CSG adsorbs to coal particles. They effectively stick to the surface of the coal. At a molecular level, there is some sharing (due to molecular bonding) of the space occupied by the atoms forming coal (C,H and O) and the atoms forming CSG (also C,H and O). Like the stars and planets in outer space, the volume of an atom occupied by protons, neutrons and electrons is only a tiny, tiny , tiny fraction of the volume of an atom. I can't state categorically what the relationship is, but there is far less void volume created relative to the mass/volume of gas removed from a CSG aquifer. Maybe someone knows this relationship. I would suggest that it is virtually negligible.
Dewatering CSG "aquifers" reduces water pressure in the seam and allows the gas to desorb from the coal, i.e. overcome the forces of the molecular bonds holding it to the coal. Dewatering does not require the aquifer to be drained of all of its water - only enough to allow the gas to overcome adsorption forces and to flow at whatever rate is needed or can be achieved from the well.
Water as a liquid or solid (ice) is virtually con-compressible. Its volume changes very little when put under pressure (try sucking water into a car engine and you'll soon know what I men). All things being equal, a vessel filled only with water needs very little water to be removed from it to reduce its internal pressure.
No doubt, understanding the geology of the seams and siting wells in "correct" positions is key to minimising water volumes extracted from seams.
It's a different process but this shows this phenomenon nicely. Take an unopened plastic bottle of soft drink and mark the height of the liquid. Then take the lid off the bottle and let it stand until all of the CO2, which is dissolved in the soft drink come out of solution. The reduction in height will be virtually nil. An even cooler experiment is to weight the bottle before and after to understand the loss of mass, which if you were really geeky could then be converted back to a volume at room temp and one atmosphere pressure. Based upon many years of watching the bubbles in my beer, I'm sure the gas occupies two to three times the volume of the liquid it is dissolved in.
Perhaps the bigger question is "What replaces the water once a CSG aquifer" is dewatered? - Ultimately, it's more water. It just might take some time - that then gets into the science of the matrix potential of all of the layers of soil and rock and strata and fissures that the water must percolate through to recharge the aquifer
Coal Seam Gas (CSG) is natural gas found in coal deposits. The coal and gas are formed from plant matter under pressure over many millions of years. Coal seam gas is used in the same way as any other form of natural gas for cooking and heating as well as in industrial processes and electricity generation.
CSG collects in underground 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 coal seams generally contain more brackish (salty) groundwater than aquifers that are usually used for agriculture.
The level of gas that can be produced from a coal bed depends on the thickness of the coal, gas content, permeability and the depth of the coal seam. In high quality CSG deposits the cleats or fractures in the coal bed are permeable enough to allow gas and water to flow freely through them.
Coal seams that can produce CSG economically are usually 200 to 1,000 metres below the surface.
In Australia, CSG is plentiful.
Coal Seam Gas is natural gas.
CSG provides 90% of Queensland's gas needs and 15% of the state's electricity generation
CSG has been known about ever since the coal mining industry began in Australia in the early 1900's.
With advances in technology, it has developed into a key transition fuel, helping to lower our carbon emissions as we move to a low carbon future. CSG now makes up a significant proportion of Australia’s natural gas supply, with the Australia Pacific LNG Project providing the largest CSG resources in Australia.
(Shame some of this doesn't apply to NSW).
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