Relationship Between Dip Angle and Deviation Force

Based on the preferential chip formation theory, the graph shown below has been derived from experimental work.

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Calculating Dip Angle

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Figure 5-49: Maximum deviation force as a function of formation dip.

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Figure 5-49: Maximum deviation force as a function of formation dip.

The effective dip angle is the angle at which the bit strikes the bedding plane. The graph predicts that when the effective dip angle is less than 45°, the direction of the deviation force is up-dip. When the effective dip angle is greater than 45° the direction of the deviation force is down-dip.

The meaning of up-dip and down-dip is illustrated in Figure 5-50. In practice, it has sometimes been observed that an up-dip tendency is observed at dip angles as high as 60°.

Down dip

Up dip

Figure 5-50: Up-dip versus down-dip.

Up dip

Figure 5-50: Up-dip versus down-dip.

The unwanted deviation in vertical wells has borne out the predictions of the graph shown in Figure 5-49. Drilling through alternately hard and soft formations with low dip angles, using a well stabilized bit and weights high enough to cause collar flexure, usually results in a course perpendicular to the bedding planes.

Figure 5-51 illustrates the tendency of the bit to deviate in the up-dip direction when the formation dip angle is low.

Figure 5-51 illustrates the tendency of the bit to deviate in the up-dip direction when the formation dip angle is low.

Dip Angle Drilling

Figure 5-51: At low dip angles, deviation tendency is up-dip.

The formation attitudes will have a similar effect on directional tendencies. For dip angles less than 45°, if the direction is up-dip, the bit will tend to maintain direction, but build angle. If the borehole direction is left of up-dip, the bit may tend to walk to the right; whereas if the direction is right of up-dip the bit tends to walk to the left. Both phenomena are just special cases of the up-dip tendency.

When the formation dip angle is greater than 60°, the usual tendency of the bit is to drill parallel to the bedding plane or down-dip.

Figure 5-52: At high dip angles, deviation tendency is down-dip.

In cases where the dip angle is greater than 60°, if the hole direction is right of down-dip direction then the bit tends to walk to the left. If the hole direction is left of down-dip direction, the bit tends to walk to the right. Again, these are simply special cases of the down-dip tendency.

There will be no deflection of the bit caused by the formation at 0° or 90° dip. This is because the bit is cutting into a structure that is essentially uniform and is constantly cutting into the same layers at the same time or constantly drilling between layers.

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Responses

  • Abdullah Robertson
    How does dip angle affects borehole deviation?
    4 years ago
  • sophia
    Why is angle of deviation formed?
    3 years ago
  • christian
    When dip angle is greater than 45 bit tends to drill parallel to bed?
    2 years ago

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