What is coal seam gas?
Coal seam gas is a form of natural gas of which Australia has relatively large supplies.
2 November 2011 | Updated 18 May 2012
Coal seam gas (CSG), also known as coal bed methane, is a form of natural gas, typically extracted from coal seams at depths of 300-1000 metres.
CSG is a mixture of a number of gases, but is mostly made up of methane (generally 95-97 per cent pure methane).
Underground, CSG is typically attached by adsorption to the coal matrix, and is held in the coal underground by the pressure of formation water in the coal cleats and fractures.
Different forms of natural gas
The different forms of natural gas are generally categorised into conventional and unconventional gas.
Conventional gas is obtained from reservoirs that largely consist of porous sandstone formations capped by impermeable rock, with the gas trapped by buoyancy.
The gas can move to the surface through the gas wells without the need to pump.
Unconventional gas is generally produced from complex geological systems that prevent or significantly limit the migration of gas and require innovative technological solutions for extraction.
The difference between conventional and unconventional gas is the geology of the reservoirs from which they are produced.
There are several types of unconventional gas such as CSG, shale gas and tight gas.
Coal Seam Gas
CSG is entirely adsorbed into the coal matrix. Movement of CSG to the surface through gas wells normally requires extraction of formation water from the coal cleats and fractures.
This reduces the pressure, allowing methane to be released from the coal matrix. Over time, water production decreases and gas production increases.
CSG production normally requires a higher density of wells than conventional gas production; however CSG wells are typically shallower than conventional wells and cost much less to drill.
Shale gas is generally extracted from a clay-rich sedimentary rock which has naturally low permeability.
The gas it contains is either adsorbed (i.e., closely to the surface matrix of the organic matter) or in a free state in the pores of the rock.
Tight gas is trapped in ultra-compact reservoirs characterised by very low porosity and permeability.
The rock pores that contain the gas are minuscule, and the interconnections between them are so limited that the gas can only migrate through it with great difficulty.
CSIRO has produced a series of factsheets relating to coal seam gas: