Borehole quality is generally related to the "smoothness" of the wellbore which manifests in a number of ways, all tied to the efficiency of the drilling process and drilling and well completion cost. Presently, several parameters, such as wellbore tortuosity, curvature, torsion, and various drilling indices, are used to quantify the well path or to estimate the difficulty of drilling a smooth well. Furthermore, there is no clear criterion for defining the quality of the wellbore. Drilling indices describe the quality of the hole more subjectively, rather than quantify the hole qualitatively. In some cases, they are used solely as a measure of how difficult it is to drill the well, rather than how smoothly it was drilled.

Another important parameter that is neglected in the calculations is wellbore torsion. Wellbore torsion depicts the rotating rate of the binormal vector with respect to curved length, or the measure of the rate at which the osculating plane changes its direction. It ensures a smooth well path and reduces the drag and torque in extended reach and ultra-extended reach wells. Because there is no industry standard for quantifying these parameters, a great deal of confusion exists between them, and they are used interchangeably and loosely without any proper substantiation and understanding. This paper provides the details of these parameters, as well as a clear definition and context in which they can be used. The simple guidelines are presented with several example calculations. This paper also provides methods for evaluating these parameters applied on a smooth well path in relative and absolute terms. An additional mathematical criterion based on non-linear curve modeling of a thin elastic beam known as the minimum energy for measuring the borehole quality is presented in this paper.

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