Drilling a borehole creates a hole in the ground — but it does not automatically create a functioning well. Before a borehole can be put into productive use, it must be developed. Borehole development is the process of removing fine particles, drilling debris, and damaged formation material from the immediate vicinity of the borehole wall and screen. Done properly, it increases hydraulic conductivity, improves water clarity, and maximises the long-term yield of the well.
Why Development Matters
During drilling, the borehole wall becomes clogged with drill cuttings, mud filtrate, and disturbed formation material. In rotary mud drilling, a filter cake forms against the formation that actively reduces water inflow. Even in air-drilled boreholes, fine particles migrate into the gravel pack and screen slots during construction. If these materials are not removed, the borehole will deliver less water than the aquifer is capable of providing, and it will deteriorate faster over its operational life.
Development works by reversing or alternating the flow of water across the borehole screen, dislodging fine particles and carrying them to the surface for removal.
- Surging
Surging is one of the oldest and most effective development methods. It involves rapidly moving a tight-fitting piston or surge block up and down inside the borehole casing. The back-and-forth motion creates alternating pressure surges that push and pull water through the screen and gravel pack, loosening fine material and drawing it into the borehole where it can be bailed or pumped out.
Surge blocks can be solid (closed-face) or valved (open-face). Valved surge blocks allow water to pass through on the downstroke, intensifying the surging action on the upstroke. The process is repeated across different depth intervals, progressively working from the bottom of the screen upward.
Surging is particularly effective in unconsolidated formations such as alluvial sands and gravels. It is low-cost, requires no specialised equipment beyond the surge block and a cable or drill string, and can be highly effective when carried out systematically with regular bailing between surge cycles.
- Jetting
Jetting uses high-velocity water directed through a nozzle to break up bridging and compaction around the borehole screen. The jetting tool is lowered into the borehole and rotated while water is pumped at high pressure through nozzles aimed horizontally at the screen perforations and surrounding gravel pack.
The hydraulic force disrupts fine particle bridges that have formed across screen slots and penetrates further into the formation than surging alone. Jetting is often combined with simultaneous pumping from the borehole, so that dislodged material is immediately removed rather than allowed to resettle.
This method is especially useful in boreholes with fine-grained formations where surging may not be aggressive enough, and in rehabilitation of older boreholes where screen blockage is a primary problem. The equipment requirements are more substantial — a high-pressure pump, rotary jetting tool, and swivel assembly — but the results in problem formations are often superior.
- Air Lifting
Air lifting introduces compressed air directly into the borehole below the water table. The air mixes with water, creating a buoyant air-water mixture that rises rapidly to the surface, drawing formation water behind it and generating a vigorous flow across the screen and into the borehole.
The surge created by intermittent air injection — pumping air in pulses rather than continuously — is particularly effective at dislodging fine material. Air lifting can be combined with a swabbing tool or with jetting for a more aggressive development programme.
Air lifting has a major practical advantage: it does not require submersible pump equipment in the hole during development, making it simple to observe and control. It is widely used in air-drilled boreholes where a compressor is already on-site from the drilling operation, making it a cost-efficient first development step.
Combining Methods for Best Results
In practice, the most effective borehole development programmes combine two or more of these methods. A typical sequence might begin with air lifting to clear the bulk of drilling debris, progress to surging to break up compacted zones, and finish with jetting to clear stubborn screen blockages. Between each stage, the borehole is pumped to remove mobilised fines, and the turbidity of the discharged water is monitored. Development is considered complete when the water runs clear and the yield stabilises.
The investment in thorough borehole development directly determines the operational performance and lifespan of the well. Skipping or shortcutting this phase is a common and costly mistake.
