Coal Seam Gas (CSG) which is also known as Coal Seam Methane (CSM), occurs naturally within coal deposits. The CSG formed as the coal matured and has been held within the coal under pressure.
The gas is largely composed of methane which is also the principal component of natural gas. Methane is held in coal seams through a number of mechanisms, principally through weak molecular surface bonding to the coal called adsorption.
The adsorption process is highly efficient, enabling large quantities of gas to be held in a coal seam. Sometimes the gas becomes dissolved in the water in the seam. Both these retention mechanisms are promoted by pressure.
The coal seams are held under pressure by a static head of water so removal of the water by pumping is the key to releasing the gas. The general approach is to drill a vertical well past the target coal seam and place a pump in the sump of the well.
The coal characteristics determine the proximity of wells to ensure there is a mutual reinforcement as the water table is lowered. Once depressurised, the gas will travel extensive distances through a coal seam to the vertical well. Where the coal is less permeable, lateral in-seam wells are drilled to provide a pathway to the vertical well.
Extraction of CSG for commercial supply developed into a significant business in the United States many years before it started in Australia. The US Department of Energy reports that CSG supplies over 7 per cent of US gas demand and accounts for more than 7 per cent of the country's natural gas reserves.
Development of Australia's CSG resources lagged progress in the USA. However, by the mid 1990's a number of companies had started developing technology and a methodology suited to Australian coal deposits.
Australia's extensive coal basin deposits contain large resources of CSG, particularly in Queensland and New South Wales. Estimates of gas-in-place in these deposits suggest that they will be proven to be larger than the combined conventional gas resources of Bass Strait, the Cooper Basin and the North West Shelf.