Well Construction Methods and Materials
This domain tests whether you can match a drilling method and a completion to the ground you are actually in, and whether you can specify casing, screen and filter pack that will still be working in twenty years.
Methods and where they fit
- Cable tool handles boulders and caving formations and gives clean, undisturbed samples, but is slow.
- Mud rotary is the standard for sand, silt and clay; the mud wall-holds the hole and carries cuttings.
- Air rotary and down-the-hole hammer are the workhorses for bedrock. Watch for dust and for air lifting fine material.
- Hollow-stem auger suits shallow unconsolidated work where a temporary casing is useful.
- Casing advancer / duplex lets you case through cobbles and boulders as you drill.
Specifying the completion
Steel casing resists driving and formation damage; PVC is lighter and corrosion-resistant but needs care with torque and heat. Match the screen slot to the formation or filter pack - too wide and the well pumps sand forever, too narrow and you throttle the yield. Centralizers keep casing off the wall so grout forms a continuous seal instead of a crescent.
Worked example
A site has 55 ft of stony till over sound granite. Recommended completion: drill an 8-inch hole through the till with a casing hammer or air rotary, seat 6-inch steel casing about 5 ft into rock (roughly 60 ft), grout the annulus, then drill a 6-inch open hole to 300 ft with a down-the-hole hammer. If the hole makes 1 gpm, you do not promise 10 gpm - you report tested yield, recommend deepening or hydrofracturing, and let the owner decide.
Traps
Sizing screen to the pump rate instead of the formation; using PVC where driving is required; omitting centralizers; and reporting a drilled depth as if it were a proven producing depth.