Why is there Water in Coal Seams?
Coals in Australia were formed up to 350 million years ago. Vegetation such as grasses, trees, leaves and other organic material fell to the earth in swamps or in areas that later became inland seas. Where the vegetation collected in sufficient quantities, and under the right conditions, the process of coal formation, or “coalification”, occurred (Bailey, 2013).
It is not unusual for the coal seams to contain water, collected either as the coal was forming “in situ” or by water entering recharge points in the coal seam in a similar manner to recharge processes for other groundwater aquifers (CSIRO, 2014).
Water trapped in situ contains salts and minerals that were a part of the inland seas in which they were formed. Water that has entered the coal seam via aquifer recharge will collect salts and minerals as it travels through the surrounding geological formations. These salts and minerals are then captured in the water within coal seams in the same way that they are found in surrounding aquifers (CSIRO, 2014).
CSG water contains various dissolved salts, and is best described as “brackish” water. For comparative purposes, seawater contains on average 35,000mg/l ppm (milligrams per litre) of salt and CSG water usually contains less than 6,000mg/l of salt (Independent Expert Scientific Committee, 2014).
Typical CSG well design
The breakdown of the vegetation during coalification also resulted in the formation of methane gas.
CSG is a mixture of gases, but is mostly made up of methane (generally 95-97 per cent pure methane), nitrogen, carbon monoxide, carbon dioxide and inert gasses (Cook, 2013; CSIRO, 2013). Methane gas is compressed within the coal inside a complex arrangement of cracks and fractures – called “cleats”. Within the cleats, the gas is bound to the surface of the coal and held in place by the water pressure in the coal seam (CSIRO, 2013). The gas is only released from the coal when the water pressure in the coal seam is reduced to less than 35 metres of Head.
To reduce the water pressure and extract the gas, a gas well is drilled through the overlying geological formations to the target coal seam.
CSG production curve, Gas production vs water for a typical CSG well against time