Bore Design and Common Types
Principle: Bore design shall:
— suit the hydrogeological conditions
— be appropriate to protect the aquifer
— be suitable for the intended purpose of the bore
— meet the client’s requirements.
The siting, design, materials, and construction method used in a bore all influence the quantity and quality of water obtained and the protection of the groundwater resource. The chosen bore design is the result of a number of considerations and decisions. These include the:
• intended purpose of the bore
• geological and hydrogeological conditions, including the groundwater quality
• drilling methods and construction methods.
Bore design for aquifer protection
The bore design should take into account the protection of the groundwater resource. Bores drilled to intersect an aquifer will disturb that aquifer by providing a vertical connection between aquifers of different head or groundwater qualities.
Where drilling intersects groundwater held under pressure, uncontrolled flowing (artesian) bores can result, causing wastage of the groundwater resource and the loss of hydrostatic pressure. The design of the typical types of water bores are discussed below, according to the protection of aquifer conditions.
All non-flowing bores can potentially provide a means of contaminating groundwater by acting as a conduit for surface run-off. Importantly, the bore design should aim to ensure the protection of the groundwater resource from surface contamination. This means that the headworks and casing are sealed so that there is no potential for flow outside the casing.
Where multiple aquifers are encountered the key element of the bore design for aquifer protection is to ensure that waters of different aquifers do not mix, either in the bore casing or in the annulus between the casing and the borehole. Sometimes two or more aquifers may be penetrated before the selected aquifer. In these instances it is often easier to ensure there is no possible mixing of waters by grouting the annulus from the production aquifer to the surface.
Often the upper aquifer contains poor-quality water, or it may be fully committed to other users. In these cases bores are drilled through the upper aquifers to allow tapping of the better quality or under allocated lower aquifers. Any unsuitable waters are excluded from the bore during casing by slotting or screening only the selected aquifer. To protect any steel casing from possible corrosive waters, annular seals are then set above the slots, or grouting of the casing to the surface is carried out, if necessary.
SINGLE AQUIFER BORE
MULTIPLE AQUIFER BORE
NOTE: Multiple aquifer bores can be completed either as flowing or nonflowing
Flowing aquifer (artesian) bores
The drilling priority for artesian bore construction is the control of artesian pressure and flow. The requirements for an artesian bore include:
• protecting the production casing from corrosive soils
• preventing any discharge up the outside of the casing by the setting and cementing of surface control casing
• preventing any intermixing of waters of different quality or pressure from one aquifer to another
• tapping only one primary aquifer
• controlling formation pressures by selective cementing of the production casing.
One example of a flowing bore construction appears below.
During the selection process for production casing and headworks materials, consideration must be given to the depth of installation, grouting pressures, well head static pressure, and water temperature, together with the corrosive nature of the water and strata. Bores must also be fitted with headworks of approved design to permit the control of flow, and for periodic maintenance and measurement.
These approved headworks must make provision for flow and pressure to be measured without having to disconnect or interfere with reticulation or surface pumping systems. The construction requirements for artesian bores tapping the Great Artesian Basin (GAB) vary from state to state, and can be different from the minimum requirements described in this article. Local water licensing authorities should be consulted concerning artesian bore construction requirements before drilling in an artesian aquifer of the GAB.
In artesian aquifers outside the GAB, alternative construction requirements may be approved by the relevant state or territory water licensing authority to meet local requirements.
Common bore types
The chosen bore design is also the result of considerations and decisions relating to the intended purpose of the bore. Common bore types include:
Stock and domestic (low-yielding) water supply bores
An example of a stock and domestic (low-yielding, non-flowing) bore in a single (consolidated) aquifer formation is shown below:
Stock and domestic (low-yielding, non-flowing) bore (incl. electric submersible pump)
Low-yielding bores are the most common type of bore. The drilling priority is usually to obtain a usable supply of water for livestock and/or domestic use. Such bores normally use 100–150 mm diameter casing.
The basic requirements for these bores are:
• the construction technique and water entry selected to allow for the long-term production of clear silt-free water
• adequate bore straightness to allow for the installation and reliable operation of the client’s preferred pump
• a usable supply of water of acceptable quality with immunity from contamination, particularly from the surface.
Commercial (higher yielding) water supply bores
Commercial bores typically include industrial, irrigation, and major water supply bores. The major objective when drilling a commercial bore is to ensure that the formation remains stable and capable of being pumped at the maximum efficient water yield. To achieve this result a test hole drilling program is usually carried out to locate the optimum production bore site.
Other important factors that must be considered include:
• selecting a casing size based on the desired or potential yield and the required pump size
• selecting a screen length appropriate to the aquifer thickness being screened
• choosing the screen slots size and gravel pack size based on analysis of the gradation of the aquifer materials
• selecting a screen diameter and length that will transmit the bore yield at low entrance velocities
• selecting a large hole diameter
• selecting gravel pack material that is well rounded and clean.
It is important in constructing a commercial bore that the long-term stability and efficiency of the operation are not compromised by imprudent cost savings.
An example of a commercial bore construction is shown:
COMMERCIAL (HIGHER YIELDING, NON-FLOWING, SCREENED, AND GRAVEL PACKED) BORE
Groundwater monitoring bores
‘Monitoring bore’ has been adopted as the standard term because it is most commonly used in hydrogeological investigations throughout Australia. Other terms often substituted are ‘observation well’ and ‘piezometer’.
Monitoring bores include bores to:
• observe water levels
• observe water quality
• intersect and monitor contaminants such as hydrocarbons, coliforms, pesticides, herbicides, and heavy metals.
Monitoring bores are drilled specifically to obtain data on groundwater. They are equipped and used for taking water samples and/or monitoring water levels. Their basic characteristic is that they are normally of low-yield construction but provide for accurate water quality sampling and water level measurements from a particular zone of interest in an aquifer.
Annulus seals and gravel packs are used where necessary to isolate the zone being monitored. Care must be taken during drilling operations and in selecting the drilling method and materials used in bore construction.
It is essential to ensure that no contaminants are introduced that may affect the monitoring or sampling results. An example of monitoring bore construction is shown below:
WATER MONITORING BORE (NON-FLOWING)
Similar requirements for sealing between aquifers are required outside the casing to prevent inter-aquifer flow.
Experience has shown that it is sometimes quicker and more secure to drill multiple holes if room permits, as any drilling time saved with a single hole can be taken up with the setting of multiple casing strings and annular cement grout seals.
Multi-port monitoring bores
Multi-port monitoring bores are specifically designed bores to obtain data from one or more aquifers or zones in a single drill hole.
Multi-port monitoring allows testing of hydraulic conductivity, monitoring of fluid pressure, and collection of fluid samples from multiple zones within a single borehole.
A single drill hole is cased and grouted over the total depth. The bore is then perforated at the testing zones through the casing and the grout sealing. Testing ports are installed with inflatable packers insulating the perforated zone so no intermixing or contamination can occur between aquifers or zones.
Groundwater injection bores
Groundwater injection bores are used to inject water (by gravity or pressure) into an aquifer. They are commonly used in managed aquifer recharge schemes or groundwater remediation. The guidance here specifically covers injection bores for managed aquifer recharge (both storage and recovery), and aquifer storage transfer and recovery, with stormwater, recycled water, and groundwater. It does not address injection or infiltration of waters into aquifers for waste disposal.
The key issues in designing injection bores are:
• ensuring the injected water reaches only the target aquifer
• protecting the aquifer and aquitards from being damaged from overpressurisation
• minimising problems such as clogging and excessive recovery of aquifer material.
Adequate development for the take of water
Injecting sediment-laden water into injection bores may result in the accumulation of particles outside the screen or within the gravel pack. The mixing of the injected water with the groundwater may cause mineral scaling if their water chemistries are not compatible.
Potential problems can be avoided at the design stage by:
• full cement sealing of surface and production casing to prevent upward migration of injection water via the annulus (This applies to both single and multiple aquifer systems.)
• using wire wound screens to improve yields and efficiencies
• applying appropriate screen apertures to minimise screen velocities and reduce encrustation potential
• ensuring gravel pack material is compatible with formation material and injected water.
Managed aquifer recharge or aquifer storage and recovery schemes can consist of either single, multi-use bores (for both injection and extraction), or multiple specific purpose bore systems where different wells are used to inject and extract water.
Construction requirements are identical to those for a multi aquifer bore.