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Coal Seam Gas Overview

Coal seam gas ( CSG ) which is also know as coal seam methene , occurs naturally within coal deposits. In Queensland’s major coal resource ‘basins’ there are many coal seams existing from close to the surface to those many hundreds of meters underground. Coal seam gas was formed when the coal itself was formed and has been held within the coal under pressure. The gas is largely composed of the gas methane which is also the principal component of natural gas. CSG generally has very little carbon dioxide content, compared to many natural gas deposits, although this is not always the case.

Methane gas is held in coal seams through a number of mechanisms, though principally through weak molecular surface bonding to the coal, called adsorbtion, which is maximised by a fractured or permeable coal offering greater surface area. A lesser effect is through the gas being dissolved in the water in the seam. Both retention mechanisms are promoted by pressure and so, as the pressure is relieved the methane gas is released.

The adsorption process is highly efficient allowing for large quantities of gas to be held in a coal seam. The key to exploiting the CSG is in relieving the pressure. The coal seams are held under pressure by the static head of water, and so it is the removal of the water by pumping that is the key to gas release. The general approach is to drill a vertical well past the target coal seam and then placing a pump in the sump of the well. The coal characteristics determine the proximity of wells so that as the subterranean water table is lowered there is a mutual reinforcement. Once depressurised, gas will travel extensive distances through a coal seam to the vertical well. Where the coal is less permeable, a technique of in-seam wells has been developed, to provide a pathway through a seam to the vertical well.

Extraction and exploitation of CSG for commercial gas supplies has been a feature of the United States market for many years where it is a significant business. The US Department of Energy reports that CSG supplies over 7% of the US gas demand and holds over 7% of their natural gas reserves. That 7% represented a US CSG production in 2004 of about 1720 PJ, compared to Australia's total natural gas demand for 2004/5 of about 700PJ. CSG has the capacity to deliver huge quantities of gas to market.Like all resource commodities demand, price and supply will be tightly linked.

Exploitation of Australian resources lagged that in the USA, however by the mid 1990s a number of developers commenced the development of technology and methodology suitable 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.