Wellbore tortuosity is an important metric of wellbore quality; however, it is not always an appropriate reflection of directional drilling performance. Drilling planned tortuous features will increase wellbore tortuosity, but this in itself says nothing about directional drilling performance. Not only is there a need for a metric of wellbore tortuosity, it isalso necessary to have a metric of "unplanned" wellbore tortuosity. The former provides information about wellbore quality, whereas the latteris reflective of directional drilling performance.

The directional drilling literature contains metrics for both wellbore and unplanned tortuosity; however, they are largely unique and difficult to relate to one another. It is desirable for the planned and unplanned tortuosity metrics to be relatable. This would allow operators to not only quantify overall wellbore tortuosity in real time, but also to understand how much of that tortuosity is unplanned and possibly avoidable. This, then, opens up avenues for directional drilling performance improvement.

In this paper, a new metric, the "Unplanned" Tortuosity Index is developed on the basis of an existing metric of wellbore tortuosity. This is done by systematically removing the effects of intended tortuous features from the wellbore tortuosity analysis, retaining only those tortuous features in the wellbore trajectory that are unplanned. The unplanned tortuosity index is then tested on two distinct sets of survey data from actual wells drilled. The results are compared between the sets and with the wellbore tortuosity metric from which the new index was derived. It is shown that thenewly developed unplanned tortuosity index canhelp operators and directional drilling companies discern their directional drilling performance, especially forwell paths with multiple, planned tortuous features.

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