Water used in gas operations is generally obtained from the area of those operations and is typically brackish (i.e. not potable).
As part of the approvals process, a company must demonstrate that using local water will not have unacceptable impacts on aquifers.
Oil and gas wells have several layers of steel casing and cement that form a continuous protective barrier between the well and the rock. Millions of oil and gas wells have been drilled through aquifers without causing problems.
Producing CSG water
Coal seam gas is natural gas trapped in a coal seam by water pressure.
Decreasing the pressure on the coals by pumping water frees the gas for production.
Treating and using CSG water
Water produced from the coal seams is mildly salty (brackish). It comes from deeper geological layers, and is generally not usable for agricultural purposes without desalination treatment or blending with fresher (less saline) water.
About 97% of the water produced is desalinised and beneficially used: 59% by agriculture, 14% by industry and 24% by reinjection in aquifers.
When treated and beneficially used, CSG production water can be an alternative supply to the water that is currently taken from the shallower, less saline aquifers of the GAB. This in itself will assist in recharging these shallow aquifers over time.
Modelling and monitoring water supply in Queensland
Water produced from CSG is a small percentage (0.03% or 3 parts in 10,000) of Australia’s Great Artesian Basin.
Regional aquifer depressurisation models predict the impacts on water supply aquifers, and continual data capture and monitoring is used to validate and improve these predictions.
In 2012, the Queensland Government commissioned the preparation of the Surat Underground Water Impact Report.
The report collated information on regional aquifers, existing water bores and petroleum wells, as well the number and location of further wells to be drilled as a part of the CSG industry’s development. It used this information to forecast the expected level of impacts. The report also identified “immediately affected areas” and “long-term affected areas”.
The cumulative model covers an area the size of Germany and is referred to as the Surat Cumulative Management Area.
Gas companies have installed monitoring wells to detect any changes in aquifer pressure (using vibrating wireline piezometers) or changes in the chemistry in the aquifers underlying their permit areas. This information is delivered to the Queensland Office of Groundwater Impact Assessment (OGIA ) on a six-monthly basis.
Of the 21,000 existing private water bores in the Surat Cumulative Management Area, it was found that only 85 (0.4%) would be immediately effected, and another 528 bores (2.51%) would be affected in the long-term. The main reason many of these wells are expected to be affected is because they are producing from the same coals that are to be used for gas production.
Tenement holders are required to “make good” on any bore level decline by providing landholders with alternative water supplies. This may include drilling new, deeper bores, or supplying treated water to the affected properties.
Shale gas operations and water use
Water use for hydraulic fracturing can vary depending on the geology and the Australian Council of Learned Academies estimates that between 4 and 22 megalitres per well is required for a shale gas well, depending on the number of fractures.
Up to 80% of the fluid used in shale gas fraccing can be recovered. Most of this can be recycled and reused for additional hydraulic fracturing or other beneficial uses such as irrigation of nearby crops.
Water that cannot be recycled is placed in specially designed ponds for evaporation. The residue from this process is tested and, if required, removed to a licensed disposal facility. At no point does this water contact or contaminate groundwater sources.
Recent proposals for hydraulic fracturing have estimated a water requirement of 1.1 megalitre per fracture treatment, with between 5 and 13 fracture treatments required, depending on the location and geology.