How coal seam gas is produced
CSG is a naturally occurring gas, mostly methane, found in the coal measures deep underground. These coal measures are geological layers that consist predominantly of impermeable mudstone and siltstone that have thin seams of coal running through them.
These coal seams are discrete and not continuous over large distances; they generally form only 10 per cent of the total thickness of the coal measures. Small amounts of gas collect in the many gaps, called cleats, within the coal deposits.
The gas is bonded to the coal and held in place by water pressure. CSG is extracted by removing some of the water and reducing the pressure holding the gas to the coal, allowing the gas to flow into the gas well.
Both the gas and associated water are extracted via the CSG well. On the surface they are separated and pumped to relevant treatment facilities via separate pipelines.
Australia Pacific LNG pumps its associated water to a water treatment facility where it is treated via reverse osmosis to remove any dissolved salts and impurities. The water is treated to better than World Health Organization standards, and is then suitable for beneficial use.
CSG is pumped to a gas processing facility. At the facility the gas is further dehydrated and any remaining water removed. Once completely dry the gas is compressed and then transported via high pressure pipeline to domestic and commercial customers such as power plants.
In upstream gas and water production activities, there are two primary pumping and pressure applications – extracting water and gas from the wells, and then compressing the gas to sales gas pressure for transport via high pressure pipelines.
At the individual well heads water is extracted using progressive cavity pumping systems. Extraction of water reduces pressure in the coal seams being targeted, allowing the gas to flow to surface inside a steel well casing.
Progressive cavity pumps are lowered inside the well casing on steel tubing to a depth of around 800 to 1000 metres – depending on the location of the coal seams – and driven by a rotating steel shaft connected to the surface.
The progressive cavity pumping shaft is rotated by either a hydraulic power unit driven by a six-cylinder gas-fired engine, or electrically using a 30 kilowatt gas-fired “microturbine” and variable frequency drive combination.
As the pump rotor spins, water is forced up inside the steel tubing, and gas released from the coal seams flows to surface between the well casing and tubing as a result of the lowering of water levels and pressure.
The extracted gas and water are separated at the well head and sent via separate low pressure (around 350kPa) gathering pipeline systems to centralised water treatment and gas processing facilities.
At the water treatment facility the water is treated via reverse osmosis to very high standards.
At the gas processing facility the gas is compressed to high pressure using a multi-stage reciprocating gas compressor.
The compressor increases gas pressure from around 350kPa to over 10MPa. Each reciprocating compressor is powered by a 12-cylinder gas-fired engine outputting around 4,000 horsepower. The gas is also dehydrated before the gas is transported via high pressure gas pipeline.
In some fields, the gas is initially compressed using a centrifugal screw compressor to raise the pressure from 350kPa to around 1400kPa, which then feeds the inlet of a multi-stage reciprocating gas compressor.
At this pressure the gas can enter sales gas pipelines for distribution to customers.