NATURAL GAS FORMATION
Natural gas is formed deep beneath the Earth’s surface. Over millions of years ago, the remains of plants and animals built up in thick layers on the Earth’s surface and ocean floors, sometimes mixed with sand, silt, and calcium carbonate. Over time, these layers were buried under sand, silt, and rock. Pressure and heat changed some of this carbon and hydrogen-rich material into coal, some into oil (petroleum), and some into natural gas.
WHERE IS NATURAL GAS FOUND?
Over the years, natural gas moved into large cracks and spaces between layers of overlying rock. The natural gas found in these types of formations is sometimes called (i)conventional natural gas. Conventional natural gas refers to natural gas that can be extracted from the Earth either through naturally occurring pressure, or pumping mechanisms. Nigeria, Algeria and Mozambique are some of the African countries that have conventional natural gas.
In other places, natural gas occurs in the tiny pores (spaces) within some formations of shale, sandstone, and other types of sedimentary rock. This natural gas is referred to as shale gas or tight gas, and it is sometimes called (ii)unconventional natural gas. Unconventional natural gas is natural gas obtained from sources of production that are considered to be new and different. Unconventional natural gas requires advanced production methods. The United States is by far the leading producer of commercial shale gas.
Natural gas also occurs with deposits of crude oil, and this natural gas is called (iii)associated natural gas. This gas can exist separate from oil in the formation (free gas) or it can be dissolved in the crude oil.
Another type of natural gas that is found in coal deposits is called (iv) coalbed methane. Coalbed methane can also be found in coal seams. It is a primary clean energy source of natural gas. There are developments of coalbed methane in Botswana.
HOW NATURAL GAS IS SOURCED
Geologists are key to locating natural gas. They study the structure and processes of the Earth. They locate the types of geologic formations that are likely to contain natural gas deposits. Geologists often use seismic surveys on land and in the ocean to find the right places to drill natural gas wells.
If the results of seismic surveys indicate that a site has potential for producing natural gas, a trial well is drilled and tested. The results of the test provide information on the quality and quantity of natural gas available in the resource.
When the results from a test well show that a geologic formation has enough natural gas to produce and make a profit, more production wells are drilled.
NATURAL GAS PROCESSING FOR SALE/MARKETING & CONSUMPTION
Natural gas withdrawn from natural gas wells is called wet natural gas because along with methane, it usually contains alkanes (ethane, propane, butanes, and pentanes) and water vapour. Wellhead natural gas may also contain nonhydrocarbons such as sulfur and carbon dioxide, most of which must be removed from natural gas before it is sold to consumers.
From the wellhead, natural gas is sent to processing plants where water vapour and nonhydrocarbons are removed. The separated alkanes are called natural gas plant liquids and the processed natural gas is called dry, consumer natural gas, sales gas, marketable gas or pipeline quality natural gas. Dry natural gas is sent through pipelines to underground storage fields or to distribution companies and then to consumers.
Due to natural gas being colourless and odourless, chemicals called odorants/mercaptan are added as a safety measure to help detect leaks in natural gas pipelines. Mercaptan is as foul-smelling chemical that is harmless.
Sources:
Shale Gas And Its Implications For Africa And The African Development Bank