QUINCE M PRO - WELL DRILLING

How to drill a well

Great skill is needed to
guide and control a water well drill

QMP equipment

Two methods typically used for drilling water wells are rotary and air hammer.

WHAT IS WHELL DRILLING?

Great skill is needed to guide and control a water well drill as it penetrates sand, gravel, clay and solid rock formations deep underground. The drill rods can weigh several tons; if the drill pushes too hard or turns too fast, the drill bit will wear out; if it does not push hard enough, it won't penetrate the rocks. There are often several rock layers in a single well; each may need different drilling pressures. Once water is encountered, the driller will need to keep a close watch on the drilling process.

For ground water monitoring, or for scientific research purposes, wells may be drilled in a way that allows the specialists to closely examine the rock formations and take frequent water samples. Two methods typically used for drilling water wells are rotary and air (DTH) hammer. There are other methods, and combinations of methods, but these are the most common.

ROTARY DRILLING

In rotary drilling, a drill bit is attached to a length of connected drill pipe. The drill bit will be made of tough metals such as tungsten, and as the drill is rotated, the bit acts to grind up the rock. The broken pieces (cuttings) are flushed upward and out of the hole by circulating a drilling fluid (sometime called drilling mud) down through the drill pipe and back to the surface. This drilling fluid also serves to cool and lubricate the drill bit, and by stabilizing the wall of the hole, it can prevent possible cave-in of unstable sands or crumbly rock before the well casing or well screen is installed.

As the drill intersects water bearing rock formations water will flow into the hole. Most drillers carefully monitor the depth of water "strikes" and keep a note of the formations in which they occur. In areas of hard rocks many drillers prefer to use a well drilling technique that uses compressed air to operate a down-hole air hammer on the end of the drill string that helps to break up the hard rocks. The compressed air also blows the crushed rock fragments out of the hole to the surface along with any water that flows in the well during drilling.

Down-the-hole drilling (DTH)

Down-the-hole drilling (DTH) essentially involves a drilling hammer at the bottom of a drill string. It relies on three elements for drilling holes: bit loading (weight), rotation and air. These active elements combine to be efficient at crushing rock. As the drill string slowly rotates, the drilling hammer is forced into the rock repeatedly. Striking power is provided by compressed air driving a piston inside the hammer.

The DTH hammer is pneumatically powered, with the compressed air propelling it forward to impact and fracture rock. Compressed air also travels through the drill bit into the hole (air exhaust), which blows the chips and dust out of the hole.

DETAILS:

No matter which method of drilling is used, the top part of the well is usually lined with steel or plastic well casing.
The diameter of the drilled hole is usually an inch or two wider than the diameter of the casing. The space between the drilled hole and the casing (the annulus) has to be filled to prevent the chance of polluted surface water from migrating downward along the outside of the casing where it might contaminate the aquifer.
This filling is called "grout" and (depending on local regulations) it may be either cement or special volcanic clay called bentonite. Sometimes most of the space is filled with the fine rock pieces from drilling and then the top 20 feet is grouted. The design of the well and its completion after the hole is drilled will be the subject of the next article in this series. A modern water well is much more than a hole in the ground.