Groundwater Systems

Improving our knowledge and understanding

Understanding the groundwater systems in the coal seam gas (CSG) producing areas of Queensland is important to ensure that existing groundwater resources are not depleted.

Aquifer connectivity describes the ease with which groundwater can flow within or between geological formations. It is controlled by the rock type, rock porosity and how well pores in the rock are connected.

Aquifer connectivity and the key factors that regulate it are important to better predictions of the groundwater impacts from future CSG development.

What determines the degree of connectivity?

Groundwater does not flow in ‘underground rivers’, it flows slowly between the connected pores and fractures or fault lines in geological formations.

The term “permeability” is used to describe the ease with which water can flow horizontally and vertically through a geological formation. These rock layers include aquifers (permeable layers that readily transmit water) and aquitards (confining layers that restrict groundwater flow).

For example, the time required for water to travel 100 metres vertically in geological formations ranges from days and decades in aquifers to millennia in aquitards.

The movement of groundwater in these layers is also determined by water pressure differences known as the hydraulic gradient. Water flows from areas of higher water level or water pressure to areas of lower water level or water pressure.

So the rate of water movement vertically between geological formations (sometimes referred to as leakage), is determined by the water pressure difference, permeability of the rock layers and the vertical separation or distance between these layers.

CSG extraction and aquifer connectivity

Methane gas is bound to the surface of the coal particles by the water pressure within the coal seam. The gas is released from the coal when the water pressure in the coal seam is reduced.

The water pressure is lowered by pumping groundwater from the coal seam. The released gas can then flow up the gas well. The lowering of the water pressure within the coal seam results in a water pressure difference between the coal seam and the aquifers above and below the coal seam.

The movement of groundwater between the formations in response to the pressure difference will depend on the connectivity between the coal seam and surrounding geological formations. The connectivity is dependent upon the vertical permeability of the rock layers and vertical separation or distance between them.

The vertical permeability of aquitards and coal measures are typically very low due to their layered sedimentary structure. The coal seams are often located at great depths (up to 800 metres below the surface) and are separated from aquifers by low permeability materials and aquitards.

This low vertical permeability and generally large vertical separation distances act together to limit aquifer connectivity between the coal measures targeted for CSG production and aquifers across most parts of the Surat and Bowen basins in Queensland.

Forecasts of groundwater impacts from CSG production have been made for aquifers in those areas.

The impacts could potentially be reduced by reinjecting treated CSG water into aquifers. Also, where the function of a bore is impaired by CSG activities, under Queensland law, CSG operators are obliged to ‘make good' the impairment so that landholders are not disadvantaged by CSG activities.

How is aquifer connectivity measured?

There are numerous methods to assess aquifer connectivity including: hydraulic assessment (pump tests and measurement of groundwater responses to pumping); laboratory tests (permeability analysis of rock cores); geophysics (electrical and other data from bore holes); and geochemical analyses (water chemistry analysis).

Current predictions about future changes to groundwater pressure are described in the Underground Water Impact Report (UWIR) for the Surat Cumulative Management Area released in 2016.