We produce gas from the Walloon Coal Measures of the Surat Basin.
In our tenements the Walloon Coal Measures are generally 300 to 400 metres thick but only 10% of the formation actually contains coal. Most of the coal seams are about only 30 to 50 centimetres thick and only a few hundred metres long.
There are a few local exceptions, but these coal seams are generally surrounded by rock such as mudstone and siltstone that is almost impervious and helps to contain the gas.
There are a number of aquifers and aquitards above and below the Walloon Coal Measures. The aquifers are typically sandstone formations and have a high porosity and permeability, allowing water movement within the formation.
The aquitards are typically mudstone and siltstone and have a low porosity and permeability. Water movement within these formations is usually very limited.
QGC is confident that our impact on the aquifers above and below the Walloon Coal Measures will be minimal and we base this on geological research as well as operational experience over the past 13 years.
For example, QGC has built sophisticated models of the Surat Basin from seismic surveys, core analysis and production-well data. As we continue to drill wells, we are continually adding to and refining our understanding of the geology.
We are also working with other natural gas companies to share our knowledge of regional water resources and a network of groundwater monitoring wells is being established.
In all QGC wells, cement is pumped into the well to form a barrier between the coal seams we are fracturing and aquifers above or below the seam. This allows us to isolate the flow of water and gas from the target coal seam.
If a fracture were to grow upwards from the coal seam and towards an aquifer, cement could be injected until any fractures are effectively sealed. This may involve filling the well with cement where it passed through the coal seam and plugging the well for abandonment.