Confined aquifers are layers of the ground that are bounded by less permeable ones. These include connate waters which behave much as a bounded black oil reservoir and aquifers that are recharged from distant sources such as the Australian Great Artesian Basin. The pressure in some of these may drive a fluid column above the ground level in which case they are described as artesian.
The key elements in determining whether a confined aquifer will deliver a water supply or not are:
- the permeability of the aquifer,
- its thickness,
- its storage behaviour,
- its area, and
- the available drawdown.
The latter may be limited by the aquifer depth or the ability of the aquifer to withstand large pressure changes without suffering damage due to increasing effective stress. This damage may include permanent compaction which leads to:
- a reduction in permeability, and
- surface subsidence.
The scale of the aquifer is also important. Ultimately, the sustainable yield of a confined aquifer is determined by its recharge.
The testing of confined aquifers benefits hugely from the technologies developed by the petroleum industry. These offer cost efficient ways in which to obtain the maximum information from a test in a single hole. What they do not do though is give any information on recharge. This must come from long term monitoring of piezometric head and well yield.
The use of purpose drilled holes fitted with piezometer installations is of extreme benefit in any important aquifer. Combined with well testing these can provide information on the storage characteristics of the aquifer and provide long term measurements do determine whether adequate recharge is taking place or whether the aquifer is being unsustainably mined of its water.
In many cases, aquifers have been developed over time and carry many production bores. There is often in these cases a need to establish if the production is sustainable and whether it can supply more water. The prior development and use of the aquifer makes the measurement of permeability, storage and pressure difficult. The approach derived from the petroleum industry of shutting wells down and measuring the pressure build up can be valuable in determining the ultimate aquifer head. So can the technique of pulsing a producing bore and monitoring pressure changes brought about by this in adjacent bores. This approach can yield information on the permeability and transmissivity of the aquifer.