A well drill is a system used to drill an artificial hole in the ground to access underground liquids. While the purpose of all well drills is essentially the same, the methods of breaking through various geologic strata may vary. A water well drill may use any of three or more methods — rotary drilling, down-hole hammer, or cable tool drilling — to penetrate sand, shale, clay, and rock to reach an aquifer or underground water source.
When using a rotary well drill, a well driller attaches a bit made of durable material such as tungsten to the end of a drill string or series of pipes extending from the well head on the surface to the bottom of the well shaft. Rotary motion is supplied to the bit via the drill string by a motor at the well head and, as the bit is rotated, it breaks rock or shale into small pieces. These small pieces, sometimes called cuttings, are brought up and out of the hole by circulating drilling fluid down the drill string pipes and up the outside of the well drill system to the surface.
The well drilling bit and pipe are cooled by the passage of drilling fluid, and the walls of the well shaft drilled through sand or silt are stabilized, preventing cave-ins. When aquifer water is encountered, the clear water passes up the shaft, permitting the well driller to identify the depth of the top of the water source. Generally, drilling continues beyond the first access to water, often as much as 100 feet (about 30.5 m) or more, to allow for fluctuations in the water table.
In areas of extensive hard rock, a down-hole air hammer is often the well drill of choice. An air line is run down the shaft, and compressed air provides power for the down-hole air hammer. The compressed air also serves to help blow debris or cuttings out of the hole.
The cable-tool well drill has typically been replaced by the rotary well drilling system. In cable-tool drilling, a heavy bit is attached to a strong cable, and the bit is raised and dropped repeatedly, pounding its way through subterranean rock strata. Debris is flushed out of the hole periodically with a water hose.
In areas where technology is limited and water tables are near the surface, a driven point or sand point may be used. A sand-point well drill is simply a series of short pipes joined with threaded ends terminating in a hardened steel point with a section of reinforced screen above the point. The screen permits water to be pumped out while blocking sand. The entire apparatus resembles a javelin or spear.
The drive pipe is simply short sections of threaded steel pipe that connect as the sand point is driven into the sand with a post driver raised and dropped by the well driller. Sand point well drills are restricted to use in soil that is essentially free of rock. Despite ease of use, the sand point has limited water flow and is primarily for residential applications. This technology has been widely used in the United States where conditions permit and is still widely used in many developing nations.