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Drilling experience for the construction of water wells of substantial depth in various Hydro geological formation all over the country reveals that the bore hole may not be perfectly straight or perfectly plumbed although, it requires to be straight and plumb.

A straight well is one in which each part of casing section welded coupled to adjacent sections in such a manner that maintains a perfect alignment.

A borehole that is plumb is one whose center does not deviate from an imaginary vertical line running from the ground surface to the center of the earth.

A borehole may be straight but not plumb, if the borehole is plumb it will be straight.  In practice some tolerance of deviation is straightness and plumpness is permissible.  In a conventional drilling a deviation from plumpness of 2/3rd the wells inside diameter per 100 ft (30.5 mts) of an anticipated pump setting is permissible, considering various difficulties of drilling in different types of formation.

Out of two factors, the straightness of the bore hole is most important, because it gives an idea whether or not particular size of pump can be lowered up to the desired depth of a constructed well.  If, the well is out of alignment beyond a certain limit, the pump cannot be lowered.  Pump can be lowered in a well without any difficulty if, it is straight but out of plumb.  Too much deviation from the vertical may affect the operation and life of the pump, however, plumpness need not to be controlled within permissible limits.

The following are the four main causes for drilling a borehole which leads to become misalign and out of plumb;

Character of the sub surface formation i.e.

faults, inclines beds, boundary etc.

Too much or too little weight on the bit.

Straightness of drill pipe and casing used.

Amount of pull down force applied to the drill pipe in rotary drilling.

While gravitational forces tends to make the drill bit cut a vertical hole, the varying hardness of different materials being penetrated deflects the bit from a truly vertical course. In percussion drilling, a boulder may deflect the well casing, causing the hole to drift increasingly as the well is deepened.        In a well drilled by a rotary method if too/much force is applied on the drill string might bend the slender column of drill pipe.  This tends to cause the bit to cut off center.  Use of heavy drill collars in the lower part of drill stem help to put weight just above the bit, which overcomes the tendencyto drift off a true vertical course.  They are also more rigid then ordinary drill pipe, and thus help in keeping the lower part of the string straight.  Stabilizers should be used to keep the hole straight. Driller on brake should check the hole alignment several times when drilling a deeop hole.  Time and money can be saved by taking steps to correct the misalignment just as soon as a deviation is noticed.  In rotary drilling, the alignment is checked at reselected intervals during drilling i.e. every 50,100,200,500 mts.  In shallow wells, the alignment may be checked after completion of well.  Any variation in the straightness of casing results in a corresponding misalignment of the well.  Sections of pipe may be slightly bowed or the center line of the threaded or beveled ends may not exactly coincide with the center line of the pipe.  Commercial tolerance and allowances permit certain deviations in the straightness of pipe and accuracy of threads. In recent years several advanced computerized special deviation instruments have been developed to measure the misalignment that occurs during drilling.  A deviation survey is conducted by the experienced surveying personnel along with the standard suite of logs after the drilling is completed to the desired depth.  This technique is very expensive as far as water well drilling is concern but useful for oil and gas drilling.