Contents preface (VII) introduction 1—37

4.9.6. Direct Rotary Drilling
Direct rotary drilling is the fastest method of drilling deep wells of diameters of up to 45 cm (or more with the use of reamers) through unconsolidated formations. The drilling bit is attached to a heavy drill pipe which is screwed to the end of the kelly which is a drill pipe of square section (Fig. 4.11). The drill collar or stabilizer helps in maintaining, straight hole in soft formations through its large wall contact. The drill pipe is turned by a rotating table which fits closely round the kelly and allows the drill rod to slide downward as the hole deepens. The drilling rig consists of a mast, a rotating table, a pump, a hoist, and an engine. The borehole is drilled by rotating a hollow bit attached to the lower end of a string of a drill pipe. Cuttings are

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Kelly

Drill pipe

Drill pipe
Drill collar
Bit

Fig. 4.11 Drill string for rotary drilling
removed continuously by pumping drilling fluid (a mixture of clay and water with some additives to make it viscous) down the drill pipe and through the orifices in the bit. The drilling fluid then flows upward through the annular space between the drill pipe and the borehole, carrying the cuttings in suspension to the surface settling pits where the cuttings settle down in the pits. The clear drilling fluid is pumped back into the borehole. The settling pits can either be portable or excavated for temporary use during drilling and then backfilled after completion of the well. Usually no casing is required during drilling because the drilling mud forms a clay lining on the borehole walls which prevents the formation materials from caving in. After drilling, the casing pipe with perforated sections opposite the aquifers is lowered into the borehole. The drilling rotary method has become the most common due to its following advantages (6):

(i) Drilling rates are relatively high.

(ii) Minimum casing is required during drilling. (iii) Rig mobilisation and demobilisation are fast.

(iv) Well screens can be set easily as part of the casing installation.

Some of the major disadvantages of the direct rotary method are as follows (6):

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(i) Drilling rigs are expensive. (ii) It is costly to maintain them.

(iii) The mobility of the rigs is restricted depending on the slope and wetness of the land surface.
(iv) The collection of accurate samples requires special procedure.
(v) The drilling fluid may cause the plugging of some aquifer formations.