The Process of Drilling Boreholes: Techniques, Challenges, and Considerations

Large drilling rigs are equipped to ensure that a borehole is started true and vertical. Maintaining verticality and straightness can be difficult during the early stages of drilling, but as the drill string weight increases, this problem tends to dissipate unless highly heterogeneous drilling conditions are encountered (in the form of boulders or cavities). Straightness is particularly important for water boreholes in which long strings of casing and screens may have to be installed with a gravel pack filter.

As drilling proceeds, drill pipes are screwed together. This allows tools and pipes to be rapidly attached and screwed together on the rig. A blast of air is sent through each pipe to remove blockages, and the string is tightened with heavy-duty spanners on the rig. Taller drill masts can obviously handle longer drill pipes – six meters is the normal length, except for smaller rigs – which speeds up bit lowering (tripping in) and raising (tripping out) times.

Reaming – the enlarging of an existing hole – can be carried out either with a drill bit of any kind or with specially designed reaming tools. Drilling companies often devise tools for special use, sometimes in the field, and they are often very ingenious. It should be borne in mind that after drilling has begun, the sides of the upper part of the borehole are likely to suffer erosion by circulation fluid and cuttings, which causes an irregular enlargement of the borehole, reducing up-hole fluid (air or mud) velocity. This can be dealt with by installing conductor pipe as described below.

As drilling proceeds, the amount of water leaving the borehole will – it is hoped – be seen to increase, reaching a point at which it becomes clear that the borehole will provide the required supply. Even then, the borehole may have to be deepened further to provide sufficient pumping
drawdown. However, if the borehole is found to be wanting, it may be advisable to stop drilling early (unless a hand-pump is acceptable at that location) or carry on in the hope of a greater water strike (here some knowledge of local geology would be very useful). If fragments of basement rocks start
appearing in the cuttings, and the penetration rate decreases significantly, the ‘hydrogeological basement’ has, in all likelihood, been reached and it would probably be futile to continue to deepen the hole.

Penetration rate through each zone or formation in the borehole may be determined simply by timing the progress of one drill pipe or a fixed distance marked by two chalk marks on the drill pipes as they pass through the table. Penetration rate can provide an estimate of formation consolidation or hardness, and also show precisely when an aquifer was crossed.

The question, then, is: When to stop drilling?
The supervisor normally has an idea, from the project specifications, of how much water is required from a borehole; a hand-pump, for instance, does not demand a large supply (0.5 litre/sec is more than enough), whereas pump supplying a storage tank for a village, a refugee camp, or a facility such as a school requires a significantly greater yield. When drilling is finally stopped by the supervisor (who normally bears this responsibility), it is advisable to allow a few minutes for the water level in the borehole to recover and to then measure it with a cable dip meter.

Field hydrogeologists and water engineers working on borehole drilling projects in the commercial or humanitarian sectors are most likely to encounter rotary drilling machines (of whatever size) using mud circulation or compressed air.

This Article is limited to those techniques most commonly used in water borehole drilling: mud rotary and air rotary, as cable-percussion drilling, auger drilling, and other methods are becoming increasingly rare.