Coal seam gas is trapped in coal formations, typically 400-1000 metres underground, although it can be found at depths up to 1200 metres.
Unlike other productive gas reservoirs, coal seams have low porosity. Natural gas collects in coal seams by bonding to the surface of coal particles. The coal seams are generally filled with water and it is the pressure of the water that keeps the gas as a thin film on the surface of the coal (the technical term for this is ‘adsorption’). The open fractures in the coal (called the cleats) can also contain free gas or can be saturated with water.
When the water pressure is reduced, gas is released. It flows through cracks (or cleats) in the coal seam to a well bore.
To produce gas, wells about the diameter of a dinner plate are drilled into the targeted coal seams which sit far below shallow aquifers commonly sourced for agricultural use.
Queensland and New South Wales contain most of Australia’s coal seam gas.