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siwes report on borehole drilling

Bore hole drilling




Ground water Part One: Drilling of boreholes.
Drilling has over the years developed to a highly specialized and technical activity. Drilling with jumper rigs as in the olden days rarely takes place and most of the boreholes drilled are drilled with the air percussion technique. This technique is fast and efficient. However, when drilling a borehole for groundwater abstraction purposes, drilling becomes much more than just making a hole in the ground.
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.



Air percussion technique

This 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 surface s specific application.

Costs

The cost of a borehole can be significantly influenced by the applied design as well as the difficulty to construct a borehole in a specific geological formation. Many times people try to save on costs and therefore they will budget for the drilling of a borehole, but not for the casing of a complete hole. Installation of casing, which might in the short term seem costly, will almost always pay off in the long run. This casing will allow for the borehole to stay open for years after completion, and correctly installed it will also assist in keeping the borehole clean and free of material that could damage your borehole pump. Thus having a professional hydrogeologist to assist in determining the optimal construction for your borehole will prove to be more cost effective in the long run.

Borehole History

Borehole drilling has a long history. Han Dynasty China (202 BC – 220 AD) used deep borehole drilling for mining and other projects. Chinese borehole sites could reach as deep as 600 m (2000 ft).[3] The practice of well logging in boreholes dates to 1927, for the French Pechelbronn oil field.[4]

For many years, the world’s deepest borehole was the Kola Superdeep Borehole. From 2011 until August 2012 the record was held by the 12,345-metre (40,502 ft) long Sakhalin-I Odoptu OP-11 Well, offshore the Russian island Sakhalin.[5] The Chayvo Z-44 extended-reach well took the title of the world's longest borehole on 27 August 2012. Z-44's total measured depth is 12,376 m (40,604 ft). However, ERD wells are more shallow than Kola Superdeep Borehole, owing to a large horizontal displacement.