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24.2 Calculating Tortuosity
If we create an additional column in a spreadsheet to hold the changes in dls from one survey to the next, we can use that to calculate unwanted curvature induced in the wellbore. We do not need to know the well plan since any well plan will require constant curvature in each section and we are measuring the deviations from constant. By multiplying the change in dls (converted to degs/ft or degs/m) by the measured depth difference, we can simply add up the unwanted curvature in the wellpath. If the dls is unchanged, this process accumulates zero unwanted curvature.
The Unwanted Curvature Method
Unwanted curvature is the sum of (dls change x md change). In the example above the unwanted curvature accumulation looks like this:
Figure 112: Unwanted curvature accumulation graph.
The design total curvature was only 27 degrees and the unwanted curvature is an additional 54 degrees or 200%. Anything over 100% would be considered tortuous and likely to cause casing and pipe problems.
Simplified Tortuosity Index
Considerable effort has gone into defining a tortuosity ‘ ndex’. There are several options for this and, of course, and no industry standard exists. But the following method can be useful in doing a like-for-like comparison between wells surveyed with similar intervals. The more frequently a well is surveyed, the greater the definition of the well position but also every twist and turn is highlighted, so it is not helpful to compare wells less frequently surveyed against wells surveyed at high resolution because the longer intervals can hide a multitude of sins.
A simple approach
Add up Total Vertical Curvature and Divide by 90° Add up Total Lateral Curvature and Divide by 90° Tortuosity Index = sqrt((TVC/90) ^2 + (TLC/90) ^2)
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