What is CSG?

What is CSG? Very simply Coal Seam Gas (CSG) is natural gas obtained directly from coal seams. Another common name for CSG is Coal Bed Methane (CBM). Like most natural gases, the chemical components of CSG are dominated by methane. Though some higher end hydrocarbons such as ethane or propane may also be present.  Carbon dioxide and nitrogen are usually significant components of CSG too and the higher the proportion of these non-hydrocarbon gases the lower the quality of the gas. This simple summary does not tell us very much, so more detail is required.

CSG is an interesting gas when compared to ‘conventional’ gases. ‘Conventional’ gas has migrated away from coal and organic rich sedimentary rocks into other porous rocks. The gas is then held in place by impermeable layers. What makes CSG different is that the gas has only migrated very small distances (if at all) to natural pore spaces such as fractures (cleats) in the coal layers. These pore spaces usually contain natural water that was left in the coal when it was laid down or water that subsequently migrated into the coal seam. The water associated with the coal seam is very important because it is actually the pressure of the water in the coal seam that keeps the gas in place. It is the hydrostatic pressure that keeps the gas in place.

In open cut or underground coal mining, CSG is a curse. It is considered a waste product and an explosion hazard. It is therefore vented as much as possible to make the coal mines safe to work in. The recent Pike River Mine explosion in New Zealand is an example where the failure to vent enough CSG caused a tragedy. As water is removed from coal mines the chances for gas mobilisation increases due to the above mentioned effect of hydrostatic pressure. This further increases the risk of explosion in coal mines.

Idealised relationship between CSG and water production

Natural gas became more popular for domestic and industrial use over the last few decades and the means to transport it economically became available (e.g. LNG). This meant gas that was often a by-product of the oil and coal industries became important in its own right. This led to people searching for gas sources in their own right, CSG included. Petroleum engineers realised if you reduce the pressure of water in a coal seam and collected the gas you could actually use the gas as a resource leaving the coal in place. This means that drill holes can be placed into coal seams and the water drawn out. The water drawn out (called formation water) is actually a ‘by-product’ of the gas extraction process. The formation water is the nuisance that needs to be removed to allow the gas to escape. This means that a new gas well will produce very little gas at first and lots of ‘waste’ water. As the water is drawn down the gas production increases and the water production decreases. Interestingly this is the opposite of that which occurs for ‘conventional’ gas, where waste water is a problem in the later stages of production but not early on.

Many people are concerned about CSG in Australia, particularly in our northern rivers region. This concern is driven by the possible effect of CSG extraction on beneficial groundwater. The use of techniques such as hydraulic fracturing that may be used to increase or prolong gas production is also raised as a concern. To keep this post short I will cover both of these issues in future. However, I will suffice to say that there is evidence that groundwater can be affected during CSG extraction despite producers trying not to have any impact. These are particularly noted in certain geological formations. There are also situations where there is no impact on important aquifers too. This matter is clearly quite complex and a one size fits all understanding does not apply very well. Hopefully, my future posts will tease the details out a little bit more.