Natural Gas

What is natural gas?

A naturally occurring mixture of hydrocarbon gases that is highly compressible and expansible called natural gas. Natural gas is primarily methane (CH4) with smaller quantities of other hydrocarbons, such as ethane (C2H6), propane (C3H8), and butane (C4H10).

Hydrocarbon Composition
Methane 70 to 98%
Ethane 1 to 10%
Propane Trace to 5%
Butane Trace to 2%

 

How did natural gas form?

Natural gas was formed million years ago when dead organisms sunk to the bottom of the ocean and were buried under deposits of sedimentary rock. Because of the intense heat and pressure, these organisms underwent a transformation in which they were converted to gas over millions of years. Natural gas is found in the underground rocks called reservoirs. The rocks have tiny spaces called pores that allow them to hold water, oil, and natural gas. The natural gas is trapped underground by impermeable called a cap rock and stays there until it is extracted.

 

Where is natural gas found?

Natural gas that comes from oil wells is typically termed ‘associated gas’. This gas can exist separate from oil in the formation (free gas) or dissolved in the crude oil (dissolved gas). Associated gas occurs in contact with crude oil in the subsurface. It occurs both as gas in the free gas cap above the oil and gas dissolved in the crude oil. It contains other hydrocarbon gases besides methane. In the past, associated gas was commonly flared or burned as a waste product. But in most places today it is captured and used.

 

  • Non-Associated Gas
    Non associated natural gas is found in reservoirs in which there is no, or at best minimum amounts of, crude oil. Non associated gas is usually richer in methane but is markedly leaner in terms of higher paraffinic hydrocarbons and condensate material. Unlike associated gas, non-associated gas could be kept underground as long as required. This is therefore discretionary gas to be tapped according to economic and technological conditions.
  • Associated Gas
    Natural gas found in crude oil reservoirs and produced during the pumping of crude oil is called associated gas. It exists as a free gas (gas cap) in contact with crude petroleum and also as a ”dissolved natural gas” in the crude oil. Associated gas is usually is leaner in methane than non-associated gas but it will be richer in the higher molecular weight hydrocarbons. Non associated gas can be produced at higher pressures whereas associated gas (free or dissolved gas) must be separated from petroleum at lower separator pressures, which usually involves increased expenditure for compression.

Natural Gas Products

  • LNG
    Natural gas that contains mostly methane (CH4) and Ethane (C2H6) called Liquified Natural Gas (LNG). LNG is colourless, odorless and non-toxic which earns it eco-friendly credentials. It is also non-corrosive, making it the fuel of choice for operating sensitive and expensive equipment and machinery.
  • LPG
    Natural gas that contains compounds such as propane and butane called Liquified Petroleum Gas (LPG). LPG is highly flammable and composed of a mix of hydrocarbon gases.

 

Physical Properties of Natural Gas

Natural gas is a mixture of light hydrocarbons including methane, ethane, propane, butanes, and pentanes (the typical composition is given elsewhere). Other compounds found in natural gas include CO2, helium, hydrogen sulfide, and nitrogen. The composition of natural gas is never constant; however, the primary component of natural gas is methane (typically at least 90%). Methane is highly flammable; it burns easily and almost completely. It emits little air pollution. Natural gas is neither corrosive nor toxic, its ignition temperature is high, and it has a narrow flammability range, which makes it an inherently safe fossil fuel compared with other fuel sources. In addition, because of its specific gravity (0.60), which is lower than that of air (1.00), natural gas rises if escaping, and thus dissipates from the site of any leak.

 

Condensate

The natural gas condensate is also called condensate, or gas condensate, or sometimes natural gasoline because it contains hydrocarbons within the gasoline boiling range, and is also referred to by the shortened name condy by many workers on gas installations. Raw natural gas may come from any one of three types of gas wells.

  • Crude oil wells: Raw natural gas that comes from crude oil wells is called associated gas. This gas can exist separate from the crude oil in the underground formation or be dissolved in the crude oil. Condensate produced from oil wells is often referred to as lease condensate.
  • Dry gas wells: These wells typically produce only raw natural gas that contains no hydrocarbon liquids. Such gas is called non-associated Condensate from dry gas is extracted at gas processingplants and is often called plant condensate.
  • Condensate wells: These wells produce raw natural gas along with the natural gas liquid. Such gas is also called associatedgas and often referred to as wet gas.

 

Measurements

Natural Gas is usually measured by volume and is stated in cubic feet. A cubic foot of gas is the amount of gas needed to fill a volume of one cubic foot under set conditions of pressure and temperature. To measure larger amounts of natural gas, a “therm” is used to denote 100 cubic feet, and “mcf” is used to denote 1,000 cubic feet. Condensate content is measured in barrels per million cubic feet (BCPMM) of gas. In the metric system, the volume of gas is measured in cubic meters. The unit used to measure heat content of fuel such as gas in the English system is the British Thermal Unit (BTU). The energy content of natural gas differs in various locations throughout the country.  For the sake of comparison, one average cubic foot of natural gas about 1,000 BTU’s of heat energy. The table below shows how much heat energy is released in various quantities of natural gas.

Unit of Measure Approx. Heat Energy
1 cubic foot 1,000 BTU’s
100 cubic feet (1 therm) 100,000 BTU’s
1,000 cubic feet (1mcf) 1,000,000 BTU’s

 

 

References

Devold, Havard. 2009. Oil and Gas Production Handbook: An introduction to oil and gas production edition 2.0. Oslo.

Energy Informaton Administration, United States. 2019. Natural gas explained. Retrieved from https://www.eia.gov/energyexplained/natural-gas/

Hyne, Norman. 2012. Nontechnical Guide Petroleum Geology, Exploration, Drilling & production. Oklahoma

 Natural Gas Condensate. Wikipedia. Wikimedia Foundation. 3 October 2020. Accessed on 12/10/2020. From: https://en.wikipedia.org/wiki/Natural-gas_condensate

Viswanathan,Balasubramanian. 2016. Energy Sources Fundamentals of Chemical Conversion Processes and Applications. Amsterdam: Elsevier.