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While drilling a pilot hole by mud rotary rig many problems and difficulties encounters in different types of Hydrogeological formation as the depth and diameter of hole increases.  These problems can be overcome by the driller on job if, he takes few precautionary measures and applies his wisdom intelligently.  Generally, the following main problems are faced in mud drilling:

a) Lost circulation

b) Verticality of hole

c) Sticking of drill rods

d) Fishing

Circulation of drilling fluid ii is losse1 generally in high porosity aquifer. There are 3 types of formation which absorb all or part of circulating fluid from minor to extreme con1itions they are:
  • Formations which contains joints and fissures such as quartzite, sand stone,    lime stone, delomite etc. These types of formations causes total loss of circulation.
  • Shales and clays which are jointed absorb the circulating fluid and causes well and filling of the hole.
  • Sand and gravel seldom absorb enough of drilling fluid.
  • Regaining of lost circulation:To regain the lost  circulation is to drill through the zone of lost circulation and set casing below  the highly porous zone.  After setting the casing, lot of water is circulated to saturate the zone further, this removes the bit cuttings which moves with water into the formation. Usually the circulation can be regained by mixing fibrous material with drilling fluid. In emergencycotton sheeds, husks, saw dust, mollases can be used to avoid the lost of fluid.
To drill a straight and vertical hole in line with plumb is the skill of the driller on job. A perfect vertical hole requires the following:

i.Proper levelling of rig

ii. Use of proper number and size of drill collars

iii. Rotational speed of rotary table

iv. Circulation of drilling fluid

Driller on job should see that the rig is levelled properly, proper size of drill collar is used which holds the drill pipes centrally and puts required weight on bit, the rotation of rotary table selected to such rpm (150 or more) that bit cuts the formation straight own preventing rotation of bit to one side. The usual method of straightening of crooked holes, is to ream the hole to largest possible diameters to give the well casing to align itself in the hole.

The sticking of drill pipe is common hazard in rotary drilling, therefore all precautionary measures should be taken before drilling is started. If the pipes are stuck up by balling up the same can be got freed by circulating fresh water and applying an upward strain on drill string.

When the pipes are stuck up by sand, gravel and accumulation of dill cuttings, fluid circulation in the bore hole should be maintained with the heaviest possible drilling mud. Lost circulation may also cause stucking or drill pipes which can be recovered with left hand drill pipes or by jacking.

Fishing tools are used for recovering of lost or broken tools from the bore holes. The recovery of lost tools from the well often requires a great skill and intelligence. The most usual fishing job in a rotary drilling is twisting off the dri1l pipe.  Fishing is done by designing of suitable appliance at site and by fishing taps and overshoots for the recovery.