Gas - Definitions and chemistry


Gas definitions

Gas refers mainly to methane. It is formed underground by the decomposition of organic material.

Conventional gas is methane formed in underground reservoirs that can be simply tapped and the gas extracted. Normally it is trapped in porous sandstone.

Dry gas is almost pure methane. 

Wet gas contains higher hydrocarbons such as ethane, propane, butane, and condensate (C5 +). Some are liquid, hence wet.

Natural gas is mainly methane, with some ethane, and some heavier hydrocarbons and contaminants such as CO2.

Liquid natural gas LNG is natural gas cooled below -160oC. At this temperature it liquefies and occupies 1/700 th the volume so can be transported in insulated ships to overseas markets. This consumes energy for refrigeration.

Liquefiable Petroleum gas LPG is propane or butane. It can be liquefied at room temperature by pressure alone. Methane and ethane cannot.

Coal Seam Gas - CSG or Coal Bed Methane - CBM is methane trapped in coal seams.

Shale gas is methane trapped in shale

Tight gas is methane trapped in impermeable rock formations that need to be fracked (hydraulically cracked) in order to release it. It needs many more wells closer together. Conventional gas needs a well every 2 KM or so. A tight gas may need a well every few hundred metres. Or up to 24 wells can be drilled on one site reaching horizontally for a few KM.  Tight gas formations are much older than conventional gas. Deposited some 248 million years ago, tight gas formations are typically found in Palaeozoic formations. Over time, the rock formations have been compacted and have undergone cementation and recrystallisation, which all reduce the level of permeability in the rock. More

Unconventional gas. There are six main types: deep gas, shale gas, coal seam gas / coal bed methane, geopressurized zones, Arctic and sub-sea methane hydrates, and tight gas.

Methane Hydrate, Methane Clathrate, Methane trapped in molecules of ice.

 Refinery gas – methane, ethane, propane and butane. 

Town gas or coal gas - Made by pyrolysis (heating without air) of coal. Mixture of CO, CH4, and H2. Coke remained. Process replaced by natural gas in 70s.



Gas vs vapour / vapor

Each stable substance has a critical temperature. Above this temperature it cannot exist as a liquid. No amount of pressure will liquefy it.

It is called a gas is above the critical temperature.

It is called a vapour below the critical temperature, so can be liquefied by pressure alone.