Drilling Of Boreholes
For many people drilling is only the activity above the ground that they can see and observe. A water borehole is however, a specially engineered hole in the ground, making provision for water to flow into this hole and allowing for a pump to be installed inside the hole to allow abstraction of water. Part of this engineered design is to prevent the surrounding geological formations to collapse and closing the borehole. This could mean cost and energy to re-drill or clean the hole to be able to use it again.
The task of the driller is to drill and construct a borehole, not to get water. This task should be left to the scientist dealing with the nature and study of the geological formations. None can however, operate in isolation and it important to see the actions of the scientist and driller as a team effort.
There are very few people that take time to consider how the drillers manage to keep a vertical borehole, which went through sand and hard rock, open until the casing is installed. How does one manage to remove the drilled pieces of rock from deep below the surface? Skill is needed to guide and control a water well drill as it penetrates sand, gravel, clay and solid rock formations underground. The drill rods are quite heavy and can weigh several tons; if the drill pushes too hard or turns too fast, the drill bit will wear out too fast; if it does not push hard enough, it won’t penetrate.
Quite often several rock layers are encountered in a single borehole, each requiring different drilling pressures and speeds. When water is encountered the drilling becomes even more complicated and the driller needs to understand exactly what forces need to deal with to further penetrate.
Two methods are typically used in South Africa for the drilling of water boreholes, these being the (mud) rotary and air percussion methods. A third method the cable tool method used to be quite popular, but due to the time it takes to drill a hole with this technique it is not quite as popular anymore. There are other methods, and combinations of methods, but these are the most common. Most home wells are drilled to 6 or 8 inches in diameter. Municipal or irrigation wells are likely to be drilled at larger diameters, sometimes as much as 24 inches or more.
The (Mud) Rotary TechniqueIn 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 grind up the rock. The drill fluids (sometimes referred to as drilling mud) are circulated through the drill string into the borehole and back to the surface and carries (flush) the broken pieces (cuttings) upwards and out of the hole. This fluid also serves as a formation stabilizer preventing possible cave-in of unstable sands or crumbly rock before the well casing or well screen is installed. Furthermore, this fluid acts as drill bit lubricant. As the drill intersects water-bearing rock formations water will flow into the hole. Drillers or geohydrologists on site will carefully monitor the depth of water “strikes” and keep a note of the formations in which they occur.
Air Percussion TechniqueThis technique utilizes compressed air to operate a down-hole air hammer on the end of the drill string that helps to break up the rock formation. This technique is currently the most commonly used technique in Southern Africa and many successful boreholes have been drilled applying this technique. The compressed air that is used to operate the down-hole air hammer also blows the crushed rock fragments out of the hole to the surface along with any water that flows into the hole during drilling. This has many times lead to the believe that this is of course not (** always) true as it is the compressed air that forces the water out of the borehole and not necessarily the relief of water pressure that allows water to freely flow on the surface.
** Artesian wells do exist where after completion of drilling, water carry on flowing on the surfaces specific application.