserves to the first category as a result of the continuing exploitation of deposits is the reason why the size and lifetime of proved reserves of natural gas continue to grow.

Probable reserves amount to 347,000 billion cubic meters. Very interesting and favorable for Europe and the Czech Republic is the geographic distribution of both categories of reserves, as shown in the following diagram. According to international gas unions, the worldwide reserves of natural gas will last 136 to 156 years (up to 200 years according to some estimates), taking into account proved and probable reserves of natural gas in the world as of year 2000.

Potential reserves include so-called unconventional sources, especially methane hydrates, a solid substance similar to snow that consists of methane (20%) and water (80%). Methane hydrates are located in the Earth's crust under the ocean bottom. These very important deposits have been known for a long time; however, extraction remains problematic. One possibility of extracting methane hydrates, which is currently being researched, is thermal breakup and depressurization. Modern geological surveying methods allow estimating the reserves of methane hydrates in a more and more precise manner.

At present, the reserves of natural gas in hydrates amount to some 21,000,000 billion cubic meters. Another source of this kind, the so-called coal bed methane (CBM), is methane assumed to originate from anthracite seams. The gas is absorbed in coal seams, bound to the microporous structure of coal. The efficiency of extracting gas from this source depends on the degree of carbonization and permeability of coal.

In most coalfields around the world, extraction of CBM is at the stage of research and pilot projects. Surveying is also underway in North Moravia where coal has been discovered to hold some 12.5 cubic meters of CBM per ton of coal, and reserves whose extraction would be economically viable amount to 70-370 billion cubic meters. One of the most important factors that has played a major role in the discovery and acquisition of new reserves is the introduction of new technologies. Three-dimensional seismic studies have identified new localities for directional boreholes with a great accuracy and reliability.

The ability to reach new deposits through low-diameter, directional, and horizontal boreholes has changed the economics of gas extraction considerably. In addition, new types of sea-based oilrigs have had a major impact on the acquisition of deposits on continental shelves.

Extraction of natural gas

Deposits of oil-based natural gas are usually located in porous rocks surrounded by impermeable materials and water, where natural gas – as a substance with lower specific weight – has accumulated over thousands of years above layers of crude oil or water. Natural gas is extracted through wells drilled directly into the porous layers of deposits, which are mostly located at depths up to three kilometers under the surface. However, gas can be extracted from much greater depths, up to eight kilometers. Natural gas comes from land deposits (Russia, Algeria, Holland) and deposits located under the sea bottom (North Sea).

Carbon-based natural gas was formed in the long process during which Paleozoic plants were transformed into anthracite. Located near anthracite deposits, gas is released during coal mining and must be suctioned off during a process called degasation. Natural gas can also be extracted directly from coal seams through wells. In some localities, carbon-based gas can be extracted on a long-term basis. The extraction stage must be preceded by a survey of the parameters of the coal deposit. Higher extraction efficiency can be achieved through an artificial increase in the permeability of gas seams.

Before long-distance transport, extracted natural gas needs to be processed to ensure that it has a sufficient quality for commercial use with no need for further processing. The refining technology depends on composition. Since natural gas is often extracted from deposits together with crude oil, it contains a considerable amount of higher hydrocarbons.

In addition, natural gas usually contains substances that can have a detrimental effect on distribution systems, such as water and sulfurous substances that can cause corrosion. High moisture can result in clogged pipelines because water and methane form solid hydrates under certain pressures and temperatures. In addition, extracted natural gas contains dust that can cause defects in compressor or regulation stations. After extraction, natural gas undergoes a process during which it is dried and solid particles (dust) are removed. If necessary, the process includes the removal of higher hydrocarbons and any sulfurous substances that may be present.