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An operation is made to determine the angle from which a bit has deviated from the vertical during drilling.

There are two basic deviation-survey instruments: one reveals the drift angle; the other indicates both the angle and the direction of deviation.

Deviation problems are much more severe and are often related to geologic structure, hole size and bottom hole drilling assembly clearances. Little is known about precisely which rock characteristics cause holes to deviate. Deviation generally becomes harder to control as rocks become harder due to the nature of the rocks.

In soft to medium-hard rocks, deviation is often the result of sidewise rather than frontal drilling by the bit. Changes in rock strength, erosion of the borehole wall, and perhaps other effects tend to cause rather abrupt changes in hole deviation. These changes may or may not be observed on deviation surveys.

The large majority of wells tend to drift up dip. In harder rocks, it is always desirable to plan deviation limitations to allow the maximum tolerable displacement. If the dip direction is known, deviation limitations can be relaxed or tightened accordingly to allow maximum penetration rates.

 
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